Combination of a Suzuki cross-coupling reaction using a water-soluble palladium catalyst with an asymmetric enzymatic reduction towards a one-pot process in aqueous medium at room temperature

Article abstract of DOI:10.1016/j.molcatb.2012.03.006

We report the combination of a palladium-catalyzed Suzuki cross-coupling reaction with an enzymatic reduction in a one-pot process in aqueous medium, in which both reaction steps are conducted at room temperature. By using a water-soluble palladium catalyst system, which is prepared from commercially available palladium chloride and TPPTS (tris(3-sulfonatophenyl)phosphine hydrate, sodium salt), the palladium-catalyzed step runs at room temperature in a mixture of isopropanol and water. After adjusting the pH value to pH 7, the resulting biaryl ketone is then directly reduced in situ via an enzymatic asymmetric reduction in the presence of an alcohol dehydrogenase (ADH), leading to the desired alcohol with up to >95% conversion (over two steps) and excellent enantioselectivities of >99% ee. Notably, both enantiomers of the desired alcohol are accessible depending on the source of applied ADH.

Full text of DOI:10.1016/j.molcatb.2012.03.006

Journal of Molecular Catalysis B: Enzymatic 84 (2012) 89–93
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Journal of Molecular Catalysis B: Enzymatic
journal homepage: www.elsevier.com/locate/molcatb
Combination of a Suzuki cross-coupling reaction using a water-soluble palladium
catalyst with an asymmetric enzymatic reduction towards a one-pot process in
aqueous medium at room temperature
Sonja Borcherta, Edyta Burdaa, Jürgen Schatza, Werner Hummelb, Harald Grögera,c,∗
a Department of Chemistry and Pharmacy, University of Erlangen-Nürnberg, Henkestr. 42, 91054 Erlangen, Germany
b Institute of Molecular Enzyme Technology at the Heinrich-Heine University of Düsseldorf, Research Centre Jülich, Stetternicher Forst, 52426 Jülich, Germany
c Faculty of Chemistry, Bielefeld University, Universitätsstr. 25, 33615 Bielefeld, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 17 March 2012
We report the combination of a palladium-catalyzed Suzuki cross-coupling reaction with an enzymatic
reduction in a one-pot process in aqueous medium, in which both reaction steps are conducted at room
temperature. By using a water-soluble palladium catalyst system, which is prepared from commercially
available palladium chloride and TPPTS (tris(3-sulfonatophenyl)phosphine hydrate, sodium salt), the
palladium-catalyzed step runs at room temperature in a mixture of isopropanol and water. After adjust-
ing the pH value to pH 7, the resulting biaryl ketone is then directly reduced in situ via an enzymatic
asymmetric reduction in the presence of an alcohol dehydrogenase (ADH), leading to the desired alcohol
with up to >95% conversion (over two steps) and excellent enantioselectivities of >99% ee. Notably, both
enantiomers of the desired alcohol are accessible depending on the source of applied ADH.
© 2012 Elsevier B.V. All rights reserved.
Keywords:
Alcohol dehydrogenase
Chemoenzymatic synthesis
Enzymatic reduction
One-pot synthesis
Suzuki cross-coupling reaction
1. Introduction
desired alcohol, but at the same time also the oxidation of the
cosubstrate isopropanol under formation of acetone, thus regen-
Multi-step one-pot processes represent an attractive synthetic
concept for the improvement of overall process efficiency by
decreasing the required number of work up and purification steps.
Although a range of successful combinations of chemo-catalytic
with biocatalytic reactions towards chemoenzymatic one-pot pro-
cesses in organic media were developed [1–3], analogous one-pot
processes in aqueous medium are still rare. Water, however, rep-
resents an attractive solvent since it is an inexpensive, readily
available, non-flammable and non toxic solvent [4,5]. Additionally
and (important from a biocatalytic perspective) water enables as
the “solvent of nature” the use of all types of enzymes. In this con-
nection there has been a recent interest by various groups in the
efficient combination of the Suzuki cross-coupling reaction as an
example for a palladium-catalyzed synthesis with an asymmetric
enzymatic reduction towards a chemoenzymatic one-pot process
in aqueous reaction media [4,6,7]. The enzymatic reduction has
been carried out under substrate-coupled cofactor regeneration
[8,9] with isopropanol as reducing agent. The alcohol dehydroge-
nase (ADH) catalyzes not only the reduction of the ketone to the
erating the required cofactor NAD(P)H, which then can be used
in catalytic amount. This type of chemoenzymatic one-pot pro-
cess suitable for the preparation of biaryl alcohols 4 is shown in
Scheme 1.
Recently the “proof of concept” for the combination of these
types of reactions in aqueous medium was reported by our group
[6], leading for example to the chiral biaryl-containing alcohol (S)-4
with 91% conversion and >99% ee in a one-pot process. Further-
more, exciting developments and improvements of this type of
chemoenzymatic one-pot process for the synthesis of chiral biaryl-
containing alcohols have been reported by other groups [4,7].
Cacchi et al. demonstrated the suitability of palladium nanopar-
ticles stabilised with DNA-binding protein (Pdnp/Te-DPS) as a
catalyst for the (phosphine-free) palladium-catalyzed Suzuki cross-
coupling reaction step in this one-pot process [4]. Kroutil and
Schmitzer set value on the engineering of the reaction medium,
and applied an advantageous biphasic solvent system consisting
of an aqueous phase and a water-immiscible ionic liquid as reac-
tion medium for this one-pot process [7]. A key feature of all of
these one-pot processes is the use of the various palladium cata-
lyst systems for the initial “bio-compatible” Suzuki cross-coupling
reaction at elevated temperatures being in the range between 70 ◦C
and 110 ◦C [4,6,7].
∗
Corresponding author at: Faculty of Chemistry, Bielefeld University, Univer-
sitätsstr. 25, 33615 Bielefeld, Germany. Tel.: +49 521 106 2057;
fax: +49 521 106 6146.
Besides this type of one-pot two-step process, in which both
steps are sequentially carried out in subsequent fashion, we also
E-mail address: harald.groeger@uni-bielefeld.de (H. Gröger).
1381-1177/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcatb.2012.03.006

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S. Borchert et al. / Journal of Molecular Catalysis B: Enzymatic 84 (2012) 89–93
Scheme 1. Concept of the chemoenzymatic one-pot process in aqueous medium.
product in CDCl3). The 1H NMR spectroscopical data of the product
are in accordance with those reported in Ref. [7].
In analogy, the synthesis of ketone 3 was carried out using
degassed and deionized water instead of degassed phosphate
buffer, resulting in >95% conversion as well.
became interested (as a long-term goal) in developing a “tandem-
like” one-pot process as a process alternative. Such a tandem-type
one-pot process, in which both reaction steps proceed simultane-
ously, requires full compatibility of both reaction steps with each
other. Among others, the prerequisite that both reactions proceed
at the same temperature has to be fulfilled. Thus, since most ADHs
operate under ambient temperature also a Suzuki process running
at room temperature would be desirable.
2.3. Enzymatic reduction of 4ꢀ-phenylacetophenone 3 (according
to Scheme 3)
In the following we report our progress on the latter issues and
the development of a chemoenzymatic one-pot process for the syn-
thesis of 4, in which now a Suzuki cross-coupling reaction running
at room temperature under basic conditions has been successfully
applied. Notably, this reaction also turned out to be highly compat-
ible with the subsequent enzymatic reduction. Further beneficial
key features of this one-pot process are (i) the application of a
water-soluble palladium catalyst which is advantageous in terms
of reaching a homogeneous reaction medium and (ii) the aqueous
reaction medium, consisting of up to 50% (v/v) of isopropanol, thus
ensuring an optimized solubility of the substrate.
2.3.1. Synthesis of (S)-biphenylethan-1-ol, (S)-4
To
a
suspension, consisting of 4ꢀ-phenylacetophenone (3;
49.1 mg; 0.25 mmol) in phosphate buffer (5.0 mL; pH 7; 50 mM)
and isopropanol (5.0 mL), NAD+ (18 mg; 0.025 mmol) and ADH
from Rhodococcus sp. (16 U; referring to the activity for 4ꢀ-
phenylacetophenone, 3) were added. After stirring for 24 h the
aqueous phase was extracted with dichloromethane (3 × 10 mL).
The combined organic layers were dried over MgSO4, filtered and
concentrated under vacuum, providing the crude product (S)-4
as a white to yellow solid (>95% conversion; determined by 1H
NMR spectroscopy of the crude product in CDCl3). The enan-
tiomeric excess of (S)-4 was determined from the crude product
by chiral HPLC chromatography using an IB column (eluent: hex-
ane/isopropanol = 95:5; flow: 0.8 mL/min; 238 nm; tR(S) = 13.1 min,
tR(R) = 14.0 min): >99% ee. The crude product was purified by flash
2. Experimental
2.1. Materials
˚
The (R)-alcohol dehydrogenase from Lactobacillus kefir has been
prepared according to Ref. [10]. The (S)-alcohol dehydrogenase
from Rhodococcus sp. is a commercially available enzyme from evo-
catal GmbH (product number 1.1.030) [11]. The ligand TPPTS (5;
tris(3-sulfonatophenyl)phosphine hydrate, sodium salt contain-
ing 10–15% oxide) was purchased from ABCR. Palladium chloride
(containing 59% Pd) was purchased from Acros Organics. The enan-
tiomeric excess was determined by chiral HPLC chromatography
either using a Chiralpak AD-H or an IB column.
chromatography (silica gel 60 A; eluent: cyclohexane/EtOAc = 4:1).
The purified alcohol (S)-4 was obtained as a colorless solid in
64% yield (31.7 mg, 0.16 mmol) and the enantiomeric excess was
determined by chiral HPLC chromatography with an AD-H col-
umn (eluent: hexane/isopropanol = 95:5; flow 0.8 mL/min; 238 nm;
tR(S) = 15.9 min, tR(R) = 16.8 min): >99% ee. The 1H NMR spectro-
scopical data of the product are in accordance with those reported
in Ref. [7].
2.3.2. Synthesis of (R)-biphenylethan-1-ol, (R)-4
2.2. Suzuki cross-coupling reaction (according to Scheme 2)
To a
suspension, consisting of 4ꢀ-phenylacetophenone (3;
76.3 mg; 0.389 mmol) in phosphate buffer (5.0 mL; pH 7; 50 mM)
and isopropanol (5.0 mL), magnesium chloride (about 1 mM),
NADP+ (18 mg; 0.037 mmol) and ADH from L. kefir (46 U; refer-
ring to the activity for 4ꢀ-phenylacetophenone, 3) were added.
After stirring for 24 h the aqueous phase was extracted with
dichloromethane (3 × 10 mL). The combined organic layers were
dried over MgSO4, filtered and concentrated under vacuum,
providing the crude product (R)-4 as a white to yellow solid (95%
conversion; determined by 1H NMR spectroscopy of the crude
product in CDCl3). The enantiomeric excess of (R)-4 was deter-
mined from the crude product by chiral HPLC chromatography
using an IB column (eluent: hexane/isopropanol = 95:5; flow:
0.8 mL/min; 238 nm; tR(S) = 13.1 min, tR(R) = 13.9 min): >99% ee.
The crude product was purified by flash chromatography (silica
At first, a suspension of palladium chloride (2.7 mg; 0.015 mmol;
4 mol%) and TPPTS (5; 11.4 mg; 0.020 mmol; 5 mol%) in degassed
and deionized water (0.5 mL) was prepared. After 30 min the result-
ing catalyst solution was then added to a mixture, consisting
of 4ꢀ-iodoacetophenone (1; 95.7 mg; 0.389 mmol), phenylboronic
acid (2; 47.4 mg; 0.389 mmol), sodium carbonate (103.1 mg;
0.973 mmol), degassed phosphate buffer (4.5 mL; pH 7; 50 mM) and
degassed isopropanol (5.0 mL). The reaction mixture was stirred
at room temperature for 24 h at 400 rpm and subsequently acid-
ified through addition of 2.5 M hydrochloric acid (in order to
stop the reaction). Then the aqueous phase was extracted with
dichloromethane (3 × 10 mL). The combined organic layers were
dried over MgSO4, filtered and concentrated under vacuum, pro-
viding the crude product 3 as a yellow to brown solid (>95%
conversion; determined by 1H NMR spectroscopy of the crude
˚
gel 60 A; eluent: cyclohexane/EtOAc = 4:1). The purified alcohol
(R)-4 was obtained as a colorless solid in 68% yield (52.2 mg,

S. Borchert et al. / Journal of Molecular Catalysis B: Enzymatic 84 (2012) 89–93
91
0.263 mmol) and the enantiomeric excess was determined by
chiral HPLC chromatography with an AD-H column (eluent: hex-
ane/isopropanol = 95:5; flow: 0.8 mL/min; 238 nm; tR(S) = 15.8 min,
tR(R) = 16.7 min): >99% ee. The 1H NMR spectroscopical data of the
product are in accordance with those reported in Ref. [7].
2.4. Preparation of (S)-biphenylethan-1-ol, (S)-4, in a one-pot
process
At first, a suspension consisting of palladium chloride (2.7 mg;
0.015 mmol; 4 mol%) and TPPTS (5; 11.4 mg; 0.020 mmol; 5 mol%)
in degassed and deionized water (0.5 mL) was prepared. After
30 min the resulting catalyst solution was then added to a mix-
ture, consisting of 4ꢀ-iodoacetophenone (1; 95.7 mg; 0.389 mmol),
phenylboronic acid (2; 47.4 mg; 0.389 mmol), sodium carbonate
(103.1 mg; 0.973 mmol), degassed phosphate buffer (4.5 mL; pH
7; 50 mM) and degassed isopropanol (5.0 mL). The reaction mix-
ture was stirred at room temperature for 24 h at 400 rpm. After
adjusting the pH-value of the reaction medium to pH 7 by addi-
tion of 2.5 M hydrochloric acid, NAD+ (26 mg; 0.037 mmol) and
ADH from Rhodococcus sp. (25 U; referring to the activity for 4ꢀ-
phenylacetophenone, 3) were added. After stirring for another
24 h the aqueous phase was extracted with dichloromethane
(3 × 10 mL). The combined organic layers were dried over MgSO4,
filtered and concentrated under vacuum, providing the crude prod-
uct (S)-4 as a white to yellow solid (>95% conversion of 1; 96%
(S)-4, 4% 3; determined by 1H NMR spectroscopy of the crude prod-
uct in CDCl3). The enantiomeric excess of (S)-4 was determined
from the crude product by chiral HPLC chromatography using an
IB column (eluent: hexane/isopropanol = 95:5; flow: 0.8 mL/min;
238 nm; tR(S) = 13.1 min, tR(R) = 14.0 min): >99% ee. The crude prod-
Scheme 2. Suzuki reaction at room temperature in aqueous medium.
determined from the crude product by chiral HPLC chromatography
using an AD-H column (eluent: hexane/isopropanol = 95:5; flow:
0.8 mL/min; 238 nm; tR(S) = 15.3 min, tR(R) = 16.6 min): ee >99%.
The crude product was purified by flash chromatography (silica gel
˚
60 A; eluent: cyclohexane/EtOAc = 4:1). The purified alcohol (R)-4
was obtained as a colorless solid in 72% yield (55.9 mg, 0.282 mmol),
and the enantiomeric excess was determined by chiral HPLC chro-
matography with an IB column (eluent: hexane/isopropanol = 95:5;
flow: 0.8 mL/min; 238 nm; tR(S) = (14.3 min), tR(R) = 15.2 min):
>99% ee. The 1H NMR spectroscopical data of the product are in
accordance with those reported in Ref. [7].
In analogy, the one-pot reaction for the synthesis of chiral alco-
hol (R)-4 was carried out using degassed phosphate buffer (pH 7;
50 mM) instead of degassed and deionized water, resulting in >95%
conversion of 1 as well (crude product: 86% (R)-4, 14% 3).
˚
uct was purified by flash chromatography (silica gel 60 A; eluent:
cyclohexane/EtOAc = 4:1). The purified alcohol (S)-4 was obtained
as a colorless solid in 72% yield (55.5 mg, 0.280 mmol) and the
enantiomeric excess was determined by chiral HPLC chromatogra-
phy with an IB column (eluent: hexane/isopropanol = 95:5; flow:
0.8 mL/min; 238 nm; tR(S) = 14.8 min, tR(R) = 15.7 min): >99% ee.
The 1H NMR spectroscopical data of the product are in accordance
with those reported in Ref. [7].
In analogy, the one-pot reaction for the synthesis of chiral alco-
hol (S)-4 was carried out using degassed and deionized water
instead of degassed phosphate buffer (pH 7; 50 mM), resulting in
>95% conversion of 1 as well (crude product: 97% (S)-4, 3% 3).
3. Results and discussion
At first we searched for a water-soluble palladium-catalyst sys-
tem suitable for the Suzuki-cross coupling reaction in aqueous
medium, favouring commercially available catalyst systems. In this
connection we focused on a combination consisting of palladium
chloride and the water-soluble tri-sulfonated phosphine ligand
TPPTS (5) as a catalytic system (Scheme 2). Sulfonated phosphines
are known as water-soluble ligands in the palladium-catalyzed
Suzuki cross-coupling reaction since 1990 when Casalnuovo and
Calabrese reported the successful cross-coupling of 4-iodotoluene
and phenylboronic acid in a mixture of water and acetonitrile
using Pd(PPh2)(m-C6H4SO3M)·(H2O)4 (M = Na+, K+) as a mono-
sulfonated phosphine palladium complex [12,13]. Another ligand
from the class of water-soluble phosphines is TPPTS (5; tris(3-
sulfonatophenyl)phosphine hydrate, sodium salt), a tri-sulfonated
phosphine ligand applied in palladium-catalyzed Tsuji-Trost reac-
tion and hydroformylation in water [14,15] as well as in Suzuki
cross-coupling reactions either in a mixture of water and acetoni-
trile [12,16–18] or pure glycerol [19] as a solvent. The latter reaction
was usually carried out in the presence of sodium carbonate or an
amine base. However the described reaction conditions, particu-
larly acetonitrile as part of the reaction medium was not suitable
for our purpose. Since we were interested to conduct the Suzuki
cross-coupling reaction in a reaction medium which we knew to
be compatible with the biotransformation, we favoured a mixture
of water and isopropanol as a reaction medium.
2.5. Synthesis of (R)-biphenylethan-1-ol, (R)-4, in a one-pot
process
At first, a suspension consisting of palladium chloride (2.7 mg;
0.015 mmol; 4 mol%) and TPPTS (5; 11.4 mg; 0.020 mmol; 5 mol%)
in degassed and deionized water (0.5 mL) was prepared. After
30 min the resulting catalyst solution was then added to a mix-
ture, consisting of 4ꢀ-iodoacetophenone (1; 95.7 mg; 0.389 mmol),
phenylboronic acid (2; 47.4 mg; 0.389 mmol), sodium carbonate
(103.1 mg; 0.973 mmol), degassed and deionized water (4.5 mL)
and degassed isopropanol (5.0 mL). The reaction mixture was
stirred at room temperature for 24 h at 400 rpm. After adjusting
the pH-value of the reaction mixture to pH 7 through addition
of 2.5 M hydrochloric acid, magnesium chloride (about 1 mM),
NADP+ (18 mg; 0.022 mmol) and ADH from L. kefir (46 U; referring
to the activity for 4ꢀ-phenylacetophenone, 3) were added. After
stirring for another 24 h the aqueous phase was extracted with
dichloromethane (3 × 10 mL). The combined organic layers were
dried over MgSO4, filtered and concentrated under vacuum, provid-
ing the crude product (R)-4 as a white to yellow solid (>95% conver-
sion of 1; 92% (R)-4, 8% 3; determined by 1H NMR spectroscopy of
the crude product in CDCl3). The enantiomeric excess of (R)-4 was
Thus, we applied the catalyst prepared from palladium chlo-
ride (4 mol%) and TPPTS (5; 5 mol%) in a mixture of isopropanol
and water (50%, v/v) as reaction medium, using an excess
of sodium carbonate as base. This catalyst system enabled us
to conduct the palladium-catalyzed cross-coupling reaction of

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S. Borchert et al. / Journal of Molecular Catalysis B: Enzymatic 84 (2012) 89–93
Table 1
Chemoenzymatic one-pot process at room temperature using a water-soluble palladium catalyst.
Entry
Reaction mediuma
ADH
Conv.b [%]
ee [%]
1
2
3
4
Water(5)/buffer(45)/i-PrOH(50)
Water(50)/i-PrOH(50)
Water(5)/buffer(45)/i-PrOH(50)
Water(50)/i-PrOH(50)
Rhodococcus sp.
Rhodococcus sp.
Lactobacillus kefir
Lactobacillus kefir
96
97
86
92
>99 (R)
>99 (R)
>99 (S)
>99 (S)
a
In parentheses the percentage of the corresponding solvent related to the overall volume of the reaction medium is given; buffer: pH 7 (50 mM KH2PO4, adjusted to pH
7 using ∼30 mM NaOH).
b
The conversion is related to the formation of the desired product (S)-4 or (R)-4 and was determined from the crude product by 1H NMR spectroscopy; amount of remaining
intermediate 3: entry 1, 4%; entry 2, 3%; entry 3, 14%; entry 4, 8% (in all these reactions, full consumption of substrate 1 was observed).
4ꢀ-iodoacetophenone (1) with phenylboronic acid (2) at room tem-
perature in the presence of a homogeneously dissolved catalyst
without the need of an inert gas atmosphere. We were pleased to
find that under these conditions the reaction proceeds with high
conversion of >95% with respect to the formation of the biaryl
ketone 3 (Scheme 2).
Thus, by changing the catalyst from [Pd(PPh3)2Cl2] to the homo-
geneously dissolved, water-soluble catalyst prepared from PdCl2
and TPPTS (5) we were now able to run the first reaction step of
the desired one-pot process at room temperature in a reaction
medium having the potential for being compatible with the bio-
catalyst, obtaining the ketone product 3 with excellent conversion
of >95% after a reaction time of 24 h. It should also be added that
this Suzuki cross-coupling reaction represents a very simple, easy
to handle and highly efficient synthetic methodology.
For the subsequent enzymatic reduction of ketone 3 we chose –
in analogy to our earlier work [6,20] – an (R)-enantioselective ADH
from L. kefir [10] and an (S)-enantioselective ADH from Rhodococ-
cus sp. [11] in order to gain access to both enantiomers of the
desired chiral alcohol 4. As biphenyl ketones like 3 are hardly
water-soluble, isopropanol also acts as a cosolvent, thus enhanc-
ing the solubility of the substrate 3. The enzymatic reduction of
the isolated biaryl ketone 3 was carried out in a mixture of phos-
phate buffer pH 7 and isopropanol (50%, v/v), and we obtained the
alcohol (S)-4 with >95% conversion when using the commercially
available (S)-enantioselective ADH from Rhodococcus sp. (Scheme 3,
Eq. (A)). In addition, the reduction proceeds with a conversion
of 95% in the presence of an (R)-enantioselective ADH from L.
kefir (Scheme 3, Eq. (B)). In both cases we used a non-optimized
amount of 10 mol% of cofactor and obtained the products (S)-4 and
(R)-4, respectively, with excellent enantiomeric excess of >99% ee
(Scheme 3). Furthermore, under the applied reaction conditions
with, e.g., high isopropanol content, we were able to optimize the
amount of enzyme. In case of the enzymatic reduction of ketone 3
with the ADH from Rhodococcus sp. the reaction proceeds with still
very high conversion at a decreased catalyst amount of 64 U/mmol
(compared to 184 U/mmol used in our previous process; see Ref.
[6]).
ligand. When we examined the potential inhibitory effect of both
palladium chloride and TPPTS (5) on the ADH from Rhodococcus
sp. in a spectrophotometric assay (analogously to a previously
reported method; see Ref. [6]) we were pleased that no signif-
icant inhibition from any of these compounds occurs. Based on
these encouraging results we then combined both reaction steps
in a one-pot process in aqueous medium. The initial Suzuki cross-
coupling reaction catalyzed by a water-soluble palladium species
prepared from palladium chloride and TPPTS (5) was carried out in
an aqueous phase containing 50% of isopropanol (v/v) in the pres-
ence of an excess of sodium carbonate. After stirring for 24 h at room
temperature the pH value was adjusted to pH 7, and subsequently
the alcohol dehydrogenase from Rhodococcus sp. was added. After
stirring for further 24 h at room temperature the desired alcohol
(S)-4 was obtained with excellent (product-based) conversion of
>95% (over 2 steps) and with excellent enantioselectivity of >99% ee
(Table 1, entries 1 and 2). This result indicates a high compatibility
of these two different types of catalysts, namely the water-soluble
palladium-TPPTS-complex and the enzyme.
In order to obtain the opposite enantiomer (R)-4 in such a type of
chemoenzymatic one-pot process we conducted the reaction anal-
ogously by means of the alcohol dehydrogenase from L. kefir. The
desired alcohol (R)-4 was obtained with up to 92% (product-based)
equation (A)
ADH from
Rhodococcus sp.,
NAD+, rt, 24 h
O
OH
phosphate buffer pH 7,
i-PrOH (50% (v/v))
3
(S)-4
>95% conversion
>99% ee
ADH from
equation (B)
O
OH
Lactobacillus kefir ,
NADP+, rt, 24 h
phosphate buffer pH 7,
i-PrOH (50% (v/v))
3
(R)-4
95% conversion
>99% ee
The next step was to combine the two transformations, namely
Suzuki cross-coupling reaction and enzymatic reduction, in a one-
pot process. To this end we studied at first if there is an inhibition
of the enzyme caused by the palladium catalyst or the phosphine
Scheme 3. Enzymatic reduction of ketone 3 using isopropanol for substrate-coupled
cofactor regeneration.

S. Borchert et al. / Journal of Molecular Catalysis B: Enzymatic 84 (2012) 89–93
93
conversion (over 2 steps) and with excellent enantioselectivity of
Acknowledgements
>99% ee (Table 1, entries 3 and 4). In this case, the conversion was
slightly lower than expected from the comparison of the conver-
sions of the isolated steps (which were >95% for the first and 95%
for the second step). After work-up, the purified products (S)-4 and
(R)-4, respectively, were obtained in 72% isolated yield.
We would like to thank Bettina Blumenröder for fruitful discus-
sions.
References
As aqueous phase in the one-pot two-step processes, pure water
gave slightly superior conversions compared to a phosphate buffer
(Table 1, entries 1–4). Thus, buffer components do not appear to
have at least a significant negative impact on the reaction course of
the Suzuki cross-coupling reaction.
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4. Conclusion
In conclusion, we successfully combined a palladium-catalyzed
Suzuki cross-coupling reaction in the presence of a water-soluble
catalyst prepared from PdCl2 and TPPTS with an enantioselec-
tive enzymatic reduction towards a one-pot process. Notably, both
steps run in aqueous medium and proceed at room tempera-
ture. The desired alcohol products of type 4 were obtained with
high conversion of up to >95%, in good isolated yields (72%) and
with excellent enantioselectivities of >99% ee. Since the Suzuki
cross-coupling reaction can be carried out in the same reaction
medium (only differing in terms of pH) and at the same reaction
temperature as the enzymatic reduction with an alcohol dehydro-
genase we consider this methodology to be a promising starting
point for current and future attempts towards a combination of a
palladium-catalyzed Suzuki cross-coupling reaction with an enzy-
matic reduction in a one-pot process running in a tandem mode
(and thus, under the same pH conditions). Addressing this excit-
ing issue, currently based on these results development of a Suzuki
cross-coupling methodology working in weakly basic media and
being compatible with enzymatic reduction is in progress. A further
research issue will also be the expansion of the substrate scope (ide-
ally after realizing the one-pot process running in a tandem mode).
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