Asymmetric hydrogenation of 1-phenylpropane-1,2-dione over cinchona-modified Pt: Role of the C-9 OH group of cinchonidine
1-Phenylpropane-1,2-dione was hydrogenated over chirally modified 5% Pt/Al 2O 3 catalyst at 10 bar H 2 and 15 °C using toluene and acetic acid as solvents. The highest enantiomeric excess for the major product ( R)-1-hydroxy-1-phenyl-2-propanone ( ee = 57 % ) was obtained using cinchonidine as the c...
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Published in | Journal of catalysis Vol. 227; no. 1; pp. 210 - 216 |
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Main Authors | , , , |
Format | Journal Article |
Language | English |
Published |
Amsterdam
Elsevier Inc
01.10.2004
Elsevier |
Subjects | |
Online Access | Get full text |
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Summary: | 1-Phenylpropane-1,2-dione was hydrogenated over chirally modified 5% Pt/Al
2O
3 catalyst at 10 bar H
2 and 15 °C using toluene and acetic acid as solvents. The highest enantiomeric excess for the major product (
R)-1-hydroxy-1-phenyl-2-propanone (
ee
=
57
%
) was obtained using cinchonidine as the chiral modifier. The presence of the hydroxyl group in the C-9 position of the modifier was important for achieving high enantioselectivity. When the C-9 hydroxyl group of cinchonidine was replaced by a methoxy group enantioselectivity was lost and a small 2% excess of (
S)-1-hydroxy-1-phenyl-2-propanone enantiomer was observed. In acetic acid the reaction with cinchonidine proceeded yielding a 7% excess of the (
R)-product. Hydrogenation of the intermediate hydroxyketones in acetic acid using cinchonidine or 9-methoxy-10,11-dihydrocinchonidine as chiral modifiers gave the corresponding (1
S,2
R)-diol in 67 and 78% enantiomeric excesses, respectively. By changing the solvent from acetic acid to toluene, an inversion of enantioselectivity took place yielding the (1
R,2
S)-diol as the main product in 38%
ee. A mechanism was proposed involving a two-step cycle (reactant–modifier) and a three-step cycle (reactant-modified acetic acid) in order to account for the observed enantioselectivities. |
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ISSN: | 0021-9517 1090-2694 |
DOI: | 10.1016/j.jcat.2004.07.001 |