Efficient and selective rhodium-catalyzed hydrophosphorylation of dienes

The hydrophosphorylation of a model 1,6-diene having a terminal and an internal alkene function has been investigated. Free radical protocols lead invariably to mixtures of cyclic phosphonate products, due to rapid cyclization of the intermediary radical species. Rhodium catalysis using a cyclic pinacol-derived phosphonate provides an efficient technique for the highly selective (> 99%) hydrophosphorylation at the terminal alkene function. In situ modification of Wilkinson's complex by addition of 2-50 equivalents (vs. Rh) of a monophosphine (PCy3 > PPh3) or carbene ligand greatly improves the catalyst performances (TON up to 2250 mol phosphonate/mol Rh). An even more efficient system was obtained with 2 equivalents (vs. Rh) of the bidentate 1,6-bis(diphenviphosphino)hexane ligand, which affords so far unprecedented high catalytic productivity (TON up to 4 550 mol phosphonate/mol Rh) and activity (TOF up to 250 h(-1)).