Design of P-Chirogenic Aminophosphine-Phosphinite Ligands at Both Phosphorus Centers: Origin of Enantioselectivities in Pd-Catalyzed Allylic Reactions

We have recently patented an unprecedented stereospecific N -> O phosphinyl migration process which transforms P-chirogenic aminophosphines into phosphinites. A fine design of aminophosphine phosphinite ligands (AMPP*) derived from ephedrine and bearing a P-chirogenic center either at the aminophosphine or phosphinite moiety was performed. The synthesis of AMPP* ligands with a P-chirogenic aminophosphine moiety was based on the well-established stereospecific reaction of oxazaphospholidine borane with organolithium reagents, followed by trapping with a chlorophosphine and borane decomplexation. Concurrently, the preparation of AMPP* ligands with a P-chirogenic phosphinite moiety was performed by N -> O phosphinyl migration of aminophosphines borane by heating at 50 degrees C with DABCO and then reaction with chlorophosphines. AMPP* ligands were studied in palladium-catalyzed asymmetric allylic alkylations, leading to enantioselectivities from 91% (R) to 95% ee (S). X-ray crystallographic data for relevant Pd-AMPP* complexes and computer modeling explained the origin of the enantioselectivities based on MO interactions of most stable conformers with nucleophiles.