We report that a regioselective reductive transposition of primary allylic bromides is catalyzed by a biphilic organophosphorus (phosphetane) catalyst. Spectroscopic, evidence supports the formation of a penta:coordinate (sigma(5)-P) hydridophosphorane as a key reactive intermediate. Kinetics experiments and computational modeling are consistent with a unimolecular docomposition of the sigma(5)-P hydridophosphorane via a conceited cyclic transition,structure that delivers the observed allylic transposition and completes a novel P-III/P-V redox Catalytic cycle. These results broaden the growing repertoire., of reactions catalyzed within the P-III/P-V redox couple and suggest additional opportunities for organophosphorus,catalysis in a biphilic mode.