Main Group Redox Catalysis: Reversible P^III/P^V Redox Cycling at a Phosphorus Platform

A planar, trivalent phosphorus compound is shown to undergo reversible two-electron redox cycling (PIII/PV) enabling its use as catalyst for a transfer hydrogenation reaction. The trivalent phosphorus compound activates ammonia-borane to furnish a 10-P-5 dihydridophosphorane, which in turn is shown to transfer hydrogen cleanly to azobenzene, yielding diphenylhydrazine and regenerating the initial trivalent phosphorus species. This result constitutes a rare example of two-electron redox catalysis at a main group compound and suggests broader potential for this nonmetal platform to support bond-modifying redox catalysis of the type dominated by transition metal catalysts.