New chiral alpha-aminophosphine oxides and sulfides: an unprecedented rhodium-catalyzed ligand epimerization

New chiral alpha -aminophosphine oxide N,P(O) and sulfide N,P(S) ligands ha ve been prepared in one-pot syntheses by addition of Ph2PH to (S)-PhCH=NCH( Ph)CH3, followed by oxidation with O-2 or S-8. Crystallization from cold me thanol leads to the isolation of an enantiomerically pure single N,P(O) dia stereomer and to a 1:1 mixture of the two N,P(S) diastereomers. The coordin ation chemistry of these ligands with [RhCl(COD)](2) and [RhCl(CO)(2)](2) h as been investigated under argon and syngas. At high temperatures, a P-C ox idative addition of the N,P(O) ligand followed by imine elimination leads t o several hydrido rhodium species. The complexes containing an N,P(S) ligan d undergo the same process at room temperature. Catalytic amounts of the rh odium complexes catalyze the epimerization of the N,P(S) ligand under argon at room temperature, the dicarbonyl complex being 14 times more active tha n the COD complex. The same catalyzed epimerization takes also place for th e N,P(O) ligand, but much more slowly, and only at 35 degreesC and under a syngas pressure. Possible mechanisms for this catalytic process are discuss ed. Catalytic tests in styrene hydroformylation have shown high activities and regioselectivities, but no enantioselectivity, for the N,P(O)-containin g rhodium complexes.