Why are α-hydroxycarboxylic acids poor chiral modifiers for Pt in the hydrogenation of ketones?

Abstract

Mandelic acid ( 1) and its derivatives and 1-naphthylglycolic acid ( 8) were used as modifiers for the asymmetric hydrogenation of ketopantolactone to pantolactone on supported Pt, Ru, and Rh catalysts. A systematic variation of the modifier structure showed that 8 was the best modifier, affording up to 28% ee on Pt; the other metals were barely efficient. The catalytic studies were completed with FTIR spectroscopic analysis of the modifier–modifier and modifier–substrate interactions in solution. Attenuated total reflection infrared spectroscopy was applied to study the adsorption of the α-hydroxy-carboxylic acid type modifiers on a Pt/Al 2O 3 model catalyst in the presence of hydrogen. The study indicates that the modifiers are present on the metal surface as monomers with an internal hydrogen bond between the hydroxyl group and the carboxyl carbonyl group, and the phenyl or naphthalene rings are in a tilted position relative to the surface. The moderate enantioselection is attributed to a hydrogen bond interaction between the carboxylic OH group of the modifier and the carbonyl O atom of the substrate. The poor enantioselectivities attained in ketopantolactone hydrogenation may be explained by the overly weak adsorption of the modifiers on Pt and by the weakness of the O H O-type H bond between the substrate and the modifiers. It seems that the carboxyl group has a double role, allowing interaction with the substrate and the Pt surface.