Enantioselective hydrogenation over cinchona-modified Pt: the special role of carboxylic acids.
The influence of acetic acid (AcOH) and trifluoroacetic acid (TFA) on the hydrogenation of ethyl-4,4,4-trifluoroacetoacetate has been investigated by using Pt/Al(2)O(3) modified by cinchonidine and O-methylcinchonidine. We have shown that the sometimes dramatic changes in enantioselectivity and rate cannot simply be interpreted by protonation of the alkaloid modifier. We propose a new three-step reaction pathway, involving interaction of the carboxylic acid with the reactant and the chiral modifier. The mechanism is supported by IR spectroscopic identification of cyclic TFA-modifier ion pairs. This new approach can rationalise the poorly understood role of acids in the enantioselective hydrogenation of activated ketones over cinchona-modified platinum metals.
von Arx
Matthias
M
Laboratory of Technical Chemistry, Swiss Federal Institute of Technology, ETH-Hönggerberg HCI, Wolfgang Pauli Strasse, 8093 Zürich, Switzerland.
Bürgi
Thomas
T
Mallat
Tamas
T
Baiker
Alfons
A