Correlation between Chiral Modifier Adsorption and Enantioselectivity in Hydrogenation Catalysis

Infrared absorption spectroscopy performed in situ at the solid–liquid interface revealed that the adsorption on platinum supported catalysts of 1‐(1‐naphthyl)‐ethylamine, which is used as a chiral modifier in hydrogenation catalysis, occurs through the amine group, not the aromatic ring as is widely believed. Comparisons were performed against a set of related modifier compounds with targeted substitutions to help identify the key moiety involved in the adsorption. It was determined that neither naphthalene‐based modifiers without amine groups nor those with tertiary amine moieties are capable of adsorbing on the metal surface to any significant extent. A direct correlation was also found between the ability of the amines to adsorb on the platinum surface and their performance as chiral modifiers that impart enantioselectivity to the hydrogenation of α‐keto esters such as ethyl pyruvate. Chiral modification of platinum hydrogenation catalysts by 1‐(1‐naphthyl)‐ ethylamine requires adsorption via the amine nitrogen atom, not bonding through the aromatic ring as is widely believed. Adsorption of the modifier requires NH2 groups (as determined by a study of a family of related compounds by using in situ infrared absorption spectroscopy) and correlates with the enantioselectivity of the catalyst.