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
