Letter Cite This: Org. Lett. 2018, 20, 1291−1294
pubs.acs.org/OrgLett
Rhodium-Catalyzed Asymmetric Cyclization/Addition Reactions of 1,6-Enynes and Oxa/Azabenzonorbornadienes Yongyun Zhou,† Lu Yu,† Jingchao Chen,† Jianbin Xu,† Zhenxiu He,† Guoli Shen,† and Baomin Fan*,†,‡ †
YMU-HKBU Joint Laboratory of Traditional Natural Medicine and ‡Key Laboratory of Chemistry in Ethnic Medicinal Resources, Yunnan Minzu University, Kunming, Yunnan 650500, People’s Republic of China S Supporting Information *
ABSTRACT: A mild, efficient, and novel rhodium catalyzed asymmetric cyclization−addition domino reaction of oxa/ azabenzonorbornadienes and 1,6-enynes is documented. Through the use of a [Rh(COD)2]BF4-(R)-An-SDP catalytic system, highly enantioenriched cyclization−addition products were obtained in good yields and with excellent enantioselectivities.
T
between oxa/azabenzonorbornadienes and 1,6-enynes and a rhodium-catalyzed cascade cyclization/addition reaction which achieved high enantioselectivities. Determination of the proper chiral ligand for the Rh catalyst was the major target of our initial experiments. Using enyne 1a and N-Boc-azabenzonorbornadiene 2a as benchmark substrates and [Rh(COD)2]BF4 as a rhodium precursor, bisphosphine ligands with different diaryl chiral backbones were screened (Table 1), and except for (R)-DTBM-segphos, (R,R)-DIOP, and (R,R)-Me-DuPhos, all of the evaluated ligands were found to be effective in this reaction and provided the cyclization− addition domino product 3aa with good enantioselectivities (entries 1−5, and 7−11). Among these reactions, (R)-An-SDP was found to be the most effective chiral ligand, resulting in 78% yield with 98% ee for 3aa (entry 11). To further improve the reaction yield, we next investigated variations in reaction conditions (Table 2). The cyclization− addition domino reactions were performed in several aprotic solvents (entries 1−6), and dichloroethane resulted in improved yield, while similar outcomes were observed using rhodium precursors other than [Rh(COD)2]BF4 (entries 7 and 8). Additionally, lowering the reaction temperature improved both yield and enantioselectivity (entries 9 and 10), while increasing the reaction temperature reduced both (entry 11). Therefore, the results of our investigations indicated that the optimal reaction conditions involved the use of [Rh(COD)2]BF4 and (R)-An-SDP as catalysts, DCE as the solvent, and a temperature of 40 °C. With the further optimized reaction conditions in hand, various 1,6-enynes were reacted with norbornene derivatives to investigate the scope of this domino reaction (Table 3). In
he desymmetrization reactions of meso organic substrates have provided efficient methods to produce chiral compounds, especially those with multiple stereocenters,1 and the transition-metal-catalyzed asymmetric reactions of oxa/ azabenzonorbornadienes with various reactants represent typical and successful examples of such a strategy. The use of different chiral catalysts with various reactions of oxa/ azabenzonorbornadienes, including asymmetric ring-opening reactions,2 hydroalkynylation reactions,3 [2 + 2] cycloaddition reactions,4 [2 + 1] cycloaddition reactions,5 Pauson−Khand reactions,6 hydroarylation reactions,7 hydroamination reactions,8 and others,9 have resulted in the successful synthesis of desired products with high enantioselectivities. Additionally, some of these reactions have been used as key steps in the synthesis of natural or bioactive molecules by the group of Lautens.10 Despite thes e various successes, it is still desirable and possible to develop new desymmetrization reactions of oxa/azabenzonorbornadienes, especially for those with complex structures. Alternatively, 1,6-enynes have been previously utilized as versatile reagents in the construction of chiral cyclic molecules.11 The asymmetric reactions of 1,6-enynes have been extensively studied over the past several decades, and various cycloadditions, additive cyclizations, and cyclic isomerizations of 1,6-enynes, with or without other reagents, have been reported. Thus, it is quite interesting to explore the transition-metal-catalyzed asymmetric reactions between oxa/ azabenzonorbornadienes and 1,6-enynes. Recent research efforts in our laboratory have focused on the desymmetrization reactions of oxa/azabenzonorbornadienes, including the development of efficient chiral catalysts or catalytic systems for the asymmetric [2 + 2] cyclization,12 hydroalkynylation,13 and ring-opening reactions14 of oxa/ azabenzonorbornadienes. To extend these previous efforts, herein we report the investigation of asymmetric reactions © 2018 American Chemical Society
Received: December 28, 2017 Published: February 13, 2018 1291
DOI: 10.1021/acs.orglett.7b04044 Org. Lett. 2018, 20, 1291−1294
Letter
Organic Letters Table 2. Optimization of the Reaction Conditionsa
Table 1. Optimization of the Reaction Conditions for the Rhodium-Catalyzed Asymmetric Reaction of 1a with 2aa
entry
chiral ligand
time (h)
yieldb (%)
eec (%)
1 2 3 4 5 6 7 8 9 10 11 12 13
(R)-synphos (R)-segphos (R)-H8-binap (R)-P-Phos (S)-MeO-biphep (R)-DTBM-segphos (R)-DM-segphos (R)-Xyl-SDP (R)-SDP (R)-Tol-SDP (R)-An-SDP (R,R)-DIOP (R,R)-Me-DuPhos
48 45 60 60 72 36 84 22 45 47 36 48 48
46 41 34 39 44 trace 36 27 64 71 78 NR NR
93 91 86 80 91
entry
Rh precursor
temp (°C)
solvent
yieldb (%)
eec (%)
1 2 3 4 5 6 7 8 9 10 11
[Rh(COD)2]BF4 [Rh(COD)2]BF4 [Rh(COD)2]BF4 [Rh(COD)2]BF4 [Rh(COD)2]BF4 [Rh(COD)2]BF4 [Rh(COD)2]OTf [Rh(COD)2]SbF6 [Rh(COD)2]BF4 [Rh(COD)2]BF4 [Rh(COD)2]BF4
60 60 60 60 60 60 60 60 25 40 80
DCE THF toluene MTBE DME 1,4-dioxane DCE DCE DCE DCE DCE
78 68 45 51 74 63 72 78 80 84 63
98 97 97 97 96 98 98 98 99 99 96
a Reaction conditions: 1a (0.2 mmol), 1a/2a/Rh/(R)-An-SDP (1:2:0.05:0.065), in solvent (2 mL) under N2 for indicated period of time. bIsolated yield after column chromatography. cThe ee values were determined by HPLC using a Chiralcel AS-H column. THF = tetrahydrofuran, DME = 1, 2-dimethoxyethane, MTBE = tert-butyl methyl ether.
elimination of the allylic hydrogen results in rhodium hydride E, and coordination of E with N-Boc-azabenzonorbornadiene 2a produced intermediate F. The alkene inserted into the Rh−C bond to generate G, and the subsequent exchange reaction of G, finally resulted in 3aa as the product of the domino reaction.15 In summary, we have successfully demonstrated the rhodium-catalyzed asymmetric cyclization−addition domino reaction of 1,6-enynes and oxa/azabenzonorbornadienes. By means of this desymmetrization reaction, the cyclization− addition products were synthesized in moderate to good yields with excellent enantioselectivities. Furthermore, various 1,6enynes and oxa/azabenzonorbornadienes with different substituents on the phenyl rings were viable within this rhodium catalytic system. Some other fascinating reactions of enynes and norbornadienes are under further investigation.
90 92 97 97 98
a
Reaction conditions: 1a (0.2 mmol), 1a/2a/[Rh(COD)2BF4]/ligand (1:1.5:0.05:0.065) in DCE (2 mL) at 60 °C under N2 for the indicated period of time. bIsolated yield after column chromatography. cThe ee values were determined by HPLC using a Chiralcel AS-H column. COD = 1,5-cyclooctadiene, DCE = 1,2-dichloroethane.
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general, most of the reactions between various 1,6-enynes and oxa/azabenzonorbornadienes proceeded smoothly to yield the corresponding cyclization−addition products, although azabenzonorbornadienes produced better enantioselectivities than oxabenzonorbornadienes (entries 1−13 vs 14−17). 1,6-Enynes with a nitrogen atom were expected to produce favorable yields, although low to moderate yields were observed for 1e, 1f, and 1g (entries 5−7). Interestingly, the presence of substituents on the phenyl rings appeared to have little effect on the enantioselectivities of the reaction, as all of the benzonorbornadienes readily reacted with 1,6- enynes to produce excellent enantioselectivities (entries 9−17). It is particularly noteworthy that the steric hindrance caused by the substituents did not have deleterious effect on the yields and ee values of the products (entries 15 and 16). The absolute configurations of the products were identified by X-ray analysis of 3ag (Figure 1). The proposed mechanism for this novel cyclization−addition domino reaction is outlined in Figure 2. The catalytic cycle is initiated by the reaction between the (R)-An-SDP/[Rh(COD)2]BF4 complex and 1, 6-enyne B to provide the complex C, which is transformed into the rhodacyclopentene species D by cyclo-isomerization. Subsequent β-hydride
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b04044. Experimental details and characterization data (PDF) Accession Codes
CCDC 1813368 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
* E-mail:
[email protected]. ORCID
Baomin Fan: 0000-0003-1789-3741 1292
DOI: 10.1021/acs.orglett.7b04044 Org. Lett. 2018, 20, 1291−1294
Letter
Organic Letters Table 3. Substrate Scope of 1 and 2a
a Reaction conditions: 1 (0.2 mmol), 1/2/Rh/(R)-An-SDP (1:1.5:0.05:0.065), in DCE (2 mL) at 40 °C under N2. bIsolated yield. cThe ee values were determined by HPLC using a Chiralcel AS-H, AD-H, or OD-H column. dThe reaction was carried out at 50 °C. eThe reaction was carried out at room temperature.
Figure 1. X-ray analysis of 3ag.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully thank the National Natural Science Foundation of China (21302162, 21572198, 21362043), the Applied Basic Research Project of Yunnan Province (2017FA004), and the Department of Education of Yunnan Province (2017ZDX046, ZD2015012) for their financial support.
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Figure 2. Proposed mechanism for cyclization−addition domino reaction of 1,6-enynes with azabenzonorbornadiene. 345, 835−848. (f) Rejzek, M.; Stockman, R. A.; van Maarseveen, J. H.; Hughes, D. L. Chem. Commun. 2005, 4661−4662. (2) (a) Cho, Y. H.; Zunic, V.; Senboku, H.; Olsen, M.; Lautens, M. J. Am. Chem. Soc. 2006, 128, 6837−6846. (b) Cho, Y. H.; Fayol, A.; Lautens, M. Tetrahedron: Asymmetry 2006, 17, 416−427. (c) Nishimura, T.; Tsurumaki, E.; Kawamoto, T.; Guo, X. X.; Hayashi, T. Org. Lett. 2008, 10, 4057−4060. (d) Fan, B.; Li, S.; Chen, H.; Lu, Z.; Liu,
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DOI: 10.1021/acs.orglett.7b04044 Org. Lett. 2018, 20, 1291−1294
