Asymmetric hydrogenation - Influence of the structure of carbohydrate derived catalysts on the relative enantioselectivity Q(H/Me) regarding acid and ester substrates and its inversion - Selectivity increase in water by amphiphiles

Article abstract of DOI:10.1016/S0040-4020(96)00953-2

4,6-O-Benzylidene protected 2,3-bis(O-diphenylphosphino)-D-glycopyranoside rhodium(I) chelate precatalysts 1-4 e,f showed for the hydrogenation of methyl (Z)2-N-acylamidoacrylates 6-8 a stepwise decrease of the enantioselectivity with increasing number of axially oriented hexopyranoside substituents. The decrease is even stronger for the analogous substrate acids 6h-8h resulting in an unusual low relative enantioselectivity Q = q(H)/q(Me) of 0.3 for the precatalysts 4e and 4f. Deprotected, 4,6-OH-group bearing catalysts 1-4 g,h generally show smaller differences of %ee in methanol or benzene, however, not in water. Under addition of amphiphiles a in comparison with blanks b the relative enantioselectivity Q = q(a)/q(b) clearly increases for both groups of catalysts in most cases to Q-values between 3 up to 8 - independent of a neutral or ionic nature of the amphiphile.