Proton-Coupled Electrochemical Reduction of a Phosphine Oxide.

The two-proton/two-electron electrochemical reduction of a phosphine oxide with elimination of water as the sole byproduct (P(V)=O + 2H+ + 2e- → P(III) + H2O) is reported. Under electrochemical (constant current electrolysis) conditions, reduction of 5-phenylphospholo[3,2-c:4,5-c']dipyridine P-oxide (1 O ) in the presence of a proton donor gives the corresponding phosphine (1) in up to 90% yield and 95% conversion. Electrokinetic data and simulations are consistent with an E r C i E r C i mechanism, in which an initial one-electron reduction brings about rate-limiting protonation of the phosphoryl bond. A regioisomeric phosphine oxide (9-phenylphospholo[2,3-c:5,4-c']dipyridine P-oxide, 2 O ) shows reversible electron transfer (ET) behavior but does not lead to proton-coupled electron transfer (PCET) P=O reduction. These results introduce the electronic design of π-substituents as a tunable mode by which to access previously challenging proton-coupled reduction of the strong P=O bond.