Influence of the nature of chiral auxiliaries on the diastereoselective hydrogenation of ortho-substituted benzoic acid derivatives

The diastereoselective hydrogenation of o-toluic acid or o-methoxy benzoic acid covalently bound to different chiral auxiliaries was performed on Rh and Ru supported catalysts. The cis-isomers were formed predominantly, with a diastereoselectivity largely influenced by the structure of the chiral inductor and the steric hindrance brought for the preferential adsorption of one face of the aromatic substrate. The effect of the functional group on the proline auxiliary (alcohol or ester groups susceptible to modify the anchoring of the aromatic substrate) was weak. Hydrogenolysis occurred rather extensively with the methoxy benzoic acid and constituted the most important hydrogenation pathway on Rh/C. The presence of the C=O group in the pyroglutamic acid methyl ester is a determining factor for obtaining good diastereoselectivity. (C) 2000 Elsevier Science Ltd. All rights reserved.