Amidophosphine−Phosphinites: Synthesis and Use in Rhodium-Based Asymmetric Hydrogenation of Activated Keto Compounds. Crystal Structure of Bis[(μ-chloro)((<i>S</i>)-2-((diphenylphosphino)oxy)-2-phenyl-<i>N</i>-(diphenylphosphino)-<i>N</i>-methylacetamide)rhodium(I)]

Abstract

Amidophosphine−phosphinite ligands (AMPP) derived from (S)-N-benzylmandelamide ((S)-R,R‘-benzylmandelNOP (S)-1 (R = R‘ = phenyl) and (S)-7 (R = phenyl, R‘ = cyclopentyl)), (S)-N-methylmandelamide ((S)-R,R‘-methylmandelNOP (S)-2 (R = R‘ = phenyl) and (S)-8 (R = phenyl, R‘ = cyclopentyl)), (S)-N-methyllactamide ((S)-R,R‘-methyllactaNOP (S)-3 (R = R‘ = phenyl) and (S)-9 (R = phenyl, R‘ = cyclopentyl)), and (S)-2-(hydroxymethyl)-2-pyrrolidinone ((S)-R,R‘-oxoProNOP (S)-4−6 and (S)-10 (R, R‘ = phenyl, cyclohexyl, cyclopentyl)) have been prepared in high yields (60−94%) and reacted with rhodium precursors to prepare neutral “Rh{AMPP}” complexes 11−26 of general formula [Rh{AMPP}X]2, where X = Cl, I, OCOCH3, OCOCF3, and OCOC3F7. The crystal structure of [Rh{(S)-Ph,Ph-methylmandelNOP}Cl]2 (12) has been determined. The rhodium atom has a cis square-planar coordination, and the seven-membered chelate ring has a boat conformation with the nitrogen atom in the mean plane RhP2. Complexes 11−26 have been used as catalyst precursors for the asymmetric hydrogenation of dihydro-4,4-dimethyl-2,3-furandione (27) and N-benzylbenzoylformamide (29) giving the corresponding optically active hydroxy compounds 28 and 30 in high yields and low to high enantiomeric excesses (28−98.7% ee and 13−87% ee, respectively). Catalytic activities (turnover frequency at 50% conversion at room temperature up to 3300 h-1) as well as the enantioselectivities depended strongly on the nature of the substituents on phosphorus as well as on the nature of the non chiral ligands. Catalyst precursor [Rh{(S)-Cp,Cp-oxoProNOP}OCOCF3]2 afforded (R)-pantolactone in 98.7% ee.