Phosphinites of carbohydrates as chiral ligands for asymmetric synthesis catalysed by complexes. IV. Influence of the solvent and the structure of cationic 4,6‐0‐benzylidene‐2,3‐0‐bis(diphenylphosphino)‐D‐hexopyranoside‐rhodium(I) chelates on enantioselectivity in Hydrogenation of Dehydroamino Acid Derivatives

Cationic rhodium(I)‐chelates of 4,6‐benzylidene‐2,3‐0‐bis(diphenylphosphino)‐D‐hexopyranosides bearing axial 0‐functions, as for instance manno‐ 5 and galactopyranosides 4c, 4d or α‐D‐glycopyranosides, show lower enantioselectivity in the hydrogenation of dehydroamino acid derivatives than chelates of the all‐equatorial phenyl 4,6‐0‐(R)‐benzylidene‐2,3‐0‐bis(diphenylphosphino)‐β‐D‐glucopyranoside 3d (abbreviated Ph‐β‐glup). Complexes of the galactopyranosides 4c and 4d give an abnormal decrease of enantiomeric excess with the prochiral dehydroamino acid 1c in contrast to the analogous substrate ester 1d. In hydrogenations with [Rh(Ph‐β‐glup)(COD)] A as precatalyst various weak coordinating anions A⊖ do not produce differences in % ee, and the solvent effect remains low. However, an analogous chelate of the methyl α‐anomer bisphosphinite Me‐α‐glup 3a gives rise to inversion of the optical induction using benzene as solvent.