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

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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.

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