(S)-selective dynamic kinetic resolution of secondary alcohols by the combination of subtilisin and an aminocyclopentadienylruthenium complex as the catalysts

Article abstract of DOI:10.1021/ja036766r

A new procedure for the dynamic kinetic resolution (DKR) of racemic alcohols into single enantiomers is described. This procedure employs surfactant-treated subtilisin as an (S)-selective resolving catalyst and an aminocyclopentadienylruthenium complex as a racemizing catalyst. The DKR is performed best in the presence of an acyl donor such as trifluoroethyl butyrate in THF at room temperature. Eight simple secondary alcohols have been efficiently resolved with high optical purities and good yields. The subtilisin-based DKR is complementary in stereoselectivity to its lipase-based counterpart. For an acyl-carrying alcohol, both subtilisin- and lipase-based DKRs have proceeded equally well to give a pair of enantiomeric products (>99.5% ee each) with opposite optical rotations in high yields (94-95%). Copyright

Full text of DOI:10.1021/ja036766r

Published on Web 08/30/2003
(S)-Selective Dynamic Kinetic Resolution of Secondary Alcohols by the
Combination of Subtilisin and an Aminocyclopentadienylruthenium Complex
as the Catalysts
Mahn-Joo Kim,* Yong Il Chung, Yoon Kyung Choi, Han Ki Lee, Daeho Kim, and Jaiwook Park*
National Research Laboratory of Chirotechnology, Department of Chemistry, DiVision of Molecular and
Life Sciences, Pohang UniVersity of Science and Technology, Pohang, Kyongbuk 790-784, Korea
Received June 19, 2003; E-mail: mjkim@postech.ac.kr
Scheme 1. Dynamic Kinetic Resolution of 1-Phenylethanol
Dynamic kinetic resolution (DKR) provides a useful methodology
for the conversion of racemic substrates to single enantiomeric
products.1 Recently, several groups, including ours, have reported
the use of enzyme-metal combinations as the catalyst system for
DKR.2 In the DKR, a metal complex acts as a racemizing catalyst,
and an enzyme, as a resolving catalyst. As the result, a racemic
mixture transforms to an enantiomerically enriched product. All
the enzyme/metal-catalyzed DKR reactions reported thus far have
employed a lipase as the resolving catalyst.3-5 Accordingly, only
the products of (R)-configuration are available in case simple
secondary alcohols are resolved. We herein wish to report for the
first time a complementary procedure using subtilisin as the
resolving catalyst and an aminocyclopentadienylruthenium complex
as the racemizing catalyst for the products of (S)-configuration.
In general, subtilisin is inferior to lipase in activity, selectivity,
and stability for nonaqueous biocatalysis.6 As a result, the use of
subtilisin in kinetic resolution (KR) has been thus far limited to a
few applications, while lipases have found wide applications. We,
however, thought that the utility of subtilisin would be expanded
by its use in DKR. In principle, DKR provides higher enantiomeric
excess (ee) and better yield than the corresponding KR. In addition,
subtilisin-based DKRs should provide a complementary stereo-
selectivity to the lipase-based DKRs since the stereospecificity of
subtilisin is opposite to that of lipase.6
Table 1. Dynamic Kinetic Resolution of 1-Phenylethanol
entry
solvent
acyl donora
mol%of3
% yieldb
%eec
1
2
3
4
5
6
7
8
2,2,4-trimethylpentane
toluene
t-butyl methyl ether
methylene chloride
1,4-dioxane
tert-butanol
THF
THF
THF
THF
THF
THF
THF
PCPB
PCPB
PCPB
PCPB
PCPB
PCPB
PCPB
IPA
TFEA
TFEB
TFEB
TFEB
TFEB
TFEB
10
10
10
10
10
10
10
10
10
10
8
88
86
93
91
98
94
98
22
60
93
95
93
95
82
80
79
82
87
84
91
89
71
52
88
91
88
92
70
9
10
11
12
13
14
A subtilisin-based DKR of 1-phenylethanol (1a) is described as
the route A in Scheme 1. The (S)-selective acylation of 1a by
subtilisin in the presence of an acyl donor takes place with the
racemization of 1a, catalyzed by the ruthenium complex 3.7 On
the other hand, the (R)-selective acylation can be achieved with
lipase as shown in the route B. We have already reported that 3 is
highly active and the (R)-selective DKR proceeds efficiently at room
temperature.4f
6
4
2
THF
a PCPB, p-chlorophenyl butyrate; IPA, isopropenyl acetate; TFEA,
trifluoroethyl acetate; TFEB, trifluoroethyl butyrate. b Determined by 1H
NMR. c Measured by chiral HPLC (Whelk-O1).
The DKR of 1a with STS was tested on 0.3 mmol scale with
varying the acyl donor, the amount of 3, and the solvent at 25 °C
for 3 days. First, the DKR was examined with STS (60 mg/mmol
substrate) and 10 mol % of 37 in the presence of p-chlorophenyl
butyrate (PCPB) as an acyl donor in seven different solvents (entries
1-7, Table 1). The highest yield (98%) was obtained in THF and
1,4-dioxane (entries 5 and 7), and the highest ee’s (89-91% ee)
were obtained in THF and tert-butyl alcohol (entries 6-7). Both
the yield and the ee were relatively lower in hydrophobic solvents
such as 2,2,4-trimethylpentane and toluene. Second, the DKR was
carried out with varying the acyl donor in THF (entries 7-10).
The DKR with PCPB showed the best efficiency, in which the yield
and the ee reached 98% and 89%, respectively (entry 7). A similar
result (93% yield and 88% ee) was obtained with a less reactive
acyl donor, TFEB (entry 10). Trifluoroethyl acetate (TFEA) was
not satisfactory, and isopropenyl acetate (IPA) was practically
inefficient. Finally, the DKR was performed with varying the
amount of 3 in the presence of TFEB in THF (entries 10-14). No
Subtilisins usually exhibit low activity in organic solvents. For
instance, a commercial subtilisin (Bacillus licheniformis, lyophilized
powder)8 showed 0.8 nmol/mg of enzyme/h of activity in the
acylation of 1a with vinyl butyrate in THF. In preliminary studies,
the DKR of 1a with the commercial subtilisin was unsuccessful
due to the low activity. To enhance the activity, the enzyme was
treated with a nonionic surfactant, polyoxyethylene(10) cetyl ether8
(C16H33(OCH2CH2)nOH, n ) ∼10; trade name, Brij 56) before use.9
The surfactant treatment resulted in a dramatic increase of the
enzyme activity in THF. It was observed that the surfactant-treated
subtilisin (STS) was 3 orders of magnitude (about 4000 times) more
active than its untreated counterpart. Furthermore, the surfactant
treatment significantly enhanced the enzyme stability. STS displayed
70% of its original activity after 5 days incubation in THF at 25
°C, while the commercial enzymes in lyophilized powder or cross-
linked crystals (CLEC) almost completely lost the activities after
5 days.10
9
11494
J. AM. CHEM. SOC. 2003, 125, 11494-11495
10.1021/ja036766r CCC: $25.00 © 2003 American Chemical Society

C O MMU N I C A T I O N S
Scheme 2. Dynamic Kinetic Resolution of
Table 2. Dynamic Kinetic Resolution of Secondary Alcohols by
Subtilisin-Ruthenium Combination
m-Butanoyloxyphenyl-1-ethanol
We have demonstrated that the (S)-selective DKR of alcohols
has been successfully achieved by the combination of subtilisin
and an aminocyclopentadienylruthenium complex. The success of
the DKR is based on three important factors: the enhanced activity
and stability of surfactant-treated subtilisin, the high activity of the
ruthenium complex at room temperature,4f and the good compat-
ibility between these two catalysts. It is now possible to transform
a wide range of racemic alcohols into their acyl derivatives
enantioselectively through a pair of complementary DKRs. The
methodology should find use, particularly in the synthesis of chiral
drugs and their building blocks.
entry
substrate
product
mol % of 3
% yielda
%ee
1
2
3
4
5
6
7
8
1b
1c
1d
1d
1e
1e
1f
2b
2c
2d
2d
2e
2e
2f
4
4
92(90)
93(91)
77(76)
89
80(78)
95
80(74)
92
77(67)
89
99b
94b
97b
92b
98b
98b
98c
98c
98c
98c
95b
4d
10e
4d
10e
4d
1f
2f
10e
4d
9
10
11
1g
1g
1h
2g
2g
2h
10e
4
90(90)
Acknowledgment. This work was supported by the Korean
Ministry of Science and Technology (NRL program) and POSCO.
We thank the Korean Ministry of Education (BK21 program) for
its support to our graduate program.
a By 1H NMR. The isolated yield is given in parentheses. b By HPLC
(Whelk-O1). c By GC (Chiraldex B-PH). d Condition A: 25 mg of STS/
mmol substrate and 4 mol % of 3. e Condition B: 60 mg of STS/mmol
substrate and 10 mol % of 3.
Supporting Information Available: General DKR procedure and
analytical data (PDF). This material is available free of charge via the
Internet at http://pubs.ac.org.
significant difference in yield and ee was observed in the range of
10 to 4 mol % (entries 10-13), but 2 mol % was too small to give
an acceptable result (entry 14).
References
On the basis of the above results, THF was chosen as the solvent
for the DKR of other substrates 1b-h to see the scope of our
catalyst system (Table 2). TFEB was chosen as the acyl donor,
because the isolation of products was easier with TFEB than with
PCPB. The DKRs (0.3 mmol scale) of 1b, 1c, and 1h were carried
out with STS (25 mg/mmol substrate) and 4 mol % of 3 in THF at
25 °C for 3-4 days. For unactivated alcohols 1d-g, their DKRs
were examined under two different conditions (condition A, 25 mg
of STS/mmol substrate and 4 mol % of 3; condition B, 60 mg of
STS/mmol substrate and 10 mol % of 3).
p-Chlorophenyl methyl carbinol (1b) was transformed in a high
yield (92%) with a high ee value (99%). The excellent ee reflects
the high enantioselectivity (E g 400) of STS toward 1b, which
was confirmed by a separate kinetic resolution experiment. p-
Methoxyphenyl methyl carbinol (1c) and phenylalkynyl methyl
carbinol (1h) were transformed also in high yields (90-93%), but
with slightly lower ee values (94-95%). The condition A for the
DKR of 1d-g gave high ee values (97-98%) but low yields
(77-80%). The yields, however, increased up to 95% under the
condition B, employing 2.5-fold more enzyme and metal catalyst.
These results reflect the relatively slow racemization of the aliphatic
substrates.11
(1) (a) Ward, R. S. Tetrahedron: Asymmetry 1995, 6, 1475.
(2) Reviews: (a) Huerta, F. F.; Minidis, A. B. E.; Ba¨ckvall, J.-E. Chem. Soc.
ReV. 2001, 30, 321. (b) Kim, M.-J.; Ahn, Y.; Park, J. Curr. Opin.
Biotechnol. 2002, 13, 578. (c) Pa`mies, O.; Ba¨ckvall, J.-E. Chem. ReV.
2003, 103, 3247.
(3) (a) Persson, B. A.; Larsson, A. L. E.; Ray, M. L.; Ba¨ckvall, J.-E. J. Am.
Chem. Soc. 1999, 121, 1645. (b) Huerta, F. F.; Laxmi, Y. R. S.; Ba¨ckvall,
J.-E. Org. Lett. 2000, 2, 1037. (c) Pamies, O.; Ba¨ckvall, J.-E. J. Org.
Chem. 2001, 66, 4022. (d) Pamies, O.; Ba¨ckvall, J.-E. J. Org. Chem. 2002,
67, 1261. (e) Dijksman, A.; Elzinga, J. M.; Li, Y.-X.; Arends, I. W. C.
E.; Sheldon, R. A. Tetrahedron: Asymmetry 2002, 13, 879.
(4) (a) Koh, J. H.; Jung, H. M.; Kim, M.-J.; Park, J. Tetrahedron Lett. 1999,
40, 6281. (b) Jung, H. M.; Koh, J. H.; Kim, M.-J.; Park, J. Org. Lett.
2000, 2, 409. (c) Lee, D.; Huh, E. A.; Kim, M.-J.; Jung, H. M.; Koh, J.
H.; Park, J. W. Org. Lett. 2000, 2, 2377. (d) Jung, H. M.; Koh, J. H.;
Kim, M.-J.; Park, J. Org. Lett. 2000, 2, 2487. (e) Kim, M.-J.; Choi, Y.
K.; Choi, M. Y.; Kim, M. J.; Park, J. J. Org. Chem. 2001, 66, 4736. (f)
Choi, J. H.; Kim, Y. H.; Nam, S. H.; Shin, S. T.; Kim, M.-J.; Park, J.
Angew. Chem., Int. Ed. 2002, 41, 1037.
(5) (a) Reetz, M. T.; Schimossek, K. Chimia 1996, 50, 668. (b) Choi, Y. K.;
Kim, M. J.; Ahn, Y.; Kim, M.-J. Org. Lett. 2001, 3, 4099.
(6) Kazlauskas, R. J.; Weissfloch, A. N. E. J. Mol. Catal. B: Enzym. 1997,
3, 65.
(7) Ruthenium complex 3 is activated by the treatment with potassium tert-
butoxide just before use (see ref 4f). It is now available from Strem.
(8) It was purchased from Sigma.
(9) The procedure for the preparation of STS: Brij 56 (1.17 g, 1.72 mmol)
in aqueous pyridine (2.3% H2O, 4.4 mL) was sonicated for 5 min, followed
by the addition of subtilisin (40 mg). The resulting mixture was stirred
for 12 h at 35 °C and then centrifuged to isolate undissolved solid. The
solid was dried in vacuo and stored at 4 °C.
To show that the subtilisin-catalyzed DKR is complementary to
its lipase-catalyzed counterpart, both DKRs were performed with
1i at 25 °C (Scheme 2). Here, no acyl donor was added because
the substrate itself carries an acyl group. Both DKRs afforded high
ee values and excellent yields. As expected, the product from the
subtilisin-catalyzed DKR showed an optical rotation opposite that
from the lipase-catalyzed DKR.
(10) For the previous reports on the low stability of commercial subtilisins in
THF, see: (a) Fernandes, J. F. A.; Halling, P. J. Biotechnol. Prog. 2002,
18, 1455. (b) Martinez, S. G.; Alvira, E.; Cordero, L. V.; Ferrer, A.;
Montanes-Clementet, I.; Barletta, G. Biotechnol. Prog. 2002, 18, 1462.
(11) It was observed in our previous studies that the racemizations of aliphatic
secondary alcohols were about two times slower than those of benzylic
alcohols. See ref 4f.
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