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Enantioselective Hydrogenation over Cinchona‐Modified Pt: The Special Role of Carboxylic Acids
Author(s) -
von Arx Matthias,
Bürgi Thomas,
Mallat Tamas,
Baiker Alfons
Publication year - 2002
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20020315)8:6<1430::aid-chem1430>3.0.co;2-k
Subject(s) - cinchonidine , enantioselective synthesis , trifluoroacetic acid , chemistry , cinchona , protonation , acetic acid , cinchona alkaloids , carboxylic acid , organic chemistry , noyori asymmetric hydrogenation , asymmetric hydrogenation , catalysis , ion
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.