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Synthesis

Deciphering Surface Enantioselectivity

by Mitch Jacoby
November 21, 2011 | APPEARED IN VOLUME 89, ISSUE 47

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.

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