Phosphorus Chemistry - American Chemical Society

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106 Structure and Reactivity of Quasiphosphonium Intermediates H. R. HUDSON, A. T. C. KOW, and K. HENRICK

Downloaded by UNIV LAVAL on July 13, 2016 | http://pubs.acs.org Publication Date: November 11, 1981 | doi: 10.1021/bk-1981-0171.ch106

Department of Chemistry, The Polytechnic of North London, Holloway Road, London N7 8DB, England

New quasiphosphonium halides derived from neopentyl diphenyl­ phosphinite and dineopentyl phenylphosphonite are reported and it is shown that the attachment of phenyl groups to phosphorus pro­ vides enhanced stability. The products, Ph2P(OR)MeX (X = Cl, Br, or I) and PhP(OR)MeX (X = Br or I) (R = Me CCH ), are resistant to moist air and can be handled in the open laboratory for the purpose of X-ray diffraction studies (1). The rate of initial reaction between the ester and an alkyl halide increases with the number of phenyl substituents on phosphorus; from the diphenyl­ phosphinite an adduct with chloromethane can be prepared. P chemical shifts (downfield from 85% H3ΡΟ4) of +72 ppm for PhP(OR)MeX (X = C l , Br, or I) and of +74 ppm for PhP(OR)MeX (X = Br or I) confirm the phosphonium structure in solution. Although phosphorus (V) intermediates have been shown to occur in reactions of certain phosphorus (III) esters with halogens (2), their role in Michaelis-Arbuzov reactions involving alkyl halides is less certain. In no cases have phosphorus (V) intermediates been detected by P nmr and the stereochemical evidence for their involvement (3) appears to require further investigation (4). In solvents such as chloroform, quasiphosphonium intermediates exist essentially as ion-pairs (5). To gain information on the kinetics and mechanism of the product-forming stage of the Michaelis-Arbuzov reaction, we have followed the decomposition of intermediates by *H nmr spectroscopy. In CDCI3 the reactions follow excellent first-order kinetics (Table I) and are in accord with rate-determining collapse to products within the undissociated ion-pair (Figure 1). Synchronous 2

3

2

31

2

2

31

L —P—0—R X I

1 >· —P=0 + RX I Figure 1. attack by halide ion and alkyl-oxygen fission are indicated by absence of rearrangement in the neopentyl group and although the process may be considered to be of the S 2-type i t is not 0097-6156/81/0171-0517$05.00/0 © 1981 American Chemical Society

Quin and Verkade; Phosphorus Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

518

PHOSPHORUS

Table I .

F i r s t - o r d e r decompositions t/°C CI

Ph P(OR)MeX

33 60 33 60 33

2

PhP(0R) MeX 2

(R0) PMeX Downloaded by UNIV LAVAL on July 13, 2016 | http://pubs.acs.org Publication Date: November 11, 1981 | doi: 10.1021/bk-1981-0171.ch106

3

i n CDCI3 1

lO^/s V

Compound

Br

1.5 92 -

CHEMISTRY

I

1.54 74 114 4670 670

2.75 105 148 5260 1100

b i m o l e c u l a r , as k i n e t i c a l l y s e p a r a t e s p e c i e s a r e n o t i n v o l v e d . A s i m i l a r p r o c e s s has b e e n r e p o r t e d f o r the d e c o m p o s i t i o n o f n e o p e n t y l o x y t r i p h e n y l p h o s p h o n i u m c h l o r i d e i n carbon t e t r a c h l o r i d e ( 6 ) . F i r s t o r d e r k i n e t i c s a r e a l s o c o n s i s t e n t w i t h a sequence w h i c h i n v o l v e s r a t e - d e t e r m i n i n g d i s s o c i a t i o n o f the i o n - p a i r ( F i g u r e 2, k__^ B r > I ) a n d t h a t w h i c h i s observed i n h y d r o x y l i c s o l v e n t s (Cl