Electrochemical enzymatic deoxygenation of chiral sulfoxides utilizing DMSO reductase

Article abstract of DOI:10.1016/S0957-4166(00)00004-5

Preparation of enantiomerically enriched sulfoxides by an electrochemical enzymatic system utilizing DMSO reductase was studied. This system consists of a glassy carbon electrode as the working electrode, methyl viologen as the mediator and DMSO reductase

Full text of DOI:10.1016/S0957-4166(00)00004-5

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 11 (2000) 823–828
Electrochemical enzymatic deoxygenation of chiral sulfoxides
utilizing DMSO reductase
Mitsuru Abo,∗ Makiko Dejima, Fumioki Asano, Akira Okubo and Sunao Yamazaki
Department of Applied Biological Chemistry, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan
Received 22 November 1999; accepted 17 December 1999
Abstract
Preparation of enantiomerically enriched sulfoxides by an electrochemical enzymatic system utilizing DMSO
reductase was studied. This system consists of a glassy carbon electrode as the working electrode, methyl viologen
as the mediator and DMSO reductase from Rhodobacter sphaeroides f. sp. denitrificans as the catalyst. The (R)-
enantiomers of chiral sulfoxides in the presence of a variety of functional groups were obtained with high e.e.
(>97%) by this system. © 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction
Enantiomerically enriched sulfoxides have been widely used in asymmetric syntheses of natural
products and their applications have been reviewed.1,2 Although various methods for the preparation
of chiral sulfoxides have been reported,3,4 there have been few which allow various chiral sulfoxides
to be prepared with high chemical yield and high enantiomeric excess. Actually, chemical preparations
such as the Andersen method,5 the modified-Sharpless method6,7 and biological preparations such as
asymmetric oxidation4,8–10 of prochiral sulfides have been used depending on the application.11–13
Previously, we have found that DMSO reductase (DMSO-R) from Rhodobacter sphaeroides f. sp.
denitrificans has broad substrate specificity and high enantioselectivity and developed an alternative
preparative method for alkyl aryl sulfoxides with high e.e. by means of DMSO-R.14 In addition, the
gram-scale preparation of methyl p-tolyl sulfoxide (MTSO) by intact cells of R. sphaeroides was also
described.15 However, the microbial preparation of chiral sulfoxides in the presence of a variety of
functional groups, which are useful for C–C chain elongation and synthetic modification, resulted in
low yield because of the undesirable side-reactions.
On the other hand, studies on electrochemical communication between an electrode and a redox
enzyme have attracted much attention and several bio-electrochemical systems utilizing redox enzymes
∗
Corresponding author. E-mail: abo@mail.ecc.u-tokyo.ac.jp
0957-4166/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(00)00004-5
tetasy 3227

824
M. Abo et al. / Tetrahedron: Asymmetry 11 (2000) 823–828
have been reported.16–19 The electrochemical enzymatic systems could be one of the best preparative
methods because the reaction is substrate-specific and it is not necessary to add the chemical reductant. In
this respect, we constructed an electrochemical enzymatic system for the preparation of enantiomerically
enriched sulfoxides using DMSO-R.
2. Results and discussion
DMSO-R is the terminal electron acceptor in DMSO respiration20 and catalyzes the deoxygenation
of DMSO to dimethyl sulfide. In vitro, DMSO-R can accept electrons directly from artificial electron
donors such as viologen dyes in the presence of a chemical reductant (sodium dithionite).21 In this
study, the system involving a glassy carbon (GC) electrode as an electron donor instead of the chemical
reductant was constructed as shown in Scheme 1. The radical cation of methyl viologen [MV(red)] carries
electron to oxidized DMSO-R and then reduced DMSO-R catalyzes the deoxygenation of sulfoxides to
the corresponding sulfides. Because of its fast reaction rate, MV was selected as an electron carrier in
this experiment.
Scheme 1.
Methyl viologen, DMSO-R and organic sulfoxides were dissolved in a phosphate buffer. This electro-
chemical cell consists of a GC electrode as the working electrode, an Ag/AgCl electrode as the reference
electrode and a Pt electrode as the counter electrode.
To confirm the electron flow from the GC electrode to the enzyme through the mediator, cyclic
voltammograms were recorded. At the applied scan rate of 5 mV/s, these voltammograms showed that
the enzyme was able to react with the radical cation MV(red) and catalyze the deoxygenation reaction
(Fig. 1A). When the substrate (MTSO) was absent, the catalytic current was not observed (Fig. 1B).
In addition, the cyclic voltammogram in the absence of DMSO-R showed that MV(red) did not react
directly with the substrate (Fig. 1C).
Utilizing the high enantioselectivity of DMSO-R, enantiomerically enriched sulfoxides were prepared
by the kinetic resolution of racemic sulfoxides as shown in Scheme 2. The amount of the substrate
remaining in the solution was measured at intervals by means of capillary electrophoresis22 and the
reaction was stopped when more than half of the substrate was deoxygenated. Sulfoxides were extracted
with chloroform from the reaction mixture and purified by silica gel chromatography. The e.e.s of the
recovered sulfoxides were determined by chiral HPLC. The reaction times, e.e.s and recovery yields of
the sulfoxides are summarized in Table 1.
As shown in Table 1, sulfoxides with a variety of functional groups were recovered with high e.e.s
by this electrochemical enzymatic reaction system. These results show that this system reflects the
high enantio- and chemoselectivity of DMSO-R. In the case of 2-phenylsulfinylethanol 6, which is a
hydrophilic compound due to the hydroxyl and sulfoxide moieties, the recovery yield was improved by
MOM (methoxymethyl ether) protection of the hydroxyl group 4. Sulfinyl esters (3 and 5) were recovered
with 99% e.e. in moderate yield; however, the recovery was lower in order to obtain sulfoxides with e.e.s

M. Abo et al. / Tetrahedron: Asymmetry 11 (2000) 823–828
825
Fig. 1. Cyclic voltammograms of the electrochemical enzymatic system. Cyclic voltammograms were recorded with a GC
electrode in 100 mM phosphate buffer (pH 7.4) at the scan rate of 5 mV/s. (A) 10 mM MV, 3.3 mM MTSO as substrate and 3
µM DMSO-R; (B) under the condition (A) without substrate; (C) under the condition (A) without DMSO-R
Scheme 2.
Table 1
Enantiomeric excesses of recovered sulfoxides by the electrochemical enzymatic reaction system
>99%. The concentrations of DMSO-R and the substrate in the reaction mixture were varied depending
on the reaction rate of the substrate.
One of the characteristics of this method is that the reaction proceeds under mild conditions. If the
compounds 3, 5 and 7 are prepared by means of the corresponding carbanions or sulfinylcarbanions,
low temperature and anhydrous conditions are required for the reaction. The mild process of the
present electrochemical enzymatic reaction could provide an advantage for the large-scale preparation
of enantiomerically pure sulfoxides.

826
M. Abo et al. / Tetrahedron: Asymmetry 11 (2000) 823–828
3. Conclusion
We have constructed the electrochemical enzymatic system utilizing DMSO-R and to the best of our
knowledge this is the first electrochemical system affording sulfoxides with high e.e.s. In asymmetric
synthesis this method could, therefore, be useful for the preparation of (R)-sulfoxides.
4. Experimental
4.1. Chemicals
Methyl viologen, phenyl vinyl sulfoxide 1, methyl p-tolyl sulfide and thiophenol were commer-
cial samples. Racemic sulfoxides were prepared by the NaIO4 oxidation23 of the corresponding sul-
fides. Methyl 2-phenylsulfenylacetate,24 t-butyl 2-phenylsulfenylacetate,25 2-phenylsulfenylethanol26
and phenylsulfenyl-2-propanone25 were synthesized from thiophenol.
4.2. Preparation of DMSO reductase
DMSO reductase was purified according to the method reported previously.21
4.3. Cyclic voltammograms
The cyclic voltammetric measurement was performed using a Model CS-1090 potentiostat (Cypress
System Inc.). The current was measured at potentials between −200 and −1100 mV, and the scan rate
was 5 mV/s. In the measurement of CVs, a small GC electrode (surface area: ca. 1 mm2) was used. The
reaction cell was 1.5 ml (internal volume) and contained a GC electrode, an Ag/AgCl electrode and a Pt
counter electrode. Before the measurement, the cell was capped with a rubber septum and the reaction
solution was de-aerated by gentle bubbling with Ar. Then the measurement was started under an Ar
atmosphere.
4.4. Capillary electrophoresis (CE) assay
CE was performed on a Waters Quanta-4000E system or Jasco CE-800 system at 25°C. The MEKC
method was used for the quantification of sulfoxides.22
4.5. Electroenzymatic reaction: general procedure
The GC rod electrode (5 mm φ) was used as the working electrode and its surface area in the solution
was ca. 250 mm2. The counter Pt electrode was separated by salt bridge to avoid undesirable side-
reactions. The applied voltage was −750 mV versus the Ag/AgCl electrode. The reaction mixture was
made up of 10 mM of MV, 0.38 µM of DMSO-R and 3.3 mM of organic sulfoxides dissolved in 18
(or 50) ml of 100 mM phosphate buffer (pH 6.8), containing methanol as co-solvent (10% v/v). Before
the reaction was started, the solution was de-aerated by gentle bubbling with Ar and then the cell was
sealed by a rubber septum. The reaction was assayed by sampling an aliquot from the reaction mixture
and measuring the amount of sulfoxide by CE. When more than half of the substrate was deoxygenated,
the reaction was stopped and the mixture was extracted with chloroform. The extract was dried over

M. Abo et al. / Tetrahedron: Asymmetry 11 (2000) 823–828
827
anhydrous magnesium sulfate and concentrated in vacuo. The recovered sulfoxide was purified over a
silica gel column (hexane:ethyl acetate=5:1–1:1 or chloroform:methanol=9:1) and its e.e. was measured
by chiral HPLC.
4.6. HPLC analysis
HPLC analysis was performed by a Jasco-900 system at 37°C, using a Daicel Chiralcel OB-H column
(0.46×25 cm) and UV detector (254 nm). The mobile phase was hexane:2-propanol=1:1 and the retention
times are listed in Table 2. Except for t-butyl 2-phenylsulfinylacetate, the absolute configuration of the
recovered substrate was determined by comparison of the values of specific rotation (data not shown)
with those reported.
Table 2
Retention time of both enantiomers of substrates
Acknowledgements
This study was supported in part by a Grant-in-Aid for Scientific Research (No. 11760087) from the
Ministry of Education, Science, Sports and Culture of Japan.
References
1. Carreno, M. C. Chem. Rev. 1995, 95, 1717–1760.
2. Walker, A. J. Tetrahedron: Asymmetry 1992, 3, 961–988.
3. Madsclaire, M. Tetrahedron 1986, 42, 5459–5495.
4. Holland, H. L. Chem. Rev. 1988, 88, 473–485.
5. Andersen, K. K. Tetrahedron Lett. 1962, 93–95.
6. Pitchen, P.; Dunach, E.; Deshmukh, M. N.; Kagan, H. B. J. Am. Chem. Soc. 1984, 106, 8188–8193.
7. Zhao, S. H.; Samuel, O.; Kagan, H. B. Org. Synth. 1989, 68, 49–55.
8. Ohta, H. Rev. Heteroatom Chem. 1991, 5, 113–135.
9. Allen, C. C. R.; Boyd, D. R.; Dalton, H.; Sharma, N. D.; Haughhey, S. A.; McMordie, R. A. S.; McMurray, B. T.; Sheldrake,
G. N.; Sproule, K. J. Chem. Soc., Chem. Commun. 1995, 119–120.
10. Colonna, S.; Gaggero, N.; Pasta, P.; Ottolina, G. Chem. Commun. 1996, 2303–2307.

828
M. Abo et al. / Tetrahedron: Asymmetry 11 (2000) 823–828
11. Holland, H. L.; Andreana, P. R.; Brown, F. M. Tetrahedron: Asymmetry 1999, 10, 2833–2843.
12. Imboden, C.; Renaud, P. Tetrahedron: Asymmetry 1999, 10, 1051–1060.
13. Rossi, C.; Fauve, A.; Madesclaire, M.; Roche, D.; Davis, F. A.; Reddy, R. T. Tetrahedron: Asymmetry 1992, 3, 629–636.
14. Abo, M.; Tachibana, M.; Okubo, A.; Yamazaki, S. Bioorg. Med. Chem. 1995, 3, 109–112.
15. Abo, M.; Okubo, A.; Yamazaki, S. Tetrahedron: Asymmetry 1997, 8, 345–348.
16. Kattz, E.; Shabtai, V. H.; Bardea, A.; Willner, I.; Rau, H. K.; Haehnel, W. Biosens. Bioelectron. 1999, 13, 741–756.
17. Blonder, R.; Katz, E.; Willner, I.; Wray, V.; Buckmann, A. F. J. Am. Chem. Soc. 1997, 119, 11747–11757.
18. Schmedding, D. J. M.; Somers, W.; Dijk, C.; Gunther, H.; Lugt, J. P.; Simon, H. Biotechol. Techniques 1992, 6, 439–444.
19. Tatsumi, H.; Takagi, K.; Fujita, M.; Kano, K.; Ikeda, T. Anal. Chem. 1999, 71, 1753–1759.
20. Satoh, T.; Hoshino Y.; Kitamura, H. Arch. Microbiol. 1976, 108, 265–269.
21. Satoh, T.; Kurihara, F. N. J. Biochem. 1987, 102, 191–197.
22. Abo, M.; Okubo, A.; Yamazaki, S. Bunseki Kagaku 1995, 44, 835–838 (124: 86489e).
23. Johnson, C. R.; Keiser, J. E. Org. Synth. 1966, 46, 78–80.
24. Tanikaga, R.; Nozaki, Y.; Tamura, T.; Kaji, A. Synthesis 1983, 134–135.
25. Colonna, S.; Banfi, S.; Fontana, F.; Sommaruga, M. J. Org. Chem. 1985, 50, 769–771.
26. Kirner, W. R.; Richter, G. H. J. Am. Chem. Soc. 1929, 51, 3409–3418.