Versatile synthesis of P-chiral (ephedrine) AMPP ligands via their borane complexes. Structural consequences in Rh-catalyzed hydrogenation of methyl #alpha#-acetamidocinnamate

摘要

An efficient and versatile synthesis of aminophosphine phosphinite (AMPP) ligands derived from ephedrine, with possible stereogenic P(III)-center(s) is described, using the borane complex methodology. The reaction of oxazaphospholidine borane with an organolithium reagent, leads to the formation of the ring-opened product, which is trapped by a chlorophosphine (borane), to afford the corresponding aminophosphine phosphinite boranes in good yields. Treatment of the borane complexes with dabco, gives the corresponding aminophosphine phosphinite ligands in 70-90% yield. These ligands are used for the preparation of Rh catalysts applied to the asymmetric hydrogenation of methyl #alpha#-acetamidocinnamate yielding the phenylalanine derivative with (R) 22% to (S) 99% e.e. These results show the importance of the structural modification at the P-stereogenic center(s), which could either amplify or cancel out the asymmetric induction resulting from the ephedrine backbone, for enantioselective catalysis.