Cooperative Catalysis of Noncompatible Catalysts through Compartmentalization: Wacker Oxidation and Enzymatic Reduction in a One-Pot Process in Aqueous Media

Article abstract of DOI:10.1002/anie.201409590

A Wacker oxidation using CuCl/PdCl₂ as a catalyst system was successfully combined with an enzymatic ketone reduction to convert styrene enantioselectively into 1-phenylethanol in a one-pot process, although the two reactions conducted in aqueous media are not compatible due to enzyme deactivation by Cu ions. The one-pot feasibility was achieved via compartmentalization of the reactions. Conducting the Wacker oxidation in the interior of a polydimethylsiloxane thimble enables diffusion of only the organic substrate and product into the exterior where the biotransformation takes place. Thus, the Cu ions detrimental to the enzyme are withheld from the reaction media of the biotransformation. In this one-pot process, which formally corresponds to an asymmetric hydration of alkenes, a range of 1-arylethanols were formed with high conversions and 98-99% ee. In addition, the catalyst system of the Wacker oxidation was recycled 15 times without significant decrease in conversion.

Full text of DOI:10.1002/anie.201409590

Angewandte
Chemie
DOI: 10.1002/anie.201409590
One-Pot Synthesis Hot Paper
Cooperative Catalysis of Noncompatible Catalysts through
Compartmentalization: Wacker Oxidation and Enzymatic Reduction in
a One-Pot Process in Aqueous Medium**
Hirofumi Sato, Werner Hummel, and Harald Grçger*
In memory of Erwin Flaschel
Abstract: A Wacker oxidation using CuCl/PdCl as a catalyst
chemoenzymatic one-pot approach, access to so-called
“dream reactions”, for which no efficient catalysts exist, is
conceivable when instead of a single catalyst a “catalyst
system”, consisting of cooperatively acting chemo- and
biocatalysts, is used.
2
system was successfully combined with an enzymatic ketone
reduction to convert styrene enantioselectively into 1-phenyl-
ethanol in a one-pot process, although the two reactions
conducted in aqueous media are not compatible due to enzyme
deactivation by Cu ions. The one-pot feasibility was achieved
via compartmentalization of the reactions. Conducting the
Wacker oxidation in the interior of a polydimethylsiloxane
thimble enables diffusion of only the organic substrate and
product into the exterior where the biotransformation takes
place. Thus, the Cu ions detrimental to the enzyme are withheld
from the reaction media of the biotransformation. In this one-
pot process, which formally corresponds to an asymmetric
hydration of alkenes, a range of 1-arylethanols were formed
with high conversions and 98–99% ee. In addition, the catalyst
system of the Wacker oxidation was recycled 15 times without
significant decrease in conversion.
One such chemoenzymatic one-pot process of current
interest is the enantioselective conversion of styrene or
a substituted derivative thereof to give the corresponding 1-
phenylethanol. This transformation corresponds to an asym-
metric hydration of a styrene, which represents a “dream
reaction” in organic chemistry. Up to now, there have been no
[
5–8]
efficient chemocatalysts for this reaction.
Recently, how-
ever, the Faber group reported the first example of an
efficient asymmetric direct hydration based on the use of
a decarboxylase as a catalyst and styrenes with a required p-
[
7]
hydroxy substituent as suitable substrates. As an alternative,
we previously developed a two-step one-pot process for the
direct transformation of styrenes into the corresponding
chiral secondary alcohols by combination of the Wacker–
Tsuji oxidation using benzoquinone as an oxidizing agent and
reduction by alcohol dehydrogenase (ADH) from Lactoba-
E
nantioselective one-pot processes have attracted much
attention as sustainable and economically viable methods.
[1]
One of the particular challenges in this field is the combina-
tion of chemical and enzymatic reactions. Often enzymes are
deactivated by components of the chemical reaction step, and
[
4g]
cillus kefir. Although this method is general with respect to
substrate range, one can see a drawback in the requirement of
a stoichiometric amount of benzoquinone. The application of
the “classic” Wacker oxidation conditions with molecular
oxygen as the oxidation agent (according to the process
shown in Scheme 1) would represent a more attractive,
economically favored, and “greener” alternative. As a catalyst
system, palladium chloride and copper chloride are suitable.
Initial studies in our group, however, unfortunately
revealed that the presence of Cu salts strongly inhibits the
subsequent enzymatic reduction, thus making an efficient
combination of the two towards a one-pot tandem process
[
2]
thus the reactions are not compatible. From a synthetic and
industrial point of view, in particular, the combination of
chemical catalysts, especially metal catalysts, and biocatalysts
in one pot would be valuable for the development of efficient
[2]
routes towards enantiomerically pure compounds. Only
a few examples of combinations of chemocatalytic and
enzymatic reactions in aqueous media have been developed
[
2–4]
by others and our group.
Furthermore, by means of this
[*] Dr. H. Sato, Prof. Dr. W. Hummel, Prof. Dr. H. Grçger
[
4g,9]
with both reactions in aqueous media impossible.
In the
Faculty of Chemistry, Bielefeld University
Universitꢀtsstrasse 25, 33615 Bielefeld (Germany)
E-mail: harald.groeger@uni-bielefeld.de
following, we report a strategy for how to overcome this
limitation, which at the same time represents a general
Dr. H. Sato
Biomaterial and Commodity Chemical Research Division
Osaka Municipal Technical Research Institute
1-6-50 Morinomiya, Joto-ku, Osaka 536-8553 (Japan)
[
**] We gratefully acknowledge generous support from the German
Federal Ministry of Education and Research (Bundesministerium
fꢁr Bildung und Forschung; BMBF) within the project “Biotechno-
logie 2020+, Nꢀchste Generation biotechnologischer Verfahren”
(
grant number 031A184A).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201409590.
Scheme 1. Combination of Wacker oxidation and enzymatic reduction
without isolation of the intermediate in a one-pot process.
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concept for combining incompatible chemo- and biocatalytic
reactions conducted in water towards one-pot processes.
To get a more detailed understanding of the impact of
metals, in particular Cu ions, on the enzyme first we
reinvestigated the one-pot synthesis of 3a through the
a membrane and encapsulation into a solid support such that
they are not in contact with each other. As a particularly
promising approach we considered the site-isolation method
for the chemocatalyts using polydimethylsiloxane (PDMS)
[
10]
thimbles developed by the Bowden group.
Due to the
combination of Wacker oxidation with PdCl /CuCl and
enzymatic reduction (Scheme 2). Whereas a quantitative
hydrophobic properties of the PDMS membrane, acetophe-
none formed in situ in the interior could flux through this
membrane into the exterior to react in the presence of the
enzyme (Figure 1). In contrast, the catalysts in the interior
2
Scheme 2. Combination of Wacker oxidation and enzymatic reduction
under “standard” conditions for a one-pot process.
conversion and formation of ketone 2a with 88% was
found for the Wacker oxidation step (when O in a balloon
2
was used, see the Supporting Information), a disappointing
conversion of only 5% was observed for the enzymatic
reduction of 2a to 3a, thus also confirming our initial results.
Next, we studied which metal component is the most
critical one for the enzyme in terms of deactivation. For the
investigation of the impact of copper ions on ADH, the
enzymatic reduction was conducted in the presence of such
metal ions. We focused on copper as the assumed critical
[4g,9]
metal component since in our previous studies
we found
Figure 1. Concept of site-isolation of catalysts using a PDMS thimble
for the combination of a Wacker oxidation and an enzymatic reduction
that PdCl has no negative impact on the enzymatic reduction.
2
However, conducting the enzymatic reduction in the presence
of a copper component revealed a moderate to strong
suppression of the biotransformation with conversions of
only 3%, 56%, and 51% in the presence of CuCl2
(
LK-ADH: alcohol dehydrogenase from L. kefir; IPA: isopropanol).
(PdCl , CuCl) as well as the exterior (enzyme, cofactors)
2
(
(
(
100 mol%), CuCl (100 mol%), and a mixture of CuCl
would be kept in their phases, and thus their undesired
interaction with each other would be avoided. In their pioneer
100 mol%) and PdCl (5 mol%) as the respective additive
2
[
10]
Scheme 3). In contrast, a high conversion of 93% was found
work the Bowden group
used a hydrophobic organic
for the enzymatic reduction in the absence of such copper
additives.
solvent in the exterior for extraction of a range of organic
hydrophobic intermediates from the interior into the exterior.
We envisioned that this compartmentalization concept might
also work with aqueous phases (with significant water
content) in both the interior and exterior phases as well as
with biocatalysts. The driving force for passing the hydro-
phobic PDMS membrane would result from the concentra-
tion gradient of acetophenone (formed in situ in the interior)
between the interior and exterior phases. In addition,
recycling of the Wacker oxidation catalyst system in the
PDMS thimble should be possible since hydrophilic catalytic
species such as metal ions (as the catalyst components for
Wacker oxidation) and enzymes are site-isolated and thus
retained by the PDMS membrane. In the following, we report
In order to achieve a combination of the Pd- and Cu-
catalyzed Wacker oxidation and the enzymatic reduction, we
envisioned a compartmentalization strategy as the most
promising. Such a compartmentalization should make it
possible to conduct both reactions, though not compatible
with each other, in a one-pot mode. Attractive options for the
compartmentalization of the catalysts include separation by
a method for the site-isolation of PdCl /CuCl and ADH from
2
L. kefir by a PDMS thimble and recycling of the metal
catalysts according to the concept shown in Figure 1 as the
first example of a chemoenzymatic one-pot process in water
under compartmentalization of water-soluble catalysts.
First, different one-pot concepts for the desired cascade
consisting of an initial Wacker oxidation of styrene as a model
Scheme 3. The impact of copper ions on the enzymatic reduction.
2
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substrate in the interior and an enzymatic reduction of in situ
formed acetophenone in the exterior (with both reactions
Wacker oxidation in the interior did not proceed efficiently in
this case, leading to only 20% conversion (Figure 2). As
a reason we found that methanol as the solvent component
for the Wacker oxidation is extracted also into the exterior,
thus decreasing the methanol content needed for an efficient
Wacker oxidation reaction in the interior.
[
11]
conducted at room temperature) were tested (Figure 2 and
Figure 3). After finding optimized conditions for the extrac-
tion of acetophenone to the exterior as well as the enzymatic
reduction there (data not shown), we examined a one-pot
process with simultaneous Wacker oxidation and enzymatic
reduction (Figure 2). Accordingly, styrene (1a) was dissolved
in a mixture of methanol and water (7:1) as the optimized
In order to suppress the methanol leaching, an alternative
one-pot concept based on conducting the Wacker oxidation
and enzymatic reduction in a sequential mode was inves-
tigated. In this process the buffer/isopropanol solution with
ADH and cofactor was added to the exterior after completion
of the initial Wacker oxidation in the interior (Figure 3). We
were pleased to find that by means of this one-pot method the
accumulated acetophenone was converted successfully into 1-
phenylethanol (R)-3a, leading to an excellent conversion also
for the second reaction step with 98% and a high overall
conversion of 85% related to the formation of product (R)-3a
(starting from 1a). This result also emphasizes that diffusion
of in situ formed ketone 2a through the PDMS thimble from
the interior to the exterior proceeds highly efficiently. In
detail, after the one-pot process was conducted 83% of 1-
phenylethanol (R)-3a and 2% of 2a was found in the exterior
as well as 2% of (R)-3a in the interior.
[
10b]
solvent
for the Wacker oxidation, and this solution was
poured into a PDMS thimble (interior chamber). The PDMS
thimble was placed in a flask filled with aqueous buffer,
isopropanol, an alcohol dehydrogenase from Lactobacillus
+
kefir as a catalyst, and NADP as a cofactor (Figure 2). In the
resulting process conducted in a “simultaneous mode” the
Wacker oxidation in the interior and enzymatic reduction in
the exterior proceed in parallel, and the formed acetophe-
none (2a) is extracted into the exterior where it is reduced by
the enzyme to alcohol (R)-3a. However, it turned out that the
Having successfully developed a one-pot procedure for
the combination of a Wacker oxidation with molecular
oxygen and an enzymatic reduction of the in situ formed
Wacker oxidation product 2a to produce alcohol (R)-3a, we
investigated the substrate scope of this methodology
(Scheme 4). Here, almost all investigated styrenes 1 (except
2
-methylstyrene (1c)) were efficiently converted into the
alcohol products 3 with high conversion (over two steps) and
excellent enantioselectivities. Thus, this one-pot method is
suitable for numerous substrates with different substituents at
the aryl moiety in 1, in particular when these substituents are
in the meta and para position. For example, all of the
examined 3- and 4-substituted styrenes with halogen, alkyl,
and alkoxy substituents were converted to the corresponding
acetophenones 2 and subsequently 1-phenylethanols (R)-3
with high conversion and excellent enantioselectivity. High
enantioselectivity was also obtained for the sterically hin-
dered 2-substituted 1-phenyl-ethanol (R)-3b with 98% ee at
Figure 2. Chemoenzymatic one-pot process in a “simultaneous
mode”.
7
9% conversion related to the formation of this product over
two steps. An exception within the broad substrate scope,
however, is the conversion of 2-methyl-substituted styrene
(1c) into product 3c: though Wacker oxidation of 1c also
proceeds successfully in this case, the resulting ketone 2c is
hardly converted by the enzyme (3% conversion).
Next we focused on recycling the catalyst system of the
Wacker oxidation (Figure 4). We were pleased to find that by
simple reuse of the (aqueous) phase of the interior of the
PDMS thimbles for further 15 cycles (which were conducted
over a period of 60 days!) no significant drop in conversion
and no decrease of enantioselectivity was observed in the one-
pot synthesis of alcohol 3a starting from substrate 1a (which
was added at each new cycle along with with methanol as
[12]
cosolvent). This indicates that the Pd/Cu catalyst system of
the Wacker oxidation in the interior is not deactivated even
after multiple recycling and that the ionic components in the
interior do not permeate the PDMS membrane.
Figure 3. Chemoenzymatic one-pot process in a “sequential mode”.
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numerous styrenes 2 into 1-phenylethanols (R)-3 with high
overall conversion and 98–99% ee. In this one-pot process
both reactions are conducted in an aqueous medium, and
tedious isolation of the ketone intermediates can be avoided
by the transport of the hydrophobic ketones 2 through the
PDMS membrane to the exterior solution, while the metal
components are retained in the interior. The expensive Pd
catalyst can be easily recovered and recycled over at least 15
cycles and remains active for at least 60 days.
Received: September 30, 2014
Published online: && &&, &&&&
Keywords: chiral alcohols · enzyme catalysis ·
.
one-pot synthesis · polydimethylsiloxane · Wacker oxidation
[
[
[
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enzymatic reduction in a one-pot process with compartmentalization
of the catalysts.
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4
0 h in some reactions for reasons of lab organization, see the
Supporting Information).
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system for a Wacker oxidation in the interior of a PDMS
thimble made it possible to combine this reaction with an
enzymatic reduction in a one-pot process although copper
chloride is highly deactivating to the enzyme. This one-pot
method under compartmentalization of the metal and enzyme
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2
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4
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[
[
[11] We chose room temperature, a relatively low reaction temper-
ature, to suppress the undesired effects that can be expected at
elevated temperatures such as loss of styrene from the reaction
mixture due to its evaporation as well as deactivation of the
enzyme and the cofactor.
[12] With respect to the recycling of other components, an attempt to
recycle methanol was not made due to its low price and readily
availability. In contrast, recycling of ADH and the cofactor is in
principle an interesting option for future work although
modified downstream-processing would be required for this
purpose. In the current process based on extractive workup,
enzyme and cofactor recycling would be tedious and might also
lead to significant enzyme deactivation. In addition, for this
process it has to be considered that the ADH can be produced
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Communications
One-Pot Synthesis
Dream reaction: A Wacker oxidation with
PdCl /CuCl was combined with an enzy-
2
H. Sato, W. Hummel,
H. Grçger*
matic reduction to convert styrenes
enantioselectively to 1-phenylethanols in
a one-pot process, although the two
reactions are not compatible with each
other due to enzyme deactivation by Cu
ions. The key to success was the com-
partmentalization of the catalysts (see
picture; PDMS=polydimethylsiloxane).
&&&& — &&&&
Cooperative Catalysis of Noncompatible
Catalysts through Compartmentalization:
Wacker Oxidation and Enzymatic
Reduction in a One-Pot Process in
Aqueous Medium
6
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Angew. Chem. Int. Ed. 2015, 54, 1 – 6
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