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

New chiral α-aminophosphine oxide N,P(O) and sulfide N,P(S) ligands have been prepared in one-pot syntheses by addition of Ph2PH to (S)-PhCHNCH(Ph)CH3, followed by oxidation with O2 or S8. Crystallization from cold methanol leads to the isolation of an enantiomerically pure single N,P(O) diastereomer and to a 1 : 1 mixture of the two N,P(S) diastereomers. The coordination chemistry of these ligands with [RhCl(COD)]2 and [RhCl(CO)2]2 has been investigated under argon and syngas. At high temperatures, a P–C oxidative addition of the N,P(O) ligand followed by imine elimination leads to several hydrido rhodium species. The complexes containing an N,P(S) ligand undergo the same process at room temperature. Catalytic amounts of the rhodium complexes catalyze the epimerization of the N,P(S) ligand under argon at room temperature, the dicarbonyl complex being 14 times more active than the COD complex. The same catalyzed epimerization takes also place for the N,P(O) ligand, but much more slowly, and only at 35 °C and under a syngas pressure. Possible mechanisms for this catalytic process are discussed. Catalytic tests in styrene hydroformylation have shown high activities and regioselectivities, but no enantioselectivity, for the N,P(O)-containing rhodium complexes.