Simultaneous iridium catalysed oxidation and enzymatic reduction employing orthogonal reagents

Article abstract of DOI:10.1039/c0cc02813d

An iridium catalysed oxidation was coupled concurrently to an asymmetric biocatalytic reduction in one-pot; thus it was shown for the first time that iridium- and alcohol dehydrogenase-catalysed redox reactions are compatible. As a model system racemic chlorohydrins were transformed to enantioenriched chlorohydrins via an oxidation-asymmetric reduction sequence.

Full text of DOI:10.1039/c0cc02813d

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Simultaneous iridium catalysed oxidation and enzymatic reduction
employing orthogonal reagentswz
Francesco G. Mutti, Andreas Orthaber, Joerg H. Schrittwieser, Johannes G. de Vries,
a
b
a
c
b
a
Rudolf Pietschnig and Wolfgang Kroutil*
Received 26th July 2010, Accepted 7th September 2010
DOI: 10.1039/c0cc02813d
An iridium catalysed oxidation was coupled concurrently to an
asymmetric biocatalytic reduction in one-pot; thus it was shown
for the first time that iridium- and alcohol dehydrogenase-
catalysed redox reactions are compatible. As a model system
racemic chlorohydrins were transformed to enantioenriched
chlorohydrins via an oxidation–asymmetric reduction sequence.
One-pot multistep cascades allow to decrease the number of
workup and purification steps and are therefore an attractive
1
synthetic concept. The main requirement for a one-pot
multistep cascade is the compatibility of all the reaction steps.
2
Especially simultaneous catalytic tandem reactions are
Scheme 1 Concurrent transition metal- and biocatalysed redox
reactions: a transition metal catalysed oxidation and a biocatalytic
asymmetric reduction run simultaneously.
demanding since multiple catalysts have to operate concurrently
in one pot. Subsequently, the combination of enzymes with
metal-catalysts in one-pot processes remains a challenge in
organic chemistry, due to the almost irreconcilable reac-
tion conditions between the metal- and the biocatalysed steps
15b,c,16
cascades,
deracemisation of halohydrins is very difficult
employing exclusively enzymes, due to limitations in the bio-
1
7
catalytic oxidation of halohydrins. For the oxidation we chose
6
iridacycle 1 as possible catalyst. In previous work it was shown
(
e.g. different solvents, inert atmosphere. . .). The probably
3c,18
that iridacycle 1 can racemise halohydrins very efficiently.
best known exception is the established dynamic kinetic
resolution (DKR), where a lipase performs an enantioselective
acylation of alcohols or amines in the presence of a metal
catalysed racemisation (using catalysts based on Ru, Rh,
Additionally, there were indications that this catalyst is
7
sufficiently stable to work in the presence of aqueous buffer.
Therefore we tested the metal catalyst at first only for the
oxidation of rac-2a via hydrogen transfer (Scheme 2). An
experiment employing 10 equivalents of acetone 3d as hydrogen
acceptor showed that catalyst 1 can indeed work as oxidative
hydrogen transfer catalyst; however the transformation led only
to 11% conversion, which might be due to an unfavourable
3
–5
Al, Pd. . .). Other simultaneous enzyme–metal combinations
6
–10
had less impact.
The few further attempts to combine
metal and enzyme catalysis simultaneously were not successful
and consequently the catalytic steps were run in a stepwise
1
1,12
fashion, but nevertheless performed in one pot.
1
7a
thermodynamic equilibrium (Table 1, entry 1).
We took the challenge to investigate whether a metal
catalysed oxidation can be combined with an enzyme catalysed
reduction, whereby the latter should preferentially be catalysed
by the alcohol dehydrogenase ADH-A from Rhodococcus
Chloroacetone 3e was expected to be a thermodynamically
more suitable hydrogen acceptor for the oxidation of 2a.
Indeed, employing 10 equivalents of the formal oxidant 3e led
to 76% conversion; employing just one equivalent of 3e led to
an equilibrium close to 50% conv. (55% conv. Table 1, entry 4).
Since the metal catalyst should be combined with an ADH
concurrently in one pot, the hydrogen acceptor for the oxida-
tion step must not be reduced by the enzyme (ADH-A),
thus an orthogonal hydrogen acceptor is required, which is
1
3
14
ruber due to its excellent tolerance toward organic solvents.
As a model system we chose the transformation of halohydrins
to perform a stereo-unselective metal catalysed oxidation and
an enantioselective bio-reduction (Scheme 1) as a first step
1
5
towards a deracemisation concept for this class of com-
pound. Although sec-alcohols can be deracemised via enzyme
a
Department of Chemistry, Organic and Bioorganic Chemistry,
University of Graz, Heinrichstrasse 28, A-8010 Graz, Austria.
E-mail: Wolfgang.Kroutil@uni-graz.at; Fax: +43 316 380 9840;
Tel: +43 316 380 5350
Department of Chemistry, Inorganic Chemistry,
University of Graz, Schubertstrasse 1, A-8010 Graz, Austria
DSM Innovative Synthesis BV, A unit of DSM Pharma Chemicals,
b
c
PO Box 18, 6160 MD Geleen, The Netherlands
w This article is part of the ‘Enzymes and Proteins’ web-theme issue for
ChemComm.
z Electronic supplementary information (ESI) available: Experimental
procedures and GC data. See DOI: 10.1039/c0cc02813d
Scheme 2 Oxidation of halohydrin 2a via hydrogen transfer.
This journal is c The Royal Society of Chemistry 2010
8
046 Chem. Commun., 2010, 46, 8046–8048

Table 1 Oxidation of rac-2a via hydrogen transfer employing 1 as
catalyst and ketones 3d–f as hydrogen acceptor
a
b
Hydrogen
acceptor
Conv.
(%)
Entry
Equiv. 3
1
2
3
4
5
6
7
8
9
3d
3e
3e
3e
3f
3f
3f
3f
3f
10
10
3
1
1
2
5
5
10
11
76
72
55
47
54
51
56
67
Scheme 3 Concurrent metal-catalysed oxidation and biocatalytic
c
c
asymmetric reduction.
c
was prolonged to four hours so that the conversion reached
8
2%. This proved that FDH was also active in the presence of
a
t
Reaction conditions: iridacycle 1 (5 mol%, activated by KO Bu)
the metal catalyst under the conditions employed. Further-
more, the ee was again less than 99% (ee 97%), showing
that also the metal catalyst 1 performed racemisation in the
presence of the two other catalyst preparations.
in toluene, under Ar, in dark glass vials. Reaction time: 16 h, 21 1C,
b
000 rpm in an Eppendorf thermomixer in vertical position. Determined
1
by GC-FID. Experiments were performed using 2 mol% of 1.
c
Finally, after all steps of the cascade were tested on their
own and adjusted to be compatible with each other, the
iridacycle catalysed oxidative hydrogen transfer was coupled
in one pot concurrently with the ADH catalysed reduction
employing FDH for NADH recycling (Scheme 3) using rac-2a
(33 mM) as a substrate. GC analysis of the product mixture
showed that the racemate was transformed to enantioenriched
(R)-2a with an ee of 30% (Table 2, entry 1), thus proving that
the metal catalyst as well as the biocatalysts of the catalyst
network were working in concert; additionally, no intermediate
ketone 3a was detectable, indicating that the enzymatic reduc-
tion was very efficient under the reaction conditions employed.
Increasing the amount of hydrogen acceptor 3f to five
equivalents and keeping the same catalyst loading did not lead
to higher ee (Table 2, entry 2). Nevertheless, increasing both
the catalyst concentration (1, 5 mol%) as well as the amount
of hydrogen acceptor 3f (10 eq.) led to an increase of the ee up
to 40% of (R)-1a (entry 3). Substrate rac-2b turned out to be a
poor substrate leading only to a low ee (6%) (entry 4),
probably due to the fact that 2b is thermodynamically very
only accepted by the metal catalyst but not by the enzyme.
Unfortunately 3e was a very good substrate for ADH-A, thus
not suitable for our purpose. Since ADH-A does not trans-
1
3a
form sterically demanding ketones,
a sterically hindered
o-chloro ketone should be suited as a formal oxidant for
the metal catalysed oxidation. Several sterically demanding
a-haloketones were investigated (see ESIz), but only 3f turned
out to be suitable. Using just one equivalent of 3f led to a
conversion close to the expected 50% (47% conv., 2 mol% 1,
entry 5). Unfortunately, reaching significant higher conver-
sions proved to be difficult; even when employing 10 equivalents
3
f and 5 mol% of the catalyst 1, just 67% conversion could be
reached (entry 9).
In a next step it was confirmed that iridacycle 1 oxidised as
efficiently in a two-phase system (toluene/Tris-HCl buffer =
3
: 7, 50 mM, pH 7.5) as exclusively in toluene leading to
comparable results as given in Table 1. Consequently, the
compatibility of ADH-A and 1 (0.5 mol%) was investigated
by performing a reduction of o-chloroacetophenone 3a
1
7b
(66 mmol) in the presence of both catalysts employing 2-propanol
stable and even more difficult to oxidise than 2a.
Indeed,
as hydride donor for the metal- as well as for the enzyme-
catalysed reduction. The highly stereoselective enzymatic
reduction of 3a by ADH-A to optically pure (R)-2a (ee 4 99%)
performing the oxidation of rac-2b with 1 (5 mol%) and
chloroacetone 3e (from 3 eq. to 50 eq.) gave 3b with only up
to 9% conversion at the equilibrium (data not shown).
Nevertheless, racemic 1-chloro-2-octanol 2c was success-
fully transformed to enantioenriched (R)-2c with ee of
29% (entry 6, Table 2). Again, no intermediate ketone 3c
could be detected.
1
9
with 2-propanol has already previously been demonstrated.
The combo-metal-biocatalytic reduction of 3a led to (R)-2a
with 35% conversion and 96% ee within one hour. Since
the reduction employing exclusively ADH-A furnished the
optically pure (R)-enantiomer (ee 4 99%, see ESIz, S6), it was
concluded that ADH-A and 1 can work in concert beside each
other: the enzyme carries out the enantioselective reduction of 3a,
whereas the metal-catalyst performs the non-stereo-selective
reduction of 3a (see ESIz, S2 and S4) and the racemisation of
The model reaction we used here, i.e. the deracemisation
of halohydrins, clearly demonstrates the feasibility of the
combination of an iridium-catalysed oxidation and an enzyme-
catalysed asymmetric reduction. However, the oxidation reac-
tion employed here is limiting in the present deracemisation
since the desired (R)-enantiomer is oxidised unnecessarily and
the employed Ir-catalyst leads to racemisation. For a further
increase of the ee our future studies will focus on the introduction
of a chiral enantioselective Ir-catalyst thereby avoiding
racemisation of the halohydrin as well as the oxidation of
both enantiomers.
(R)-2a, thus leading in combination to an ee less than 99%.
Since 2-propanol as hydrogen donor is not only accepted by
the enzyme but also by the metal catalyst, an orthogonal
reducing agent for the biocatalytic reduction is required.
Employing formate and formate dehydrogenase (FDH)
seemed to be a good option, since in a test experiment the
metal catalyst 1 did not accept formate. To check for compati-
bility of all three catalysts (FDH, ADH, 1 mol% iridacycle 1)
the reduction of 3a was performed in the presence of all
catalysts and formate as reducing agent. The reaction time
Summarizing, the first simultaneous combination of an
iridium-catalysed oxidation and an enzymatic reduction was
demonstrated in one pot. As a metal catalyst iridacycle 1
was employed to perform the oxidation of the halohydrin
This journal is c The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 8046–8048 8047

Table 2 Simultaneous metal catalytic oxidation–enzymatic reduction: combining iridacycle 1 and ADH-A in one pot to work concurrently
(
a
Scheme 3)
Product mixture
b
ee (%)
Entry
Substrate
Ir-cat. 1 (%)
Equiv. 3f
Ketone 3a–c (%)
Alcohol 2 (%)
c
1
2
3
4
5
6
rac-2a
rac-2a
rac-2a
rac-2b
rac-2c
rac-2c
2
2
5
5
2
5
2
5
10
10
2
o0.1
o0.1
o0.1
o0.1
o0.1
o0.1
499.9
499.9
499.9
499.9
499.9
499.9
30
31
40
6
17
29
10
a
Reaction conditions: racemic substrate 2a–c (33 mmol), iridacycle 1 (2 or 5 mol%), ADH-A (500 U) and FDH (30 U), NADH (1 mM), sodium
formate (260 mM) and 3f (2, 5 or 10 eq.) as an oxidant in Tris-HCl buffer (50 mM, pH 7.5)/toluene (1 mL, 7 : 3, v : v), under Ar in dark glass vials,
b
c
1
6 h, 30 1C, 150 rpm on a rotary shaker. Measured by GC on a chiral phase. Performing the same experiment with stirring the two phase system
and taking care to maintain phase separation, lower ee was obtained (20%) and a significant amount of o-chloroacetophenone was formed (26%).
via hydrogen transfer. The obtained a-chloro ketone inter-
mediate was then stereoselectively reduced to the corresponding
optically enriched halohydrin employing the alcohol dehydro-
genase ADH-A from R. ruber (ADH-A). The key for the
success of the combo-oxidation–reduction cascade was to find
orthogonal reaction conditions for the oxidation and the
reduction, thus an orthogonal hydrogen acceptor which is
exclusively accepted by the metal catalyst but not by the
biocatalyst, and an orthogonal reducing system for the bio-
catalytic reduction which does not interfere with the metal
catalyst. The sterical demanding ketone 3f turned out to be a
suitable hydrogen acceptor for the oxidation and formate/
FDH was used for the biocatalytic reduction.
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048 Chem. Commun., 2010, 46, 8046–8048
This journal is c The Royal Society of Chemistry 2010