Page 129 - Catalysts for Fine Chemical Synthesis Vol 1 - Robert & Poignant

116 hydrolysis, oxidation and reduction

Enantioselective catalysis using chiral metal complexes provides a flexible
method for asymmetric hydrogenation. The metallic elements possess a variety
of catalytic activities and their combination with organic ligands or auxiliaries
that direct the steric course can give very efficient catalytic complexes. Well-
designed chiral metal complexes can discriminate precisely between enantiotopic
groups, or faces, and catalyse the formation of a wide range of natural
and unnatural substances with high enantiomeric purity. Asymmetric reduc-
tion with a transition metal can use molecular hydrogen (hydrogen gas) as
the source of hydrogen or nonhazardous organic molecules as donors of hydro-
gen such as formic acid or 2-propanol. This last method, hydrogen transfer,
[1]
can provide a complement to the catalytic reduction using molecular hydrogen .
The first enantioselective hydrogenation of unsaturated compounds was
[2]
using metallic catalysts deposited on chiral supports in the 1930s . In the
[3]
1950s, using this method an enantioselectivity exceeding 60 % was obtained .
Knowles [4] and Horner [5] in 1968 reported homogeneous asymmetric hydrogen-
ation using rhodium±chiral tertiary phosphine complexes.
Nonmetallic systems (Chapter 11) are efficient for catalytic reduction and are
complementary to the metallic catalytic methods. For example lithium alumin-
ium hydride, sodium borohydride and borane±tetrahydrofuran have been modi-
[6]
fied with enantiomerically pure ligands . Among those catalysts, the chirally
modified boron complexes have received increased interest. Several ligands, such
[7]
as amino alcohols , phosphino alcohols [8,9] and hydroxysulfoximines [10] , com-
plexed with the borane, have been found to be selective reducing agents.
In 1969, Fiaud and Kagan [11] tested ephedrine boranes but achieved only
3.6±5 % enantiomeric excess in the reduction of acetophenone. Itsuno et al. [12]
reported in 1981 an interesting enantioselective reduction of a ketone using an
amino alcohol±borane complex as a catalyst. Buono [13] investigated and de-
veloped the reactivity of phosphorus compounds as ligands in borane com-
plexes for asymmetric hydrogenation.
Enzyme reductions of carbonyl groups have important applications in the
synthesis of chiral compounds (as described in Chapter 10). Dehydrogenases
are enzymes that catalyse, for example, the reduction of carbonyl groups; they
require co-factors as their co-substrates. Dehydrogenase-catalysed transform-
ations on a practical scale can be performed with purified enzymes or with
whole cells, which avoid the use of added expensive co-factors. Bakers' yeast is
the whole cell system most often used for the reduction of aldehydes and
ketones. Biocatalytic activity can also be used to reduce carbon±carbon double
bonds. Since the enzymes for this reduction are not commercially available, the
majority of these experiments were performed with bakers' yeast [14] .
In summary, the asymmetric hydrogenation of olefins or functionalized
ketones catalysed by chiral transition metal complexes is one of the most prac-
tical methods for preparing optically active organic compounds. Ruthenium±
and rhodium±diphosphine complexes, using molecular hydrogen or hydrogen
transfer, are the most common catalysts in this area. The hydrogenation of
simple ketones has proved to be difficult with metallic catalysts. However,