Alkoxyphosphonium Salts. 4. Kinetics and Mechanisms of Arbuzov Reactions

Article abstract of DOI:10.1021/jo00160a017

The kinetics of the Arbuzov reactions of trivalent phosphorus esters of the type ABPOCH3 catalyzed by CH3X have been studied.The alkoxyphosphonium ion ABCH3POCH3⁺ is always intermediate but often undetected.In all cases studied using methyl iodide, the rate-determining step is the alkylation of the trivalent phosphorus and the intermediate alkoxyphosphonium ion does not accumulate detectibly by NMR.In all cases with methyl triflate, the alkylation is fast and complete, and the resulting alkoxyphosphonium salt then has a constant concentration.It methylates the triv alent compound (if any remains) in a measurably slow reaction, the only observed process.This reaction represents the pure "autocatalytic" mechanism.With dimethyl sulfate the intermediate accumulates but does not ordinarily persist, and the reaction course is best fit by a combination of the "autocatalytic" mechanism with attack on the intermediate phosphonium ion by ABPOCH3 and the "textbook" mechanism in which the intermediate alkoxyphosphonium ion is dealkylated by attack of the methyl sulfate anion.