Flavin-catalyzed aerobic oxidation of sulfides in aqueous media

Article abstract of DOI:10.1016/j.tetlet.2012.11.133

An aerobic, organocatalytic, and aqueous method for the oxidation of sulfides is described. Synthetic flavin, 5-ethyl-3,7,8,10- tetramethylisoalloxazinium perchlorate, acts as an efficient catalyst for the oxidation of sulfides in water under an oxygen atmosphere (1 atm) with the assistance of ascorbic acid as a reductant. This is an inexpensive, convenient, and environmentally benign method for the selective oxidative transformation of sulfides into sulfoxides.

Full text of DOI:10.1016/j.tetlet.2012.11.133

Tetrahedron Letters 54 (2013) 621–624
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Flavin-catalyzed aerobic oxidation of sulfides in aqueous media
a,b,
⇑
a
a
a
a,
⇑
Yasushi Imada
, Takahiro Kitagawa , Han-Kun Wang , Naruyoshi Komiya , Takeshi Naota
a
Department of Chemistry, Graduate School of Engineering Science, Osaka University, Machikaneyama 1-3, Toyonaka, Osaka 560-8531, Japan
Department of Chemical Science and Technology, Institute of Technology and Science, The University of Tokushima, Minamijosanjima 2-1, Tokushima 770-8506, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
An aerobic, organocatalytic, and aqueous method for the oxidation of sulfides is described. Synthetic fla-
vin, 5-ethyl-3,7,8,10-tetramethylisoalloxazinium perchlorate, acts as an efficient catalyst for the oxida-
tion of sulfides in water under an oxygen atmosphere (1 atm) with the assistance of ascorbic acid as a
reductant. This is an inexpensive, convenient, and environmentally benign method for the selective oxi-
dative transformation of sulfides into sulfoxides.
Received 20 October 2012
Revised 21 November 2012
Accepted 28 November 2012
Available online 5 December 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Aerobic oxidation
Sulfides
Organocatalyst
Water
Flavin catalyst
Sulfoxides
The development of environmentally benign processes is an
important subject with significant urgency in view of the move to-
ward green and sustainable chemistry. Two major strategies have
via anaerobic dehydrogenation of the substrate (AH
ration to the reduced flavin (Flred), and the elimination of H
(path a). These processes provide various dehydrogenative trans-
2 2
), O incorpo-
2 2
O
1
8
,13
14
15
been adopted to bestow sustainability on existing molecular trans-
formations; the substitution of transition metal catalysts with
organocatalysts, and of organic solvents with water.3 In the field
of oxidation chemistry, aerobic method, that is, the substitution
of stoichiometric metal oxides or organic peroxides with molecular
oxygen, has been recognized as an additional important strategy
for enhanced sustainability.4 Organophoto5 and organoradical
catalysts have been employed for the aerobic oxidation of hydro-
formations of alcohols,
amines, and thiols. The organocata-
lytic aerobic oxidation of alcohols in water has been achieved via
this dehydrogenative process with a neutral flavin catalyst.8
The simulation of oxygenases is examined as an alternative
strategy for organocatalytic oxidative transformation. The reaction
proceeds via a similar redox process that includes the reduction of
2
6
2 2
Flox with AH as a reductant, the incorporation of O to afford flavin
1
6
hydroperoxide (FlEtOOH), oxygen atom transfer to the substrate
(S) to form SO (path b), and dehydration to complete the catalytic
cycle. Various heteroatom compounds, including amines,1
5
a,b,6a,d,e,g
5d,6b
5c,6c,f,h
carbons,
sulfides,
and alcohols
under mild con-
7a,c
ditions and proceeding exclusively in organic media. Aerobic
7
a,b
oxidation in water can be performed with water-soluble Cu,
7
c
7d
7e,f
Ru, Rh, and Pd metal catalysts, although organocatalytic aer-
obic oxidation that has been reported to proceed in water is limited
to flavin-catalyzed reactions.8
SO, H O
2
AH2
Flox
9
10
Flavin-containing oxidases and oxygenases catalyze a variety
of oxidative transformations in biological processes.11 Simulations
(b)
2 2
H O
(a)
of these processes have been studied extensively using various
synthetic flavin organocatalysts.12 Scheme 1 shows a typical cata-
S
A
lytic cycle for flavin-catalyzed oxidative transformation. Model
reactions of flavin-containing oxidases have been extensively
studied with synthetic flavin catalysts (Flox). The reaction proceeds
FlOOH
Flred
⇑
Corresponding authors. Tel.: +81 88 656 7407 (Y.I.); tel.: +81 6 6850 6220; fax:
81 6 6850 6222 (T.N.).
O2
+
E-mail addresses: imada@chem.tokushima-u.ac.jp (Y. Imada), naota@chem.es.
Scheme 1. Redox process for oxidative transformation with flavoenzymes and
osaka-u.ac.jp (T. Naota).
synthetic flavins.
0
040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.11.133

6
22
Y. Imada et al. / Tetrahedron Letters 54 (2013) 621–624
Me
N
Table 2
Various reaction parameters for aerobic oxidation of 2 with ascorbic acida
Me
Me
N
O
O
·
–
ClO
4
Yield of 3b (%)
N
Entry
Concentration (M)
Atmosphere
Time (h)
N
Et
Me
1
2
AA
1
(cat)
O
S
R1
R2
1
2
3
4
5
0.0025
0
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.2
0.2
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
O2
O2
Ar
Air
Air
6
6
6
6
24
6
6
6
12
6
12
99
0
0
S
ascorbic acid
(1 atm)
phthalate buffer / H O
R1 R2
O
2
0.0025
0.0025
0.0025
0.0025
0.00125
0.0025
0.0025
0.0025
0.0025
41
98
98
99
80
81
41
47
2
c
6
O
2
O2
Scheme 2. Flavin-catalyzed aerobic oxidation of sulfides.
7
8
9
10
O
O
O
O
2
2
2
2
sulfides,17a,c and ketones17b undergo oxygenation reactions by this
catalytic process under atmospheric pressure O
During the course of our systematic studies on the simulation of
enzymatic functions with synthetic flavin catalysts,
have achieved the highly efficient aerobic oxygenation of sul-
11
2
.
a
A mixture of sulfide 2, flavin catalyst 1, and ascorbic acid (AA) in 0.05 M
1
2a,17–19a,e,h
we
phthalate buffer aqueous solution with EtOH (4:1 v/v) was stirred at 30 °C for 6 h.
b
Determined by GLC analysis based on 2.
The reaction was conducted under light-shielding conditions.
c
1
7a,c
fides
acid is employed as a reductant (AH
as depicted in Scheme 2. Metallo-21 and organocatalytic oxida-
tion has been performed using H as an oxidant for the environ-
under mild conditions in aqueous media, when ascorbic
2
0
2
)
for the catalytic system,
Table 3
22
a
Flavin-catalyzed aerobic oxidation of sulfides with ascorbic acid
2 2
O
Entry
Sulfide
Sulfoxide
Yieldb (%)
mentally benign oxidation of sulfides. This is a very green method
for the oxidation of sulfides that does not require special oxidants,
organic solvents, and transition metal catalysts.
The effect of the acidity and basicity of the aqueous media was
examined for the aerobic oxidation of 4-(methylthio)toluene (2)
with 5-ethyl-3,7,8,10-tetramethylisoalloxazinium perchlorate (1,
.5 mol %) and ascorbic acid (3 equiv) in various aqueous buffer
solutions. Yields of methyl 4-methylphenyl sulfoxide (3) were
evaluated by gas–liquid chromatography (GLC) analysis after stir-
1
2
(n-C
(n-C
4
H
H
9
)
2
S
(n-C
(n-C
4
H
H
9
)
2
S?O
) S?O
90 (80)c
96
8
17
2
) S
8
17 2
O
3
n-C
8
H
17SMe
87
8
n-C H17SMe
S
O
Me
2
3
2
S
4
Me
Me
89 (86)c
2
Me
3
ring for 6 h at 30 °C in an O
are given in Table 1. The 0.05 M phthalate buffer solution (pH
.0) gave the best result among the buffer solutions examined (en-
try 3). The reaction was much more accelerated when a small
amount of EtOH was used as a co-solvent (entry 3, in parentheses).
More basic solutions of acetate (pH 5.0) and phosphate buffers (pH
2
atmosphere (1 atm), and the results
S
O
Me
S
5
Me
88
MeO
HO
4
MeO
HO
S
O
S
Me
6
7
Me
96
93
7
.4) afforded moderate reactivities, while no reaction occurred in
the acidic solutions with citrate buffer (pH 3.3, entry 2) and with-
out a buffering agent (pH 2.0).
Other reaction parameters were examined for the oxidation of 2
with ascorbic acid at 30 °C in an aqueous solution of phthalate buf-
fer with EtOH (4:1 v/v). The results are listed in Table 2. The reac-
tion does not proceed without flavin catalyst (entry 2) or without
S
O
S
a
A mixture of sulfide (0.5 mmol), flavin catalyst 1 (0.0125 mmol, 2.5 mol %), and
ascorbic acid (1.5 mmol) in an aqueous 0.05 M phthalate buffer (4.0 mL) with EtOH
(1.0 mL) was stirred at 30 °C for 6 h in an O
O
2
(entry 3). Although the reaction proceeds more slowly in air
than in an O atmosphere (entries 1 and 4), the reaction in air pro-
duced 3 quantitatively with extension of the reaction time (entry
). The reaction can be performed smoothly under both daylight
2
atmosphere (1 atm).
b
Isolated yield.
2
c
Reaction conducted without EtOH for 24 h.
5
Me
and under light-shielding conditions (entry 6). Similar treatment
with 1.0 and 1.5 equiv of ascorbic acid affords 3 with yields of
Me
Me
N
N
O
O
N
2 AH–
4
7% (entry 11) and 81% (entry 9), respectively after 12 h. These
2
H O
N
Et
Me
Flox
H+
2 AH·
Me
N
Me
N
H
N
Table 1
Me
Me
N
O
O
Me
Me
O
Flavin-catalyzed aerobic oxidation of 2 with ascorbic acid in an aqueous buffer
solutiona
N
N
N
Me
N
Et
Me
Yield of 3 (%)b
Et
O
Entry
Buffer solution
pH
OH FlOH
Flred
O
1
2
3
4
5
None
2.0
3.3
4.0
5.0
7.4
0
0
O
2
0.1 M Citrate buffer
0.05 M Phthalate buffer
0.1 M Acetate buffer
0.1 M Phosphate buffer
S
Me
N
R1
R2
R1
81 (99)c
47
Me
N
O
O
S
R2 Me
N
29
N
Me
Et
a
A mixture of sulfide 2 (0.1 M), flavin catalyst 1 (2.5 mM), and ascorbic acid
OOH
(
0.3 M) in aqueous buffer solution was stirred under O
2
atmosphere at 30 °C for 6 h.
FlOOH
b
Determined by GLC based on 2.
c
Ethanol (20 vol %) was used as a co-solvent.
Scheme 3. Mechanism for the aerobic flavin-catalyzed oxidation of sulfides.

Y. Imada et al. / Tetrahedron Letters 54 (2013) 621–624
623
results indicate that 2 equiv of ascorbic acid are required for the
Acknowledgment
formation of 1 equiv of sulfoxide.
Table 3 shows representative results for the flavin-catalyzed
aerobic oxidation of sulfides in aqueous media.2 A variety of ali-
phatic and aromatic sulfides were selectively converted into the
corresponding sulfoxides when the reaction was performed with
flavin catalyst 1 (2.5 mol %) and ascorbic acid (3 equiv) in 0.05 M
aqueous phthalate buffer solution containing 20 vol % of EtOH at
This work was supported by a Grant-in-Aid for Scientific Re-
search from the Ministry of Education, Culture, Sports, Science
and Technology of Japan.
4
References and notes
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Commun. 2008, 3352–3365.
3
0 °C for 6 h in an O
moieties, such as phenol and olefin, are tolerated by the reaction
entries 6 and 7). Overoxidation of the product sulfoxides to sulf-
2
atmosphere (1 atm). Other reactive functional
(
2.
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ones was not observed in any of these reactions. Although the
addition of a small amount of water-soluble organic co-solvent,
such as EtOH, generally accelerates the reaction rate significantly
due to an increase in solubility, the reaction can still proceed
slowly yet efficiently without an organic co-solvent (entries 1
and 4).
4
55, 304–308.
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The present reaction can be rationalized by assuming the redox
process of the flavin species shown in Scheme 3.12 The oxidized
form of flavin (Flox) undergoes a single electron transfer from the
5.
(a) Ohkubo, K.; Fukuzumi, S. Org. Lett. 2000, 2, 3647–3650; (b) Kotani, H.;
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H.-J.; Lin, Y.-C.; Wan, X.; Yang, C.-Y.; Feng, Y.-S. Tetrahedron 2010, 66, 8823–
À
ascorbate anion (AH ) to form a flavin radical anion intermediate,
which is then converted into reduced flavin (Flred) by repeated sin-
gle electron transfer from a second ascorbate anion. Two equiva-
lents of ascorbic acid are required in this catalytic system
8827.
6.
(a) Ishii, Y.; Nakayama, K.; Takeno, M.; Sakaguchi, S.; Iwahama, T.; Nishiyama,
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(
Table 2), probably due to low reducing ability of the resulting
Å
25
ascorbate radical species (AH ). In addition, the reaction does
not proceed via a photooxidation process involving AH , because
Å 26
the reaction proceeded efficiently under light-shielding conditions
(
entry 6, Table 2). Incorporation of molecular oxygen into the re-
duced flavin affords 4a-hydroperoxyflavin (FlOOH), which effects
oxygen transfer to the sulfides to afford the corresponding sulfox-
ides. Dehydration of the resulting FlOH regenerates Flox to com-
plete the catalytic cycle.
The pH dependence observed for the present reaction (Table 1)
can be explained by the proton dissociation of ascorbic acid, as
7. (a) Gamez, P.; Arends, I. W. C. E.; Reedijk, J.; Sheldon, R. A. Chem. Commun. 2003,
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B. P.; Clarkson, J. P.; Belitz, N. L.; Kundu, A. J. Mol. Catal. A: Chem. 2005, 225, 111–
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shown in Scheme 4, where three species, including neutral (AH
2
),
monoanionic (AH ), and dianionic (A ) forms, are in equilibrium
in aqueous solution with acid dissociation constants of 4.0 (pKa1
and 11.3 (pKa2). The reducing activity of the ascorbic acid species
8
9
.
.
Schmaderer, H.; Hilgers, P.; Lechner, R.; König, B. Adv. Synth. Catal. 2009, 351,
163–174.
Fitzpatrick, P. F. Acc. Chem. Res. 2001, 34, 299–307.
–
2À
)
1
0. (a) Ballou, D. P. In Flavins and Flavoproteins; Massey, V., Williams, C. H., Eds.;
Elsevier: New York, 1982; pp 301–310; (b) van Berkel, W. J. H.; Kamerbeek, N.
M.; Fraaije, M. W. J. Biotech. 2006, 124, 670–689.
À
2À 25
is known to increase in the order of AH
acidic conditions greater than pKa1, the fully protonated, less reac-
tive ascorbic acid AH species is present as the major component of
2
< AH < A
.
Under
1
1. (a) Palfey, B. A.; Massey, V. In Comprehensive Biological Catalysis; Sinnott, M.,
Ed.; Academic Press: San Diego, 1996; Vol. III, pp 83–154; (b) Ghisla, S.;
Massey, V. Eur. J. Biochem. 1989, 181, 1–17.
2
ascorbic acid, and this would not reduce Flox to Flred, which leads to
complete inhibition of the reaction (entries 1 and 2, Table 1). Under
less acidic conditions with phthalate buffer (pH 4.0), the stronger
reductant AH species contributes to the reduction of Flox to drive
the catalytic reaction. The slower reaction with acetate and the
phosphate buffer (entries 4 and 5, Table 1) can be ascribed to a
12. (a) Imada, Y.; Naota, T. Chem. Rec. 2007, 7, 354–361; (b) Bäckvall, J.-E. In
Modern Oxidation Methods; Bäckvall, J.-E., Ed.; Wiley-VCH: Weinheim, 2004; pp
193–222; (c) Gelalcha, F. G. Chem. Rev. 2007, 107, 3338–3361.
1
3. (a) Fukuzumi, S.; Kuroda, S.; Tanaka, T. J. Am. Chem. Soc. 1985, 107, 3020–3027;
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À
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negative pH effect toward the dehydration step from FlOH to Flox
,
which is retarded under more basic conditions.
15. Yano, Y.; Ohshima, M.; Yatsu, I.; Sutoh, S.; Vasquez, R. E.; Kitani, A.; Sasaki, K. J.
Chem. Soc., Perkin Trans. 2 1985, 753–758.
In summary, we have found an environmentally benign method
for the aerobic, organoocatalytic oxidation of sulfides in water that
employs a water-soluble reductant. This principle provides a safe,
facile, and economic strategy for the catalytic oxygenation of vari-
ous heteroatom compounds. Efforts are currently underway to
investigate the full scope of the reaction.
16. (a) Kemal, C.; Bruice, T. C. Proc. Nat. Acad. Sci. U.S.A. 1976, 73, 995–999; (b)
Kemal, C.; Chan, T. W.; Bruice, T. C. J. Am. Chem. Soc. 1977, 99, 7272–7286.
17. Aerobic oxidation: (a) Imada, Y.; Iida, H.; Ono, S.; Murahashi, S.-I. J. Am. Chem.
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1
27, 14544–14545; (b) Imada, Y.; Kitagawa, T.; Ohno, T.; Iida, H.; Naota, T. Org.
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1989, 111, 5002–5003; (b) Mazzini, C.; Lebreton, J.; Furstoss, R. J. Org. Chem.
996, 61, 8–9; (c) Bergstad, K.; Bäckvall, J.-E. J. Org. Chem. 1998, 63, 6650–6655;
2
OH
OH
OH
H
H
H
2 2
O
HO
O
O
HO
O
O
HO
O
O
H+
–H+
–
1
H+
H+
HO
OH
O
OH
O
O
(d) Minidis, A. B. E.; Bäckvall, J.-E. Chem. Eur. J. 2001, 7, 297–302; (e) Murahashi,
S.-I.; Ono, S.; Imada, Y. Angew. Chem., Int. Ed. 2002, 41, 2366–2368; (f) Lindén, A.
A.; Krüger, L.; Bäckvall, J.-E. J. Org. Chem. 2003, 68, 5890–5896; (g) Lindén, A. A.;
Hermanns, N.; Ott, S.; Krüger, L.; Bäckvall, J.-E. Chem. Eur. J. 2005, 11, 112–119;
–
A2–
AH2
AH
pKa1 = 4.0
pKa2 = 11.3
(
h) Imada, Y.; Ohno, T.; Naota, T. Tetrahedron Lett. 2007, 48, 937–939; (i) Zˇ urek,
J.; Cibulka, R.; Dvo rˇ áková, H.; Svoboda, J. Tetrahedron Lett. 2010, 51, 1083–
086; (j) Marsh, B. J.; Carbery, D. R. Tetrahedron Lett. 2010, 51, 2362–2365.
Scheme 4. Interconversion of ascorbate species.
1

6
2
24
Y. Imada et al. / Tetrahedron Letters 54 (2013) 621–624
0. Ascorbic acid as a water-soluble reductant: Fe: (a) Udenfriend, S.; Clark, C. T.;
Axelrod, J.; Brodie, B. B. J. Biol. Chem. 1954, 208, 731–739; Mn: (b) Mansuy, D.;
Fontecave, M.; Bartoli, J.-F. J. Chem. Soc., Chem. Commun. 1983, 253–254; Cu: (c)
Aihara, K.; Urano, Y.; Higuchi, T.; Hirobe, M. J. Chem. Soc., Perkin Trans. 2 1993,
24. Typical procedure for aerobic oxidation of sulfides. Dioctyl sulfoxide: A mixture
of dioctyl sulfide (0.128 g, 0.50 mmol), 1 (0.005 g, 0.0125 mmol) and ascorbic
acid (0.264 g, 1.5 mmol) in a mixture of an aqueous 0.05 M phthalate buffer
solution (4.0 mL) and ethanol (1.0 mL) was stirred vigorously at 30 °C under O
atmosphere for 6 h. The reaction mixture was extracted with ether, and the
combined extracts were washed with brine, dried over MgSO and evaporated
2
2
6
165–2170; V: (d) Shul’pin, G. B.; Lachter, E. R. J. Mol. Catal. A: Chem. 2003, 197,
5–71.
4
2
2
1. Kaczorowska, K.; Kolarska, Z.; Mitka, K.; Kowalski, P. Tetrahedron 2005, 61,
315–8327.
2. (a) Baxová, L.; Cibulka, R.; Hampl, F. J. Mol. Catal. A: Chem. 2007, 277, 53–60; (b)
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