Kinetic study on the reaction of tributylphosphine with methylviologen. Reactivity of the phosphine radical cation intermediate towards nucleophiles

Abstract

Tributylphosphine, Buⁿ₃P (BP), was reacted with 1,1′-dimethyl-4,4′-bipyridinium (methylviologen; MV²⁺) in the presence of an alcohol or thiol (RXH; X = O, S) in acetonitrile under an argon atmosphere at 50 °C, which resulted in the gradual formation of the one-electron reduced form of the MV²⁺, MV⁺. Meanwhile, BP was oxidized to tributylphosphine oxide (BP-O). The increase in the amount of MV⁺, which was followed spectrophotometrically with BP and RXH being in large excess, did not obey first-order kinetics. The observation, along with the results from product analysis, shows that single-electron transfer (SET) takes place from BP to MV²⁺ to generate tributylphosphine radical cation BP^˙+, as well as MV⁺, and the resulting BP^˙+ undergoes ionic reaction with RXH and back electron transfer from MV⁺ in comparable efficiency. A regression analysis of the kinetic data gave the relative value of the second-order rate constant, k_Nu^rel, for the ionic reaction of BP^˙+ with RXH. Comparison of the k_Nu^rel values thus obtained for reactions with various RXH’s shows that the reaction of BP^˙+ with nucleophile RXH is governed by both steric and electronic factors of RXH. The activation energy E_a of the reaction was found to be significantly large, which is in contrast to previous observations that ionic reactions of carbon radical cations with nucleophiles usually have very small values of E_a.