Chemoenzymatic Dynamic Kinetic Resolution of Secondary Alcohols Using an Air- and Moisture-Stable Iron Racemization Catalyst

Article abstract of DOI:10.1002/chem.201605754

Herein, we report on a metalloenzymatic dynamic kinetic resolution of sec-alcohols employing an iron-based racemization catalyst together with a lipase. The iron catalyst was evaluated in racemization and then used in dynamic kinetic resolution of a numbe

Full text of DOI:10.1002/chem.201605754

A Journal of
Accepted Article
Title: Chemoenzymatic Dynamic Kinetic Resolution of Secondary
Alcohols using an Air and Moisture stable Iron Racemization
Catalyst
Authors: Jan-E. Bäckvall, Karl.Gustafson@su.se Gustafson, Arnar
Guðmundsson, and Kayla Lewis
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To be cited as: Chem. Eur. J. 10.1002/chem.201605754
Link to VoR: http://dx.doi.org/10.1002/chem.201605754
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COMMUNICATION
Chemoenzymatic Dynamic Kinetic Resolution of Secondary
Alcohols using an Air and Moisture stable Iron Racemization
Catalyst
[a]
[a]
[a]
[a]
Karl P. J. Gustafson, Arnar Guðmundsson, Kayla Lewis and Jan-E. Bäckvall*
Abstract: Herein, we report on a metalloenzymatic dynamic
kinetic resolution of sec-alcohols employing an iron-based
racemization catalyst together with a lipase. The iron catalyst
was evaluated in racemization and then used in dynamic kinetic
resolution of a number of sec-alcohols to give enantiomerically
pure products in good to high yields. The iron catalyst is air and
moisture stable and it is readily accessible.
Asymmetric synthesis is an active and important area of
organic chemistry that has had a profound influence on several
other fields of science.[1] The most common method today for
obtaining enantiomerically pure compounds on an industrial
scale is still the resolution of a racemic mixture.[2] Resolution
protocols suffer from one major intrinsic drawback, which is that
only a maximum yield of 50% of a single desired enantiomer can
be obtained. To enable quantitative yield, the undesired
Figure1. a) Ruthenium transfer hydrogenation catalyst used in chemo-
enantiomer has to be converted into the desired one, which can
be achieved by coupling the resolving protocol to a racemization.
This combination leads to a so-called dynamic kinetic resolution
enzymatic DKR of alcohols. b) iron transfer hydrogenation catalysts.
More recently, research has been focused on the use of
complexes of more non-toxic and inexpensive metals for the
racemization of alcohols. Therefore, work has been dedicated
towards the development of iron-based racemization catalysts.
(
DKR), where an in situ racemization catalyst maintains the
starting material racemic during the resolution.[3]
Racemization of secondary alcohols can be achieved with
different methods. One of the most studied approaches relies on
We have recently reported on
a pincer type iron-based
a reversible transfer hydrogenation strategy, where metals such
as Ru,[4] Ir[5] and Rh,[6,7] and Al [8] have been shown to catalyze
the racemization. Racemization of alcohols can also be
accomplished without metals, for instance with the use of acidic
resins[9] or zeolites[10] in a dehydration/hydration mechanism.
racemization catalyst 1, being the first reported iron-based
racemization catalyst.[16] This catalyst was able to fully racemize
a wide range of benzylic alcohols at 50 oC within 10-15 min.
However, catalyst 1 was not able to racemize aliphatic alcohols
nor was it compatible with the reaction conditions required for a
chemoenzymatic DKR.
These results led us to pursue studies on other iron cata-
lysts.[17] Catalyst 3, first isolated in 1999 by the group of
Knölker,[18] found its first catalytic applications when Casey and
Guan demonstrated its use in hydrogenation of ketones to alco-
hols.[19] More recently, several reports on in-situ generation of A
Several other substrate-specific catalysts have been reported
utilizing a similar concept as the dehydration/hydration pathway
[11]
(
with C-O cleavage), for racemization of allylic alcohols
or
allylic esters.[
12]
The first practical metalloenzymatic DKR of secondary alco-
[
13]
hols was reported in 1997, where the dimeric ruthenium Shvo
[14]
catalyst and Candida antarctica lipase B were employed at 70
o
and 3 by activation of precursor 2, for dehydrogena-
C. Continued efforts by several groups have led to the develop-
tion/hydrogenation reactions have been reported.[20] We
therefore decided to explore the use of 2 as a precursor for the
potential racemization catalytic species A and 3. In these
racemization reactions species A dehydrogenates the alcohol to
a ketone and hydride 3 hydrogentaes the ketone back to alcohol.
We now report on the use of stable catalyst precursor 2 in
combination with a lipase for the chemoenzymatic DKR of
alcohols. During the completion of our work the group of
Rueping reported on the direct use of air- and moisture-sensitive
catalyst 3 in DKR of alcohols.[21] An advantage with our
procedure over the one reported by Rueping, is that stable
complex 2 can be used as pre-catalyst, which avoids handling of
the highly sensitive catalyst 3.
ment of a number of efficient ruthenium-based racemization
catalysts compatible with enzymatic processes (see Figure 1).[15]
Several of these catalysts give a fully racemic alcohol at ambient
conditions with short reaction times (<30 min). Numerous
enzymes have been shown to be compatible with these Ru
racemization catalysts leading to efficient DKR of a large
number of substrates in high yields and excellent enantiomeric
excess.[3f]
[
a]
Department of Organic Chemistry, Arrhenius Laboratory, Stockholm
University, SE-10691 Stockholm, Sweden
Fax: +46-8-154908; Phone: +46-8-674 7178
E-mail: jeb@organ.su.se
We commenced the screening of reaction conditions for
catalyst 2 with attempts to racemize (S)-1-phenylethanol (4a)
Supporting information for this article is given via a link at the end of
the document.
using 10 mol% of activator Me
delight, we observed racemization of the alcohol; however, the
3
NO (TMANO) in toluene. To our
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COMMUNICATION
catalyst was not stable over more than a few minutes. This
phenomenon can most likely be ascribed to that trimethylamine
esters such as vinyl or isopropenyl acetate afforded
acetaldehyde or acetone, respectively, which interferes with the
transfer hydrogenation catalyst.23 The DKR reaction was highly
concentration dependent and using a substrate concentration of
1M in the presence of the enzyme terminated the racemization
catalyst. For a successful result it was necessary to dilute the
deactivates the catalyst, which was previously observed by Funk
and Moyers.[
20c]
In our recently published iron racemization
protocol[16] we observed that etheric solvents increased the
stability of the catalyst. Thus, when subjecting catalyst 2 to the
use in etheric solvents such as anisole or CPME an increased
stability of the catalytic system was observed (Table 1, entry 1),
To increase the efficiency of the system, additives were
screened, and the addition of 50 mol% of ketone increased the
rate of racemization dramatically (entry 2). This effect is in
agreement with the observation by Casey and Guan,[19] that the
slow step in the transfer hydrogenation is the re-addition of
hydrogen to ketone. Inorganic bases also had a profound effect
on the system (entries 3-5), which can be explained by the fact
that the iron hydride becomes more reactive in the presence of
base, leading to a facilitated re-addition.[22] We then continued to
explore the ketone amount (see Supporting Information, Table
S1), where 20 mol% proved to be sufficient to maintain the
catalytic efficiency (entry 6). Lowering the temperature to 60 °C
reduced the rate (entry 7), but the lower temperature would
allow the catalyst to be compatible with a broader set of
enzymes.
2 3
reaction mixture to a substrate concentration of 0.25 M. Na CO
was used as base to facilitate the hydride re-addition as well as
to remove traces of free acid.24 Simple phenyl acetate as
acyldonor worked well together with CalB and iron catalyst 2 in
the DKR of 4a (Figure 2), and 5a could be isolated in 84% yield
with 98% ee. Other substrates also gave an efficient DKR, and
the 4-chloro and 4-bromo substituted benzylic alcohols 4b and
4c afforded enantiomerically pure (>99% ee) 5b and 5c in 87%
and 84% yield, respectively. The electron-rich substrate 4d
afforded 5d in 89% yield (99% ee). Furthermore, the protocol
tolerated the more electron rich 4-methoxy substituted benzylic
alcohol 4e, which furnished acetate 5e in 85% yield and 97% ee.
Surprisingly, for the strongly electron deficient alcohol 4f,
acetate 5f was only isolated in 62% yield and with an ee of 96%.
Naphtyl derivative 4g furnished acetate 5g in 93% yield and
98% ee. The DKR system also tolerated an ethyl group as the
small group of the alcohol (4h, R’ = Et) and acetate 5h was
obtained in 75% yield and 98% ee. For the DKR of aliphatic
alcohol 4i Burkholderia cepacia lipase (PS-C) was used as
enzyme and acetate 5i was isolated in 78% yield and 95% ee.
The latter enzyme has proven to be versatile in the resolution of
numerous sec-alcohols and amines.[
25]
Table 1. Racemization of alcohol (S)-4a using catalyst 2, screening of
[
a][b]
conditions.
Amount
of
ketone
T
°C)
Additive[c]
ee-value of 4[d]
Entry
(
3
0 min
60 min
(mol%)
1
2
3
4
5
6
7
90
90
90
90
90
90
60
-
-
-
76%
43%
52%
50
50
50
50
20
20
22%
4%
Na
2
CO
3
14%
K
2
CO
3
10%
2%
Na
2
SO
4
84%(62%)[d]
26%
74%(32%)[e]
4%
K
K
2
CO
3
2
CO
3
76%
58%
[
a] General reaction conditions: 1.0 mmol of (S)-4a, 0.10 mmol of 2, 0.1 mmol
of Me NO, 0.2 mmol of acetophenone, 1 mL of anisole. [b] for complete
3
optimization see Supporting Information. [c] 1 mmol of additive. [d] ee-values
were determined by chiral GC. [e] 0.8 equiv. of triethylamine was added.
The catalytic activity of catalyst 2 under DKR conditions
using Candida antarctica lipase B (CalB) was next evaluated.
After accessing a set of acyldonors in the DKR it became
evident that aryl acetates worked best since other activated
Figure 2. Substrate scope. General reaction conditions: 1.0 mmol of 4, 0.1
mmol of 2, 0.1 mmol of Me NO, 0.2mmol of the corresponding ketone
3
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(
RCOR’), CalB (12 mg of Novozym 435) and 2.0 mmol of phenyl acetate in 4
Keywords: iron catalysis • enzyme catalysis• racemization •
secondary alcohols • dynamic kinetic resolution
mL of anisole [a] The lipase used was PS-C (25 mg/mmol).
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Figure 3. DKR reaction scaled up to 10 mmol.
Herein, we have reported on a chemoenzymatic dynamic
kinetic resolution of secondary alcohols utilizing an air- and
moisture-stable iron catalyst for the racemization of alcohols.
This procedure allows a set of enantiomerically pure benzylic
and aliphatic acetates to be prepared in good to high yields and
with excellent ee. Two enzymes were shown to be compatible
with the racemization catalyst, CalB and PS-C. It was also
demonstrated that the reaction could be scaled up to 10 mmol
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Standard procedure for the Fe-catalyzed racemization of secondary
alcohols. Into a flame dried Schlenk flask were added Fe catalyst 2
[
[
[
14]
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16]
2 3
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flask was evacuated and refilled with argon repeated times. A stock
solution of alcohol and ketone in anisole was added. The reaction
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[
[
24]
25]
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Text:
K. P. J. Gustafson, Arnar Guðmund-
Iron Thrown: Utilization of
an in-situ activated iron
complex as racemization
catalyst in the chemo-
enzymatic dynamic kinetic
resolution of sec-alcohols is
reported. The iron catalyst was
evaluated in the racemization
and then used in dynamic
kinetic resolution of a number
of sec-alcohols to give
enantiomerically pure pro-
ducts in good to high yields.
sson, Kayla Lewis, J.-E. Bäckvall*
Page No. – Page No.
Chemoenzymatic Dynamic Kinetic
Resolution of Secondary Alcohols
using an Air and Moisture stable Iron
Racemization Catalyst
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