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Catalytic Asymmetric Cyanation Reactions Nobuhito Kurono, and Takeshi Ohkuma ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02184 • Publication Date (Web): 28 Dec 2015 Downloaded from http://pubs.acs.org on December 30, 2015
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Catalytic Asymmetric Cyanation Reactions Nobuhito Kurono‡ and Takeshi Ohkuma*,† Division of Applied Chemistry and Frontier Chemistry Center, Faculty of Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan, and Department of Chemistry, Hamamatsu University School of Medicine, Hamamatsu, Shizuoka 431-3192, Japan
ABSTRACT: Catalytic asymmetric cyanations of prochiral unsaturated compounds affording the corresponding nitrile products in high enantiomeric excess (≥ 90% in general) are summarized in this review. The nucleophilic cyanide addition onto aldehydes, ketones, and imines is promoted by chiral metal complexes and organocatalysts. Recent progress in asymmetric conjugate cyanation of α,β-unsaturated carbonyl compounds is also discussed. The asymmetric cyanation of unactivated alkenes is catalyzed by chiral transition-metal complexes. Current topics of intramolecular carbocyanation and aminocyanation in addition to the traditional hydrocyanation are reviewed.
KEYWORDS: asymmetric cyanation, catalytic reaction, catalyst efficiency, chiral nitrile, cyanide reagent, enantioselectivity, metal complex, organocatalyst
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1. INTRODUCTION Catalytic asymmetric cyanations of prochiral unsaturated compounds affording a variety of enantiomerically enriched nitriles have been extensively studied in recent years due to the significant utility of the products, which are readily convertible to the corresponding chiral carboxylic acids, ketones, amines, and so on (Scheme 1).1–3 These reactions are roughly divided into two categories. The first consists of nucleophilic additions to polarized compounds, such as aldehydes, ketones, and imines, utilizing the acid/base catalysis (Scheme 1-1)).1,2 Conjugate cyanation of α,β-unsaturated carbonyl compounds is included in this category. The protocol of bifunctional catalysis (vide infra) is currently playing a vital role in the application of these cyanations.4 Trimethylsilylcyanide ((CH3)3SiCN) is frequently used as a cyanide source, and ethyl cyanoformate, acetyl cyanide, and acetone cyanohydrin are typical organo-cyanide reagents. Hydrogen cyanide (HCN) is the simplest reagent, and has been utilized in the industrial processes. However, because HCN is highly toxic and volatile, HCN formed in-situ by hydrolysis of (CH3)3SiCN or ethyl cyanoformate is often applied to these reactions rather than isolated HCN. As another safe alternative, non-volatile KCN is selected in some cases. Main group metal and early transition metal complexes as well as lanthanide compounds act as efficient catalysts. Homo and hetero bimetallic systems have also been devised. Addition of nucleophilic co-catalyst activating the cyanide source sometimes increases the reaction rate. Application of a variety of organocatalysts derived from peptides, (thio)ureas, cinchona alkaloids, etc. has notably improved and expanded this chemistry. Enzymatic cyanation reactions are important practical processes, although they are outside the scope of this review.5
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The other type of reactions are transition-metal catalyzed asymmetric cyanations of unactivated alkenes (Scheme 1-2)).3 Studies on these reactions are relatively rare yet, but hydrocyanation of styrenes as well as intramolecular carbocyanation and aminocyanation of nitrile compounds carrying alkenyl groups with high enantioselectivity have been reported. Nickel and palladium complexes with chiral phosphinite, phosphoramidite, or phosphine-based ligands are typical efficient catalysts. The H–CN, C–CN, and N–CN bonds are oxidatively cleaved by these metal complexes to add onto the less-polar olefinic moieties. This review covers enantioselective cyanations promoted by artificial (not enzymatic) catalysts affording the corresponding nitrile products in high enantiomeric excess (ee >90% in general). Previous reviews have elegantly presented the pioneering studies in this field.1–3 In the present review, the features of each catalytic reaction are introduced with a representative reaction scheme involving the catalyst or ligand structure. For the range of applicable substrates, the individual articles should be consulted. The catalytic efficiency is primarily estimated by the activity reflecting the amount of catalyst loading and enantioselectivity. Following the introductory chapter, we discuss the nucleophilic cyanation of aldehydes, ketones, and imines in Chapter 2. Among these cyanations, cyanosilylation has been the most intensively studied, although the Strecker-type reaction (hydrocyanation of imines) is also a major topic. In Chapter 3, recent progress in the asymmetric conjugate cyanation of α,β-unsaturated carbonyl compounds is disclosed. Finally, enantioselective cyanation of unactivated alkenes is discussed. The current topics of intramolecular carbocyanation and aminocyanation in addition to the traditional hydrocyanation are shown here.
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1) Nucleophilic cyanation of polar unsaturated compounds O
NC OX R1 ∗ R2
R2
R1
X NC NPG
cyanide reagent
NPG R1
R2
O
R1
chiral metal or organocatalyst
R1 ∗ R2
X = H, (CH3)3Si, C2H5OCO, CH3CO, etc. O NC R1 R2
Y
∗ R2
Y
ketones or carboxylic-acid derivatives cyanide reagent: NC OH R
R
(CH3)3SICN
HCN
O
KCN O
CN
EtO
etc.
CN
2) Cyanation of unactivated alkenes through oxidative bond cleavage HCN chiral Ni catalyst
Ar
NC H Ar ∗ R
CN Z
chiral Ni or Pd catalyst
CN
∗ n
Z
n
R Z = CH2, NCH3 Boc N CN
chiral Ni catalyst
Boc N ∗
CN
Scheme 1. Catalytic Enantioselective Cyanation 2. NUCLEOPHILIC CYANATION OF CARBONYL COMPOUNDS AND IMINES 2-1. Cyanosilylation of Carbonyl Compounds 2-1-1. Aldehydes In 1993, a pioneering work on the enantioselective cyanosilylation of aldehydes using a chiral titanium catalyst was reported by Oguni and co-workers (Scheme 2).6 Some aromatic and α,βunsaturated aldehydes reacted with (CH3)3SiCN with high enantioselectivity in the presence of a catalyst prepared in situ from Ti(Oi-Pr)4 and a chiral Schiff base. Introduction of the tert-butyl
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group on the phenolic moiety of the auxiliary was crucial to achieve high enantioselectivity. The reaction rate was remarkably enhanced by addition of the Schiff base to titanium alkoxide. The chemical structure of the chiral Schiff base–titanium alkoxide complex was discussed based on the 13C NMR spectra, field desorption mass spectra, and molecular weight measurement. The 1:1 chiral Schiff base–titanium alkoxide monomeric complex was considered to be the active species. Cyanide anion approaches the aldehyde coordinated to the titanium of the complex from the siface side to avoid the bulky tert-butyl group covering the carbonyl re-face.
O H
TMSCN (2 equiv) Ti(Oi-Pr)4 (20 mol%) chiral Schiff base (22 mol%) CH2Cl2, –80 ºC, 40 h
(H3C)3SiO CN H 68%, 96% ee (R)
chiral Schiff base:
N
OH
OH
H i-PrO N O Ti O Oi-Pr O H CN
R
proposed working model
Scheme 2. Asymmetric Cyanosilylation of Aldehydes with Chiral Schiff Base–Titanium Catalyst Following on from Oguni’s report, Yoshinaga and Nagata found that the catalytic activity was notably increased by using a partially hydrolyzed titanium alkoxide as a catalyst precursor (Scheme 3).7 The titanium species prepared from Ti(On-Bu)4, water, and a chiral Schiff base
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catalyzed cyanosilylation of aldehydes with high enantioselectivity in the low catalyst loading (0.2–1.0 mol%). Ti(OEt)4 and Ti(On-Pr)4 were also efficient titanium sources but bulky Ti(OiPr)4 was less efficient. Secondary alkyl aldehydes, 2-ethylbutanal and cyclohexanecarbaldehyde, as well as o-fluorobenzaldehyde were suitable for this catalytic reaction.
O
TMSCN (1.5 equiv) Ti(On-Bu)4 (0.2 mol%) H2O (0.15 mol%) H chiral Schiff base (0.2 mol%) CH2Cl2, 20 ºC, 2 h
(H3C)3SiO CN H 99%, 97% ee (S)
chiral Schiff base: N OH
OH
Scheme 3. Asymmetric Cyanosilylation of Aldehydes with Chiral Schiff Base–Titanium Catalyst Camphor-derived Schiff base–titanium complexes were investigated by Bosiak and co-workers. Among
five
ligand
candidates,
the
Schiff
base
prepared
from
2-hydroxy-3-
isopropylbenzaldehyde was the most efficient for this reaction (Scheme 4).8 An excellent enantioselectivity of 99% was achieved in the reaction of cinnamaldehyde.
O
TMSCN (2.3 equiv) Ti(Oi-Pr)4 (5 mol%) chiral Schiff base (20 mol%) H CH2Cl2, rt, 48 h
H
93%, 99% ee (S)
chiral Schiff base:
N OH
(H3C)3SiO CN
OH
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Scheme 4. Asymmetric Cyanosilylation of Aldehydes with Chiral Schiff Base–Titanium Catalyst Belokon' and North and their co-workers have investigated the use of chiral (salen)titanium complexes to induce the asymmetric addition of (CH3)3SiCN to aldehydes (Scheme 5).9 The optimal
catalyst
derived
from
(R,R)-1,2-diaminocyclohexane
and
3,5-di-tert-butyl-2-
hydroxybenzaldehyde had a dimeric structure of [(salen)Ti(μ-O)]2 in which the two Ti atoms were bridged by two oxygen atoms. Addition of water was crucial to generate the dimeric structure. The dimeric complex was more active than that prepared from (salen)TiCl2. The reaction of m-methoxybenzaldehyhde using 0.1 mol% of this complex quantitatively gave the cyanohydrin silyl ether in up to 92% ee at ambient temperature. The dimeric structure of a Ti complex derived from (R,R)-1,2-diaminocyclohexane and 2-hydroxybenzaldehyde was confirmed by an X-ray crystallographic analysis. O
TMSCN (1.1 equiv) H chiral Ti catalyst (0.1 mol%)
(H3C)3SiO CN H
CH2Cl2, rt, 1 h CH3O
CH3O
92% ee (S) chiral Ti catalyst: O
OO O N
O
Ti
Ti
O
O
N
N N
t-Bu
N
O = N
N
O
O
t-Bu
N t-Bu
t-Bu
Scheme 5. Asymmetric Cyanosilylation of Aldehydes with Chiral Salen–Titanium Catalyst The active dimeric species [(salen)Ti(μ-O)]2 reported by Belokon' and North equilibrated with the less active monomeric form. In order to stabilize the dimeric structure, Ding and co-workers synthesized a (salen)titanium complex in which the two monomeric components were linked by
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a cis-5-norbornene-endo-2,3-dicarboxylate bridge (Scheme 6).10 The linked complex acted as an excellent cyanation catalyst in terms of both activity and enantioselectivity. Various aldehydes, including aromatic, α,β-unsaturated, and aliphatic substrates reacted with (CH3)3SiCN under catalyst loadings of 0.01–0.02 mol% at ambient temperature to afford the corresponding products in 87–99% yields and in 64–97% ees. The catalyst was particularly effective for the reaction of aromatic aldehydes regardless of the presence of an electron-withdrawing or electron-donating group at either the para, meta, or ortho position of the aromatic ring. O
TMSCN (1.1 equiv) catalyst (0.01 mol%)
H
(H3C)3SiO CN H
CH2Cl2, 25 ºC, 0.5 h 99%, 96% ee (S) catalyst: N
N Ti O
O
O
t-Bu
O t-Bu
t-Bu O
O t-Bu
t-Bu O O
O
O
t-Bu
Ti N
N
Scheme 6. Asymmetric Cyanosilylation of Aldehydes with Dimeric Chiral Salen–Titanium Catalyst The in situ formed titanium complexes bearing pyrrolidine-based chiral salen ligands derived from natural L-tartaric acid were evaluated as catalysts in the reaction of aromatic aldehydes (Scheme 7).11 The catalysts with N-benzyl and N-cyclohexyl pyrrolidine ligands showed higher activity and selectivity.
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O H
TMSCN (2 equiv) Ti(Oi-Pr)4 (20 mol%) chiral ligand (22 mol%)
(H3C)3SiO CN H
CH2Cl2, –30 ºC, 24 h 97%, 92% ee (S) chiral ligand:
Bn N N
t-Bu
N
OH HO t-Bu
t-Bu
t-Bu
Scheme 7. Asymmetric Cyanosilylation of Aldehydes with Chiral Salen–Titanium Catalyst Choi and co-workers synthesized N-sulfonylated β-aminoalcohols with one or two stereocenters as chiral ligands. Titanium complexes prepared in situ from these ligands and Ti(Oi-Pr)4 efficiently catalyzed enantioselective cyanosilylation of aldehydes including benzaldehyde, 2-naphthaldehyde, cinnamaldehyde, and isobutylaldehyde (Scheme 8).12 Feng and co-workers used chiral β-aminoalcohol ligands prepared through reduction of Schiff bases for the titanium-catalyzed reaction (Scheme 9).13 The optimal complex showed high activity and enantioselectivity in the cyanation of aromatic aldehydes. The reaction pathway was proposed based on Oguni’s report6 shown in Scheme 2. Benzaldehyde coordinates to the titanium to avoid phenyl (complex)–phenyl (aldehyde) repulsion. The cyanide anion preferentially attacks the carbonyl si-face over the shielded re-face by the phenyl moiety of the complex.
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TMSCN (3 equiv) Ti(Oi-Pr)4 (10 mol%) chiral N-sulfonylated aminoalcohol (10 mol%)
O H
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(H3C)3SiO CN H
CH2Cl2, MS 4Å, –65 ºC, 48 h 93%, 96% ee (R)
chiral N-sulfonylated aminoalcohol:
O2S NH Cl
OH
OH Cl
Scheme 8. Asymmetric Cyanosilylation of Aldehydes with Chiral N-Sulfonylated βAminoalcohol–Titanium Catalyst
O H
TMSCN (2 equiv) Ti(Oi-Pr)4 (5 mol%) chiral aminoalcohol (5 mol%)
(H3C)3SiO CN H
CH2Cl2, –20 ºC, 22 h 99%, 94% ee (S)
chiral aminoalcohol:
NH
OH
OH CH3 H Ph
O Ph Ph
N O
CN
Ti
O Oi-Pr
CH3 proposed working model
Scheme 9. Asymmetric Cyanosilylation of Aldehydes with Chiral β-Aminoalcohol–Titanium Catalyst
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Shibasaki and co-workers devised a bifunctional chiral catalyst consisting of aluminum as a Lewis acid and phosphine oxide as a Lewis base moiety, which activated both aldehydes (substrate) and (CH3)3SiCN (reagent) (Scheme 10).14 Phosphine oxide moieties were introduced into the 3,3'-positions of the binaphthyl framework to avoid internal acid–base interaction with the aluminum part. Addition of tributylphosphine oxide was important to change the geometry of aluminum from tetrahedral to trigonal bipyramidal in which the phosphine oxide moiety with (CH3)3SiCN could be placed in a more favorable position to react with the aldehyde. Cyanosilylation of aromatic, aliphatic and α,β-unsaturated aldehydes with this catalyst afforded the products in excellent yields and enantioselectivities.
O H
TMSCN (1.8 equiv) chiral Al catalyst (9 mol%) Bu3P(O) (36 mol%)
(H3C)3SiO CN H
CH2Cl2, –40 ºC, 40 h 99%, 98% ee (S) chiral Al catalyst:
(CH3)3Si O Ph P Ph
Ph Ph P O O Cl Al O O Ph P Ph
CN R H
O
O
Al
O
Ph Ph P O
Cl O PR3 proposed working model
Scheme 10. Asymmetric Cyanosilylation of Aldehydes with Chiral Bifunctional Aluminum Catalyst
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A bimetallic (salen)aluminium catalyst in the presence of a phosphine oxide co-catalyst reported by North and co-workers also efficiently promoted enantioselective cyanosilylation of aldehydes (Scheme 11).15 Under optimized conditions, enantioselectivity of 96% was obtained by using 2 mol% of the catalyst in the reaction of m-anisaldehyde. An analysis of the reaction kinetics revealed that the reactions exhibited first-order kinetics in which the rate of reaction was independent of the aldehyde concentration.
O H
TMSCN (3 equiv) chiral Al catalyst (2 mol%) Ph3P(O) (10 mol%)
(H3C)3SiO CN H
CH2Cl2, rt, 24 h 96% ee (S) chiral Al catalyst:
t-Bu
t-Bu
t-Bu
t-Bu N Al N
N
O
O
Al
O O
O t-Bu
N
t-Bu
t-Bu
t-Bu
Scheme 11. Asymmetric Cyanosilylation of Aldehydes with Chiral Bimetallic Salen–Aluminum Catalyst Based on the mechanistic understanding of asymmetric cyanohydrin synthesis catalyzed by the chiral (salen)titanium complex, Belokon', North, and Parsons developed a chiral (salen)vanadium oxide complex (Scheme 12).16 The reaction of m-methylbenzaldehyde with only 0.1 mol% of the catalyst loading gave the cyanated product in 95% ee. Propylene carbonate, a green solvent, was also usable for this reaction.
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O H
(H3C)3SiO CN
TMSCN (1.1 equiv) chiral V catalyst (0.1 mol%) CH2Cl2, rt, 24 h
H
95% ee (S) chiral V catalyst:
t-Bu
N O N V O O t-Bu
t-Bu
t-Bu
Scheme 12. Asymmetric Cyanosilylation of Aldehydes with Chiral Salen–Vanadium Catalyst Uang and co-workers prepared a tridentate Schiff base ligand from tert-butyl substituted salicylaldehyde and inexpensive (S)-valine (Scheme 13).17 The vanadium oxide complex with the chiral Schiff base and the salicylaldehyde ligand catalyzed cyanosilylation of 2-naphthaldehyde in the presence of tetrabutylammonium fluoride (TBAF) to afford the product in 95% yield and 90% ee.
O H
TMSCN (2 equiv) chiral V catalyst (5 mol%) TBAF (0.5 mol%)
(H3C)3SiO CN H
CH3CN, –20 ºC, 24 h 95% 90% ee (S) chiral V catalyst: O N O O V O O O t-Bu H
t-Bu
Scheme 13. Asymmetric Cyanosilylation of Aldehydes with Chiral Schiff Base–Vanadium Catalyst
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Corey and Wang invented a sophisticated catalyst system (Scheme 14).18 The two kinds of chiral bisoxazoline derived from (S)-phenylglycinol were used as a pair of synergistic chiral reagents: The first one (bisoxazoline a) and HCN formed from (CH3)3SiCN and a trace amount of water provided the equivalent of a chiral cyanide source. The second one (bisoxazoline b) coordinated to magnesium chloride acting as the chiral Lewis acid. Thus, the enantioselective reaction may have occurred between the chiral cyanide source and the aldehyde activated by the chiral magnesium Lewis acid as shown in the scheme. Effective catalysis was shown in the reaction of aliphatic aldehydes such as heptanal. TMSCN (2 equiv) chiral bisoxazoline a (12 mol%) chiral bisoxazoline b– Mg complex (20 mol%)
O H
(H3C)3SiO CN H
CH2Cl2-CH3CH2CN (3/1), –78 ºC, 10 h
chiral bisoxazoline a:
88%, 95% ee (S)
chiral bisoxazoline b–Mg complex:
O
O N
CN O
O
N
N
N Mg Cl
CN O H
H O O
N N
N Cl N Mg
H Ph H
O
H
Ph N C
O H
R
proposed working model
Scheme 14. Asymmetric Cyanosilylation of Aldehydes with Chiral Bisoxazoline–Magnesium Catalyst
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Corey’s group later revealed that the oxazaborolidinium bistriflimidate described in Scheme 15 behaved as a highly enantioselective catalyst.19 The coordination of aldehyde onto the chiral oxazaborolidinium resulting in high enantioselectivity and was suggested to have several key features: (1) Nucleophilic attack on the formyl carbon is expected to occur at the si face because the opposite face is shielded by the neighboring π-eletctron-rich m-xylyl ring of the catalyst. (2) The complex structure was fixed with regard to rotation by using coordination links of the C=O•••B and the formyl C–H•••O hydrogen bond. Therefore, the absolute configuration of the silylated cyanohydrin was predicted to be R. Addition of a Lewis basic phosphine oxide activated (CH3)3SiCN and formed the corresponding isocyanide. The reaction of a variety of aromatic and aliphatic aldehydes was demonstrated and the silylated cyanohydrins were obtained in high yield and in >90% ee under mild conditions.
O H NC
TMSCN (1.1 equiv) oxazaborolidinium cat (10 mol%) Ph3P(O) (0.2 equiv) toluene, 0 ºC, 144 h
(H3C)3SiO CN H NC 99%, 94% ee (R)
oxazaborolidinium cat.: H Ar Ar CH3
N O H B Tf2N H3C
Ar = CH3 H3C CH3
H3C N H
B O H3C
O H CH3
R proposed working model
Scheme 15. Asymmetric Cyanosilylation of Aldehydes with Chiral Oxazaborolidinium Catalyst
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Kagan and Holmes studied cyanosilylation of aldehydes using fundamental chiral lithium phenolate catalysis (Scheme 16).20 The reaction of m-tolaldehyde conducted by a catalyst with a chiral salen structure afforded the cyanated product in 97% ee. The binaphtholate catalyst resulted in 59% ee. O H
TMSCN (1 equiv) chiral Li catalyst (1 mol%)
(H3C)3SiO CN H
Et2O, –78 ºC, 20 min 88%, 97% ee (R)
chiral Li catalyst:
N t-Bu
N
OLi HO t-Bu
t-Bu
t-Bu
Scheme 16. Asymmetric Cyanosilylation of Aldehydes with Chiral Lithium Phenolate Catalyst Ishihara and co-workers improved the generation method for the binaphthol–Li catalyst system (Scheme 17).21 A simple 1:1 mixture of (R)-BINOL and LiOi-Pr efficiently catalyzed asymmetric cyanosilylation of aromatic aldehydes. A transition state model consisting of the lithium binaphtholate, (CH3)3SiCN, and benzaldehyde was proposed. The aldehyde coordinates to the hypervalent silicon atom in a manner that avoids steric hindrance. The attractive π–π interaction between the phenyl ring of the aldehyde and binaphthyl plane of the catalyst was shown to be the major factor for high enantioselectivity.
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TMSCN (1.0 equiv.) (R)-BINOL (10 mol%) LiOi-Pr (10 mol%)
O H
(H3C)3SiO CN H
toluene, –78 ºC, 1 h
O
O 96%, 98% ee (S)
N C Me H H
Me
Si
O MeO O Li
R O H
O H R n–1
proposed working model
Scheme 17. Asymmetric Cyanosilylation of Aldehydes with BINOL–Lithium Catalyst Ohkuma and co-workers devised a highly active, robust, and enantioselective catalyst consisting of Ru(phgly)2(binap) and Li2CO3 (phgly = phenylglycinate) (Scheme 18).22 The reaction could be conducted with catalyst loading of 0.01 mol% to convert the aldehydes quantitatively into the silylated cyanohydrins in up to 98% ee. The ruthenium complex was recognized as a chiral template and combination with lithium cyanide formed the active catalytic species. The structure of the Ru–Li combined complex was determined by a single crystal X-ray analysis.
O Cl
TMSCN (1.2 equiv) Ru complex (0.01 mol%) H Li2CO3 (0.01 mol%) t-BuOMe, –78 ºC, 12 h
NC OSi(CH3)3 Cl
H 98%, 98% ee (R)
Ru complex:
O Ph2 O P
NH2
Ru
NH2
P Ph2 O O
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Scheme 18. Asymmetric Cyanosilylation of Aldehydes with Chiral Ruthenium–Lithium Combined Catalyst Duan and co-workers examined the catalyst efficiency of their homochiral porous metal– organic frameworks (MOFs) in asymmetric reactions (Scheme 19).23 The MOFs, Ce–MDIP1, were prepared through crystallization from a mixture of methylenediisophthalic acid (H4MDIP), celium nitrate, and L-N-t-butoxycarbonyl-2-(imidazole)-1-pyrrolidine as a chiral inducing agent in the presence of H2O and Et3N. The CD spectrum of bulk crystals of Ce-MDIP2, which were prepared from the opposite enantiomer of the chiral agent, exhibited Cotton effects that were precisely opposite those of Ce-MDIP1, indicating that the homochiral crystallization was achieved by the chiral induction. Ce-MDIPs worked as heterogeneous catalysts and exhibited high catalytic activity and enantioselectivity for asymmetric cyanosilylation of four aromatic aldehydes.
O
NC OSi(CH3)3
TMSCN (2.4 equiv) Ce-MDIP (2 mol%)
H
∗ H
CH3CN, –20 ºC, 24 h 98% ee
Ce(NO3)3•6H2O + HOOC
COOH
COOH
COOH
H2O, Et3N, 100 ºC, 3 d
Ce-MDIP
+ N Boc
N
N
Scheme 19. Asymmetric Cyanosilylation of Aldehydes with Chiral Metal–Organic Framework Catalyst
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A homochiral metal−organic framework (MOF) with an enantiopure 2,2’-dihydroxy-1,1’biphenyl unit was constructed by Cui’s group (Scheme 20).24 Replacing one proton of the phenoxy groups with Li cations made the MOF a highly efficient and recyclable heterogeneous catalyst for asymmetric cyanation of aldehydes with enantioselectivity as high as 99%. The catalytic activity and enantioselectivity of the MOF were enhanced over the original homogeneous system, especially at a low catalyst loading.21 The rigid framework seemed to stabilize the catalytically active monolithium salt of biphenol, avoiding the formation of inactive and/or less active assemblies in the reaction.
O H
TMSCN (1 equiv) chiral MOF-Li catalyst (0.5 mol%) H2O (0.15 mol%) toluene, –78 ºC, 45 min
N
NC OSi(CH3)3 H N 97%, >99% ee (S)
chiral MOF–Li catalyst:
OLi OH
Zn4O(O2C)6
Scheme 20. Asymmetric Cyanosilylation of Aldehydes with Chiral Metal–Organic Framework Catalyst 2-1-2. Ketones In 2000, Shibasaki and co-workers developed the first and highly enantioselective cyanosilylation of ketones by designing bifunctional catalysts (Scheme 21).25 The carbohydrate-
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derived chiral ligand A with a phosphine oxide moiety and Ti(Oi-Pr)4 efficiently catalyzed the reaction of 2’-acetonaphthone, providing the corresponding quaternary cyanohydrin derivative in 95% ee. The NMR studies suggested that the actual catalyst was a titanium monocyano monoisopropoxide complex with chiral ligand A which activated ketonic substrate with titanium as well as (CH3)3SiCN with the phosphine oxide. The titanium complex derived from chiral ligand B showed higher catalytic activity and enantioselectivity in the cyanation of ketones. The reaction of acetophenone using 1 mol% of the catalyst gave the product in 94% ee (10 mol%, 92% ee with ligand A). Steric hindrance of the benzoyl group on the ligand B was expected to reduce undesired coordination with the ketones, and the electron-withdrawing ability seemed to stabilize the reaction intermediate.
TMSCN (2 equiv) Ti(Oi-Pr)4 (10 mol%) chiral ligand A (10 mol%)
O
(H3C)3SiO CN
THF, –40 ºC, 80 h 82%, 95% ee (R) chiral ligand B:
chiral ligand A: Ph Ph P O
Ph Ph P O HO
HO
O
O
O
HO
HO
Me3Si CN Ph O Ph P
RL RS
O
O O O
Ti O CN
proposed working model
Scheme 21. Asymmetric Cyanosilylation of Ketones Catalyzed by Titanium Complexes with Carbohydrate-derived Chiral Ligands
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The titanium complex prepared from a partially hydrolyzed titanium alkoxide and a chiral tridentate Schiff base ligand catalyzed cyanosilylation of ketones in the low catalyst loading (Scheme 22).7 The reaction of aldehydes with this catalyst was discussed in the previous section.
O
TMSCN (1.5 equiv) Ti(On-Bu)4 (0.2 mol%) H2O (0.15 mol%) chiral ligand (0.2 mol%)
(H3C)3SiO CN
CH2Cl2, 20 ºC, 24 h 97%, 90% ee (S) chiral ligand:
N OH
OH
Scheme 22. Asymmetric Cyanosilylation of Ketones with Chiral Schiff Base–Titanium Catalyst Feng and co-workers developed an efficient tetraaza ligand, (2S)-N-{(1R,2R)-2-[(S)pyrrolidine-2-carboxamido]-1,2-diphenylethyl}pyrrolidine-2-carboxamide, for the addition of (CH3)3SiCN to ketones (Scheme 23).26 This tetraaza ligand was readily synthesized in two steps from commercial compounds. The titanium complex formed in situ seemed to have a monometallic Ti(Oi-Pr)2 structure with the chiral ligand. It could activate both ketones and (CH3)3SiCN with titanium (also an amino proton) and an appropriately positioned pyrrolidine nitrogen, respectively. The reaction of ketones was catalyzed by this complex with enantioselectivity as high as 94%.
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O
TMSCN (2.5 equiv) Ti(Oi-Pr)4 (30 mol%) chiral ligand (30 mol%)
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(H3C)3SiO CN
CH2Cl2, –45 ºC, 100 h 77%, 92% ee (S) chiral ligand:
O
O NH HN NH
HN
O
N O C
O N
NH
N H
Ti O
N O
Si proposed working model
Scheme 23. Asymmetric Cyanosilylation of Ketones Catalyzed by Titanium Complex with Chiral Amide Ligand Feng’s group also reported another bifunctional catalyst system composed of (S)-prolinamide, Ti(Oi-Pr)4 (Lewis acid), and phenolic N-oxide (Lewis base), which exhibited high catalytic efficiency in the enantioselective cyanosilylation of ketones (Scheme 24).27 In the presence of 2.5 mol% catalyst, a variety of aromatic ketones were converted into the corresponding cyanohydrin silyl ethers in high yields and enantioselectivities (up to 96%).
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TMSCN (1.5 equiv) Ti(Oi-Pr)4 (2.5 mol%) chiral ligand (2.5 mol%) achiral N-oxide (2.5 mol%)
O
(H3C)3SiO CN
THF, –45 ºC, 60 h 89%, 96% ee (R) chiral ligand:
achiral N-oxide: O N
N O Ph
N
OH
HN
NH Ph
Ph
OH O Ph
Scheme 24. Asymmetric Cyanosilylation of Ketones Catalyzed by Titanium Complex with Chiral Amide Ligand Shibasaki and co-workers devised a chiral gadolinium catalyst prepared from Gd(Oi-Pr)3 and a D-glucose-derived
ligand for asymmetric cyanosilylation of ketones (Scheme 25).28 The reaction
of acetophenone provided the corresponding (S)-cyanohydrin silyl ether in 92% ee. As described above, a titanium catalyst with the same ligand selectively afforded the R enantiomer in this reaction. The features of these two catalyst systems were quite different, although both utilized the same chiral ligand. The active species of the titanium catalyst seemed to be monomeric, but the gadolinium one consisted of gadolinium and the ligand in a ratio of 2:3. A bimetallic catalysis was proposed.
O
TMSCN (1.5 equiv) Gd(Oi-Pr)3 (5 mol%) chiral ligand (10 mol%)
(H3C)3SiO CN
THF, –40 ºC, 2 h chiral ligand:
92%, 92% ee (S) Ph Ph P O
HO O HO
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Scheme 25. Asymmetric Cyanosilylation of Ketones Catalyzed by Gadolinium Complex with Carbohydrate-derived Chiral Ligand The chiral gadolinium catalyst was applied to a 100 gram-scale reaction of cyclohexyl phenyl ketone, affording the cyanohydrin silyl ether in 94% ee without loss of enantioselectivity.29 The chiral ligand could be recovered with a silica-gel short column in the purification of the product. The enantioselective cyanosilylation of 3',5'-difluorophenacyl chloride was promoted by the gadolinium catalyst bearing a modified chiral ligand. The functionalized product was applied to the synthesis of several chiral medicines.30 Snapper and Hoveyda and their co-workers reported an aluminum-catalyzed reaction of ketones using a peptide-based chiral ligand (Scheme 26).31 A variety of aromatic (cyclic and acyclic) and aliphatic ketones (saturated and unsaturated) were converted to the cyanated products in high ees. The high level of asymmetric induction in the reaction of aliphatic substrates (3-none-2-one: 95%; 2-nonanone: 86%) was noteworthy. The chiral ligand had the benefits of being readily prepared and easily modified.
O
TMSCN (2 equiv) Al(Oi-Pr)3 (20 mol%) chiral ligand (20 mol%) CH3OH (20 mol%)
NC OSi(CH3)3
toluene, –78 ºC, 48 h, MS 3Å, 98%, 95% ee (R) chiral ligand: H3CO
N OH
O
H N
OCH3
N H
O O
O
NHTr
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Scheme 26. Asymmetric Cyanosilylation of Ketones Catalyzed by Aluminum Complex with Peptide-based Chiral Ligand Feng and co-workers utilized a bifunctional catalyst system composed of a chiral (salen)aluminum complex and an achiral N-oxide achieving high catalytic turnovers (200 for aromatic ketones, 1000 for aliphatic ones) (Scheme 27).32 A wide range of aliphatic and aromatic ketones were converted under mild conditions into the cyanation products in high ees (acetophenone: 94%; 1-tetralone: 90%; methyl isopropyl ketone: 90%). A double-activation catalysis model was proposed. (CH3)3SiCN is activated by the tertiary aniline N-oxide to form the hypervalent-silicon isocyanide, and it reacts with the ketone coordinated to the chiral (salen)aluminum complex on the less hindered-side.
O
TMSCN (2 equiv) AlEt3 (0.5 mol%) chiral ligand (0.5 mol%) N-oxide (0.25 mol%)
NC OSi(CH3)3 ∗
THF, –20 ºC, 48 h, 99%, 92% ee chiral ligand:
N-oxide: O N N
Br
N Br
OH HO H3C
N
O
C N Si
O Al O
O N
N
proposed working model
Scheme 27. Asymmetric Cyanosilylation of Ketones with Chiral Salen–Aluminum Catalyst
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Zhou and co-workers reported a tandem process involving a Wittig reaction that provided enone substrates and cyanosilylation using a chiral (salen)aluminum catalyst (Scheme 28).33a Phosphine oxide, a byproduct generated in the first Wittig step, was utilized to activate (CH3)3SiCN as a Lewis base, and the following enantioselective cyanosilylation of enones afforded the silylated cyanohydrins in up to 93% ee. The tandem transformation using meta- and para-substituted benzaldehydes showed high enantioselectivity. The ee value of the product was somewhat decreased in the reaction of o-chlorobenzaldehyde and n-butanal. Very recently they developed an improved ternary catalyst system for cyanosilylation of ketones consisting of (salen)aluminum chloride (10 mol%), phosphorane (10 mol%), and triphenyl phosphine oxide (50 mol%) through mechanistic studies on the above tandem reaction (Scheme 28).33b This catalyst system succeeded in the achievement, for the first time, of excellent enantioselectivity up to 95% in the reaction of linear aliphatic ketones and α,β,γ,δ-unsaturated ketones. They proposed a reaction mechanism according to their experimental observations and theoretical calculations: Phosphorane acts as an efficient Lewis base that interacts with (salen)aluminum chloride to form a cationic aluminum complex. The ketonic substrate, which coordinates to the electrophilic aluminum species, reacts with (CH3)3SiCN activated by phosphine oxide.19
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O O
H
PPh3 +
CH2Cl2, 80 ºC
Cl
(H3C)3SiO CN
TMSCN (2 equiv) (salen)Al cat. (5 mol%) CH2Cl2, –30 ºC, 2 d
Cl 93%, 93% ee (S)
O
TMSCN (2 equiv) (salen)Al cat. (10 mol%) O (10 mol%) Ph3P (H3C)3SiO CN Ot-Bu Ph3P(O) (50 mol%) CH2Cl2, –30 ºC, 72 h 90%, 95% ee (S) (salen)Al cat.:
N
N Al
t-Bu
O
Cl
t-Bu
t-Bu
O t-Bu R1
t-Bu Cl
N O t-Bu
Al O
Ph3P
R2
O
OTMS CN PPh3
N t-Bu
O t-Bu
R
proposed working model
Scheme 28. Tandem Process of Wittig Reaction/Asymmetric Cyanosilylation with a Chiral Salen–Aluminum Catalyst and Cyanation with an Improved Ternary Catalyst System Corey and Ryu found that the chiral oxazaborolidinium triflate with diphenylmethyl phosphine oxide was an excellent catalyst for the reaction of methyl ketones and (CH3)3SiCN (Scheme 29).34 The sense of enantioface selection of this reaction was the same as that in the case of aldehydes described in the former section. The cyanation of p-nitroacetophenone gave an enantioselectivity of 96%, higher than that of 83% with acetophenone itself. On the other hand, p-methoxyacetophenone was transformed to the product in 32% ee with the opposite sense of
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enantioselection. The reaction mechanism seemed to be similar to that of the aldehyde cyanation shown in Scheme 15.19 The ketonic substrate was fixed on the oxazaborolidinium with coordination links of the C=O•••B and the α-C-H•••O hydrogen bond. The π,π-interaction of the binded ketonic carbonyl with the neighboring π-electron-rich mexyl group of oxazaborolidinium was a major factor in determining the enantioselectivity. Then, the intermediate complex reacted with the in situ formed Ph2MePOTMS(N=C:).
O
TMSCN (1.1 equiv) oxazaborolidinium cat. A (10 mol%)
(H3C)3SiO CN
MePh2P(O) (0.11 equiv) toluene, 45 ºC, 10 d
O2N
O2N 83%, 96% ee (R)
O O O
TMSCN (1.1 equiv) oxazaborolidinium cat. B (10 mol%)
(H3C)3SiO CN O O
Ph3P(O) (0.11 equiv) toluene, 25 ºC, 48 h
92%, 96% ee (S)
oxazaborolidinium cat. A: R = CH3 cat. B: R = H H Ar Ar N O H B
CH3
TfO Ar =
R
CH3 H3C CH3
H3C N H
B
O
O TfO
H3C R
H
H CH H 3 C
N TMSO
CH3 Ph
Ph
proposed working model
Scheme 29. Asymmetric Cyanosilylation of Ketones with Chiral Oxazaborolidinium Catalyst
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Feng and co-workers reported a simple and highly enantioselective cyanosilylation of ketones catalyzed by phenylglycine sodium salt (Scheme 30).35 The corresponding lithium and potassium salts as well as proline sodium salt were much less selective. The reaction of benzalacetone in the presence of 30 mol% of the catalyst gave the cyanated product in 96% yield and 97% ee. The reaction was proposed to proceed through the hypervalent silicate intermediate.
O
TMSCN (1.5 equiv) i-PrOH (0.5 equiv) phenylglycine Na salt (30 mol%)
NC OSi(CH3)3 ∗
–45 ºC, 27 h 96%, 97% ee phenylglycine Na salt: NH2 COONa
Scheme 30. Asymmetric Cyanosilylation of Ketones with Phenylglycine Sodium Salt Ishihara and co-workers examined the catalytic enantioselective cyanosilylation of aromatic ketones using chiral lithium salts of (R)-BINOL-derived phosphoric acid compounds (Scheme 31).36 In the presence of 10 mol% of the chiral lithium salt, the corresponding tertiary cyanohydrins were obtained in high yields with moderate to high enantioselectivities (up to 86%). This catalyst was not appropriate for the aliphatic substrates. The reaction was proposed to proceed through a cyclic transition state in which the chiral lithium phosphate interacts with both ketone and (CH3)3SiCN. The direction of ketonic substrate is controlled in a manner that diminishes steric repulsion toward the phenyl group on the BINOL skeleton.
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TMSCN (1 equiv) BuLi (10 mol%) chiral phosphoric acid (10 mol%)
O
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(H3C)3SiO CN
toluene, –40 ºC, 6 h 96%, 86% ee (R) chiral phosphoric acid: Ph O
O P O
OH
Ph (R)-axis Ph
N O C O si-face attack O Li P Si O Ar Ph O proposed working model
Scheme 31. Asymmetric Cyanosilylation of Ketones with Chiral Phosphoric Acid Lithium Salt Ohkuma and co-workers demonstrated that the ruthenium complex–lithium phenoxide systems efficiently catalyzed enantioselective cyanosilylation of various ketones, such as simple aromatic ketones, α-keto esters, α-alkoxy ketones, and α,α- and β,β-dialkoxyketones (Scheme 32).37 The Ru(phgly)2(binap)–lithium phenoxide system showed high enantioselectivity for the reaction of acetophenone derivatives and α-keto esters to afford the cyanated products in up to 99% ee. The reaction features were similar to those of the aldehyde cyanation as discussed in the previous section. For the cyanosilylation of dialkoxy ketones and α-alkoxy ketones, the Ru(tleu)2(BINAP)–lithium phenoxide system exhibited the best catalyst performance to produce the cyanohydrin derivatives in up to 99% ee and 98% ee, respectively (t-Leu = tert-leucinate). The excellent catalytic activity resulted in complete conversion in the reaction with 0.01 mol% in the best cases.
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TMSCN (2 equiv) Ru complex A (0.1 mol%) LiOPh (0.1 mol%)
O
NC OSi(CH3)3
t-BuOMe, –60 ºC, 18 h 97%, 90% ee (R) TMSCN (2 equiv) Ru complex A (0.1 mol%) LiOPh (0.1 mol%)
O O
NC OSi(CH3)3 O O
t-BuOMe, –60 ºC, 18 h
O
97%, 99% ee (R) TMSCN (2 equiv) Ru complex B (0.1 mol%) LiOPh (0.1 mol%)
O O
H3CO
t-BuOMe, –40 ºC, 6 h
O
H3CO
NC OSi(CH3)3 O O 96%, 99% ee (R)
O R
Ph2 O P
NH2
Ru
NH2
P Ph2 O
Ru complex A: R = C6H5 Ru complex B: R = t-Bu
R O
Scheme 32. Asymmetric Cyanosilylation of Ketones with Chiral Ruthenium–Lithium Combined Catalysts A bifunctional thiourea–amine derivatives-catalyzed enantioselective cyanosilylation of ketones was developed by Jacobsen and Fuerst (Scheme 33).38 High enantioselectivities were obtained in the reaction of aromatic and α,β-unsaturated ketones. Some α-heterosubstituted ketones were cyanated with usable enantioselectivities. HCN formed in situ from (CH3)3SiCN and CF3CH2OH was necessary for the reaction to proceed. The mechanism of this cyanation was investigated using a combination of experimental and theoretical methods. The kinetic analysis was consistent with a cooperative mechanism in which both the thiourea and the tertiary amine of the catalyst were involved in the rate-limiting cyanide addition step. Density functional theory
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calculations indicated the most favorable transition structure involving addition of the aminebound HCN to the thiourea-bound ketone with two hydrogen bonds.
CH3O
O
TMSCN (2.2 equiv) CF3CH2OH (1 equiv) chiral thiourea (5 mol%)
CH3O NC OSi(CH3)3
CH2Cl2, –78 ºC, 36 h 96%, 98% ee (S) chiral thiourea: S
H N
Me
N H
O
N H N n-Pr Pr-n
Scheme 33. Asymmetric Cyanosilylation of Ketones with Chiral Thiourea Catalyst A cyanosilylation of α,α-dialkoxy ketones catalyzed by an organic chiral Lewis base, (DHQ)2AQN, was demonstrated by Deng and co-workers (Scheme 34).39 The catalyst was commercially available and recyclable. A series of α,α-dialkoxy ketones with substituents, such as aromatic, vinylic, acetylenic, and aliphatic groups, was converted to the cyanated products in >90% ee. Some amino alcohols with quaternary stereocenters were synthesized by using this reaction. O
NC OSi(CH3)3 O ∗
TMSCN (3 equiv) (DHQ)2AQN (2 mol%)
O O
O
CHCl3, –50 ºC, 18 h
96%, 98% ee (DHQ)2AQN:
N
N
O N
O
O
O
O
O N
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Scheme 34. Asymmetric Cyanosilylation of Ketones Catalyzed by An Organic Chiral Lewis Base Feng and co-workers reported that the bifunctional N,N’-dioxide compounds formed in situ catalyzed enantioselective cyanosilylation of α,α-dialkoxy ketones (Scheme 35).40 This catalyst seemed to activate both (CH3)3SiCN with two N-oxides and the ketone by hydrogen bonding with the amide proton.
TMSCN (2 equiv) chiral diamine (10 mol%) Ph m-CPBA (20 mol%)
O O O
Ph
NC OSi(CH3)3 Ph O ∗ O
t-BuOMe/DCE, –45 ºC, 10 h
Ph
Cl
Cl
99%, 93% ee chiral diamine: N
O
N
NH
O HN
O N H
N O
N
Si
O O
N
H
CN
O O
Ph
O Ph
proposed working model
Scheme 35. Asymmetric Cyanosilylation of Ketones with Chiral Diamine Catalyst 2-2. Carbocyanation 2-2-1. Carbonyl Compounds
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Shibasaki and co-workers developed an asymmetric reaction of aldehyde and ethyl cyanoformate with a catalyst system consisting of YLi3[tris(binaphthoxide)] (YLB), water, n-butyllithium, and [2,6-(CH3O)2C6H3]3P=O (Scheme 36).41 Achiral additives had a key role in the construction of a proper chiral environment of this catalyst system. No reaction was observed without water. A variety of aromatic, aliphatic, and α,β-unsaturated aldehydes were converted to the adducts in high enantioselectivity. The catalyst system was able to perform an asymmetric tandem reaction of the cyanation and nitro aldol reaction by tuning with the achiral additives in each step. O
O H
(1.2 equiv) NC OEt (S)-YLB (10 mol%) H2O (30 mol%) BuLi (10 mol%) [2,6-(CH3O)2C6H3]3P=O (10 mol%) THF, –78 ºC, 2 h
O NC O
OEt
H 88%, 98% ee (R)
O (1.2 equiv) NC OEt (S)-YLB (10 mol%) H2O (30 mol%) BuLi (10 mol%) O [2,6-(CH3O)2C6H3]3P=O (10 mol%) H THF, –78 ºC, 3 h
O NC O
OEt
H
98%, 93% ee (R) (S)-YLB:
O O Li
Y O
Li O O
O Li
Scheme 36. Asymmetric Reaction of Aldehydes and Ethyl Cyanoformate with YLB-Based Catalyst
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The optically active allylic cyanohydrin carbonates prepared from the asymmetric cyanation of α,β-unsaturated aldehydes were converted into the γ-oxy-α,β-unsaturated nitriles through [3,3]sigmatropic rearrangement without racemization but with partial E/Z isomerization. This method was applied to an enantioselective total synthesis of (+)-patulolide C. Detailed mechanistic studies suggested that the active species was a 1:1:1:1 mixture of YLB:H2O:LiCN:phosphine oxide derivative, and the initiation step to generate nucleophilic LiCN was rather slow. To accelerate the initiation step, the addition of a catalytic amount of acetone cyanohydrin was effective and completed the reaction within 9 min using 5 mol% of the catalyst at –78 °C. Feng and co-workers found that 10 mol% (S)-aluminum lithium bis(binaphthoxide) (ALB) with 10 mol% cinchonine catalyzed the asymmetric cyanoethoxycarbonylation of aldehydes to afford the cyanohydrin ethyl carbonates in high yields (up to 99%) with moderate to high enantioselectivities (up to 95%) under mild conditions (at –20 ºC) according to Shibasaki’s heterobimetallic catalysis concept (Scheme 37).42 The ALB prepared from LiAlH4 in THF poorly catalyzed the reaction. The use of solid ALB free of THF obtained from (S)-bi(2-naphthol), aluminum 2-propoxide, and n-butyllithium in dichloromethane, which was insensitive to air and moisture, was important to perform this reaction efficiently. Aluminum of ALB could activate aldehyde and cinchonine coordinated with the lithium could activate ethyl cyanoformate.
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O CH3O
(1.5 equiv) NC OEt (S)-ALB (10 mol%) cinchonine (10 mol%) CH2Cl2, –20 ºC, 1.5 h
O H
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O CH3O NC O
OEt
H 96%, 95% ee (S)
cinchonine:
(S)-ALB: Li O O
Al
O O
HO
N H N
Scheme 37. Asymmetric Reaction of Aldehydes and Ethyl Cyanoformate with ALB–Cinchonine Catalyst Belokon' and North and co-workers found that the bimetallic titanium complex [(salen)Ti(µO)]2, in which salen derived from (R,R)-1,2-cyclohexanediamine and 3,5-di-tert-butylsalicylaldehyde was the ligand, catalyzed the asymmetric reaction of aromatic and α,βunsaturated aldehydes and ethyl cyanoformate to give the cyanated products in high enantiomeric excesses (up to 99%) (Scheme 38).43 Aliphatic aldehydes were reacted with moderate to good enantioselectivities. This catalyst was also effective for the asymmetric cyanosilylation as discussed in the former section.
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O O
(1.2 equiv) NC OEt catalyst (2 mol%)
O H
NC O
KCN (10 mol%)
H
CH2Cl2, –40 ºC, 17 h
MeO
OEt
MeO 94%, 99% ee (S)
t-Bu
catalyst:
t-Bu
t-Bu t-Bu N
O Ti
N
O
O O
N
O Ti O
N
t-Bu t-Bu
t-Bu
t-Bu
Scheme 38. Asymmetric Reaction of Aldehydes and Ethyl Cyanoformate with Bimetallic Salen– Titanium Catalyst Moberg and co-workers reported that the bimetallic Lewis acid [(salen)Ti(µ-O)]2 combined with the amine Lewis base behaved as an efficient catalyst system for the addition of acetyl cyanide or ethyl cyanoformate to aldehydes, affording the enantio-enriched O-acylated or Oethoxycarbonylated cyanohydrins in high yields, although the titanium complex or amine alone poorly catalyzed the cyanation with acetyl cyanide (Scheme 39).44 The reaction of benzaldehyde, p-Me, p-MeO or p-Cl substituted benzaldehyde, cinnamaldehyde, and hexanal proceeded with >90% enantioselectivity, but sterically hindered pivalaldehyde was cyanated with a moderate selectivity. Experimental data supported a mechanism involving attack of the Lewis base on the carbonyl carbon of acetyl cyanide coordinating to titanium of the complex, thereby providing cyanide and the acylated ammonium compound reacted with the aldehyde activated by the Lewis acid.
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O (1.2 equiv) Me NC catalyst (5 mol%)
O
O NC O
Et3N (10 mol %)
H
H
CH2Cl2, –40 ºC, 6 h
H3C
Me
H3C 90%, 96% ee (S)
t-Bu
catalyst:
t-Bu
t-Bu t-Bu N
O
N
Ti
O
O
N
O
O
Ti
O
N
t-Bu t-Bu
t-Bu
t-Bu
(salen)Ti
O
O
O R'
Ti(salen)
CN
R
N proposed mechanism of addition of ketonitriles to aldehydes
Scheme 39. Asymmetric Reaction of Aldehydes and Ethyl Cyanoformate with Bimetallic Salen– Titanium Catalyst Feng and co-workers studied several Ti catalyst systems for the cyanoethoxycarbonylation of aldehydes. The (salen)titanium complex was already known to form the dimeric structure based on the report of Belecon’ and North. The dimerization was avoided by the addition of 2-propanol in the preparation of the titanium complex, and the obtained mononuclear complex catalyzed the asymmetric cyanation (Scheme 40).45 The cyanohydrin ethyl carbonates in 76–91% ees were obtained in the reaction of a series of aromatic, aliphatic, and α,β-unsaturated aldehydes with 5 mol% catalyst in a 1:4 mixture of 2-propanol and chloroform at –20 °C. A transition state model was proposed in which the aldehyde coordinates to the titanium complex to avoid steric
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repulsion toward an imino moiety of the complex, and the cyanide approaches from the lessshielded carbonyl re-face. O
O H
(1.5 equiv) NC OEt Ti(Oi-Pr)4 (5 mol%) chiral ligand (5 mol%)
O NC O
OEt
H
i-PrOH/CHCl3, –20 ºC, 16 h
99%, 91% ee (S) chiral ligand: N t-Bu
N t-Bu
OH HO t-Bu
re-face
t-Bu H
Ph
O N
CN O
N Ti O
O
proposed working model
Scheme 40. Asymmetric Reaction of Aldehydes and Ethyl Cyanoformate with Salen–Titanium Catalyst The multicomponent catalyst system prepared from Ti(Oi-Pr)4, (S)-6,6'-dibromo-1,1'-bi-2,2'naphthol, cinchonine, and (1R,2S)-N-methyl-ephedrine promoted the asymmetric reaction to afford the cyanated products with moderate to high enantioselectivity (up to 94%) (Scheme 41).46 This system was regarded as a Lewis acid–Lewis base bifunctional catalyst.
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O NC O H
OEt (1.3 equiv)
O NC O
combined catalyst (10 mol%)
OEt
∗ H
CHCl3, –45 ºC, 55 h 82%, 94% ee combined catalyst: Ti(Oi-Pr)4 N
OH Br OH OH Br
N
HO H
N
Scheme 41. Asymmetric Reaction of Aldehydes and Ethyl Cyanoformate with Multicomponent Titanium Catalyst Two types of ternary titanium catalyst systems were also reported. One was the tridentate Schiff base–cinchonine–Ti system affording the cyanated products in up to 94% ee (Scheme 42).47 The other was prepared from a nitrogen-containing (R)-3,3'-bis((methyl((S)-1phenylethyl)amino)methyl)-1,1'-binaphthyl-2,2'-diol
(a
BINOL
derivative),
N-[(1S,2R)-2-
hydroxy-1,2-diphenyl ethyl]acetamide (a β-aminoalcohol derivatives), and Ti(Oi-Pr)4, and gave the adducts in up to 92% ee (Scheme 43).48 The chiral titanium complex formed in situ might activate ethyl cyanoformate with the tertiary amine group as a Lewis base, and then the liberated cyanide anion might attack the aldehyde coordinated to the titanium atom.
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O
(1.5 equiv) NC OEt Ti(Oi-Pr)4 (5 mol%)
O
O NC O
H chiral ligands (5 mol%) CH2Cl2, –20 ºC, 2.5 h
OEt
H
OCH3
OCH3 99%, 94% ee (R)
chiral ligands: i-Pr N
OH
HO
N H
OH
t-Bu t-Bu
N
Scheme 42. Asymmetric Reaction of Aldehydes and Ethyl Cyanoformate with Ternary Titanium Catalyst O
CH3O
O (1.5 equiv) NC OEt O CH3O NC O OEt Ti(Oi-Pr)4 (5 mol%) H chiral ligands (5 mol%) H CH2Cl2, –15 ºC, 48 h 82%, 92% ee (S)
chiral ligands: CH3 Ph (S) N (R)
OH OH
HN
(S) CH3 Ph N
OH
O Ph
N O O
Ti N
EtO
O CN
O-iPr Ac N H O Ph Ph O
Ph R
H
proposed working model
Scheme 43. Asymmetric Reaction of Aldehydes and Ethyl Cyanoformate with Ternary Titanium Catalyst
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The dimeric titanium complex [(salen)Ti(µ-O)]2 bridged by cis-5-norbornene-endo-2,3dicarboxylate devised by Ding and co-workers catalyzed the cyanoacetylation of aryl and α,βunsaturated aldehydes using NaCN as a cyanide source and acetic anhydride as an acetyl source (Scheme 44).10 The reactions were conducted with very low catalyst loading of 0.05–0.005 mol% to afford the adducts in up to 96% ee. This catalyst was also efficient for asymmetric cyanosilylation of aldehydes (see the former section). O O H
NaCN (3 equiv) Ac2O (3 equiv) catalyst (0.01 mol%)
NC O H
CH2Cl2, 25ºC, 10 h 99%, 96% ee (S) catalyst: N
N Ti O
O
O
t-Bu
O t-Bu
t-Bu O
O t-Bu
t-Bu O O
O
O
t-Bu
Ti N
N
Scheme 44. Asymmetric Cyanoacetylation of Aldehydes Catalyzed by Dimeric Titanium Complex Khan and co-workers carried out cyanoethoxycarbonylation of aromatic, aliphatic, and α,βunsaturated aldehydes catalyzed by the chiral (salen)vanadium(V) complex with imidazole as a co-catalyst (Scheme 45).49 Imidazole catalytically activated ethyl cyanoformate to react with aldehyde, which in turn interacted with vanadium complex. The cyanation seemed to occur on the less-hindered side in order to avoid repulsion with an imino moiety of the complex. The
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products were obtained with good to high enantioselectivity. Hydrocinnamaldehyde was converted to the adduct in a notably high ee of 97%. O NC O H
(2 equiv)
OEt
O NC O
catalyst (2.5 mol%) imidazole (10 mol%)
OEt
H
CH2Cl2, –20 ºC, 48 h 92%, 97% ee (S) catalyst: EtSO4
H H N O N V O O O H H t-Bu t-Bu
t-Bu
t-Bu
H Ph
O N
CN H O
N O H
V O
proposed working model
Scheme 45. Asymmetric Reaction of Aldehydes and Ethyl Cyanoformate with Salen–Vanadium Catalyst An organocatalyst with two cinchonidine ammonium salts linked with an anthracenyldimethyl
group
showed
high
catalytic
activity
for
the
enantioselective
cyanomethoxycarbonylation of aromatic aldehydes (Scheme 46).50 Chinchilla and Nájera and coworkers found that the reaction proceeded with 1 mol% of the cinchonidine derivative in the presence of 20 mol% of triethylamine to provide the cyanated products in up to 96% ee. The organocatalyst was almost quantitatively recovered by ether-promoted precipitation without any loss of activity.
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O
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O
(1.5 equiv) NC OEt O catalyst (1 mol%) H Et3N (20 mol%) CH2Cl2, –78 ºC, 14 h
NC O
OEt
H 99%, 92% ee (R)
catalyst :
2 Cl
N OH
N
N OH N
Scheme 46. Asymmetric Reaction of Aldehydes and Ethyl Cyanoformate Catalyzed by Cinchonidine Ammonium Salt Deng and Tian developed enantioselective cyanoethoxycarbonylation of ketones catalyzed by dihydroquinidyl phenanthrene (DHQD-PHN), a cinchona alkaloid derivative that was a commercially available and recyclable catalyst (Scheme 47).51 Cyclic and sterically hindered dialkyl ketones as well as α,α-dialkoxy ketones were converted to the tertiary cyanohydrin derivatives in up to 97% ee. The chiral acyl-ammonium cyanide derived from the Lewis-basic catalyst and the acyl cyanide induced the cyanation of ketones to give the enantio-enriched cyanohydrin alkoxides reversibly. Besides, the dynamic kinetic resolution through enantioselective acylation of the alkoxides increased the enantiomeric purity of the products.
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O O EtO EtO
O (3 equiv)
NC OEt DHQD-PHN (10 mol%)
NC O EtO EtO
CHCl3, –24 ºC, 7 d
OEt
∗
99%, 94% ee DHQD-PHN:
CH3O O N N
H
Scheme 47. Asymmetric Reaction of Ketones and Ethyl Cyanoformate Catalyzed by A Cinchona Alkaloid Derivative Sakakura
and
Ishihara
and
co-workers
demonstrated
the
enantioselective
cyanoethoxycarbonylation of isatins by using the Lewis base–Brønsted acid cooperative catalyst (Scheme 48).52 N-p-Nitrobenzyl protected isatins having various substituents at the aromatic ring were converted into the cyanated products with up to 99% ee. The reaction proceeded in two steps. The first step was enantioselective but reversible cyanation at the carbonyl group, and the second one was a rate-determining acylation with kinetic resolution, resulting in high ee of the products. The structures of two intermediates at the first step involving the R and S cyanohydrin alkoxides, respectively, based on theoretical calculation were proposed as shown in the scheme. The intermediate of the R alkoxide interacting with three hydrogen-bonds is more stable than that of the S alkoxide with two hydrogen-bonding, and the former is preferentially converted to the product.
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O
O
(2 equiv) NC OEt catalyst (5 mol%) MeOH (50 mol%)
O
Br
O N PNB
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Br NC O
OEt O
N
CHCl3, rt, 9 h
PNB 96%, 99% ee (R)
catalyst:
PNB :
N
MeO
Ph
S
N
N H
N H
CF3 Ph
HN
S O2
N
N
N H O
CF3
Ph
S
MeO N
NO2
CF3
N H
HN S O CN O2
EtO O
CF3
N
R
HN S O CN O2
O more stable
Ph
N H
EtO
R
N
N H O
CF3
Ph
S
MeO
Ph
CF3
S
N R
less stable
proposed intermediates
Scheme 48. Asymmetric Reaction of Isatins and Ethyl Cyanoformate with Chiral Thiourea Catalyst 2-2-2. Imines List and co-workers developed an efficient Brønsted acid-catalyzed reaction of imines with acetyl cyanide (Scheme 49).53 The desired N-acetyl α-aminonitriles were formed from a wide range of aromatic, aliphatic, and unsaturated imines with 1–5 mol% of the Jacobsen-type thiourea catalyst in high yields and enantioselectivities of >98% in the best case. The reaction was proposed to proceed through the N-acetyl iminium intermediate shown in the scheme. The thiourea catalyst interacts with the cyanide to react in the catalyst’s chiral environment. This acetylcyanation was applied to a three-component Strecker-type reaction of in situ generated
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imines from the aldehydes and benzylamine in the presence of MS 5Å to give the cyanated products in up to 94% ee. O
(1.5 equiv) CN thiourea catalyst (1 mol%)
N H
O N
toluene, –40 ºC, 20 h
Cl
CN
Cl 87%, 98% ee (S) thiourea catalyst: t-Bu S N N N H H O
N
HO t-Bu S
t-Bu N
N H
O Ph Ac
N H CN
N
OPiv
N
H Ar
H
Ph proposed working model
Scheme 49. Asymmetric Reaction of Imines and Acetyl Cyanide with Chiral Thiourea Catalyst 2-2-3. N-Containing Hetroaromatics The enantioselective addition of cyanide to quinolines and isoquinolines (Reissert-type reaction) using the BINOL-derived bifunctional catalysts was developed by Shibasaki and co-workers (Scheme 50).54 The use of (CH3)3SiCN and 2-froyl chloride gave successful results. The reaction of (CH3)3SiCN with the acyl quinolinium intermediate was promoted by the bifunctional chiral aluminum catalyst—that is, the phosphine oxide moiety activated (CH3)3SiCN and the aluminum center interacted with the acyl quinolinium intermediate. The high Lewis basicity of the o-tolylsubstituted phosphine oxide group in the catalyst was important in terms of reactivity and
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enantioselectivity. The bulkiness of the o-tolyl moiety prevented formation of the intra-molecular coordination of the phosphine oxide onto the aluminum, resulting in low catalytic activity. The reaction of 4-diallylamino-5,7-dichloroquinoline gave the corresponding N-furoyl 2cyanoquinoline, which was a synthetic intermediate of bioactive compound, in 96% ee. Less reactive substrates (e.g., 6-dichloroquinoline) resulted in medium enantioselectivity.
Cl (2 equiv)
O Cl
Cl
O N(allyl)2 TMSCN (2 equiv) chiral Al catalyst (9 mol%) N
CH2Cl2, –40 ºC, 40 h
Cl
N(allyl)2
N
Cl
CN O
O 80%, 96% ee (R) chiral Al catalyst:
Ar P Ar O O Al Cl O O P Ar Ar Ar
Ar P
Ar =
CH3
Me3 Si CN O N
O O
O
Al Cl
R Cl
P(O)Ar2 proposed working model
Scheme 50. Asymmetric Reissert-type Reaction with Chiral Bifunctional Aluminum Catalyst Shibasaki's group expanded the applicability of the catalytic Reissert-type reaction into the 1substituted isoquinolines to provide the N-acyl 1-cyanodihydroisoquinolines with a quaternary stereocenter (Scheme 51).55 The catalyst was tuned to have two electron-withdrawing bromo groups at the 6- and 6'-position of the BINOL skeleton as well as a triflate group on the aluminum, and thereby exhibited high Lewis-acidic behavior. Use of vinyl chloroformate gave
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the highest enantioselectivity. The 1-alkyl, -alkenyl, and -aryl isoquinolines were cyanated with enantioselectivity as high as 98%. A dual-activation transition state, in which (CH3)3SiCN and acyl isoquinolinium are activated by the Lewis base (phosphine oxide) and the Lewis acid (aluminum), respectively, was proposed.
N Br
CH2=CHOCOCl (1.8 equiv) TMSCN (2 equiv) chiral Al catalyst (9 mol%) CH2Cl2, –40 ºC, 72 h
N
O
CN O Br 63%, 98% ee (R)
chiral Al catalyst: Ph P Ph O O Al OTf O O P Ph Ph
Br
Br
Ph O Ph PMe 3Si N CN O
O
Al O Y
Br
R O
P(O)Ph2
proposed working model
Scheme 51. Asymmetric Cyanation Reaction of Isoquinoline Derivatives with Chiral Bifunctional Aluminum Catalyst Shibasaki's group also developed the enantioselective cyanation of nicotinic amides using the dual-activation protocol (Scheme 52).56 In this case, a sulfoxide group was utilized as a Lewisbasic part of the catalyst instead of a phosphine oxide group as described above. The reaction of N,N-diisopropyl nicotinamide using (CH3)3SiCN and fluorenylmethoxycarbonyl chloride proceeded with high 1,6-regioselectivity to afford the corresponding product in 98% yield and in 96% ee.
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O N(i-Pr)2
Fmoc-Cl (1.4 equiv) TMSCN (2 equiv) Et2AlCl (5 mol%) chiral ligand (10 mol%) CH2Cl2, –60 ºC, 5 h
N
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O N(i-Pr)2 NC ∗ N Fmoc 98%, 96% ee
chiral ligand: S
Ph O
OH OH S
O Ph
Scheme 52. Asymmetric Cyanation Reaction of Nicotinic Amides with Chiral Bifunctional Aluminum Catalyst 2-3. Hydrocyanation 2-3-1. Aldehydes Inoue and co-workers found that a chiral cyclic dipeptide, cyclo[(S)-phenylalanyl-(S)-histidyl] (cyclo[(S)-Phe-(S)-His]), efficiently catalyzed the transformation (Scheme 53).57 (R)Mandelonitrile was obtained with enantiomeric excess of 97% in high yield in the reaction of benzaldehyde with 2 mol% of cyclo[(S)-Phe-(S)-His] in toluene at –20 ºC. Some benzaldehydes with
an
electron-rich
substituent
and
2-naphthaldehyde
were
cyanated
with
high
enantioselectivity. The reaction of heteroaromatic and aliphatic aldehydes proceeded with medium selectivity. The origin of enantioselectivity was ascribed to bifunctionality of the dipeptide catalyst: The peptide hydrogen of the histidine residue activated benzaldehyde with a hydrogen bond, and the imidazolyl moiety of the histidine residue interacted with HCN to form cyanide ion, promoting
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the intramolecular si-face-selecting nucleophilic addition. The re-face attack was prevented by the aromatic ring of the phenylalanine residue. O H
HO CN
HCN (2 equiv) chiral dipeptide (2 mol%)
H
toluene, –20 ºC, 8 h 97%, 97% ee (R) chiral dipeptide: HN
N
O NH H
NH O
Ph
H
CN O HN
H NH
O N H
H
NH H
O
proposed working model
Scheme 53. Asymmetric Hydrocyanation of Aldehydes Catalyzed by A Chiral Cyclic Dipeptide Ohkuma and co-workers revealed that bimetallic complexes [Li{Ru[(S)-phgly]2[(S)-binap]}]X (X = Cl, Br) acted as excellent catalysts for asymmetric hydrocyanation of aldehydes (Scheme 54).58 The reaction was successfully conducted with 0.2 mol% of the Ru•Li catalyst. A series of aromatic, heteroaromatic, and α,β-unsaturated aldehydes as well as pivalaldehyde were converted to the corresponding cyanohydrins in up to 99% ee. The structure of [Li{Ru[(S)phgly]2[(S)-binap]}]Br was determined by an X-ray crystallographic analysis: A lithium cation coordinated with the carbonyl oxygen of PhGly and the bromide ion was located between two amino protons interacting with the hydrogen bonds.
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O Br
HCN (3 equiv) Br H chiral Ru•Li cat. (0.2 mol%)
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HO CN H
t-BuOMe, –78 ºC, 18 h 98%, 99% ee (R) chiral Ru•Li cat.: Li
X
O
Ph2 O P
NH2
Ru
NH2
P Ph2 O
X = Cl, Br
O
Scheme 54. Asymmetric Hydrocyanation of Aldehydes with Chiral Ruthenium–Lithium Combined Catalyst 2-3-2. Imines Snapper and Hoveyda and co-workers reported that tripeptide Schiff base–titanium complexes catalyzed the addition of cyanide to a series of aldimines with high enantioselectivity (Scheme 55).59 The ligand structure was systematically optimized to proceed in the order of Schiff base– (S)-tert-leucine–(S)-O-tert-butyl threonine–glycine terminated with methyl ester. The Schiff base structure was also tuned to have 5-methoxy, 3,5-dichloro, or 3,5-dibromophenyl groups. NBenzhydryl (diphenylmethyl)-protected aldimines derived from aromatic aldehydes and pivalaldehyde were cyanated with up to 97% enantioselectivity. The titanium complex with the tridentate peptide ligand was proposed as a chiral Lewis acid catalyst. The regio- (1,2- over 1,4-) and enantioselective cyanation of α,β-unsaturated imines was also achieved by using the titanium–tripeptide Schiff base catalyst.
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Ph N H
TMSCN (2 equiv) i-PrOH (1.5 equiv) Ti(Oi-Pr)4 (10 mol%) Ph tripeptide Schiff base (10 mol%)
Ph NC HN H
toluene, 4 ºC
H3CO
Ph
H3CO 99%, 94% ee (S) tripeptide Schiff base: t-Bu Cl
H N
N Cl
OH
O
O
N Ti
O N H Ot-Bu
OCH3 O
NH O
NP (Oi-Pr)2 RCN R = TMS or H Ph proposed working model
Scheme 55. Asymmetric Cyanation of Aldimines with Chiral Tripeptide Schiff Base–Titanium Catalyst Feng and co-workers found that a titanium complex with two different ligands, one of cinchonine and one of achiral 3,3'-bis(naphth-2-yl)-2,2'-biphenol, catalyzed cyanation of N-Ts protected aldimines and ketimines enantioselectively (Scheme 56).60 A range of aromatic aldimines and cyclohexyl aldimine were cyanated with 5 mol% of catalyst with up to 97% selectivity. A titanium complex with two chiral ligands was proposed to be the catalytic species. The imine substrate coordinates to this complex to avoid the large RL–2-naphthyl (complex) repulsion, and HCN activated by the tertiary amine of cinchonine reacts with the imine on the reface. The catalyst system was applied to the reaction of various aromatic ketimines as well as cyclohexyl and α,β-unsaturated imines to afford the α-aminonitriles with quaternary stereocenters in up to 99% ee.
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TMSCN (1.2 equiv) i-PrOH (1.2 equiv) Ts Ti(Oi-Pr)4 (6 mol%) N cinchonine (5 mol%) H biphenol ligand (6 mol%)
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NC HN Ts H
toluene, –20 ºC, 2.5 h >99%, 97% ee (S)
N
TMSCN (1.2 equiv) i-PrOH (1.2 equiv) Ti(Oi-Pr)4 (6 mol%) cinchonine (5 mol%) biphenol ligand (6 mol%)
Ts
NC HN Ts CH3
toluene, –20 ºC, 8 h >99%, 99% ee (S) cinchonine:
biphenol ligand:
N H
HO
OH OH
N
N RS
N C H N
RL
TsN
O O i-PrO Ti
O
O
proposed working model
Scheme 56. Asymmetric Cyanation of Imines Catalyzed by Chiral Titanium Complex with Two Different Ligands Vilaivan and coworkers demonstrated that titanium complexes with tridentate N-salicyl-βaminoalcohols acted as effective catalysts for enantioselective Strecker reaction of aromatic aldimines (Scheme 57).61 The configuration as well as the bulkiness of the β-substituent significantly influenced the enantioselectivity. Use of the most efficient ligand, (S)-N-(2hydroxybenzyl)alaninol, allowed the reaction to proceed, affording the α-arylaminonitriles in >98% ee in the best cases.
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Ph
Ph
TMSCN (2 equiv) N Ph i-PrOH (1 equiv) Ti(Oi-Pr)4 (2.5 mol%) chiral aminoalcohol (2.5 mol%) H toluene, 0 ºC, 72 h Br
NC HN
Ph
H Br 97%, >98% ee (S)
chiral aminoalcohol:
N H OH
OH
Scheme 57. Asymmetric Cyanation of Aldimines with Chiral Aminoalcohol–Titanium Catalyst Seayed and Chai and co-workers reported that the partially hydrolyzed titanium alkoxide (PHTA) coordinated with N-salicyl-t-leucinol catalyzed the cyanation of N-benzyl, N-benzhydryl, or N-Boc aldimines (Scheme 58).62 Aromatic and heteroaromatic N-benzhydryl aldimines were converted to the a-aminonitriles in up to 98% ee at room temperature. The homogeneous titanium complex was converted to the heterogeneous cluster, which acted as a recyclable catalyst 10 times. The catalyst cluster was utilized in the flow reactor system for the threecomponent Strecker reaction. Ph
Ph
TMSCN (1.5 equiv) n-BuOH (1.0 equiv) N Ph PHTA (5 mol%) chiral aminoalcohol (5 mol%) H toluene, rt, 15 min Cl
NC HN
Ph
H Cl 95%, 98% ee (S)
PHTA: partially hydrolyzed titanium alkoxide chiral aminoalcohol:
t-Bu N H OH
OH
Scheme 58. Asymmetric Cyanation of Aldimines with Chiral Aminoalcohol–Titanium Catalyst
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A chiral (salen)aluminum complex-catalyzed enantioselective hydrocyanation of N-allyl aldimines was reported by Jacobsen and Sigman (Scheme 59).63 They were able to suppress the uncatalyzed cyanation that gave racemic products by the reaction at –70 ºC, which occurred rapidly at room temperature. Under the optimized conditions, the hydrocyanation of an aldimine prepared from benzaldehyde and allyl amine followed by the amidation with trifluoroacetic anhydride (TFAA) gave the corresponding product in 91% yield and 95% ee. The reaction of aromatic aldimines resulted in high enantioselectivity, but medium selectivity was observed by using aliphatic substrates. F3C N H
HCN (1.2equiv) (salen)AlCl (5 mol%)
O
NC N
TFAA
toluene, –70 ºC, 15 h
H 91%, 95% ee (S)
(salen)AlCl: N
N Al
t-Bu
O t-Bu
Cl
O
t-Bu
t-Bu
Scheme 59. Asymmetric Cyanation of Aldimines with Chiral Salen–Aluminum Catalyst Shibasaki and co-workers developed an asymmetric Strecker reaction catalyzed by the bifunctional Lewis acid–Lewis base catalyst (Scheme 60).64 The reaction of N-fluorenyl aromatic and α,β-unsaturated aldimines was catalyzed by the chiral aluminum complex with high enantioselectivity, although the aliphatic aldimines reacted with medium selectivity.
The
cyanation occurred with (CH3)3SiCN followed by slow addition of phenol or HCN to afford the protonated products. A bifunctional catalysis was proposed in which the Lewis basic phosphine oxide activates (CH3)3SiCN and the Lewis acidic aluminum interacts with the imine.
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N
Flu TMSCN (2 equiv) chiral Al catalyst (9 mol%) H PhOH (20 mol%)
NC HN Flu H
CH2Cl2, –40 ºC, 40 h 80%, 96% ee (R) chiral Al catalyst: Ph Ph P O O Cl Al O O Ph P Ph
Flu:
Ph P Ph O
Si N
C H
Flu N
R
Al Cl
O O
O P Ph Ph proposed working model
Scheme 60. Asymmetric Cyanation of Aldimines with Chiral Bifunctional Aluminum Catalyst Yamamoto and Abell achieved the asymmetric Strecker reaction of aldimines and ketimines based on a dual-activation protocol in which the tethered bis(8-quinolinolato) aluminum complex described in Scheme 61 and Et3N were employed as a chiral Lewis acid and achiral Lewis base, respectively.65 Ethyl cyanoformate was the cyanide source of choice in terms of yield, enantioselectivity, toxicity and availability. Under the optimized conditions, this catalyst system exhibited high enantioselectivity in the reaction of aromatic, heteroaromatic, and tert-butyl Ndiarylphosphinoyl aldimines. In addition, various ketimines, such as acetophenone-, propiophenone-, butyrophenone-, and α-tetralone derived ketimines, were converted to the quarternary α-aminonitriles in up to 96% ee with the same catalytic system.
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O O Ar P Ar N H
CN (1.5 equiv)
O
O Ar P Ar NC HN
i-PrOH (1.5 equiv) NEt3 (10 mol%) Al catalyst (10 mol%)
H
toluene, rt, 6 h CH3O
CH3O
Ar = 2,6-Me2-C6H3
99%, 98% ee (R) O
O O CN (2.5 equiv) Ar P i-PrOH (2.5 equiv.) Ar N NEt3 (10 mol%) Al catalyst (10 mol%) CH3 Ar = 2-Me-C6H3
toluene–hexane (1:1), rt, 12 h
Al catalyst:
O Ar P Ar NC HN H 82%, 96% ee (R)
Br
N Cl Al
CH3 O O
N
CH3
Br
Scheme 61. Asymmetric Cyanation of Imines with Chiral Bifunctional Aluminum Catalyst Li and co-workers established an enantioselective Strecker reaction of N-phosphonyl aldimines with nonvolatile (C2H5)2AlCN and a catalytic amount of primary amino acid (Scheme 62).66 Phenylglycine, which was the optimized amino acid, connected in a bidentate manner to the Al– CN moiety, forming the catalytically active species (phgly)AlCN. (C2H5)(i-C3H7O)AlCN formed from (C2H5)2AlCN and 2-propanol acted as the cyanide source. The reaction of N-phosphonyl imines derived from benzaldehydes gave the corresponding products in excellent ee of 99.7% in the best case. The N-phosphonyl group was readily removed from the products by treatment with aqueous HCl, and N,N'-bis(naphthalen-1-ylmethyl)ethane-1,2-diamine could be quantitatively recovered.
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N
N P
N Ph
O
Et2AlCN (1.5 equiv) i-PrOH (1 equiv) Ph O (10 mol%) H2N OH toluene, MS 4Å, –78 ºC, 5 h
H N
N P
O NC NH Ph
H
95%, 99.7% ee (S)
Scheme 62. Asymmetric Cyanation of Aldimines Using (C2H5)2AlCN and An Amino Acid Catalyst Khan's group synthesized macrocyclic dinuclear (salen)manganese linked with two triethylene glycol tethers and utilized it as a catalyst of enantioselective Strecker reaction with 4-phenyl pyridine N-oxide (4PPyNO) as a co-catalyst (Scheme 63).67 4PPyNO was considered to be an axial ligand on the (salen)manganese complex. The macrocyclic manganese catalyst system worked with a catalyst loading of 5 mol% on the cyanation of N-benzhydryl aryl and tert-butyl aldimines. The substituent on the aromatic ring of the substrate affected the enantioselectivity (pH, 94%; p-CH3O, >99%; p-Br, 91%). The reaction using ethyl cyanoformate as a cyanide source catalyzed by the diethyl tartrate linked macrocyclic (salen)manganese was also reported.
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Ph N
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TMSCN (1.5 equiv) Ph macrocyclic Mn cat. (5 mol%) 4PPyNO (10 mol%)
H
toluene, –40 ºC, 8 h Ph
H3CO NC HN
Ph
H H3CO 93%, >99% ee (R) macrocyclic Mn cat.: O
O
tBu
tBu
N O Cl Mn N O
O
N Mn N
O
O
Cl O But
tBu
O
O
O
O
4PPyNO: N O
Scheme 63. Asymmetric Cyanation of Aldimines with Chiral Macrocyclic Dinuclear Manganese Catalyst Kobayashi and co-workers developed a zirconium catalyzed asymmetric reaction of aldimines and tributyltin cyanide (Scheme 64).68 The unique Zr complex shown in the scheme was formed from Zr(Ot-Bu)4, (R)-6,6'-dibromo-BINOL, (R)-3,3'-dibromo-BINOL, and N-methyl imidazole in a 2:2:1:2 ratio. N-(2-Hydroxy)phenyl aromatic and heteroaromatic aldimines were cyanated in up to 92% enantioselectivity. The hydroxyl group on the protective group was important to achieve high reactivity and enantioselectivity. The zirconium catalyst was also effective for the asymmetric three-component reaction of aldehyde, 2-amino-3-methylphenol, and HCN. A range of aromatic and aliphatic aldehydes was
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converted to the α-aminonitriles in up to 94% ee. This is the pioneering study showing the onepot three-component protocol. HO
HO
Bu3SnCN (1.1 equiv) chiral Zr cat. (10 mol%)
N
NC HN
toluene/benzene (1/1) 0 ºC, 12 h –65 ºC
H
H
S
S 89%, 92% ee (R)
HCN (2 equiv) chiral Zr cat. (5 mol%)
HO
O + H
CH2Cl2, –45 ºC, 5 h
H2N
HO
CH3
NC HN H
CH3
99%, 94% ee (R) chiral Zr cat.: Br O O
Br Ot-Bu O Zr
Br L O
L
Br
O
Zr
Br
O Ot-Bu Br
L: N-methylimidazole
Scheme 64. Asymmetric Three-component Strecker Reaction with Chiral Zirconium Catalyst A chiral lanthanum binaphthyldisulfonate complex-catalyzed Strecker reaction of aldimines was reported by Ishihara's group (Scheme 65).69 The catalyst prepared in situ from La(OPh)3 and the chiral disulfonate promoted the reaction of N-benzhydryl aryl and heteroaryl aldimines in up to 92% enantioselectivity. Addition of 3-methylbutanoic acid was important in terms of catalytic activity. A transition state model was proposed as shown in the scheme. The lanthanum complex with a chiral binaphthyldisulfonate and a carboxylate interacts with the imine substrate avoiding remarkable repulsive interactions, and a sulfonate moiety activates HCN, resulting in the internal re-face attack of the cyanide.
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TMSCN (1.5 equiv) i-PrCO2H (50 mol%) La(OPh)3 (10 mol%)
Ph N
SO3H SO3H
Ph
Ph NC HN
(10 mol%)
H S
EtCN, –20 ºC, 86 h
Ph
H S 97%, 92% ee (R)
O O OO
R O
La
S S O
O O
H N NC [NCEt]n H proposed working model
Scheme 65. Asymmetric Cyanation of Aldimines Catalyzed by Chiral Lanthanum Binaphthyldisulfonate Complex Jacobsen’s group reported that the asymmetric hydrocyanation of hydrazones derived from aromatic aldehydes was catalyzed by Er(Ph-pybox) to furnish the corresponding hydrazino nitrile in up to 97% ee (Scheme 66).70 Europium was selected as the metal center in terms of yield and enantioselectivity according to the lanthanide source screening. The substrates with electron-rich phenyl rings were cyanated with high reactivity and selectivity, although the electron-deficient ones reacted slowly.
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N
H N
Ph TMSCN (2 equiv) MeOH (2 equiv) O [(S)-Ph-pybox]ErCl3 (5 mol%) H CHCl3, 0 ºC, 3 d
(H3C)2N
O
NC
HN NH
Ph
H (H3C)2N 85%, 97% ee (R) (S)-Ph-pybox: O
O
N N
N
Scheme 66. Asymmetric Cyanation of Hydrazones with Ph-pybox–Europium Catalyst Ohkuma and co-workers found that the Ru(phgly)2(binap)–PhOLi system or the bimetallic complex [Li{Ru(phgly)2(binap)}]Cl acted as an efficient catalyst for enantioselective hydrocyanation of N-carbamoyl-protected aldimines (Scheme 67).71 A variety of N-Cbz aryl and heteroaryl aldimines as well as primary-, secondary-, and tertiary-alkyl substrates were quantitatively converted to the α-amino nitriles in up to 98% ee with 0.2 mol% of the catalyst under 0 ºC in 30 min. In some cases the reaction was carried out with catalyst loading of 0.02 mol% with maintenance of the enantioselectivity.
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O N Br
O
Ph
H
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(CH3)3SiCN (3 equiv) CH3OH (3 equiv) Ru complex (0.2 mol%) PhOLi (0.2 mol%) t-BuOMe, 0 ºC, 30 min O NC HN Br
Ph
O
H
98%, 98% ee (R) Ru complex:
O Ph2 O P
NH2
Ru
NH2
P Ph2 O O
Scheme 67. Asymmetric Cyanation of Aldimines with Chiral Ruthenium–Lithium Combined Catalyst Lipton and co-workers devised a chiral dipeptide catalyst prepared from (S)-phenylalanine and (S)-α-amino-γ-guanidinobutyric acid for asymmetric Strecker reaction (Scheme 68).72 The Nbenzhydryl aryl aldimines with electron-donating substituents were reacted with excellent enantioselectivity. The enantioface selection did not work well for the electron-deficient aryl imines and the aliphatic substrates.
Ph N H
Ph Ph HCN (10 equiv) chiral dipeptide (2 mol%) MeOH, –25 ºC, 12 h
NC HN
Ph
H
97%, >99% ee (S) chiral dipeptide:
H N
O
NH2 NH
HN NH O
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Scheme 68. Asymmetric Cyanation of Aldimines with Chiral Dipeptide Catalyst Jacobsen's group developed urea or thiourea derivatives containing a substituted salicylimine group linked by a chiral 1,2-cyclohexyldiamine and a chiral N-substituted t-leucine terminal for the Strecker reaction of aldimines and ketimines (Scheme 69).73 The catalyst structure was optimized by using a combinatorial method. Presence of two tert-butyl groups, one each at the amino acid position and the C3-position of the salicylimine group of catalyst A and B was required for the high enantioselectivity. After the elaboration, a pivaloyl group was introduced into the C5-position of the salicylimine moiety. A proposed transition state using catalyst C was shown in the scheme. The amido–thiourea-induced imine protonation by HCN generates the catalyst interacting with the iminium–cyanide ion pair, and this interaction promotes the smooth carbon–carbon bond formation.
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F3C N
HCN catalyst A (2 mol%)
H
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O
NC N TFAA
H
toluene, –70 ºC, 20 h
89%, 97% ee (R) N Br
CH3
NC HN
HCN catalyst B (1 mol%)
∗ CH3
Br
toluene, –78 ºC, 15 h 92%, 96% ee Ph
Ph
TMSCN (2 equiv) N Ph CH3OH (2 equiv) catalyst C (2 mol%) H toluene, –30 ºC, 20 h
H3CO catalyst A:
H3CO 99%, 99% ee (R)
catalyst B:
O
S N H
N H
N
N H
N H
O
N
N
HO
HO
catalyst C:
O
N Ph
S
CF3
Me
O
H
S N H
N H
O
O O
Ph
Ph
H
O
H N
Ph
NC HN
CF3
t-Bu Me N Ph Ph Ph
N H O H N
Ph N H N C t-Bu
H proposed working model
Scheme 69. Asymmetric Cyanation of Imines with Chiral Urea and Thiourea Catalyst The urea catalyst A was suggested to have a wide substrate scope.74 A variety of N-allyl or Nbenzyl aldimines with aromatic, aliphatic, and 1-cyclohexenyl substituents was cyanated with high enantioselectivity at –70 °C. The reaction of imines with less bulky alkyl groups afforded the products in somewhat lower ee. This problem was solved by using the thiourea catalyst B. The catalyst B also effectively catalyzed the cyanation of ketimines derived from acetophenone and pinacolone, yielding the products in 96% ee and 86% ee, respectively.75
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The simpler amido–thiourea catalyst C promoted the Strecker reaction of N-benzhydryl aldimines at –30 to 0 °C. The catalyst was applicable to the gram-scale reactions using HCN formed in situ from KCN and acetic acid.76 Based on Jacobsen’s report,73–75 Kunz and co-workers synthesized the chiral urea catalysts using glucosamine derivatives instead of chiral 1,2-cyclohexyldiamine for enantioselective hydrocyanation of aldimines (Scheme 70).77 The optimized catalyst showed high enantioselectivity of 95% in the reaction of the imine derived from benzaldehyde. Other aryl imines were reacted with moderate to good stereoselectivity.
N H
TMSCN (2 equiv) CH3OH (2 equiv) catalyst (2 mol%)
NC HN H
toluene, –70 ºC 86%, 95% ee (S)
catalyst: OAc H N
AcO AcO N
H N
O OH
O
t-Bu
N H
t-Bu PivO
Scheme 70. Asymmetric Cyanation of Aldimines with Chiral Urea Catalyst Palacios and co-workers found that cinchonidine catalyzed the enantioselective reaction of αketiminophosphonates and acetyl cyanide to give the α-phosphono-α-amino nitriles in up to 92% ee (Scheme 71).78 Substitution on the phenyl ring of substrates somewhat decreased the enantioselectivity. Cinchonidine was considered to activate the imine with a hydrogen bond of the hydroxyl group, and also to activate acetyl cyanide with the Lewis basic tert-amine moiety.
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Ts
O N Oi-Pr P Oi-Pr O
(2 equiv) NC CH3 cinchonidine (10 mol%)
Ts
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NH CN Oi-Pr P Oi-Pr O
CHCl3, –45 ºC, 72 h
80%, 92% ee (S) cinchonidine: H
N OH
N
N N H
O
N C
N
Ar
R2O
P O OR1
Ts
O
proposed working model
Scheme 71. Asymmetric Cyanation of α-Ketiminophosphonates Catalyzed by Cinchonidine Chiral thiourea-catalyzed asymmetric Strecker reaction of N-aryl α-trifluoromethylated ketimines was developed by Enders and co-workers (Scheme 72).79 The reaction rate was relatively slow, but several aryl, alkenyl, and alkyl ketimines were converted to the α-quaternary α-trifluoromethylated amino nitriles in 83–95% ee. Selection of the thiourea-catalyst structure was crucially important for achieving high chemical yield and enantioselectivity.
N
PMP CF3
PMP HN CN
TMSCN (2 equiv) i-PrOH (2 equiv) catalyst (5 mol%)
CF3
DCE, rt, 5 d 90%, 95% ee (S) catalyst:
CF3 S
F3C
N H
N H
N
Scheme 72. Asymmetric Cyanation of N-Aryl α-Trifluoromethylated Ketimines Catalyzed by Chiral Thiourea
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Zhou and co-workers reported that dihydroqunine-derived urea catalyzed the asymmetric Strecker reaction of N-aryl α-CF3- and α-CF2H-substituted ketimines (Scheme 73).80 The bifunctional character of Brønsted-acid (urea moiety) and Lewis base (quinidine part) in the catalyst remarkably increased the enantioselectivity. Addition of a stoichiometric amount of (CF3)2CHOH (HFIP) speeded up the reaction rate. Under the optimized conditions the α-CF3and α-CF2H-substituted amino nitriles were obtained in up to 96% ee and 92% ee, respectively.
N
PMP TMSCN (2 equiv) HFIP (1 equiv) CF3 catalyst (10 mol%)
NC
NH
PMP
CF3
toluene, 25 ºC 97%, 96% ee (R) catalyst: Ar O
N
H N
H N
N OCH3
Ar = 3,5-(CF3)2C6H3
Scheme 73. Asymmetric Cyanation of N-Aryl α-CF3- and α-CF2H-Substituted Ketimines with Chiral Dihydroqunine/Urea Catalyst Ma's group and Wang's group independently reported an asymmetric Strecker reaction of cyclic N-acyl trifluoromethyl ketimines with the cinchona alkaloid-based thiourea catalysts (Scheme 74).81 Although their experiments were carried out under different conditions, the stereoselective outcomes were similar, achieving enantioselectivity as high as 97%. It was proposed that the thiourea moiety interacted with the N-acyl ketimine through two hydrogen bonds, and the acid-base interaction between the tert-amine moiety of the catalyst and HCN increased the nucleophilicity.
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TMSCN (2 equiv) cinchona alkaloidthiourea cat. A (1 mol%)
CF3 CH3O
N N PMB
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NC CF3 CH3O
NH
O toluene, 0 ºC, 24 h
N O PMB 93%, 97% ee (S)
TMSCN (2 equiv) EtOH (2 equiv) cinchona alkaloidthiourea cat. B (10 mol%)
CF3 I
N N O PMB
NC CF3 I
NH
CH2Cl2, –78 ºC, 2 d
N O PMB 96%, 97% ee (R)
cinchona alkaloid–thiourea cat. A: cinchona alkaloid–thiourea cat. B: CH3O H
CH3O
N
H
CF3
N
NH N
S
CF3
NH
N H
N
CF3 N
S
N H
CF3
CF3
H3CO
S N H CN F3C
N H
N H
N
O N
CF3
R
proposed working model
Scheme 74. Asymmetric Cyanation of Cyclic N-Acyl Trifluoromethyl Ketimines with Cinchona Alkaloid-based Thiourea Catalysts Cinchona alkaloid-based thioureas also catalyzed the reaction of N-Boc isatin-derived ketimines, which were independently reported by Zhou's group and Yan and Wang's group (Scheme 75).82 The quinine-derived thiourea (10 mol%) catalyzed the cyanation at –25 °C with medium to high enantioselectivity. The reaction with the cinchonidine-derived catalyst (5 mol%) at –70 °C gave the products in >99% ee in the best case, although a long reaction time (2–4 days)
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was required. Addition of (CF3)2CHOH (HFIP) increased the reactivity. The one-pot aza-Wittig– Strecker reaction sequence from the corresponding keto-amide substrate was also examined. O t-BuO
TMSCN (2 equiv) i-PrOH (1.5 equiv) quinine-thiourea cat. (10 mol%)
N
NC NH
O DCE, –25 ºC, 10 h N CH3
Cl
Ot-Bu
O
O N
Cl
CH3 98%, 94% ee (R)
O t-BuO
TMSCN (1.5 equiv) HFIP (1 equiv) cinchonidinethiourea cat. (5 mol%)
N
Ot-Bu
O
NC NH
O DCE, –70 ºC, 2–4 d N CH3
O N CH3 95%, >99% ee (R)
quinine-thiourea cat.: R = OCH3 cinchonidine-thiourea cat.: R = H R N
CF3
NH N
S
N H
CF3
Scheme 75. Asymmetric Cyanation of N-Boc Isatin-derived Ketimines with Cinchona Alkaloidbased Thiourea Catalysts Tian and Shao reported that the quinine-based thiourea also catalyzed the hydrocyanation of cyclic (Z)-aldimines such as 3H-indoles and 2H-benzo[b][1,4]thiazines (Scheme 76).83 The cyclic aminonitriles in up to 98% ee were obtained in the reaction using HCN generated from ethyl cyanoformate with 10 mol% of the catalyst at 10 °C. The catalyst and HCN seemed to form the ammonium salt shown in the scheme. The cyclic imine was activated by the ammonium with hydrogen bonding, and the cyanide interacted with the thiourea group, which in turn reacted with imine in an intramolecular manner.
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O (1.2 equiv) NC OEt CH3OH (1.2 equiv) quinine-thiourea cat. (10 mol%) N
DCE, MS 3Å, 10 ºC, 60 h
N H
CN
99%, 98% ee (R)
quinine-thiourea cat.: CH3O N
CF3
NH N
N H
S
CF3 MeO
Ar
N N
H
H H N
C N
S
N
H N R R = 3,5-(CF3)2C6H3 proposed working model
Scheme 76. Asymmetric Cyanation of Cyclic Aldimines with Quinine-based Thiourea Catalyst Du's group devised the quinine-squaramides to catalyze the enantioselective cyanation of N(benzothiazol-2-yl)aldimines (Scheme 77).84 The N-protected benzaldimines were converted to the α-aminonitriles in up to 98% ee. The catalyst was considered to activate the substrate with the two NH groups through hydrogen bonding, and HCN formed in situ from (CH3)3SiCN and ethanol interacted with the quinuclidine moiety as shown in the scheme. Then the reaction occurred under the catalyst’s chiral environment.
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TMSCN (2 equiv) EtOH (2 equiv) cinchona alkaloidsquaramide cat. (5 mol%)
N N H3CO
S
CH2Cl2, –15 ºC, 48 h
H
N
H3CO
NC HN H3CO
S
H
H3CO 80%, 98% ee (S) cinchona alkaloid-squaramide cat.: OMe N NH
H N
N
CF3
O O CF3 OCH3 N
H N
O H
N O F3C
C
H N
H
N N
N Ph
S
CF3 proposed working model
Scheme 77. Asymmetric Cyanation of Aldimines with Quinine-Squaramide Catalyst Feng and co-workers synthesized several chiral N,N’-dioxides derived from proline derivatives and
found
that
the
N,N’-dioxide
of
trans-4-hydroxy-l-proline
linked
with
1,2-
diphenylethylenediamine showed catalytic activity for the one-pot three-component Strecker reaction of an aldehyde, benzhydrylamine, and (CH3)3SiCN (Scheme 78).85 A series of aromatic and aliphatic aldehydes were converted to the α-aminonitriles in around 90% ee. High enantioselectivity of 95% was obtained in the reaction of 2-ethylbutanal. The catalysis was shown to proceed in a bifunctional manner. The imine generated in situ was activated through
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the hydrogen bond with amide N–H, and (CH3)3SiCN was simultaneously activated with two Noxides with fixing of the chiral structure.
CHO + H2N
Ph
TMSCN (2 equiv) chiral N,N'-oxide Ph (10 mol%)
NC HN
Ph
∗ H
Ph CH2Cl2, –45 ºC, 36 h
86%, 95% ee chiral N,N'-oxide:
Ph
Ph
O
O NH
N O
HO
H re
HN O
N
OH
Ph N
Ph
Ph
H O NH CN N O O Si N N
O
OH
HO
proposed working model
Scheme 78. Asymmetric Three-component Strecker Reaction with Chiral N,N’-Dioxide Catalyst Khan's group developed chiral amide-based or chiral oxazoline-based sulfonamide catalysts for the Strecker reaction of aldimines (Scheme 79).86 Catalyst A and B showed good to high enantioselectivity in the reaction of a series of N-benzhydryl aryl, alkenyl, and alkyl aldimines. The electronic properties of substituents and their positions on the phenyl ring of aryl substrates affected the enantioselectivity. Catalyst C showed a broader substrate scope than the other two catalysts. Every sulfonamide catalyst was considered to activate imine substrates strongly through the hydrogen bond of the sulfonamide moiety.
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O
Ph N
Ph (1.5 equiv)
OEt Ph NC i-PrOH (2 equiv) H chiral amide A (10 mol%)
∗ H
toluene, –10 ºC, 24 h
NO2
NO2 95%, 99% ee
Ph N
Ph Ph
H OCH3
TMSCN (1.2 equiv) i-PrOH (2 equiv) chiral amide B (5 mol%)
∗ H OCH3 93%, 96% ee
O
Ph
(1.5 equiv)
OEt Ph NC i-PrOH (2 equiv) chiral amide C (5 mol%) H
NC HN
chiral amide A:
Ph
∗ H
toluene, 0 ºC, 24 h
CH3
Ph
NC HN
THF, –20 ºC, 24 h
Ph N
Ph
NC HN
CH3 88%, 99% ee
chiral amide B: O
O
O HN S O
N
O NH HN
O
NH S
O
HN O S O
chiral amide C: CH3 O N H
= half of the catalyst O NH O CN O O H N N O S O Ph Ph
H N
S O
Ph
(S) NH CN O N O H S O (S)
N
proposed working models using chiral amide A (left) and C (right)
Scheme 79. Asymmetric Cyanation of Aldimines with Chiral Sulfonamide Catalysts Dughera and co-workers synthesized a sulfonimide derivative with an atropisomeric pterphenyl structure (Scheme 80).87 The C2 chiral compound catalyzed the three-component Strecker reaction of aromatic aldehydes or methylketones, aniline derivatives, and (CH3)3SiCN to give the aminonitrile products in moderate yield and in up to 97% ee.
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O CH3
NH2 + CH3O
H3C
OCH3 TMSCN (1.1 equiv) chiral sulfonimide (5 mol%)
NC NH CH3
0 ºC, 5 h
H3C 68%, 97% ee (S) chiral sulfonimide: H3C H3C
∗
H3C H3C
∗
O2 S NH S O2
Scheme 80. Asymmetric Three-component Strecker Reaction with Chiral Sulfonimide Catalyst Rueping and co-workers reported the chiral BINOL phosphate-catalyzed enantioselective Strecker reaction of N-benzyl aromatic aldimines (Scheme 81).88 The phosphate with 9phenanthryl groups at the 3,3’-positions showed high enantioselectivity with medium to high chemical yield. The phosphate derivative was considered to be a Brønsted acid catalyst.
N
Ph H
HCN (1.5 equiv) chiral binol phosphate (10 mol%)
NC HN
Ph
H
toluene, –40 ºC, 6 h
85%, 99% ee (S) chiral binol phosphate:
O O P OH O
Scheme 81. Asymmetric Cyanation of Aldimines with Chiral BINOL Phosphate Catalyst
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Tsogoeva's group used 3,3'-bis(p-nitrophenyl)-substituted BINOL-phosphate as a catalyst for the asymmetric hydrocyanation of aliphatic hydrazones (Scheme 82).89 The α-hydrazinonitriles were obtained in up to 93% ee. The 4-nitro substituent on the benzoyl protective group was important in terms of both reactivity and enantioselectivity. The active catalyst was proposed to be the O-TMS BINOL-phosphate generated in situ, in which the silicon atom interacted with the nitrogen or oxygen atom of hydrazone to activate it.
NO2 N
H N H
O
TMSCN (2 equiv) t-BuOH (20 mol%) chiral binol phosphate (5 mol%) CH2Cl2, –10 ºC, 72 h NO2 NC HN H
H N O
95%, 93% ee (R) chiral binol phosphate: NO2
O O P OH O
NO2
Scheme 82. Asymmetric Cyanation of Hydrazones with Chiral BINOL Phosphate Catalyst A chiral ammonium salt-catalyzed enantioselective Strecker reaction was reported by Corey's group (Scheme 83).90 The cinchona alkaloid-derived ammonium salt may have activated the imine substrate with the ammonium proton in the U-shaped chiral environment of the catalyst, and the cyanide attacked on the less-shielded re-face. A series of N-allylbenzaldimines was cyanated with 10 mol% of catalyst loading to give the aminonitriles in >99% ee in the best cases.
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HCN (2 equiv) chiral ammonium salt (10 mol%)
N H3C
H
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TFAA
CH2Cl2, –70 ºC, 40 h F3C NC H3C
O
N H
98%, >99% ee (S) chiral ammonium salt: OCH3 N
N
H N
H
H
O
N N N
O CF3CO2
OCH3 N N H
H O N H
H
O
N N H N N CN
proposed working model
Scheme 83. Asymmetric Cyanation of Aldimines Catalyzed by A Chiral Ammonium Salt Maruoka and Ooi's group developed the asymmetric Strecker reaction of aldimines in a toluene–water two-phase system using aqueous KCN with chiral phase transfer catalysts (Scheme 84).91,92 The quaternary ammonium salt with a tetranaphthyl backbone possessing an axial chirality catalyzed the reaction of N-mesitylenesulfonyl aliphatic aldimines with 1 mol% catalyst loading to give the cyanated products in high ee. Among them the tert-alkyl aldimines were reacted with up to 98% enantioselectivity.91 This catalysis protocol was expanded into the cyanation of in situ-generated aldimines from the N-mesitylenesulfonyl α-amido sulfones. This method improved the yield and enantioselectivity
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(up to 99% enantioselectivity) by diminution of the imine hydrolysis and background cyanation under the biphasic conditions. Reducing the amount of KCN to 1.05 equiv was appropriate for the practical usage.92
N
SO2Mes H
2 M aq. KCN (1.5 equiv) chiral quarternary ammonium salt (1 mol%) toluene–H2O, 0 ºC, 8 h NC HN
SO2Mes
∗ H
95%, 98% ee HN
SO2Mes SO2(p-Tol)
Cbz
N
2 M aq. KCN (1.5 equiv) chiral quarternary ammonium salt (1 mol%) toluene–H2O, 0 ºC, 8 h NC HN
SO2Mes
∗ H Cbz
N 99%, 99% ee
chiral quarternary ammonium salt: Ar Ar Me N
Me
I
Ar Ar
Ar = p-CF3-C6H4
Scheme 84. Asymmetric Cyanation of Aldimines with Chiral Phase Transfer Catalyst Lee and co-workers devised chiral BINOL-based bishydroxy polyethers for the asymmetric Strecker reaction by using KCN as a reagent (Scheme 85).93 The 3,3’-diiodo-substitution, threeether tether unit, and phenolic hydroxy groups of the catalyst were all important in terms of reactivity and enantioselectivity. The reaction of in situ-generated benzaldimines from the corresponding N-tert-butoxycarbonyl α-amido sulfones afforded the α-amino nitriles in up to 99% ee. The reaction of primary alkyl imines was less enantioselective. KCN coordinated by the
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polyether may have reacted with the α-amido sulfone to generate the aldimine, HCN, and potassium benzenesulphinate. Then activated imine and HCN reacted via the polyether catalyst through hydrogen bonding under the chiral circumstance as shown in the scheme. KCN (1.05 equiv) chiral bishydroxy polyether (10 mol%)
NHBoc SO2Ph
NC NHBoc H
toluene, 0 ºC, 60 h CH3O
CH3O 91%, 99% ee (S)
chiral bishydroxy polyether: I
I O
O
O OH
O HO
I
I
I
O
O
O O
I
O H O H I I H O O CN N S
O t-BuO
K
R
Ph
proposed working model
Scheme 85. Asymmetric Cyanation of In Situ-generated Benzaldimine with Chiral Bishydroxy Polyether Catalyst Kunz and co-workers synthesized a chiral N-galactosyl[2,2]paracyclophane aldimine bearing a methoxycarbonyl group and applied the aldimine as a catalyst for the enantioselective Strecker reaction (Scheme 86).94 The aldimine moiety of this catalyst was sterically shielded from nucleophilic reactions. N-benzyl or ally aldimines with isopropy, cyclohexyl, or isoamyl groups were cyanated with high enantioselectivity (up to 99%). The reaction of aromatic aldimines was somewhat less stereoselective. Although the aldimine catalyst contained neither a hydrogenbond-donor nor a Brønsted acidic site, the proton of HCN was trapped with the Lewis basic
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imine nitrogen and the pivaloyl oxygen at the C2 position of the galactose moiety, providing the Brønsted acidic site. Then the substrate imine was dragged electrophilically into the chiral pocket of the catalyst.
N
Ph H
TMSCN (1.2 equiv) MeOH (1.2 equiv) galactopyranosecyclophane cat. (10 mol%) toluene, –50 ºC, 24 h
TFAA NC HN
Ph
H 84%, 99% ee (S) galactopyranose-cyclophane cat.: PivO PivO
OPiv O O N PivO H3CO H N C
R t-Bu t-Bu O O O O t-Bu
O
N
R1
H
O O N CH3O H O t-Bu
O
O proposed working model
Scheme 86. Asymmetric Cyanation of Aldimines with Galactopyranose-cyclophane Catalyst
3. CONJUGATE CYANATION OF α,β-UNSATURATED CARBONYL COMPOUNDS Jacobsen and Sammis developed the (salen)aluminum-catalyzed conjugate addition of HCN to α,β-unsaturated imides (Scheme 87).95 The use of HCN generated in situ from (CH3)3SiCN and 2-propanol was important for the reaction to proceed. No reaction was observed with the use of HCN alone as a cyanide source. The reaction of the imide substrates bearing aliphatic βsubstituents achieved a good result, but the substrates with unsaturated substituents such as aryl,
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vinyl, and alkynyl groups were unreactive. The enantioselectivity was far less sensitive to the steric properties of the substituent, affording the cyanide adducts in up to 98% ee.
TMSCN (2.5 equiv) i-PrOH (2.5 equiv) (salen)Al cat. (10 mol%)
O
O N H
O
O N H NC
toluene, 24 ºC, 26 h
92%, 98% ee (R) (salen)Al cat.:
N
N Al
t-Bu
O t-Bu
Cl
O
t-Bu
t-Bu
Scheme 87. Asymmetric Conjugate Cyanation of α,β-Unsaturated Imides with Salen–Aluminum Catalyst Weck and Madhavan synthesized a polymer-supported (salen)aluminum catalyst showing comparable enantioselectivity with the original one, and demonstrated that the catalyst could be recycled 5 times.96 Jacobsen's group also devised a cooperative heterobimetallic catalyst system for the same reaction (Scheme 88).97 The two chiral metal complexes, (salen)aluminum and (pybox)europium, were considered to activate imides and HCN, respectively, and promoted the conjugate addition in a highly enantioselective manner. The dual catalyst system improved the reactivity over the (salen)aluminum catalyst described above with similar or better enantioselectivity.
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TMSCN (2 equiv) i-PrOH (2equiv) [(salen)Al]2O cat. (2 mol%) (pybox)Eu cat. (3 mol%)
O
O N H
O
O
N H NC
toluene, 23 ºC, 8 h
88%, 97% ee (S) [(salen)Al]2O cat.:
(pybox)Eu cat.: O
N
N
N
Al t-Bu
O
X
O
t-Bu
t-Bu
O
N Cl
Er Cl
N Cl
t-Bu
X = OAl(salen)
Scheme 88. Asymmetric Conjugate Cyanation of α,β-Unsaturated Imides with Heterobimetallic Catalyst System The chiral (salen)aluminum catalyst was applied to asymmetric conjugate cyanation of nitroalkenes by Khan and co-workers (Scheme 89).98 The β-nitronitriles in up to 91% ee were obtained at –25 °C in the presence of 4-phenylpyridine N-oxide (4-PPNO) as a co-catalyst. A series of nitroalkenes with β-alkyl substituents was cyanated with good to high enantioselectivity. Based on the spectroscopic studies, the N-oxide seemed to act as an axial ligand to the (salen)aluminum complex as well as a Lewis basic activator of (CH3)3SiCN. TMSCN ( 1.5 equiv) NO2 (salen)AlCl (5 mol%) 4-PPNO (10 mol%)
CN NO2
toluene, –25 ºC, 24 h 70%, 91% ee (S) (salen)AlCl:
4PPNO: N
N O
N Al
t-Bu
O t-Bu
Cl
O
t-Bu
t-Bu
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Scheme 89. Asymmetric Conjugate Cyanation of Nitroalkenes with a Salen–Aluminum Catalyst System Shibasaki and Kanai and co-workers developed enantioselective conjugate addition of cyanide to various β-substituted α,β-unsaturated N-acylpyrroles catalyzed by a chiral complex prepared in situ from Gd(Oi-Pr)3 and an electronically tuned D-glucose-derived ligand (Scheme 90).99 Use of the combination of (CH3)3SiCN (0.5–1 equiv) and HCN (2 equiv) gave the best result. Various α,β-unsaturated N-acylpyrroles with β-alkyl, aryl, and vinyl substituents were converted to the 1,4-adducts in up to 98% ee. Mechanistic studies suggested that the active species had poly-metallic structures of gadolinium and the chiral ligand with hydroxide moieties as proton donors. The reaction proceeded through an intramolecular cyanide transfer from the gadolinium cyanide to the activated N-acylpyrrole substrate by the Lewis acidic gadolinium. (CH3)3SiCN participated in the regeneration of the active species.
O N
HCN (2 equiv) TMSCN (0.5 equiv) Gd(Oi-Pr)3 (5 mol%) chiral ligand (10 mol%)
CN
N
EtCN, –20 ºC, 42 h
91%, 98% ee (R)
chiral ligand: Ph Ph P O
O
O HO O
F
HO
F
Scheme 90. Asymmetric Conjugate Cyanation of α,β-Unsaturated N-Acylpyrroles Catalyzed by Gadolinium Complex with D-Glucose-derived Ligand
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The modified gadolinium complex successfully catalyzed the conjugate cyanation of α,βunsaturated ketones (Scheme 91).100 The combination of (t-C4H9)(CH3)2SiCN (TBSCN, 2 equiv) and 2,6-dimethylphenol (2 equiv) was effective in this reaction. A range of linear and branched alkyl ketones as well as cyclic ketones was quantitatively converted to the 1,4-aducts in high ee (up to 98% ee). The gadolinium complex also catalyzed transformation of the allylic cyanohydrin (1,2-aduct) into the 1,4-adduct, which could assist the excellent 1,4-addition over 1,2-addition selectivity. The proposed mechanism was somewhat different from the previous reaction of α,βunsaturated N-acylpyrroles. The poly-metallic gadolinium complex with the (t-C4H9)(CH3)2SiO moiety (not with hydroxide) was the active species. HCN (not silyl cyanide) was used for regeneration of the active species with release of the product.
O
TBSCN (2 equiv) 2,6-dimethylphenol (2 equiv) Gd(Oi-Pr)3 (10 mol%) chiral ligand (15 mol%)
CN
O
∗
THF, 0 ºC, 18 h
91%, 98% ee chiral ligand:
Ph
Ph P
O HO O
F
HO
F
Scheme 91. Asymmetric Conjugate Cyanation of α,β-Unsaturated Ketones Catalyzed by Gadolinium Complex with D-Glucose-derived Ligand Shibasaki's group extended the conjugate cyanation to β,β-disubstituted α,β-unsaturated carbonyl compounds using a strontium catalyst (Scheme 92).101 A variety of β,β-
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disubstituted α,β-unsaturated ketones and N-acylpyrroles bearing alkyl and aryl groups reacted with the combination of (t-C4H9)(CH3)2SiCN (2 equiv) and 2,6-dimethylphenol (2 equiv) using 0.5 mol% of the catalyst to afford the β-cyano carbonyl compounds with quaternary carbon centers in up to 99% ee. The E/Z stereoisomers of the substrate were converted to the opposite enantiomers of one another’s products. The strontium complex also catalyzed asymmetric rearrangement of the cyanohydrin (1,2-adduct) to the 1,4-cyanation product. The catalytic species was the Sr/ligand = 3:5 complex, which was observed as a single species in the ESI-MS analysis. It activated both enone and HCN formed in situ, and intramolecular conjugate cyanation occurred irreversibly. This catalyst may also promote the 1,2-cyanation, but the 1,2-adduct readily returned to the enone, and then was converted to the 1,4-adduct. TBSCN (2 equiv) 2,6-dimethylphenol (2 equiv) Sr(Oi-Pr)2 (0.5 mol%) chiral ligand (0.8 mol%)
O
toluene, 40 ºC, 2 h (E)- or (Z)-enone
O
CN ∗
(E)-enone: >99%, 99% ee (+) (Z)-enone: >99%, 99% ee (–) (+) and (–): signs of the optical rotation chiral ligand:
Ar
Ar
i-BuO HO O Ar = p-tol
HO
Scheme 92. Asymmetric Conjugate Cyanation of β,β-Disubstituted α,β-Unsaturated Carbonyl Compounds with Chiral Strontium Catalyst
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Feng and co-workers devised enantioselective conjugate cyanation of alkylidene malonates catalyzed by an in situ formed titanium complex coordinated by cinchonidine and 3,3'-(9phenanthrenyl)-substituted biphenol (Scheme 93).102 The axial chirality of the biphenol ligand could be induced at the formation of the complex. Alkylidene malonates with β-alkyl and aromatic groups were cyanated with good to high enantioselectivity (up to 94%) under a solventfree condition. Use of ethyl cyanoformate as a cyanide source resulted in better enantioselectivity than that with (CH3)3SiCN. O (5 equiv) NC OEt i-PrOH (5 equiv) Ti(Oi-Pr)4 (10 mol%) ligand 1 (10 mol%) COOEt ligand 2 (10 mol%) COOEt neat, 0 ºC, 72 h
CN COOEt
∗
COOEt 94%, 94% ee
ligand 2:
ligand 1:
R H
N
OH OH
OH N cinchonidine
R R = 9-phenanthryl
Scheme 93. Asymmetric Conjugate Cyanation of Alkylidene Malonates Catalyzed by Chiral Titanium Complex with Two Ligands According to the bifunctional catalyst concept, Wang and co-workers designed a dinuclear bis(prophenol)–magnesium catalyst for conjugate cyanation of α,β-unsaturated ketones (Scheme 94).103 The dual-mode coordination in the dinuclear catalyst could activate both enone and cyanide, and then the cyanation occurred in a stereospecific manner while avoiding the substrate rotation expected in a mononuclear catalysis. Chalcone derivatives reacted with (CH3)3SiCN (2
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equiv) and 2,6-di-tert-butylphenol (1.5 equiv) at 30 °C with 20 mol% of catalyst to afford the 1,4-adducts in up to 97% ee.
O
TMSCN (2 equiv) Bu2Mg (20 mol%) chiral ligand (20 mol%) 2,4-di-tert-butylphenol (1.5 equiv)
O
THF, MS 4Å, 30 ºC, 10 h
O
O
CN O O
92%, 97% ee (R) chiral ligand: Ph
OH
HO
Ph N
OH
Ph Ph
N
Scheme 94. Asymmetric Conjugate Cyanation of α,β-Unsaturated Ketones with Dinuclear Bis(prophenol)–Magnesium Catalyst A ruthenium–lithium combined system devised by Ohkuma and co-workers exhibited remarkable catalyst performance in asymmetric conjugate cyanation of α,β-unsaturated ketones and the N-acylpyrroles with various alkyl, heterosubstituted alkyl, and aryl groups at the β position to afford the 1,4-adducts in up to 98% ee and 99% ee, respectively (Scheme 95).104 In both cases, even 0.2 mol% of the ruthenium complex and LiOPh worked effectively under mild conditions. The ruthenium complex was so robust that it could be recovered with a silica-gel column and was reused 5 times without loss of catalytic efficiency. Based on the X-ray and NMR spectroscopic analyses, a reaction mechanism was proposed in which Li+ interacting with the carbonyl oxygen of phenylglycine on the ruthenium complex-activated substrate enone and
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CN– located between two amino protons with hydrogen bonding attacked the β-position of the enone. HCN (3 equiv) Ru complex (0.2 mol%) LiOPh (0.2 mol%)
O
CN
O
t-BuOMe, 0 ºC, 5 h 99%, 94% ee (S) HCN (3 equiv) Ru complex (0.2 mol%) LiOMe (0.2 mol%)
O N
CN
O N
t-BuOMe, 0 ºC, 12 h 97%, 91% ee (S) Ru complex: O Ph2 O P
NH2
Ru
NH2
P Ph2 O O Li O H
O N
P
O P
N H H
Ru O H C N O
P N
P
:
P H H
P
proposed working model
Scheme 95. Asymmetric Conjugate Cyanation of α,β-Unsaturated Carbonyl Compounds with Chiral Ruthenium–Lithium Combined Catalyst Chen and co-workers reported asymmetric conjugate cyanation of less reactive chalcone analogues with benzophenone cyanohydrin in the presence of a ternary catalyst system of the 6,6'-adamantyl-substituted BINOL-derived phospholic acid, NaNH2, and 2-tert-butylphenol (Scheme 96).105 A series of substituted chalcones was transformed to the β-cyano ketones in 92– 98% ee with 5–10 mol% of the catalyst at 80 °C. The sodium BINOL-derived phosphate was proposed as the catalytic species activating the HCN formed in situ.
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HO CN
(2 equiv) Ph Ph chiral phosphoric acid catalyst (10 mol%) NaNH2 (10 mol%) 2-t-Bu-C6H4OH (5 mol%)
O
OCH3
toluene, 80 ºC, 2 h O
CN
OCH3 90%, 98% ee (R) chiral phosphoric acid: Ad O O P O OH Ad
Scheme 96. Asymmetric Conjugate Cyanation of Chalcones with BINOL-derived Phosphate Catalyst Shibata and co-workers reported that the ammonium salt of cinchona alkaloid methyl ether, N[2,5-bis(trifluoromethyl)phenyl]
methyl-O-methylquinidinium
bromide,
catalyzed
the
enantioselective conjugate cyanation of β-aryl-β-trifluoromethyl-substituted enones (Scheme 97).106 The O-methylquinidinium catalyst was more enantioselective than the conventional hydroxy cinchona alkaloids. Use of diisopropyl ether as a solvent was also important. The reaction using acetone cyanohydrin with 10 mol% of the catalyst at 0 °C gave the β-cyano ketones bearing a quaternary stereocenter in up to 97% ee.
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HO CN O CF3
(3 equiv) O NC
Cs2CO3 (1.2 equiv) catalyst (10 mol%)
CF3
i-Pr2O, 0 ºC, 12 h Cl
Cl
99%, 96% ee (R) catalyst:
CH3O Br
CH3O N
CF3
N F3C
Scheme 97. Asymmetric Conjugate Cyanation of β-Aryl-β-trifluoromethyl-substituted Enones Catalyzed by Ammonium Salt of Cinchona Alkaloid Chiral phase transfer catalysis has been developed as an effective strategy for the activation of practical cyanation reagents, such as KCN and acetone cyanohydrin. Deng's group utilized accessible cupreine- or cupreidine-derived salt as the catalyst and acetone cyanohydrin as a cyanide source for asymmetric conjugate cyanation of β-alkylsubstituted α,β-unsaturated ketones and N-acylpyrroles with enantioselectivities as high as 97% and 98%, respectively (Scheme 98).107 They designed the catalyst based on the bifunctional (quaternary ammonium moiety and hydrogen-bond-donor group) catalyst concept and optimized the structure with careful analysis of the X-ray data.
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HO CN O
(2 equiv) O
Cs2CO3 (1.2 equiv) PTC (5 mol%) toluene/CHCl3, rt, 72 h HO CN O
CN
80%, 97% ee (S)
(2 equiv) O
Rb2CO3 (1.2 equiv) PTC (10 mol%)
N
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CN
N
toluene/CHCl3, rt, 48 h 79%, 97% ee (S)
PTC (Phase Transfer Catalyst): OH
PYR: OPYR N
N
Ph
N
Ph Cl
N
Ph
Br Ph
Scheme 98. Asymmetric Conjugate Cyanation of α,β-Unsaturated Carbonyl Compounds with Chiral Phase Transfer Catalyst Maruoka and co-workers devised an enantioselective conjugate cyanation of alkylidene malonates with a simple cyanide source, KCN, in the presence of a chiral bifunctional ammonium bromide under cyclopentane–water biphasic conditions (Scheme 99).108 Addition of Brønsted acids such as NH4Cl and HCl was essential to accelerate the slow ammonium cyanide regeneration from the intermediate malonyl anion ammonium salt. The reaction of di-tert-butyl alkylidene malonates and KCN was promoted with 2 mol% of the phase-transfer catalyst to provide the cyanated products in up to 93% ee.
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KCN (2 equiv) PTC (2 mol%) COOt-Bu NH4Cl (1 equiv) cyclopentane/H2O, COOt-Bu 0 ºC, 24 h
CN COOt-Bu COOt-Bu 68%, 93% ee (R)
PTC (Phase Transfer Catalyst): Ar Ar OH N
CH3 CH3
Br
OH Ar Ar
Ar = 3,5-(CF3)2-C6H3
Scheme 99. Asymmetric Conjugate Cyanation of Alkylidene Malonates with Chiral Phase Transfer Catalyst 4. CYANATION OF UNACTIVATED ALKENES 4-1. Hydrocyanation Casalnuovo and RajanBabu and co-workers found that C2-chiral diarylphosphinites derived from D-glucose
were efficient ligands for the nickel-catalyzed asymmetric hydrocyanation of
vinylarenes (Scheme 100).109 The enantioselectivity was highly dependent on electronic features of the phosphinite ligands. When the ligand had electron-deficient 3,5-(CF3)2C6H3 as a P-aryl substituent, the hydrocyanation of 6-methoxy-2-vinylnaphthalene (MVN) afforded the Sconfigured branched nitrile in up to 91% ee. The catalyst with the P-3,5-(CH3)2C6H3 moiety gave the product in only 16% ee. Moderate enantioselectivities were obtained in the reaction of styrenes under the optimized conditions. The nickel catalysts with C1-chiral diarylphosphinites prepared from D-fructofranoside with two electronically different P-aryl groups promoted the hydrocyanation to yield the R products in up to 95% ee.
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Mechanistic studies of the reaction with MVN suggested that the chiral nickel(0) complex suffered an HCN oxidative addition and MVN coordination followed by formation of an (η3benzyl)nickel cyanide complex with a nickel-hydride migration onto MVN, and then an irreversible reductive elimination of the product nitrile regenerated the catalytic cycle intermediate. HCN (1 equiv) Ni(COD)2 (1 mol%) Ligand (1 mol%) heptane, 0 ºC
H3CO
CN
H3CO >99% conv., 91% ee (S)
Ligand: Ph
O O O Ar2P
O
Ar = OPh
O Ar2P
F3C
CF3
Scheme 100. Asymmetric Hydrocyanation of Vinylarenes with Nickel Catalyst Bearing a Chiral Diarylphosphinite Ligand Nickel-catalyzed hydrocyanation of 1,3-cyclohexadiene using a chiral phosphite ligand with a binaphthol backbone was investigated by Vogt and co-workers (Scheme 101).110 The reaction with 0.2 mol% of the catalyst afforded 2-cyclohexene-1-carbonitrile in moderate yield and in high ee of 86%. Styrene derivatives were cyanated with moderate enantioselectivity. Mechanistic studies including deuterium-labeling experiments reveled that the enantioselectivity was determined at the reductive elimination process.
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HCN Ni(COD)2 (0.2 mol%) chiral Ligand (0.2 mol%)
CN
toluene, 4 h, 0 ºC 45%, 86% ee (R) chiral Ligand:
O CH3
O
O P O O P O
2
2
O O
Scheme 101. Asymmetric Hydrocyanation of 1,3-Cyclohexadiene with Nickel Catalyst Bearing a Chiral Phosphinite Ligand Schmalz and co-workers devised taddol-derived phosphine–phosphite as a tunable chiral ligand, and utilized it in the nickel-catalyzed asymmetric hydrocyanation of vinylarenes, yielding chiral nitriles with good to high enantioselectivity (Scheme 102).111 The reaction of indene gave the highest enantioselectivity of 97%. p-Isobutyl-substituted styrene was also cyanated in 92% enantioselectivity. The ligand structure, reaction conditions, and cyanide source should be carefully chosen. Slow addition of (CH3)3SiCN to the methanol solution while maintaining a low concentration of HCN through the reaction was important to obtain high enantioselectivity. TMSCN (1.5 equiv) Ni(COD)2 (5 mol%) chiral Ligand (5 mol%)
CN ∗
THF/MeOH, rt, 2 h 73%, 97% ee chiral ligand:
O O
O
P O
O
PPh2
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Scheme 102. Asymmetric Hydrocyanation of Vinylarenes with Nickel Catalyst Bearing a Chiral Phosphine–Phosphite Ligand 4-2. Intramolecular carbocyanation Jacobsen and a co-worker accomplished the enantioselective intramolecular arylcyanation of benzonitriles linked with an alkenyl moiety through a tether via Caryl−CN bond activation using 5–10 mol% of nickel catalyst and 10–20 mol% of BPh3 co-catalyst (Scheme 103).112 The chiral alkyl diphosphine TangPHOS gave the highest enantioselectivity. This method generated two new C−C bonds and one new quaternary carbon stereogenic center in a single synthetic step, affording the 1,1-disubstituted indanes in 49−85% yield and 92−97% ee. The corresponding cyanated benzopyran was obtained in lower ee, and the benzofuran analogue could not be obtained due to the formation of an inactive π-allyl–nickel complex. The addition of Lewis acidic BPh3 contributed to oxidative cleavage of the Caryl–CN bond by the nickel(0) complex, and the identity of the Lewis acid affected the enantioselectivity. Subsequent migratory insertion resulted in construction of an indane structure with a quaternary chiral center, and then reductive elimination formed the Csp3–CN bond with regeneration of the nickel(0) species.
CN
NiCl2•DME (5 mol%) (S,S,R,R)-TangPHOS (9 mol%) BPh3 (10 mol%) Zn (10 mol%)
NC
toluene, 105 ºC, 40 h 85%, 93% ee (R) (S,S,R,R)-TangPHOS: H P H P t-Bu t-Bu
Scheme 103. Asymmetric Intramolecular Arylcyanation with TangPHOS–Nickel Catalyst
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Nakao, Hiyama, Ogoshi, and co-workers successfully utilized a nickel catalyst having a chiral phosphinoxazoline ligand, i-Pr-Foxap or i-Pr-Fox, and AlMe2Cl as a co-catalyst for the enantioselective intramolecular arylcyanation (Scheme 104).113 The reaction occurred in an exclusive exo-dig manner to give a range of cyanated indoline derivatives bearing a benzylic quarternary stereogenic center in high yields and in up to 97% ee. The Chiraphos–nickel complex effected the enantioselective formation of a six-membered ring. AlMe2Cl seemed to promote the formation of the η2-nitrile–nickel intermediate. This procedure was applied to the asymmetric synthesis of the bioactive compound and the intermediate.
CH3O
Ni(cod)2 (10 mol%) (R,R)-i-Pr-Foxap (20 mol%) AlMe2Cl (40 mol%)
CN
DME, 100 ºC, 10 h
N
CN
CH3O N
88%, 96% ee (S)
CH3O
Ni(cod)2 (5 mol%) (R,R)-ChiraPhos (6 mol%) AlMe2Cl (20 mol%)
CN
neat, 120 ºC, 1 h CN
98%, 92% ee (R)
(R,R)-i-Pr-Foxap: O N Ph2P Fe
(R,R)-ChiraPhos: PPh2 PPh2
Scheme 104. Asymmetric Intramolecular Arylcyanation with Nickel Catalysts Bearing Chiral Phosphine-based Ligands
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Takemoto
and
co-workers
reported
a
chiral
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palladium-catalyzed
intramolecular
cyanoamidation of alkenyl cyanoformamides to give the enantio-enriched 3,3-disubstituted oxindoles (Scheme 105).114 The reaction using a catalyst system consisting of Pd(dba)2, a chiral phosphoramidite ligand, and N,N-dimethylpropylene urea (DMPU) in decalin at 100 °C provided a series of 3,3-disubstituted oxindoles in up to 86% ee. The reaction appeared to proceed through oxidative addition of the CO–CN bond to palladium, followed by amidopalladation of the olefin and reductive elimination.
CN N Bn
O
Pd(dba)2 (2 mol%) phosphoramidite ligand (8 mol%) DMPU (100 mol%) decalin, 100 ºC, 24 h
CN O N Bn >99%, 81% ee (S)
CN N Bn
O
Pd(dba)2 (5 mol%) phosphoramidite ligand (10 mol%) DMPU (100 mol%) decalin, 100 ºC, 24 h
CN
∗
O
N Bn 44%, 86% ee
phosphoramidite ligand: Ph O P N O Ph
Scheme 105. Asymmetric Intramolecular Cyanoamidation with Palladium Catalyst Bearing a Chiral Phosphoramidite Ligand 4-3. Aminocyanation Recently, two groups independently reported the intramolecular aminocyanation of alkenes with N–CN bond cleavage.
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Douglas and co-workers devised a Lewis acid-promoted reaction of alkenes including an Ncyano-N-phenyl-p-toluene sulfonamide group.115 The reaction with a stoichiometric amount of B(C6F5)3 in toluene at 90 °C afforded the racemic cyanomethyl-substituted indolines and tetrahydroquinolines in high yield. The reaction was proposed to proceed through nucleophilic addition of the alkene to the nitrile carbon, which was activated by coordination of the nitrile onto B(C6F5)3, followed by the C–N bond cleavage. Nakao's group developed the aminocyanation of alkenes with an N-Boc-N-cyano-aniline moiety catalyzed by a palladium/triorganoboron cooperative system with N–CN bond cleavage to give the indoline derivatives.116 The high chemo- and regioselectivities were achieved with an optimized catalyst consisting of CpPd(allyl), 4,5-Bis(diphenyl phosphino)-9,9-dimethylxanthene (Xantphos), and triethyl- or triphenylborane. A series of substituted indolines and pyrrolidines with both tetra- or trisubstituted carbon and cyano functionalities were readily obtained by this procedure. In regard to the mechanism, it was considered that oxidative addition of the N−CN bond to palladium(0) catalyst was promoted by the cyano-group coordination to the boron Lewis-acid co-catalyst. Aminopalladation in an exo-trig manner was followed by reductive elimination with formation of the C−CN bond to release the boron-bound indoline product, and then transfer of the boron compound to the unreacted cyanamide substrate regenerated the palladium catalyst and the cyanamide−boron complex. An asymmetric version of this reaction was furnished by using a chiral bisphosphine ligand (R,R,R)-Ph-SKP (Scheme 106).116 The indoline product in 93% ee was obtained successfully.
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Boc
N CN
CpPd(allyl) (10 mol%) (R,R,R)-Ph-SKP (10 mol%) BPh3 (40 mol%) toluene, 80 ºC, 3 h
Boc N
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CN
∗ 82%, 93% ee
(R,R,R)-Ph-SKP:
O O PPh2 Ph2P
Scheme 106. Asymmetric Intramolecular Aminocyanation with Palladium Catalyst Bearing a Chiral Bisphosphine Ligand 5. SUMMARY We have reviewed the asymmetric cyanation of unsaturated compounds, such as aldehydes, ketones, imines, and unactivated alkenes, as well as the conjugate cyanation of α,β-unsaturated carbonyl compounds. A variety of enantiomerically enriched nitrile compounds are obtained by the use of appropriately designed catalysts. For nucleophilic cyanation, Lewis- or Brønsted-acid catalysts interract with the polar substrates to increase the reactivity toward nucleophiles, and the basic catalysts activate cyanide reagents to increase their nucleophilicity. The acid/base bifunctional catalysts activate both substrates and reagents, conducting the cyanation in an intramolecular manner. Many metal-complex catalysts and organocatalysts successfully promote this type of cyanation. Some titanium and ruthnium–lithium combined catalysts have achieved complete conversion in the reaction with a 0.01 mol% catalyst loading. Close to perfect enantioselectivity has been achieved by constructing a well organized chiral environment of the catalysts, as shown in the main text. To improve the catalyst efficiency and expand the substrate scope, novel concepts for the catalysis and the reaction field might be required. A recently
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devised catalyst connected in a metal-organic framework (MOF) would be a candidate for realizing these goals. The cyanation of unactivated alkenes was efficiently catalyzed by chiral transition metal complexes. The reaction proceeds through oxidative cleavage of H–CN, C–CN, and N–CN bonds followed by insertion of carbon–carbon double bonds. Less-polar olefinic substrates are appropriate for this reaction. Addition of Lewis acidic compounds interacting with the CN: moiety promotes activation of the C–CN and N–CN bonds. Chiral nitrile products with tertiary or quaternary stereocenters are available in high ee. Studies on this type of reaction are relatively rare, and may constitute a novel class of chemical research as in the case of catalytic C–CN and N–CN bond cleavage. We expect that the catalyst efficiency, substrate scope, and variety of reactions conld be improved in the near future. AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected] Notes The authors declare no competing interest. ACKNOWLEDGMENT This work was supported in part by Grants-in-Aid from the Japan Society for the Promotion of Science (JSPS) (No. 15H03802 and No. 25410031) and the MEXT (Japan) program "Strategic Molecular and Materials Chemistry through Innovative Coupling Reactions" of Hokkaido University.
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REFERENCES †
Hokkaido University
‡
Hamamatsu University School of Medicine
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