Merging Iron Catalysis and Biocatalysis—Iron Carbonyl Complexes as Efficient Hydrogen Autotransfer Catalysts in Dynamic Kinetic Resolutions

Article abstract of DOI:10.1002/anie.201606197

A dual catalytic iron/lipase system has been developed and applied in the dynamic kinetic resolution of benzylic and aliphatic secondary alcohols. A detailed study of the Kn?lker-type iron complexes demonstrated the hydrogen autotransfer of alcohols to proceed under mild reaction conditions and allowed the combination with the enzymatic resolution. Different racemic alcohols were efficiently converted to chiral acetates in good yields and with excellent enantioselectivities.

Full text of DOI:10.1002/anie.201606197

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201606197
German Edition: DOI: 10.1002/ange.201606197
Kinetic Resolution
Merging Iron Catalysis and Biocatalysis—Iron Carbonyl Complexes as
Efficient Hydrogen Autotransfer Catalysts in Dynamic Kinetic
Resolutions
Osama El-Sepelgy, Nurtalya Alandini, and Magnus Rueping*
Abstract: A dual catalytic iron/lipase system has been
developed and applied in the dynamic kinetic resolution of
benzylic and aliphatic secondary alcohols. A detailed study of
the Knçlker-type iron complexes demonstrated the hydrogen
autotransfer of alcohols to proceed under mild reaction
conditions and allowed the combination with the enzymatic
resolution. Different racemic alcohols were efficiently con-
verted to chiral acetates in good yields and with excellent
enantioselectivities.
In an early study, Williams et al. reported the combination
of a rhodium(II)acetate dimer and lipase to achieve the DKR
of 1-phenylethanol.[6] The groups of Bꢀckvall, Kim, and Park
have pioneered the concept of ruthenium-catalyzed racemi-
zation of alcohols and amines via either reversible transfer
hydrogenation or b-hydride elimination.[7] The relatively high
cost and limited availability of ruthenium-based racemization
catalysts have triggered the development of more cost-
effective and readily accessible metal catalysts. For instance,
Berkessel et al. have developed an AlMe3/binol/lipase system
to catalyze the DKR of secondary alcohols. In this case the
corresponding enol acetates have to be used as the acylating
agent which may limit the practicality of this protocol.[8]
Furthermore, Akai et al. have reported highly efficient
cooperative vanadium/lipase systems for deracemization.
This method is limited to allylic alcohols. Moreover, hetero-
geneous acids have been reported as racemization catalysts.[9]
The racemization in these protocols proceeds via a dehydra-
tion pathway, which limits the substrate scope to alcohols that
can only form a stable carbocation intermediate.[10] Despite
these advances, DKR protocols with alternative cheap, read-
ily available, and efficient catalysts would be highly desirable.
Inspired by nature and the fact that the Fe-hydrogenases
catalyze the reversible heterolytic cleavage of hydrogen,[11] we
decided to investigate iron-based catalysts. These iron cata-
lysts must fulfil the requirements of both enzyme compati-
bility and redox activity (Scheme 1). Therefore the use of an
efficient and compatible iron dehydrogenation–hydrogena-
tion catalyst would be key to the development of such a dual
enzyme and iron catalysis protocol.
O
ver the last decades, transition metal catalysis has been
further developed and plays a crucial role in the chemical and
pharmaceutical industries. Its significance was recognized by
three Nobel Prizes for the use of noble metal catalysis in
organic synthesis.[1] In recent years, efforts have been made to
develop processes that can be catalyzed by more earth-
abundant metals including iron.[2]
Enantiomerically pure alcohols and amines are among the
important key intermediates used in both academia and
industry.[3] The kinetic resolution via either esterification or
hydrolysis is one of the most important methods used in
industrial production of optically pure alcohols and amines.[4]
Its major drawback is the limited yield which cannot exceed
50%. Metal-catalyzed racemization of the slow-reacting
enantiomer has emerged as a powerful tool to increase the
theoretical yield to 100% via dynamic kinetic resolution
(DKR).[5]
The iron-catalyzed hydrogenation was reported by Casey
and Guan.[12] They employed the bifunctional Knçlker
complex 8 for the reduction of carbonyl compounds.[13]
Beller and co-workers combined the iron-based catalyst 8
=
and Brønsted acids for the enantioselective reduction of C N
bonds whereas, Quintard and Rodriguez realized the combi-
nation of iron complex 1 with iminium catalysis for the
functionalization of primary allylic alcohols.[14] Taking into
account the recent advances in iron cyclopentadienone
catalysis,[15] we here report our preliminary results on the
iron-catalyzed dehydrogenation–hydrogenation of secondary
alcohols and the challenging combination with biocatalysis.
We began our studies with the racemization of (R)-1-
phenylethanol [(R)-10a] and tested several tricarbonyl iron
complexes bearing different innocent ligands 1–6 (Figure 1).
The precatalysts were activated in situ by partial oxidative
decarbonylation with trimethylamine N-oxide (Me3NO).
Unfortunately, they did not show high catalytic activity and
no complete racemization was realized even after prolonged
Scheme 1. Iron-catalyzed racemization of alcohols for dynamic kinetic
resolution with enzymes.
[*] Dr. O. El-Sepelgy, N. Alandini, Prof. Dr. M. Rueping
Institute of Organic Chemistry
RWTH Aachen University
Landoltweg 1, 52074 Aachen (Germany)
Prof. Dr. M. Rueping
King Abdullah University of Science and Technology (KAUST)
KAUST Catalysis Center (KCC)
Thuwal, 23955-6900 (Saudi Arabia)
E-mail: magnus.rueping@Kaust.edu.sa
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201606197.
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Figure 1. Iron complexes used in this study.
Figure 2. Reaction profile of the racemization of (R)-10a (0.2m) using
iron complex 8.
Table 1: Iron-catalyzed racemization of (R)-1-phenylethanol [(R)-10a].[a]
found to be highly dependent on temperature. While only
90 minutes at 908C and 6 hours at 608C suffice for complete
racemization, we observed lower reactivity at 408C
(Figure 2).
Entry
Cat.
(mol%)
Additiev
(mol%)
rac-10a/9a[b]
t
[h]
ee
[%][b]
With the good racemization catalyst 8 in hand we decided
to combine the iron-catalyzed dehydrogenation–hydrogena-
tion sequence with the enzyme-catalyzed resolution via
acetylation using lipase from Candida antarctica. This
enzyme typically shows good thermostability and activity.
After reaction optimization complete conversion of the rac-
10a was obtained when vinyl acetate or isopropenyl acetate
was employed as the acyl donor (Table 2, entries 1 and 2).
1
2
3
4
5
6
7
8
9
10
11[c]
12[d]
13[e]
14[f]
1 (10)
2 (10)
3 (10)
4 (10)
5 (10)
6 (10)
7 (10)
8 (10)
8 (5)
8 (10)
8 (10)
8 (10)
8 (10)
8 (10)
Me3NO (15)
Me3NO (15)
Me3NO (15)
Me3NO (15)
Me3NO (15)
Me3NO (15)
–
–
91:09
84:16
90:10
83:17
83:17
83:17
92:08
77:23
80:20
87:13
78:22
80:20
91:09
80:20
18
18
18
18
18
18
18
6
9
5
8
7
93
86
36
18
13
41
56
0
0
0
0
0
–
9a (20)
–
–
–
–
Table 2: Iron/enzyme-catalyzed DKR of rac-10a.[a]
18
5
26
0
[a] The reactions were performed on a 0.2 mmol scale with the iron
complex and additive in 1 mL of toluene at 608C in a Schlenk tube under
an inert atmosphere. [b] The ratios of products and ee of 10a were
determined by gas chromatography using a b-dex column. [c] Benzene as
the solvent. [d] Cyclohexane as the solvent. [e] tert-Butanol as the solvent.
[f] n-Hexane as the solvent.
Entry
Acetylating reagent
11a/10a/9a[b]
ee[b] [%]
1
2
3
4
5
vinyl acetate
isopropyl acetate
1-ethoxyvinyl acetate
acetyl isopropyl carbonate
p-chlorophenyl acetate (PCPA)
56:0:44
72:0:38
80:20:0
64:26:10
100:0:0
99
99
74
34
99
heating (Table 1, entries 1–6). This is in agreement with Funk
and Moyerꢁs observation of catalyst poisoning during the
oxidation of alcohols.[15d]
[a] The reactions were performed on a 0.2 mmol scale with iron complex
8, 3 mg of Novozyme-435, and 3 equiv of acyl donor in 1 mL of toluene at
608C for 18 h in a Schlenk tube under an inert atmosphere. [b] The ratios
of products and ee of 11a were determined by gas chromatography with
a b-dex column.
We then turned our attention to the use of the mono-
acetonitrile dicarbonyl analogue, which also did not fully
racemize the alcohol (R)-10a even after 18 hours (Table 1,
entry 7). Interestingly when we used the isolated air-sensitive
iron hydride complex 8, complete racemization was realized
after only 6 hours in toluene at 608C (Table 1, entry 8).
Reducing the catalyst loading to 5 mol% increased the
reaction time to 9 hours (Table 1, entry 9), while the presence
of 20 mol% of acetophenone enhanced the reaction rate and
the racemate was formed after only 5 hours (Table 1,
entry 10). The racemization could also be carried out in
several nonpolar solvents but the racemization rate slowed
down in the polar protic tert-butanol and 26% ee was detected
after 18 hours (Table 1, entries 11–14). The racemization was
However, despite high stereocontrol, the formation of
considerable amounts of acetophenone 9a was observed
which can be explained by the generation of the hydrogen
acceptors acetaldehyde or acetone. To overcome this limi-
tation, we used 1-ethoxyvinyl acetate which produces ethyl
acetate as a byproduct. Unfortunately, only 80% yield and
74% ee resulted (Table 2, entry 3). The reaction with acetyl
isopropyl carbonate showed also lower reactivity and selec-
tivity (Table 2, entry 4). Importantly, p-chlorophenyl acetate
(PCPA) was found to be compatible for the iron- and enzyme-
catalyzed DKR of alcohols (Table 2, entry 5).
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Table 3: Reaction scope for the iron/lipase-catalyzed DKR.[a]
Next, we investigated the inversion of configuration by
employing (S)-10a under the optimized reaction conditions.
Pleasingly, the reaction proceeds with complete inversion of
configuration to give the acetate (R)-10a in 88% yield and
99% ee (Scheme 2). Thus the procedure may be a good
alternative to the Mitsunobu reaction which is often applied
for inversion of chiral alcohols in organic synthesis.
Scheme 2. Iron/enzyme-catalyzed inversion of (S)-10a.
In conclusion, we have developed the first example of
a cooperative nonnatural iron/lipase catalytic system.[16] The
key to success is the unprecedented combination and
compatibility of a reactive iron dehydrogenation–hydrogena-
tion catalyst with a lipase. The iron-catalyzed hydrogen
autotransfer proceeds under very mild reaction conditions
which facilitates the combination with the enzyme. Various
racemic alcohols including benzylic, aliphatic, and heteroar-
omatic alcohols have been transformed into the enantioen-
riched acetates without the use of precious metal catalysts and
expensive chiral ligands. Given the compatibility of iron
catalysis with enzymatic resolutions demonstrated here and
the fact that the iron catalyst is inexpensive and readily
available, we believe that these initial promising findings will
open up new avenues for further exploration of this effective
dual catalysis system.
[a] Reaction conditions: 10 (1 mmol), catalyst 8 (10 mol%), Novozyme-
435 (15 mg), and PCPA (3 mmol) in toluene (5 mL) were stirred at 608C
for 24 h in a Schlenk tube under an inert atmosphere; yields after column
chromatography. ee values were determined by GC with b-dex columns.
[b] Toluene (2.5 mL). [c] Novozyme-435 (10 mg).
Keywords: chiral alcohols · hydrogenases · kinetic resolution ·
lipases · reduction
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Received: June 26, 2016
Published online: && &&, &&&&
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Communications
Kinetic Resolution
Iron can: Green and inexpensive iron
carbonyl complexes are efficient catalysts
for the racemization of chiral alcohols by
mimicking the action of Fe-hydrogenases.
The combination of the iron-based cata-
lyst with lipase provides a platform for the
enzymatic resolution of secondary alco-
hols. TMS=trimethylsilyl.
O. El-Sepelgy, N. Alandini,
M. Rueping*
&&&& — &&&&
Merging Iron Catalysis and Biocatalysis—
Iron Carbonyl Complexes as Efficient
Hydrogen Autotransfer Catalysts in
Dynamic Kinetic Resolutions
Angew. Chem. Int. Ed. 2016, 55, 1 – 5
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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