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pH-Rate Profiles for the Oxygen-18 Exchange and Epimerization of a Phosphetane Oxide. Rate-Limiting Pseudorotation David G. Gorenstein Contribution,from the Department of Chemistry, University of Illinois at Chicago Circle, Chicago, Illinois 60680. Received August 16, 1971 Abstract: pH-rate profiles for epimerization a n d I 8 0 exchange of l-pheny1-2,2,3-trimethylphosphetane 1-oxide are presented. The profiles show that the rates of epimerization a n d exchange are proportional to hydroxide ion concentration at p H 9-12 and are independent of acidity between p H 4 and 8. Epimerization is slower than exchange by factors of 5-140 in this p H range. In the acid region, exchange is much faster (lo4-fold at 4 M H C 1 ) than epimerization, and only the former reaction is proportional to the hydrogen ion concentration. A rate maximum observed in dilute acid for the slower epimerization reaction is interpreted as evidence that pseudorotation of the pentacovalent intermediates is rate limiting. A rate maximum observed for exchange in strong acid is ascribed to a decrease in the activity of water.
R
ecent attention has been focused upon the interpretation of the rate maxima observed in the acid hydrolyses of phosphate esters and phosphinate esters. * Rate maxima in 1-5 N acid have been generally ascribed to a decrease in the activity of water1f3while the rate maximum in dilute acid for a strained bicyclic phosphinate ester has been interpreted as evidence for ratelimiting p s e u d ~ r o t a t i o nof~ a pentacovalent intermediate.2,j We now wish to report the first example of a compound which shows both a rate maximum in strong acid due to a decrease in the water activity and a rate maximum in dilute acid due to rate-limiting pseudorotation. This study also presents the first pH-rate profiles for epimerization and l80exchange of a phosphine oxide, l-phenyl-2,2-3-trimethylphosphetane 1-oxide (1). It is hoped that the study will further help to unravel
cis-1
the factors responsible for the disparate results previously observed in the reactions of phosphine oxides6-12 and firmly establish the important role played by the pentacovalent intermediate in the reactions of these compounds. (1) C. A . Bunton, Accounts Chem. Res., 3,257 (1970), and references therein. (2) R. Kluger and F. H. Westheimer, J . Amer. Chem. Soc., 91, 4143 (1969). (3) P. Haake and G. Hurst, ibid., 88, 2544 (1966). (4) F. H. Westheimer, Accounts Chem. Res., 1, 70 (1968). (5) The dilute acid maximum in the pH-product profile for the hydrolysis of methyl ethylene phosphate represents the only other example of such dilute acid maxima: R. Kluger, F. Covitz, E. Dennis, L. D. Williams, and F. H. Westheimer, J . Amer. Chem. Soc., 91,6066 (1969). (6) K. L. Marsi, ibid., 4724 (1969). (7) W. Hawes and S . Trippett, Chem. Commun., 295 (1968). (8) I
