Kinetics of the autoxidation of trimethyl phosphite, methyl diphenyl-phosphinite, and triphenylphosphine

Abstract

Auotoxidation of trimethyl phosphite, methyl diphenylphosphinite, and triphenylphosphine has been studied kinetically in o-dichlorobenzene at 60·0 °C with 2,2′-azobisisobutyronitrile (AIBN) as a radical initiator. The autoxidation proceeded after a short induction period to give predominantly the corresponding quinquecovalent phosphorus compounds. The rates are always independent of oxygen pressure and the rate law for the first two compounds is –d[O₂]/dt=k₂′[AIBN][substrate], which was analogous to that for dimethyl phenylphosphonite. The rate law for triphenylphosphine was –d[O₂]/dt=k₁′[AIBN](1 +k₂′[substrate]). The autoxidation may have a radical chain mechanism involving chain carriers R·, RO·, and RO₂·. The different kinetic behaviour of triphenylphosphine is attributable to the stability of a radical carrier, Ph₃PO₂R, and resulting lower reactivity of RO₂· with Ph₃P. The rate constants decrease in the order: methyl diphenylphosphinite > trimethyl phosphite > triphenylphosphine. The application of Taft equation, log (k/k₀)=ρ*Σσ*, to trimethyl phosphite, methyl diphenylphosphinite, and dimethyl phenylphosphonite gives a ρ* value of –0·380, which implies a mechanism involving a nucleophilic attack of phosphorus atom.