Flavin-catalyzed aerobic oxidation of sulfides and thiols with formic acid/triethylamine

Article abstract of DOI:10.1039/c4cc05216a

An efficient and practical catalytic method for the aerobic oxidative transformation of sulfides into sulfoxides, and thiols into disulfides with formic acid/TEA in the presence of a new, readily available, and stable flavin catalyst 5d is described. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc05216a

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Flavin-catalyzed aerobic oxidation of sulfides and
thiols with formic acid/triethylamine†
Cite this: Chem. Commun., 2014,
50, 10295
Shun-Ichi Murahashi,*a Dazhi Zhang,a Hiroki Iida,b Toshio Miyawaki,c
Masaaki Uenaka,c Kenji Muranoc and Kanji Meguroc
Received 7th July 2014,
Accepted 18th July 2014
DOI: 10.1039/c4cc05216a
www.rsc.org/chemcomm
An efficient and practical catalytic method for the aerobic oxidative
transformation of sulfides into sulfoxides, and thiols into disulfides
with formic acid/TEA in the presence of a new, readily available, and
stable flavin catalyst 5d is described.
Sulfoxides are important intermediates in the synthesis of biologically
important compounds;1 therefore, chemoselective, environmentally
benign catalytic oxidation of sulfides under mild conditions without
overoxidation is much needed. Although the transition-metal-
Scheme 1 Flavin-catalyzed oxidation of sulfides and thiols with molecular
oxygen.
catalyzed aerobic oxidation of sulfides has been well documented, 5-ethyl-3-methyllumiflavin is shown in Scheme 2. 4a-Hydroperoxy-
the aerobic oxidation of sulfides by organocatalysts is limited to a few flavin (FlEtOOH, 1) participate in the monooxygenation of a sub-
cases.2 Over the last decade, compounds containing disulfide bonds strate (S), to give an oxidized product (SO) and 4a-hydroxyflavin
have been used in various fields, including their application as (FlEtOH, 2), which undergoes dehydration to give oxidized flavin
biologically active compounds. There is still a need for an improved, (FlEt+, 3). FlEt+ (3) is reduced by the hydrogen donor ZH
general, versatile, efficient, and green methodology for the synthesis (NADPH) to give reduced flavin (FlEtH, 4), which undergoes a
of disulfides from thiols.3
We wish to report that oxidation reactions of sulfides and thiols complete the catalytic cycle.
reaction with molecular oxygen to generate FlEtOOH (1), to
with molecular oxygen can be performed with high efficiency using
In 1989, based on the kinetic study of the reactivity of
a new flavin catalyst 5d in the presence of formic acid and 4a-hydroxyflavin using the stopped-flow technique, the first
triethylamine (TEA) (Scheme 1). These methods are very useful, catalytic oxidation with hydrogen peroxide was reported.7 Thus,
because formic acid is an inexpensive and key compound that is the oxidation of sulfides and secondary amines by hydrogen
obtained by catalytic hydrogenation of CO2 as an attractive option of peroxide proceeds in the presence of the FlEt+ClO4 (5a) catalyst
reuse of CO2.4 Moreover, catalyst 5d is stable and can be readily with high efficiency;7 then, various flavin-catalyzed oxidation
obtained from commercially available riboflavin (vitamin B2).
reactions with hydrogen peroxide have been reported for secondary
Simulation of the functions of enzymes using simple organo- amines,7 tertiary amines,8 sulfides,8b–10 ketones,11 and aryl alde-
catalysts may lead to the discovery of biomimetic, catalytic oxida- hydes12,13 under mild conditions (Scheme 2). Asymmetric flavin-
tion reactions.5,6 Flavoenzymes are responsible for the oxidation of catalyzed oxidation of sulfides5a,10e,14 with hydrogen peroxide,
substrates via the activation of molecular oxygen and transfer of and asymmetric Baeyer–Villiger oxidation reactions15 have also
one oxygen to the substrate. The catalytic cycle of flavin adenine been reported using the advantage of the easy design of chiral
dinuleotide-containing monooxygenase (FADMO) using simplified flavin catalysts.
The next challenge was the attainment of more environmen-
a Department of Chemistry, Okayama University of Science, Ridai-cho, 1, Kita-ku,
Okayama, 700-0005, Japan. E-mail: murahashi@high.ous.ac.jp
b Department of Molecular Design and Engineering, Graduate School of Engineering,
Nagoya University, Chikusa-kuNagoya, 464-8603, Japan
c CT Laboratory, Hamari Chemicals Ltd., 1-4-29, Kunijima, Higashiyodogawa-ku,
Osaka, 533-0024, Japan
tally benign flavin-catalyzed oxidation with molecular oxygen
under mild conditions as shown in Scheme 3. Flavin-catalyzed
oxidation with molecular oxygen requires a reducing reagent
(ZH), which may correspond to NADPH. We reported the first
aerobic oxidation reaction using hydrazine hydrate in 2,2,2-
trifluoroethanol as a reducing reagent. The aerobic catalytic
oxidation of sulfides and secondary amines can be performed
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cc05216a
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was not very different compared with that of 5a by examining
the aerobic oxidation of sulfides by hydrazine hydrate in trifluoro-
ethanol (see Table S1, ESI†). Therefore, we decided to use OTf as a
safer and more convenient counter ion.
A flavin catalyst was prepared simply and easily from commer-
cially available riboflavin (vitamin B2). Thus, the oxidation of ribo-
flavin with sodium periodate in water, treatment with NaBH4, and
N-methylation gave 10-(2-hydroxylethyl)-3,7,8-trimethylisoalloxazine.
A mixture of this compound, Pd–C, conc HCl, and acetaldehyde was
allowed to react, and subsequent treatment with NaNO2 and HOTf/
NaOTf gave 5-ethyl-10-(2-hydroxylethyl)-3,7,8-trimethylisoallozazium
triflate (5d). This compound is stable and can be stored in the
refrigerator (À10 1C) for half a year. Similarly, 5e was prepared
after treatment with NaNO2 and HClO4/NaClO4. It was found
that compounds 5a, 5d, and 5e are excellent catalysts for the
aerobic oxidation of sulfides (see Table S1, ESI†), therefore, we
selected 5d for further oxidation reactions. The hydroxyl group
did not retard any catalytic activity.
Scheme 2 Catalytic cycle of flavin-catalyzed oxidation.
We looked for an efficient and practical method to achieve
aerobic oxidation of sulfides using flavin as an organocatalyst.
We focused on formic acid as a reducing reagent instead of
hydrazine in 2,2,2-trifluoroethanol, because formic acid is a key
compound in the construction of low-carbon society and is an
excellent hydrogen source for catalytic-transfer hydrogenation
reactions.20 Our initial study targeted aerobic oxidation of
dibutyl sulfide (8) using the flavin catalyst 5d. The representa-
tive results of the aerobic oxidation of 8 with formic acid are
summarized in Table 1. The catalytic oxidation of 8 in the
presence of formic acid alone did not give the corresponding
sulfoxide 9 (Table 1, entry 1); however, aerobic oxidation took
place with formic acid in the presence of the corresponding
Scheme 3 Flavin-catalyzed oxidation reactions with H2O2 and molecular
oxygen.
at room temperature.16 Aerobic flavin-catalyzed Baeyer–Villiger
oxidation was performed using zinc dust as a reducing reagent.17
Recently, aerobic oxidation of sulfides and aryl aldehydes was also
carried out using ascorbic acid18 and Hantzsch ester as reducing
reagents.19 These aerobic flavin-catalyzed oxidation reactions are
not practical and not suitable for large-scale production, because
the reducing reagents are specific and cannot be obtained at a
reasonable cost. 2,2,2-Trifluoroethanol is very expensive and not
environmentally friendly solvent.
We reinvestigated flavin catalysts to discover stable, efficient,
and readily available catalysts. There are three types of flavin
catalysts, as shown in Scheme 4; i.e., flavinium ion catalysts (5),7,15
such as 5a7 and 5c,17 flavin catalysts (6),9 such as 6a and 6b, and
bridged flavin catalysts (7),10 such as 7a10a and 7b.10b
Table 1 Flavin-catalyzed aerobic oxidation of dibutyl sulfidea
Since we reported the high efficiency of the flavinium
perchlorate catalyst (FlEt+ClO4) (5a),7 the ClO4 anion has been
used as a counter ion; however, it is desirable to use a safer
counter ion in large-scale preparation. We confirmed that the
efficiency of catalysts bearing other counter ions (OTf, BF4, and PF6)
Entry Cat. Reductant
Ratio Equiv. Solvent Time (h) Yieldb (%)
1
2
3
4
5d HCO2H
5d HCO2H/HCO2Na 4 : 1 50
5d HCO2H/HCO2Na 8 : 1 50
—
50
—
—
—
—
14
21
7
Trace
79
59
74
5d HCO2H/
8 : 1 50
14
HCO2NH4
5
6
7
8
9
5d HCO2H/TEA
5d HCO2H/TEA
5d HCO2H/TEA
5d HCO2H/TEA
5d HCO2H/TEA
5 : 2 50
8 : 1 50
—
—
7
7
95
99
(94)
99
99
98d
99
99
99
88
99
99
8 : 1
8 : 1
3.12 MeCNc 24
6.25 MeCNc
7
7
8 : 1 12.5 MeCN
10
11
12
13
14
15
5d HCO2H/TEA
5d HCO2H/TEA
5d HCO2H/TEA
5e HCO2H/TEA
5b HCO2H/TEA
5a HCO2H/TEA
8 : 1 12.5 DMA
8 : 1 12.5 DME
8 : 1 12.5 DMF
8 : 1 12.5 MeCN
8 : 1 12.5 MeCN
8 : 1 12.5 MeCN
7
7
7
7
7
7
a
Unless otherwise noted, the reactions were carried out using dibutyl
sulfide (0.5 mmol) in a solvent (0.4 mL) in the presence of catalyst
b
(5 mol%) at 60 1C under molecular oxygen (1 atm, balloon). Yields of
dibutyl sulfide were determined by 1H NMR or HPLC (parentheses).
c
d
Scheme 4 Structures of representative flavin catalysts.
10296 | Chem. Commun., 2014, 50, 10295--10298
The amount of solvent used was 0.8 mL. Isolated yield.
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salts, such as HCO2Na and HCO2NH4, to give the sulfoxides reaction with molecular oxygen to form FlEtOOH and complete
with a yield of 59–79% (entries 2–4). The screening of the the catalytic cycle.
combination of formic acid with a base revealed that HCO2H/
triethylamine (TEA) gave excellent results (entries 5 and 6).
Next, we applied this method to the aerobic catalytic oxidative
transformation of thiols into disulfides. Generally, the aerobic
We examined the HCO2H/TEA ratio and found that a ratio oxidation of thiols is performed in the presence of a base or heavy-
of 8/1 gave a higher yield compared with a ratio of 5/2 metal ions; however, metal-free conditions are often required in
(entries 5 and 6).
the synthesis of biologically active compounds and medicines.
Flavin-promoted oxidation of thiols takes place under basic
The amount of HCO2H and reaction time influenced the
conversions (entries 7–9). Regarding the solvent, MeCN, DMA, conditions via the nucleophilic attack of the thiolate at the C(4a)
DME and DMF were used similarly (entries 9–12). The oxidation position, to form a covalent adduct, followed by the nucleophilic
of 8 in the presence of the flavin catalyst 5d (5 mol%) in MeCN attack of the second thiol anion, to afford the corresponding
under molecular oxygen (1 atm, balloon) with HCO2H/TEA disulfides, with very slow rates of oxidation.21
(8 : 1) at 60 1C gave 9 with a yield of 99%. The isolated yield of 9
Flavin-catalyzed aerobic oxidation of thiols under the present
was over 98% (entry 9). As no overoxidation product was detected, acidic conditions gave the corresponding disulfides with excellent
it was not necessary to control reaction conditions to avoid yields, as shown in Table 3. Aerobic oxidation tolerates various
overoxidation. We examined the catalytic activity of the related functional groups (entries 2–7). The oxidation of 2-mercapto pyridine
flavin catalysts. The catalytic activity of 5e was slightly lower than (12) gave the corresponding disulfide with a yield of 83% (entry 5).
that of 5d (entry 13). Catalysts 5a7 and 5b17 exhibited high catalytic
activity, as did catalyst 5d (entries 14 and 15).
This method is highly useful for the direct synthesis of
disulfides from S-protected thiols, because some thiols are very
To demonstrate the generality and the scope of this method- sensitive and difficult to handle from a synthetic perspective.
ology, a variety of sulfides bearing various functional groups Typically, the aerobic oxidation of S-[(acetylamino)methyl]-N-
were oxidized. Representative results are shown in Table 2. [(9H-fluoren-9-ylmethoxy)carbonyl]-l-cysteine (13)22 in the presence
As expected, electron-rich sulfides (entries 1–6) were oxidized of the flavin catalyst 5d gave the corresponding N,N-bis[(9H-fluoren-
faster than electron-deficient sulfides (entry 7). The oxidation 9-ylmethoxy)carbonyl]-l-cysteine (14) selectively with an isolated
proceeds smoothly, when tertiary amine (entry 8) and olefin yield of 84% without formation of the corresponding overoxidation
(entry 9) (allylsulfide 10) are present. Disulfide 11 was converted products (Scheme 5). It is noteworthy that a control experiment
to the corresponding monosulfoxide selectively (entry 10).
performed in the absence of catalyst 5d showed that compound 13
The reaction mechanism can be rationalized by assuming was recovered without any loss under the reaction conditions. The
the following pathway. The oxidation of sulfides with FlEtOOH present method is highly useful for the synthesis of some complex
occurs electrophilically to give sulfoxides and FlEtOH, which disulfides to be used as drugs.
undergoes a pseudo first order reaction to give FlEt+ and
In conclusion, we have developed an efficient and practical
water.7,16 The reaction of FlEt+ with HCOOÀ would give FlEtH, catalytic method for the aerobic oxidative transformation of
together with carbon dioxide. FlEtH thus formed would undergo sulfides into sulfoxides, and thiols into disulfides with formic
Table 2 Flavin-catalyzed aerobic oxidation of sulfidesa
Table 3 Flavin-catalyzed aerobic oxidation of thiolsa
Yieldb (%) Entry
Yieldb (%)
Substrate
Entry
Substrate
1
2
R = C12H25
R = Bu
93
96
1
98c
t
2
94
3
4
R = CO2Me
R = CO2H
93
95
3
4
5
R = Me
R = OMe
R = OH
95
99
97
6
7
8
R = NHCOMe
R = CI
R = N(CH3)2
98
76
85
5
6
83
9
65d
77
79
83
10
7
a
The aerobic oxidation of sulfides (0.5 mmol) was carried out in the
presence of a mixture of HCO2H and Et3N (8 : 1, 12.5 mmol) and 5d
(5 mol%) in MeCN (4 mL) under molecular oxygen (1 atm, balloon) at
60 1C for 24 h. Isolated yield. The reaction was run for 7 h. The
reaction was run for 16 h.
a
The oxidation of thiols (0.5 mmol) was carried out in the presence of a
mixture of HCO2H and TEA (8 : 1, 12.5 mmol) and catalyst 5d (5 mol%)
in MeCN (0.4 mL) under molecular oxygen (1 atm, balloon) at 60 1C for
4 h. Isolated yield.
b
c
d
b
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