AN ISOTOPE EFFECT STUDY OF ACETOLYSES OF SOME TRIPHENYLCARBALKOXYMETHYLPHOSPHONIUM SALTS

13C and 18O kinetic isotope effects in the carbon dioxide formed during acetolysis of three triphenylcarbalkoxymethylphosphonium salts have been determined. Accompanying formation of carbon dioxide from triphenylcarbobenzhydryloxymethylphosphonium bromide (I) are 13C and 18O isotope effects of 1.045 and 1.015, respectively. Triphenylcarbooctyloxymethylphosphonium bromides (II and III) under similar conditions, yield carbon dioxide attended by no observable isotopic oxygen fractionation and a smaller 13C isotope effect (1.020) than that observed during acetolysis of I. Exact calculations with various transition-state models support solvent-assisted heterolysis of I into benzhydryl carbonium ion, carbon dioxide, and methylenetriphenylphosphorane. Similar considerations lend support to a mechanism whereby II or III in acetic acid-acetic anhydride undergoes rate-controlling acetyl anhydride formation followed by the production of carbon dioxide, triphenylmet ylphosphonium bromide, and acetic anhydride. © 1969, American Chemical Society. All rights reserved.