Iron-catalyzed direct C-H thiolation of trimethoxybenzene with disulfides

Article abstract of DOI:10.1080/00397911.2011.569867

(Chemical Equation Presented) An iron-catalyzed thiolation access to sulfides from disulfides via arene C-H bond cleavage of trimethoxybenzene is described. The procedure tolerates methoxyl, fluoro, chloro, bromo, nitro, and heterocyclic groups, using air

Full text of DOI:10.1080/00397911.2011.569867

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Iron-Catalyzed Direct C-H Thiolation of
Trimethoxybenzene with Disulfides
Manli Zhang a , Shouhui Zhang a , Changduo Pan b & Fan Chen a
a College of Chemistry and Materials Engineering , Wenzhou
University , Wenzhou , P. R. China
b Wenzhou Institute of Industry and Science , Wenzhou , P.R. China
Accepted author version posted online: 17 Nov 2011.Published
online: 29 May 2012.
To cite this article: Manli Zhang , Shouhui Zhang , Changduo Pan & Fan Chen (2012) Iron-Catalyzed
Direct C-H Thiolation of Trimethoxybenzene with Disulfides, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 42:19, 2844-2853,
DOI: 10.1080/00397911.2011.569867
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Synthetic Communications1, 42: 2844–2853, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2011.569867
IRON-CATALYZED DIRECT C-H THIOLATION
OF TRIMETHOXYBENZENE WITH DISULFIDES
Manli Zhang,1 Shouhui Zhang,1 Changduo Pan,2 and
Fan Chen1
1College of Chemistry and Materials Engineering, Wenzhou University,
Wenzhou, P. R. China
2Wenzhou Institute of Industry and Science, Wenzhou, P.R. China
GRAPHICAL ABSTRACT
Abstract An iron-catalyzed thiolation access to sulfides from disulfides via arene C-H bond
cleavage of trimethoxybenzene is described. The procedure tolerates methoxyl, fluoro,
chloro, bromo, nitro, and heterocyclic groups, using air as the clean and terminal oxidant.
Keywords C-H activation; disulfides; Fe-catalyzed; thiolation; trimethoxybenzene
INTRODUCTION
The scope and application of organosulfur compounds have increased tremen-
dously because sulfur-containing groups serve important auxiliary functions in syn-
thetic sequences.[1] Aryl sulfides have been widely used as intermediates in synthetic
chemistry and products in the pharmaceutical industry.[2] The transition-metal-
catalyzed cross-coupling reactions of ArX [X ¼ Cl, Br, I, OTf, and B(OH)2] with
thiol are powerful tools for the formation of C-S bonds (Scheme 1, Eq. (1)).[3] How-
ever, the thiols are easily formed with S-S coupling disulfides as the by-product.[4]
Furthermore, the combination of sulfur compounds with metal could deactivate
the catalyst in the reaction system.[5] Employing ArSSAr may solve these drawbacks
(Scheme 1, Eq. (2)).[6] However, in most cases, one or more equivalents of reductants
such as Zn or Mg was required in the reaction.
The transition-metal-catalyzed direct C-H bond functionalization has become
a versatile synthetic method for organic synthesis,[7] which affords valuable transfor-
mations of C-H bonds to C-C,[8] C-X,[9] C-O,[10] and C-N[11] bonds. However, direct
Received February 10, 2011.
Address correspondence to Fan Chen, College of Chemistry and Materials Engineering, Wenzhou
University, Wenzhou 325027, China. E-mail: fanchen@wzu.edu.cn or Changduo Pan, Wenzhou Institute
of Industry and Science, Wenzhou 325000, China. E-mail: changduo@yahoo.cn
2844

C-H ACTIVATION
2845
Scheme 1. Formation of C-S bond.
C-S bond-forming reaction via C-H bond cleavage is rarely reported. Recently, pal-
ladium- or copper-catalyzed formations of C-S bonds via 2-phenylpyridine C-H
bonds with different sulfur sources have been reported (Scheme 1, Eq. (3)).[12] From
the synthetic point of view, the use of cheap catalysts instead of expensive catalysts is
highly desirable for the aforementioned transformation. Very recently, Zhang et al.
developed a copper-catalyzed thiolation of trimethoxybenzene C-H bond with disul-
fides using oxygen as the oxidant (Scheme 1, Eq. (4)).[13]
Iron represents a cheap and promising catalyst[14] in the C-H functionalization
to form the C-C and C-heteroatom bonds.[15] In 2009, Fang and coworkers reported
an iron-catalyzed sulfenylation of indoles C-H bond with diaryl disulfides, where a
catalytic amount of iodine was necessary to promote the reaction.[16] On the basis
of the previous work, herein we report a non-chelation-assisted iron(III)-catalyzed
direct C-H thiolation of trimethoxybenzene with ArSSAr (Scheme 1, Eq. (4)).
Initially, we began to examine the phenylthiolation of 1,3,5-trimethoxybenzene
withPhSSPhusingironcatalysts,asshowninTable1.Toourdelight,thethiolationreac-
tion catalyzed by FeF3 (20 mol%) took place in DMF in 25% yield (Table 1, entry 1).
Afterthescreeningofironsalts,thethioether3acouldbegainedin82% yieldusingFeBr3
as the catalyst under air in dimethylformanlide (DMF) at 145 ꢀC for 48h (Table 1, entry
8). Decreasing the amount of iron led to poor yields. The use of other iron salts such as
FeCl2, FeCl3, FeBr2, FeI2, Fe(OAc)2, and Fe(acac)3 as catalysts resulted in lower yields
(Table 1, entries 2–7). Other oxidants, such as oxone, K2S2O8, PhI(OAc)2, BQ,
Cu(OAc)2, CuBr2, and tert-butyl hydroperoxide (TBHP), were inferior to air for this
transformation (Table 1, entries 9–15). Notably, the use of two RS in (RS)2 in the reac-
tion represents an atom-economical procedure. Under an O2 atmosphere, the yield
decreased to 64%. Only 25% of the product was isolated when the reaction was per-
formedunderN2 atmosphere(Table1,entry8). Thechoiceofsolventhaddramaticeffect
onthereaction, and(DMF)turnedouttobethebest(Table1,entries17–20). Noproduct
was observed in the absence of metal catalyst (Table 1, entry 16). The reaction conducted
on a 10- mmol scale and formed the thiolation product in an acceptable 65% yield.

2846
M. ZHANG ET AL.
With the optimal parameters established, the thiolation reactions of trimethox-
ybenzene with various disulfides are summarized in Fig. 1. A wide range of functional
groups, such as methoxy, 2-naphthyl, fluoro, chloro, bromo, nitro, and heterocyclic,
were tolerated well, and good product yields were obtained (Fig. 1, 3c, 3d, 3e, 3f, 3 h,
3i, 3j, and 3n). Generally, the electron-withdrawing groups on the phenyl ring of ArS-
SAr produced thiolation products in better yields (Fig. 1, 3i and 3j). The reaction is
applicable to the synthesis of a wide variety of substituted disulfides and does not
seem to be extremely sensitive to hindrance effects. For example, the ortho-substituted
substrate 2 g provided a useful 68% yield of product 3 g, while 3f was obtained in 70%
yield. Importantly, the halo groups on the phenyl ring of disulfides survived in the
procedure (Fig. 1, 3d, 3e, 3f, and 3g), which is attractive because halo products could
be further synthetic elaboration.
Notably, PhSeSePh was subjected to the procedure, and the mono-selenation
product 3o was obtained in 30% yield, along with 20% of the diselenation product
3p. Fortunately, the compatibility of benzyl and allyl groups is synthetically useful
Table 1. Screening for the optimum conditionsa
Entry
Fe source
Solvent
Oxidant
Yield (%)
1
2
FeF3
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
Xylene
DMSO
NMP
THF
Air
Air
25
45
FeCl2
FeCl3
FeBr2
FeI2
3
Air
Air
40
30
4
5
Air
Air
63
30
6
Fe(OAc)2
Fe(acac)3
FeBr3
FeBr3
FeBr3
FeBr3
FeBr3
FeBr3
FeBr3
FeBr3
7
Air
Air
35
82 (64)b (25)c
8
9
Oxone (0.2 mmol)
K2S2O8 (0.2 mmol)
PhI(OAc)2 (0.2 mmol)
BQ (0.2 mmol)
Cu(OAc)2 (0.2 mmol)
CuBr2 (0.2 mmol)
TBHP (0.2 mmol)
Air
40
55
45
78
77
70
60
0
10
11
12
13
14
15
16
17
18
19
20
FeBr3
FeBr3
FeBr3
FeBr3
Air
Air
15
63
60
10
Air
Air
aAll reactions were run with 1a (0.2 mmol), PhSSPh 2a (21.8 mg, 0.1 mmol), Fe sources (20 mol %), dry
solvent (2 mL), under air, 145 ꢀC in a sealed tube, 48 h.
bUnder O2.
cUnder N2.

C-H ACTIVATION
2847
Figure 1. Reaction conditions: 1a (0.2 mmlo), RSSR 2 (0.1 mmol), FeBr3 (20 mol %), dry DMF (2 mL),
air, 145 ꢀC in a sealed tube, 48 h. a60 h.
because they can be further functionalized, although the thiolation products were
isolated in poor yields (Fig. 1, 3k and 3 m). Furthermore, the alkyl ester gave the pro-
duct 3 l in 53% yield. Disappointingly, under the current condition, MeSSMe would
not be compatible.
Next, we investigated the substrate scope with regard to other electron-rich are-
nes, as shown in Fig. 2. 1,3-Dimethoxysubstituted and the 1,2,4-trimethoxysubstituted
benzenes underwent the reaction to generate the products 3s and 3t, albeit in poor
yields, where no other isomer were observed by analysis of gas chromatography–mass

2848
M. ZHANG ET AL.
Figure 2. Reaction conditions: 1 (0.2 mmol), PhSSPh 2a (0.1 mmol), FeBr3 (20 mol %), dry DMF (2 mL),
air, 145 ꢀC in a sealed tube, 48 h. a60 h.
spectrometry (GC-MS). Particularly, the indole could also deliver the corresponding
thiolation products in moderate yields (Fig. 2, 3 u and 3v).
In conclusion, we have demonstrated an efficient and straightforward method
for the iron-catalyzed thiolation of trimethoxybenzene C-H bond with disulfide. The
employment of cheap iron as the catalyst and air as the clean oxidant significantly
improved the practicality of this C-H functionalization reaction.
EXPERIMENTAL
Chemicals were either purchased or purified by standard techniques without
special instructions. The FeBr3 (99%), which contained less than 25 ppm transition
metals such as palladium or copper through analysis of inductively coupled plasma
(ICP), was purchased from J&K. 1H NMR and 13C NMR spectra were measured on
a 500 MHz (1H 500-MHz, 13C 125 MHz) spectrometer using CDCl3 as the solvent
with tetramethylsilane (TMS) as the internal standard at room temperature. Chemi-
cal shifts (d) are given in parts per million (ppm) relative to TMS, and the coupling
constants J are given in hertz (Hz).
General Procedure
Under an air atmosphere, a sealed reaction tube was charged with 1,3,5-tri-
methoxybenzene (33.6 mg, 0.2 mmol), RSSR (0.1 mmol), FeBr3 (11.6 mg, 20 mol %),
and dry DMF (2 mL). The mixture was stirred at 145 ꢀC for 48 h. After the completion
of the reaction, as monitored by thin-layer chromatography (TLC), the solvent was
concentrated in vacuo, and the residue was purified by flash column chromatography
on silica gel (300–400 mesh) with petroleum ether–EtOAc as eluent to give the
product.

C-H ACTIVATION
2849
Phenyl(2,4,6-trimethoxyphenyl)sulfane (3a)[17]
1H NMR (CDCl3, 500 MHz): d 7.17–7.14 (m, 2H), 7.03–7.01 (m, 3H), 6.22 (s,
2H), 3.87 (s, 3H), 3.80 (s, 6H). 13C NMR (CDCl3, 125 MHz): d 162.9, 162.5, 138.7,
128.5, 125.7, 124.3, 98.9, 91.2, 56.3, 55.4.
4-Tolyl(2,4,6-trimethoxyphenyl)sulfane (3b)[13]
1H NMR (CDCl3, 500 MHz): d 6.99–6.95 (m, 4H), 6.21 (s, 2H), 3.86 (s, 3 H),
3.81 (s, 6H), 2.25 (s, 3H). 13C NMR (CDCl3, 125 MHz): d 162.8, 162.5, 135.0, 134.1,
129.2, 126.0, 99.4, 91.1, 56.4, 55.6, 20.9.
(3-Methoxyphenyl)(2,4,6-trimethoxyphenyl)sulfane (3c)[13]
1H NMR (CDCl3, 500 MHz): d 7.08–7.05 (m, 1H), 6.62–6.57 (m, 3H), 6.21 (s,
2H), 3.86 (s, 3H), 3.81 (s, 6H), 3.71 (s, 3H). 13C NMR (CDCl3, 125 MHz): d 163.0,
162.6, 159.7, 140.2, 129.3, 118.0, 111.2, 110.0, 98.4, 91.2, 56.3, 55.4, 55.1.
(4-Fluorophenyl)(2,4,6-trimethoxyphenyl)sulfane (3d)[13]
1H NMR (CDCl3, 500 MHz): d 7.04–7.01 (m, 2H), 6.86 (d, J ¼ 8.6 Hz, 2H),
6.20 (s, 2H), 3.86 (s, 3H), 3.81 (s, 6H). 13C NMR (CDCl3, 125 MHz): d 162.9,
162.4, 160.8 (d, JC-F ¼ 241.8 Hz), 133.6, 127.9 (d, JC-F ¼ 7.6 Hz), 115.5 (d, JC-F
¼ 21.9 Hz), 99.6, 91.3, 56.3, 55.4.
-
(4-Chlorophenyl)(2,4,6-trimethoxyphenyl)sulfane (3e)[13]
1H NMR (CDCl3, 500 MHz): d 7.11 (d, J ¼ 8.5 Hz, 2H), 6.94 (d, J ¼ 8.5 Hz,
2H), 6.21 (s, 2H), 3.87 (s, 3H), 3.80 (s, 6H). 13C NMR (CDCl3, 125 MHz): d
163.1, 162.4, 137.4, 130.0, 128.5, 126.9, 98.3, 91.2, 56.3, 55.4.
(4-Bromophenyl)(2,4,6-trimethoxyphenyl)sulfane (3f)[13]
1H NMR (CDCl3, 500 MHz): d 7.18 (d, J ¼ 8.1 Hz, 2H), 6.80 (d, J ¼ 7.8 Hz,
2H), 6.14 (s, 2H), 3.80 (s, 3H), 3.73 (s, 6H). 13C NMR (CDCl3, 125 MHz): d
163.1, 162.4, 138.1, 131.4, 127.2, 117.8, 98.1, 91.2, 56.3, 55.4.
(2-Bromophenyl)(2,4,6-trimethoxyphenyl)sulfane (3g)[13]
1H NMR (CDCl3, 500 MHz): d 7.45 (d, J ¼ 7.9 Hz, 1H), 7.04–7.01 (m, 1H),
6.90–6.87 (m, 1H), 6.50 (d, J ¼ 8.0 Hz, 1H), 6.23 (s, 2H), 3.88 (s, 3H), 3.79 (s, 6H).
13C NMR (CDCl3, 125 MHz): d 163.3, 162.6, 139.8, 132.4, 127.2, 125.4, 125.1,
120.1, 97.8, 91.2, 56.3, 55.4.

2850
M. ZHANG ET AL.
Naphthalen-2-yl(2,4,6-trimethoxyphenyl)sulfane (3h)[13]
1H NMR (CDCl3, 500 MHz): d 7.72 (d, J ¼ 8.0 Hz, 1H), 7.65–7.60 (m, 2H),
7.38–7.32 (m, 3H), 7.21 (dd, J ¼ 9.0, 7.0 Hz, 1H), 6.26 (s, 2H), 3.90 (s, 3H), 3.81
(s, 6H). 13C NMR (CDCl3, 125 MHz): d 163.0, 162.6, 136.3, 133.8, 131.2, 127.9,
127.6, 126.8, 126.1, 124.8, 124.7, 123.0, 98.7, 91.3, 56.3, 55.4.
(3-Nitrophenyl)(2,4,6-trimethoxyphenyl)sulfane (3i)[13]
1H NMR (CDCl3, 500 MHz): d 7.87 (d, J ¼ 8.0 Hz, 1H), 7.77 (s, 1H), 7.36–7.26
(m, 2H), 6.24 (s, 2H), 3.89 (s, 3H), 3.82 (s, 6H). 13C NMR (CDCl3, 125 MHz): d
163.6, 162.4, 148.6, 142.0, 131.3, 129.0, 120.0, 119.3, 96.6, 91.4, 56.3, 55.5.
(4-Nitrophenyl)(2,4,6-trimethoxyphenyl)sulfane (3j)[13]
1H NMR (CDCl3, 500 MHz): d 8.00 (d, J ¼ 9.0 Hz, 2H), 7.04 (d, J ¼ 9.0 Hz,
2H), 6.23 (s, 2H), 3.89 (s, 6H), 3.80 (s, 3H). 13C NMR (CDCl3, 125 MHz): 163.8,
162.4, 149.4, 144.6, 124.8, 123.7, 96.0, 91.3, 56.3, 55.5.
Benzyl(2,4,6-trimethoxyphenyl)sulfane (3k)[13]
1H NMR (CDCl3, 500 MHz): d 7.19–7.13 (m, 5H), 6.09 (s, 2H), 3.88 (s, 2H),
3.81 (s, 3H), 3.74 (s, 6H). 13C NMR (CDCl3, 125 MHz): d 162.2, 161.9, 138.9,
128.9, 127.9, 126.5, 101.5, 91.0, 56.1, 55.3, 39.2.
Methyl 3-(2,4,6-Trimethoxyphenylthio)propanoate (3l)[13]
1H NMR (CDCl3, 500 MHz): d 6.15 (s, 2H), 3.86 (s, 6H), 3.83 (s, 3H), 3.63 (s,
3H), 2.96 (t, J ¼ 7.5 Hz, 2H), 2.50 (t, J ¼ 7.6 Hz, 2H). 13C NMR (CDCl3, 125 MHz):
d 210.3, 172.7, 162.0, 126.7, 91.0, 56.1, 55.4, 51.6, 34.6, 22.7.
Allyl(2,4,6-trimethoxyphenyl)sulfane (3m)[13]
1H NMR (CDCl3, 500 MHz): d 6.14 (s, 2H), 5.84–5.75 (m, 1H), 4.88–4.84 (m,
2H), 3.86 (s, 6H), 3.82 (s, 3H), 3.35 (d, J ¼ 7.3 Hz, 2H). 13C NMR (CDCl3,
125 MHz): d 162.1, 161.7, 134.6, 116.3, 93.3, 91.0, 56.1, 55.3, 37.6.
2-(2,4,6-Trimethoxyphenylthio)benzo[d]thiazole (3n)[13]
1H NMR (CDCl3, 500 MHz): d 7.83 (d, J ¼ 8.0 Hz, 1H), 7.59 (d, J ¼ 8.0 Hz,
1H), 7.37–7.34 (m, 1H), 7.19 (d, J ¼ 8.0 Hz, 1H), 6.23 (s, 2H), 3.89 (s, 3H), 3.83 (s,
6H). 13C NMR (CDCl3, 125 MHz): d 164.4, 162.6, 154.5, 140.3, 135.4, 127.2, 125.6,
121.4, 120.9, 97.2, 91.3, 56.3, 55.5.

C-H ACTIVATION
2851
Phenyl(2,4,6-trimethoxyphenyl)selane (3o)[13]
1H NMR (CDCl3, 500 MHz): d 7.20–7.18 (m, 2H), 7.15–7.07 (m, 3H), 6.21 (s,
2H), 3.87 (s, 3H), 3.79 (s, 6H). 13C NMR (CDCl3, 125 MHz): d 163.0, 162.0, 133.6,
128.9, 128.7, 125.3, 93.0, 91.3, 56.3, 55.4.
1,3,5-Trimethoxy-2,4-bis(phenylselanyl)benzene (3p)[13]
1H NMR (CDCl3, 500 MHz): d 7.25–7.24 (m, 4H), 7.17–7.10 (m, 6H), 6.41 (s,
1H), 3.83 (s, 6H), 3.77 (s, 3H). 13C NMR (CDCl3, 125 MHz): d 164.8, 163.1, 133.3,
129.4, 128.8, 125.7, 104.6, 92.5, 62.1, 56.4.
2,4-Dimethoxyphenyl)(phenyl)sulfane (3s)[13]
1H NMR (CDCl3, 500 MHz): d 7.54–7.10 (m, 5H), 6.53–6.47 (m, 3H), 3.81 (s,
3H), 3.79 (s, 3H). 13C NMR (CDCl3, 125 MHz): d 161.9, 160.9, 136.7, 129.9, 129.1,
128.8, 127.8, 125.4, 106.2, 100.5, 55.5, 55.3.
Phenyl(2,4,5-trimethoxyphenyl)sulfane (3t)[13]
1H NMR (CDCl3, 500 MHz): d 7.24–7.21 (m, 2H), 7.15–7.12 (m, 3H), 7.11 (s,
1H), 6.95 (s, 1H), 3.93 (s, 3H), 3.82 (s, 3H), 3.78 (s, 3H). 13C NMR (CDCl3,
125 MHz): d 154.4, 150.4, 143.6, 138.1, 128.9, 127.2, 125.7, 119.0, 110.9, 98.2,
57.0, 56.3, 55.9.
3-(Phenylthio)-1h-indole (3u)[18]
1H NMR (CDCl3, 500 MHz): d 8.48 (s, 1H), 7.62 (d, J ¼ 8.0 Hz, 1H), 7.50 (d,
J ¼ 2.5 Hz, 1H), 7.45 (d, J ¼ 8.5 Hz, 1H), 7.25–7.23 (m, 1H), 7.18–7.05 (m, 6H). 13
C
NMR (CDCl3, 125 MHz): d 139.2, 136.5, 130.6, 129.1, 128.7, 125.8, 124.7, 123.0,
120.9, 119.7, 111.5, 102.8.
1-Methyl-3-(phenylthio)-1h-indole (3v)[19]
1H NMR (CDCl3, 500 MHz): d 7.53 (d, J ¼ 8.0 Hz, 1H), 7.31 (d, J ¼ 8.5 Hz,
1H), 7.24–7.18 (m, 1H), 7.10–7.06 (m, 3H), 7.02–7.00 (m, 2H), 6.98–6.95 (m, 1H),
3.77 (s, 3H). 13C NMR (CDCl3, 125 MHz): d 139.7, 137.5, 135.0, 129.8, 128.6,
125.7, 124.6, 122.5, 120.5, 119.7, 109.7, 100.5, 33.1.
ACKNOWLEDGMENTS
We thank the Key Project of Chinese Ministry of Education (No. 209054) and
the Major Technology Program of Zhejiang Province (2008C11062-1) for financial
support.

2852
M. ZHANG ET AL.
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