Pyroglutamic acid as a chiral auxiliary in the diastereoselective hydrogenation of disubstituted aromatic rings on Rh(111): a periodic density functional theory approach

In the present work, the role of the pyroglutamic acid as a chiral auxiliary in the hydrogenation of o-toluic acid and 2-methylnicotinic acid on the Rh(111) surface has been investigated by using periodical density functional theory. The pyroglutamic acid, indeed, allows the adsorption of the aromatic ring by only one side of the ring plane. This effect is observed for both molecules. In the case of the benzoyl ring, the selectivity should be high on a perfectly flat metallic surface. It is, however, limited in practice by the presence of edges, especially in the case of small catalytic particles. On the other hand, in the case of a nicotinyl ring, the presence of a lone pair of the nitrogen atom has a great influence on the reduction of the diastereomeric excess. The NH group, formed from the partial hydrogenation of the nicotinyl ring, interacts strongly with the surface, preventing the influence of the chiral auxiliary.