Deciphering Surface Enantioselectivity

By combining scanning tunneling microscopy and computational methods, researchers have determined key steps in the chirality transfer mechanism that governs an enantiospecific surface reaction (Science, DOI: 10.1126/science.1208710). Nearly all stereoselective reactions are carried out in solution with liquid-phase reagents and catalysts. Solid-phase catalysts would make separating the catalyst from the product simpler. Yet only a few such stereoselective catalytic systems, which are formed by treating a catalytic metal such as platinum with a chiral modifier, are available. Furthermore, progress in developing such systems is hampered by a lack of mechanistic information. Peter H. McBreen of Laval University, in Quebec; Bjørk Hammer of Aarhus University, in Denmark; and coworkers have discovered several mechanistic details that are critical to a model enantioselective surface reaction. The team studied the room-temperature asymmetric hydrogenation of 2,2,2-trifluoroacetophenone (TFAP) to (R)-2,2,2-trifluorophenylethanol over Pt that had been modified with (R)-1-(1-naphthyl)ethylamine (R-NEA). The team determined that R-NEA adopts two adsorption conformations in a 7-to-3 ratio. They also determined that TFAP reversibly forms dimers and a large variety of short-lived diastereomeric complexes with the modifier. By analyzing more than 900 such complexes, the team identified the geometries, relative abundances, and molecular forces that guide the reaction to the R product with a 34% enantiomeric excess.