Modular chemoenzymatic one-pot syntheses in aqueous media: Combination of a palladium-catalyzed cross-coupling with an asymmetric biotransformation

Article abstract of DOI:10.1002/anie.200801341

(Chemical Equation Presented) Two breeds of cat: A palladium-catalyzed Suzuki cross-coupling and an enzymatic reduction with an alcohol dehydrogenase from Rhodococcus sp. together enable the efficient and highly enantioselective synthesis of chiral biaryl alcohols in a one-pot process (see scheme).

Full text of DOI:10.1002/anie.200801341

Angewandte
Chemie
DOI: 10.1002/anie.200801341
Aqueous Catalysis
Modular Chemoenzymatic One-Pot Syntheses in Aqueous Media:
Combination of a Palladium-Catalyzed Cross-Coupling with an
Asymmetric Biotransformation**
Edyta Burda, Werner Hummel, and Harald Grꢀger*
Dedicated to Professor Dr. Jꢁrgen Martens on the occasion of his 60th birthday
Multistep one-pot processes are an attractive synthetic
concept for the improvement of overall process efficiency
through a decrease in the required number of workup and
purification steps. By avoiding such time-, effort-, and solvent-
intensive steps, multistep one-pot syntheses contribute to a
significantly improved process economy as well as to more
sustainable synthetic routes.[1] A key criteria for multistep
one-pot processes is the compatibility of the individual
reaction steps with one another. Accordingly, most of the
multistep one-pot processes known today are based on either
chemocatalytic multistep reactions[2] or “pure” biotechnolog-
ical processes,[3] such as fermentation. In contrast, few
successful combinations of chemo- and biocatalytic reactions
are known.[4] Remarkable breakthroughs include, in partic-
ular, the dynamic kinetic resolutions developed by Williams
and co-workers,[5] by Bꢀckvall and co-workers,[6] and recently
by Berkessel et al.[7] These synthetic processes are based on
lipase-catalyzed resolution in combination with a simulta-
neous metal-catalyzed racemization of the substrate in an
organic solvent.
d-mannitol.[8] In general, however, chemoenzymatic one-pot
processes in aqueous media are still a largely unexplored area
of research.[8,9] As palladium-catalyzed cross-coupling reac-
tions[10] are of particular importance in the field of metal
catalysis, and enzymatic reduction[11] is very important in
biocatalysis, we were interested in the compatibility of these
types of reactions in water. As the first example of a one-pot
process in which a palladium-catalyzed cross-coupling reac-
tion is combined with a biotransformation in an aqueous
reaction medium, we report herein the synthesis of chiral
biaryl alcohols 4 through Suzuki cross-coupling and subse-
quent asymmetric enzymatic reduction (according to the
synthetic concept shown in Scheme 1).
Preliminary experiments showed the general difficulty in
the development of such a one-pot two-step process, in
particular with respect to the compatibility of metal catalysis
and biocatalysis. When the Suzuki cross-coupling and enzy-
matic reduction were carried out separately, both reactions
proceeded smoothly (Scheme 2A). We chose the palladium
However,
as
most
enzymes are incompatible,
or at best poorly compatible,
with organic solvents, the
development of chemoenzy-
matic multistep one-pot pro-
cesses in aqueous media is
highly desirable. Pioneering
studies in this field involved
the combination of a glucose
isomerase with a heterogene-
ous platinum catalyst for the
conversion of a mixture of
d-glucose and d-fructose into
Scheme 1. Concept of the chemoenzymatic one-pot synthesis. NAD+ =nicotinamide adenine dinucleotide;
NADH is the reduced form of NAD+.
complex 5 and phosphane 6 as the catalyst system for the
[*] E. Burda, Prof. Dr. H. Grꢀger
Department of Chemistry and Pharmacy
University of Erlangen–Nuremberg
Henkestrasse 42, 91054 Erlangen (Germany)
E-mail: harald.groeger@chemie.uni-erlangen.de
cross-coupling step, as these catalyst components had been
applied previously in a Suzuki coupling in an aqueous
medium.[12] The Suzuki cross-coupling of the boronic acid
2a (1.75 equiv) with 1a gave the biaryl ketone 3a with a
conversion of greater than 95% (Scheme 2A, step 1). In a
second step, enzymatic reduction of the isolated and purified
ketone 3a led to the formation of the desired alcohol (S)-4a
with 93% conversion and > 99% ee after adjustment of the
pH value to pH 7 (Scheme 2A, step 2). This reaction was
catalyzed by an alcohol dehydrogenase (ADH) from Rhodo-
Prof. Dr. W. Hummel
Institute of Molecular Enzyme Technology
Heinrich Heine University of Dꢁsseldorf
Research Centre Jꢁlich (Germany)
[**] We thank Sonja Borchert for excellent technical support, and Evonik-
Degussa, Amano Enzymes, and Oriental Yeast Company, Japan for
generous support with chemicals.
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Scheme 2. Synthesis in two separate steps (A) versus one-pot two-step
synthesis (B).
coccus sp.[13] with in situ substrate-coupled cofactor regener-
ation with 2-propanol (which is oxidized to acetone). How-
ever, when these two processes were combined in a one-pot
two-step synthesis with adjustment of the pH value to pH 7
prior to the biotransformation, the desired product (S)-4a was
formed with a significantly decreased conversion of only 44%
(Scheme 2B).
Figure 1. Influence of the components of the Suzuki cross-coupling on
To find an explanation for this unsatisfying result, we
studied spectrophotometrically the influence of the compo-
nents of the Suzuki cross-coupling reaction on enzyme
activity at concentrations up to the solubility limit, with 3a
as the reference substrate (Figure 1). We first investigated the
potential inhibition of the ADH by the palladium complex, as
enzyme inhibition by heavy metals is a known phenomenon.
To our surprise, however, we found that the palladium
complex 5 had only a minor negative impact on the enzyme
activity (Figure 1, part A). Even at a metal-complex concen-
tration of 0.63 mm, the enzyme activity remained high at
79%. In contrast, a more significant decrease in the enzyme
activity was observed in the presence of the phosphane. For
example, the residual activity was only 56% at a 4 mm
concentration of triphenylphosphane (6; Figure 1, part B).
The boronic acid, however, had the strongest negative
influence on enzyme activity. In the presence of phenyl-
boronic acid (2a) at a concentration of 0.17m, the residual
activity of the enzyme was only 14% (Figure 1, part C). The
borate salt formed from the boronic acid in the Suzuki cross-
coupling has a much less negative impact on enzyme activity:
At a borate-salt concentration of 0.17m, the residual enzyme
activity was 66%.
enzyme activity.
developed such a Suzuki cross-coupling for the synthesis of
the biaryl ketone 3a as a model reaction. In the presence of
the catalyst [Pd(PPh3)2Cl2] (5) and exactly one equivalent of
phenylboronic acid (2a), the reaction proceeded successfully
in water to give 3a with a conversion of greater than 95%
(Scheme 3).
Scheme 3. Optimized Suzuki cross-coupling in water.
We were pleased to find that the resulting reaction
mixture was compatible with a subsequent ADH-catalyzed
reaction. When this Suzuki cross-coupling was followed by an
enzymatic reduction (after adjustment of the pH value to
pH 7) with substrate-coupled cofactor regeneration with
2-propanol, the desired biaryl-substituted alcohol (S)-4a
was formed with 91% conversion and excellent enantiose-
lectivity (> 99% ee; Table 1, entry 1). This conversion of 91%
corresponds almost exactly to the calculated overall con-
From these experiments, we deduced the following
prerequisites for an enzyme-compatible Suzuki cross-cou-
pling reaction: a) No phosphane additive may be used, b) the
boronic acid may not be used in excess, c) conversion must be
quantitative with complete consumption of the boronic acid,
and d) water must be used as the reaction medium. We
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Table 1: Substrate spectrum of the one-pot two-step synthesis.[a]
Entry
1
Product
Conversion [%]
91
ee [%][b]
>99
Scheme 4. Synthesis of the enantiomerically pure diol (S,S)-7.
2
3
83
67
>99
>99
In conclusion, we have described the one-pot synthesis of
chiral biaryl alcohols through Suzuki cross-coupling and
subsequent enzymatic reduction. The products were obtained
with up to 91% conversion and excellent enantioselectivities
(> 99% ee). To the best of our knowledge, this one-pot two-
step synthesis is the first example of the combination of a
palladium-catalyzed cross-coupling reaction with an (asym-
metric) biotransformation in an aqueous medium. We are
currently investigating further one-pot multistep syntheses
that combine chemocatalytic and biocatalytic reactions in
aqueous media.
[a] For the reaction conditions, see the Experimental Section. [b] The
ee value was determined by HPLC on a chiral phase with a mixture of
hexane and 2-propanol (95:5) as the eluent (4a: Daicel chiracel OD
column; 4b: Daicel chiracel OJ-H column; 4c: Daicel chiracel AD-H
column).
version of the two reactions when carried out separately
(according to Scheme 2A). Thus, in the one-pot process, the
reaction mixture of the Suzuki cross-coupling has minimal
negative impact on the subsequent biotransformation, in
particular with respect to conversion. Furthermore, this one-
pot two-step synthesis is suitable for a broad range of
substrates. For example, with the substrate 3-bromoaceto-
phenone, a combination of Suzuki cross-coupling and ADH-
catalyzed reduction in an aqueous medium gave the product
(S)-4b with 83% conversion and > 99% ee (Table 1, entry 2).
The boronic acid component can also be varied, as demon-
strated by the synthesis of (S)-4c from 4-methylphenylbor-
onic acid with 67% conversion and > 99% ee (Table 1,
entry 3).
An additional challenge is the synthesis of biaryl diols,
such as (S,S)-7. Chiral diols are valuable (monomeric)
building blocks for the construction of enantiomerically
pure polymers. To date, the only known asymmetric
approaches to bis(a-hydroxyethyl)biphenylenes involve a
multistep synthesis from (R)-3-bromophenylethan-1-ol as a
chiral auxiliary,[14] a diastereoselective synthesis,[15] or an
enzymatic resolution.[16] In the first asymmetric (bio-)catalytic
synthesis of such a diol, we prepared (S,S)-7 via a diacetylbi-
phenyl intermediate (synthesized in situ through Suzuki
cross-coupling) with our one-pot two-step synthesis: The
Suzuki cross-coupling of the prochiral substrates 4-bromoa-
cetophenone (1a) and 3-acetylphenylboronic acid (2c) in an
aqueous medium, followed by in situ enzymatic reduction of
the formed diacetylbiphenyl intermediate, produced the
desired diol (S,S)-7 with high diastereoselectivity (d.r. =
25:1) and excellent enantioselectivity (> 99% ee; Scheme 4).
Experimental Section
Spectrophotometric assay for the measurement of enzyme activity
(see Figure 1): In analogy with a previous protocol,[17] the consump-
tion of NADH through oxidation to NAD+ was measured spectro-
photometrically at a wavelength of 340 nm in the presence of
4-phenylacetophenone (3a) as the substrate and the corresponding
additive (e340 = 6.3 mmÀ1 cmÀ1). The additives tested and their con-
centrations are given in Figure 1. A cuvette (1 mL) was filled with
960 mL of a buffered solution of 4-phenylacetophenone (3a: 10 mm;
phosphate buffer: pH 7.0, 50 mm), which also contained the additive
in various concentrations, and 20 mL of a buffered solution of NADH
(NADH: 12.5 mm; phosphate buffer: pH 7.0, 50 mm). A solution
(20 mL, dilution: 1:100) of (S)-ADH from Rhodococcus sp. (partially
purified; NADH-dependent; volumetric activity: 116 UmLÀ1) was
then added. The relative activities were determined by comparison of
the enzyme activities (in UmLÀ1) measured spectrophotometrically
with the enzyme activity in the experiment in the absence of an
additive (regarded as the reference experiment with a relative activity
of 100%). U always refers to 3a as the standard substrate.
One-pot synthesis of biaryl alcohols (S)-4 (Table 1): The aryl
boronic acid 2 (0.25 mmol), the bromoacetophenone component 1
(0.25 mmol), and bis(triphenylphosphane)palladium(II) chloride (5,
0.005 mmol, 2 mol%) were added sequentially to a solution of
sodium carbonate (10 mmol) in water (7.5 mL) in a 25 mL round-
bottomed flask. The reaction mixture was stirred for 17 h at 708C and
then cooled to room temperature. After adjustment of the pH value
to pH 7 by the addition of hydrochloric acid, 2-propanol (2.5 mL),
NADH[18] (0.02 mmol), and the ADH from Rhodococcus sp. (Table 1,
entries 1 and 2: 46 U; Table 1, entry 3: 69 U) were added, and the
reaction mixture was stirred for 48 h at room temperature. The
aqueous phase was then extracted with dichloromethane (3 ꢁ 20 mL).
The combined organic phases were dried over magnesium sulfate,
filtered, and concentrated under vacuum. The crude product was
purified by flash chromatography (silica gel 60 ꢂ; 1: 1.5 cm; length:
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22 cm; eluent: n-hexane/ethyl acetate (5:1)). Alcohols (S)-4a,c were
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Received: March 19, 2008
Revised: July 21, 2008
Published online: October 22, 2008
Keywords: chemoenzymatic reactions · chiral diols ·
.
cross-coupling · homogeneous catalysis · one-pot synthesis
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