Chemistry Reference
In-Depth Information
reduced with high enantioselectivities of
99% ee in most cases. The recombinant
expression of the
R. ruber
ADH has also been reported, thus even further expanding
the scope of this versatile enzyme [186]. Besides mono- and biphasic aqueous-organic
solvent media, the substrate-coupled regeneration with isopropanol in the presence of
ADH from
R. ruber
has been successfully applied in microaqueous organic systems with
99% (v/v) of an organic solvent. Notably, high substrate concentrations of up to
>
∼
2 M
were realized [187]. Starting from
-chloro ketones, the corresponding halohydrins were
obtained with enantioselectivities of up to
α
99% ee when using
R. ruber
as a lyophilized
catalyst [188]. For example, (
R
)-octanol was formed with
>
99% conversion and 99% ee.
Furthermore, highly enantio- and diastereoselective reduction of diketones under for-
mation of the corresponding diols with
>
>
99% ee and
>
99% de has been reported by the
Kroutil group [189] .
The high effi ciency of enzymatic asymmetric ketone reduction with substrate-coupled
cofactor regeneration is also underlined by commercial applications thereof, as has been
reported, for example, by Wacker.
Very recently, the Gröger group reported the combination of an ADH-catalyzed
reduction of ketones under substrate-coupled cofactor regeneration with a palladium-
catalyzed Suzuki cross-coupling reaction in a one-pot synthesis in aqueous media [190].
When carrying out the Suzuki cross-coupling reaction in the initial step starting from
aromatic boronic acids and a halogenated acetophenone, subsequent biocatalytic reduc-
tion gave enantiomerically pure biaryl alcohols with conversions of up to 91%.
When applying an enzyme-coupled cofactor regeneration for asymmetric biocatalytic
reduction processes, the use of a formate dehydrogenase (FDH) represented a popular
approach. The FDH catalyzes the oxidation of formate into carbon dioxide, while reduc-
ing the oxidized form of the cofactor into its reduced form, NAD(P)H. The most widely
applied FDH is probably the FDH from
C. boidinii
and optimized mutants thereof
[191] developed in the Kula group who are--jointly with the Hummel and Wandrey
groups- - pioneers in the fi eld of FDH - based applications [192,193] in addition to the
Whitesides group [194]. A key advantage when using FDH for cofactor regeneration
certainly is the irreversible step of carbon dioxide formation and removal, thus shifting
the equilibrium toward (complete) product formation. In addition, downstream process-
ing is simplifi ed since (ideally) no organic by-product remains in the reaction mixture.
The initial work on enzymatic reduction of ketones has been carried out based on the
use of isolated enzymes in homogeneous aqueous media. Due to the low solubility of
the hydrophobic ketones in water, the reactions were carried out at low substrate con-
centrations for a long time, typically in the range of 5-20 mM or below. In the 1990s,
Hummel et al. as well as the Kula group studied in detail the suitability of different types
of ADHs in combination with an FDH for asymmetric reduction of a broad range of
ketones comprising keto esters, aromatic ketones, and aliphatic 2-alkanones [195-197].
The Kula group also carried out preparative transformations based on these enzymes
by coupling the ADH reduction reactions with FDH regeneration [198,199]. As enzymes,
ADHs from
R. erythopolis
and
C. parapsilosis
were used in combination with the FDH
from
C. boidinii
. Carrying out reductions of several keto esters and a keto dialkyl acetal
at a substrate concentration of 100 mM furnished the desired alcohols in most cases with
high conversion (up to 100%) and high enantioselectivities of
>
99%. A selected example
is given in Scheme 6.51.
The issue of high space-time yields in spite of the limitation of low ketone solubil-
ity has been successfully addressed by the Wandrey group, who developed elegant
