Page 130 - Catalysts for Fine Chemical Synthesis Vol 1 - Robert & Poignant

asymmetric reduction of ketones 117

asymmetric borane complexes are used as nonmetallic catalysts for the hydro-
genation of simple ketones, like acetophenone. They can give the corresponding
alcohol in high yield and enantiomeric excess. The use of oxidoreductases
contained in bakers' yeast can give good results for reduction of carbonyl and
carbon±carbon double bonds.
In contrast to the enantioselective reduction of alkenes or ketones, few
catalytic systems have been described for the enantioselective reduction of
imines. The nature of the N-substituent and the E/Z-isomerism caused by the
carbon±nitrogen double bond of the substrate are important parameters for the
control of the enantioselectivity [15] . The enantioselective hydrogenation of
carbon±nitrogen bonds has been reported to occur in the presence of iridium,
ruthenium, [1,16,17] titanium, [18±20] zirconium or cobalt [21] , with chiral dipho-
sphine ligands. Iridium catalysts have been successfully employed in the hydro-
genation of N-arylimines. [22±25] Rhodium catalysts have been used for the
reduction of imines and nitrones. [26,27] The employment of chiral auxiliaries
for the activation of borane reagents in carbon±nitrogen double bond reduction
has been shown to induce high enantioselectivities. However, it is quite difficult
to maintain the high enantiomeric excesses, obtained by using stoichiometric
amounts of such auxiliaries, in a catalytic version of the reduction [28] . The
substrates which lead to the best results are oxime ethers. The oxazaborolidine
derived from valinol is a very efficient chiral auxiliary for oxime ether reduction
with borane. The oxazaphospholidine derived from prolinol (developed by
[8]
Buono et al.) induce enantioselectivity in the reduction of imines .
In this chapter and in Chapters 10±12, we will review and validate some
methods for asymmetric (transfer) hydrogenation of carbon±oxygen and
carbon±carbon double bonds catalysed by non-metallic systems, homogeneous
transition metal catalysts and biocatalysts. Reduction of carbon±nitrogen
double bond systems will be reported in another volume of this series.

9.2 ASYMMETRIC HYDROGENATION USING A METAL CATALYST:
[RU((S)-BiNAP)]

Few catalysts have been found to produce chiral alcohols from a range of
ketones with both high levels of absolute stereocontrol and high catalytic
efficiencies [29] . Metallic catalysts are generally used for the asymmetric hydro-
genation of functionalized ketones. High enantioselectivities (>98 % ee) have
been observed in the hydrogenation of a variety of b-keto esters using 2,2 -
0
0
bis(phosphino)-1,1 -biaryl (BINAP)-derived catalysts such as ruthenium±
BINAP [30] . For the hydrogenation of simple ketones, nonmetallic catalysts
like chiral borane complexes are often preferred (Chapter 11). [31,32] Biocatalytic
reduction with bakers' yeast is used for reduction of ketones such as b-ketoe-
sters, b-diketones, a-hydroxy ketones, aliphatic and aromatic ketones [33]
(Chapter 10).