Catalytic Asymmetric Hydrogenation of α-(Acetamido)acrylates Using TRAP Trans-Chelating Chiral Bisphosphine Ligands: Remarkable Effects of Ligand P-Substituent and Hydrogen Pressure on Enantioselectivity

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Abstract The catalytic asymmetric hydrogenation of α-(acetamido)acrylates was carried out with the rhodium complexes prepared from [Rh(cod)2]BF4 and trans-chelating chiral bisphosphine ligands, (S,S)-2,2′-bis[(R)-1-(dialkylphosphino)ethyl]-1,1′-biferrocenes [(R,R)-(S,S)-TRAPs]. In the reaction of β-unsubstituted or β-monosubstituted α-(acetamido)acrylates [(E)-RCH=C(NHAc)CO2Me], the selectivity for (R)-hydrogenation product increased with decreasing steric demand of the substrate β-substituent and the ligand P-substituent as well as decreasing hydrogen pressure. The selectivity for the (R)-product in the reaction with EtTRAP-rhodium catalyst at 60 °C and 0.5 kg cm-2 of hydrogen pressure was as follows: R = H, 96% ee; R = Me, 92% ee; R = Ph, 77% ee; R = i-Pr, 57% ee. The remarkable steric and pressure effects caused a dramatic reversal of enantioselectivity in the reaction of methyl 2-(N-acetylamino)cinnamate (R = Ph). For example, the selectivity of 87% (R) with EtTRAP at 60 °C and 0.1 kg cm-2 of hydrogen pressure has reversed to 92% (S) with i-BuTRAP at 15 °C and atmospheric hydrogen pressure. Hydrogenation of the β,β-disubstituted α-(acetamido)acrylates proceeded smoothly at 15 °C and atmospheric hydrogen pressure with high enantioselectivity in favor of the 2S-isomers when EtTRAP or BuTRAP was employed as the chiral ligand.