Electrophilic Phosphonium Cation-Mediated Phosphane Oxide Reduction Using Oxalyl Chloride and Hydrogen.

The metal-free reduction of phosphane oxides with molecular hydrogen (H2 ) using oxalyl chloride as activating agent was achieved. Quantum-mechanical investigations support the heterolytic splitting of H2 by the in situ formed electrophilic phosphonium cation (EPC) and phosphane oxide and subsequent barrierless conversion to the phosphane and HCl. The reaction can also be catalyzed by the frustrated Lewis pair (FLP) consisting of B(2,6-F2 C6 H3 )3 and 2,6-lutidine or phosphane oxide as Lewis base. This novel reduction was demonstrated for triaryl and diaryl phosphane oxides providing access to phosphanes in good to excellent yields (51-93 %).