Electroreduction of Triphenylphosphine Oxide to Triphenylphosphine in the Presence of Chlorotrimethylsilane

Electroreduction of triphenylphosphine oxide to triphenylphosphine in an acetonitrile solution of tetrabutylammonium bromide in the presence of chlorotrimethylsilane was performed successfully in an undivided cell fitted with a zinc anode and a platinum cathode under constant current. A plausible mechanism involving, (1) one-electron reduction of triphenylphosphine oxide generating the corresponding anion radical [Ph(3)P(center dot)-O(-)], (2) subsequent reaction with chlorotrimethylsilane affording the (trimethylsiloxy)triphenylphosphorus radical [Ph(3)P(center dot)-OSiMe(3)], and (3) further one-electron reduction followed by P-O bond fission leading to triphenylphosphine is proposed. In a similar manner, electroreduction of some triarylphosphine oxides and alkyldiarylphosphine oxides was executed to give the corresponding phosphine derivatives in good to moderate yields.