Ag-Cu copromoted direct C2-H bond thiolation of azoles with Bunte salts as sulfur sources

Article abstract of DOI:10.1039/d1ob00823d

A direct C2-H thiolation of azoles with Bunte salts was achieved under the combined action of copper and silver salts. This protocol could furnish various substituted 2-thioazoles in moderate to good yields. This method has a broad substrate scope and shows good functional group tolerance.

Full text of DOI:10.1039/d1ob00823d

Organic &
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Ag–Cu copromoted direct C2–H bond thiolation
of azoles with Bunte salts as sulfur sources†
Cite this: Org. Biomol. Chem., 2021,
19, 5899
Rui Wang,a Hongyan Xu,a Ying Zhang,a Yuntao Hu,a Yingsu Wei,a Xiao Dua and
a,b
Huaiqing Zhao
*
Received 28th April 2021,
Accepted 4th June 2021
A direct C2–H thiolation of azoles with Bunte salts was achieved under the combined action of copper
and silver salts. This protocol could furnish various substituted 2-thioazoles in moderate to good yields.
This method has a broad substrate scope and shows good functional group tolerance.
DOI: 10.1039/d1ob00823d
rsc.li/obc
group developed an efficient tetra-n-butylammonium iodide
(TBAI) promoted procedure for C3–H sulfenylation of indoles
Introduction
Sulfur-containing organic molecules play a vital role in pharma- with Bunte salts under metal-free and oxidant-free conditions.11
ceuticals, biologically important molecules, and organic func- A similar synthesis of C3–H sulfenylated 4-anilinocoumarins via
tional materials. The construction of the C–S bond is an effective KI-catalyzed sulfenylation of 4-anilinocoumarins with Bunte
way to synthesize sulfur-containing compounds.1 Therefore, salts was realized by Yang’s group.12 Recently, Wu’s group has
developing methodologies for the synthesis of the C–S bond is a achieved the reaction of CuI-catalyzed thioamination of malei-
persistently attractive research topic among synthetic chemists. mides with secondary amines and Bunte salts with the for-
Classically, the well-known nucleophilic substitution reaction2 mation of C–N, C–S bonds in one pot.13
and the Ullmann reaction3 could provide approaches for C–S
Substituted 2-thioazoles as an important class of sulfur-
bond formation using halides with thiol compounds as starting containing heterocyclic compounds have received widespread
materials, but these reactions are often accompanied by some attention because of their biological activities (Fig. 1).14 Over
side-reactions generating redundant compounds and the reac-
tant halides also need to be prefunctionalized.4
Direct C–H functionalization has provided a streamlined syn-
thetic route, and this strategy has been proven to be a promising
synthetic approach for chemical bond construction with atom-
and step-economy.5 How to achieve the transformation of the
C–H bond to the C–S bond is an attractive topic in synthetic
chemistry.6 Traditionally, thiol7 and disulfide compounds8 are
often introduced as sulfur sources to construct the C–S bond.
The use of these sulfur reagents usually comes with some limit-
ations, namely, lower reaction yields, unpleasant odor, and air-
sensitivity. Therefore, alternatives to these compounds have
always been explored. Bunte salts are stable, relatively easy to
prepare and odorless and have therefore gradually been pre-
ferred for C–S bond formation.9 Over the past few years, we have
noticed that research on the construction of the C–S bond via
C–H activation using Bunte salts as sulfur sources has been
widely developed.9a–c,10 For example, in 2016, Ji and Yang’s
aSchool of Chemistry and Chemical Engineering, University of Jinan, Jinan,
Shandong, 250022, China. E-mail: chm_zhaohq@ujn.edu.cn
bState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1ob00823d
Fig. 1 Representative substituted 2-thio-azole molecules.
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the past few decades, considerable progress has been made in employed (Table 1, entry 1). Then various copper salts were
the synthesis of substituted 2-thioazoles.15 Liu and co-workers investigated, and the results of different reactions indicated
realized the copper-mediated reaction of thiazoles and thiols that other copper salts could not provide a better yield of 3a
to afford substituted 2-thioazoles in the presence of Na2CO3 as
a base (Scheme 1a, route 1).15b Gao’s group achieved the coup-
ling of azoles and thiols without adding additional bases, by
Table 1 Optimization of the reaction conditionsa
adding Lewis acids to increase the activity of the thiazole C2–H
bond (Scheme 1a, route 2).15e Bolm and co-workers found that
disulfide compounds have the ability to couple with azoles
(Scheme 1b).15f In order to replace thiols and disulfides,
Dong’s group utilized tetramethylthiuram (TMTD) as the
sulfur reagent and developed a one-pot synthesis of substi-
tuted 2-thioazoles through the cyclization of 2-aminothiophe-
nols, 2-aminophenols, and 1,2-phenylenediamines with
TMTD to give azoles followed by coupling with halides
(Scheme 1c).15j However, in this reaction, the raw materials are
complex and difficult to obtain. Therefore, it is necessary to
develop an efficient and novel protocol for preparing substi-
tuted 2-thioazoles from simple, easily available and odorless
reagents. Herein, we report a direct C–H thiolation of azoles
with Bunte salts as the source of sulfur.
Entry
Cu salt
Lewis acid Solvent
Temp. (°C)
Yieldb (%)
1
Cu(OAc)2
CuCl2
CuI
CuBr
—
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
—
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMSO
MeCN
120
120
120
120
120
120
120
120
120
120
120
120
120
62
2
Trace
None
None
None
53
50
51
60
57
60
32
50
Trace
53
27
Trace
43
56
77
46
52
3c
4c
5
6
7
8
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
AgOAc
Ag2SO4
AgCF3SO3
AgSbF6
AgBF4
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
FeCl3
9
10
11
12
13
14
15d
16e
17f
18
19
20g
21
22
23
24g,h
25g,i
26c,i
Dioxane 120
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
120
120
120
110
130
120
120
120
120
120
120
120
Results and discussion
At the outset of our study, the reaction of benzothiazole (1a)
and sodium S-benzyl thiosulfate (2a) was exploited as a model
reaction to optimize the reaction conditions. According to the
reported research,9,10 we chose a cheap copper salt as the
oxidant and accelerator, N,N-dimethylformamide (DMF) with
good solubility for Bunte salts as the solvent, and 120 °C as
the reaction temperature. We recognized that the silver salt
could coordinate with the nitrogen atom of azoles to increase
the activity of the C2–H bond.15e,16 So we initially introduced
silver nitrate in this reaction. Gratifyingly, the desired 2-(ben-
zylthio)benzo[d]thiazole (3a) was obtained in 62% isolated
yield when Cu(OAc)2 (2.0 equiv.) and AgNO3 (20 mol%) were
AlCl3
Zn(OAc)2
AgNO3
AgNO3
AgNO3
58
75
74
41
a Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), Cu(OAc)2
(2 equiv.), Lewis acid (20 mol%) in DMF (2 mL) at 120 °C for 22 h under
a nitrogen atmosphere in a sealed tube. b Isolated yields. c Under air.
d Cu(OAc)2 (1.5 equiv.). e Cu(OAc)2 (1.2 equiv.). f Cu(OAc)2 (1.0 equiv.).
g 1a (0.2 mmol), 2a (0.3 mmol). h Run for 24 h. i Run for 20 h.
Scheme 1 Synthesis routes of substituted 2-thioazoles.
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(Table 1, entries 2–5). We noted that CuCl2 could only provide entries 24 and 25). Finally, we carried out the reaction under
a trace amount of the product, and this may be attributed to an air atmosphere, and the yield of 3a was reduced to only
the chlorine anion which impedes the C2–H activation of 41% (Table 1, entry 26).
azoles. Subsequently, an examination of diverse silver salts
With the optimized conditions in hand, the generality of
revealed that AgNO3 in combination with copper salts was still this protocol was further investigated. The scope of Bunte salts
a better choice (Table 1, entries 1 and 6–11). Afterwards, was first evaluated (Scheme 2). Sodium S-benzyl thiosulfate
different solvents were tested in this reaction, and it was found with –Me and –Cl at the 4-position of the benzene ring could
that DMF could furnish a more satisfactory result (Table 1, afford the desired compounds in yields of 41% and 60% (3b
entries 1 and 12–14). Then other influencing factors of the and 3c). Phenethyl Bunte salt could react well in this system to
reaction were examined in this system. Reducing the amount provide the expected product (3d). Alkyl Bunte salts also could
of Cu(OAc)2 could evidently reduce the yield of the target furnish the target products in moderate to good yields, and we
product (Table 1, entries 15–17). Interestingly, when one equi- noted that the yield of the corresponding products increased
valent of Cu(OAc)2 was employed in this reaction, only a trace with the growth of the carbon chain of Bunte salts (3e–3i). We
amount of the product was observed (Table 1, entry 17). Next, believe that this could be attributed to the improved solubility
raising or lowering the reaction temperature could not improve
the reaction (Table 1, entries 18 and 19). We found that the
yield of 3a could increase to 77% when the amount of 2a was
reduced to 1.5 equivalents (Table 1, entry 20). We speculated
that the concentration of the Bunte salt could affect the self-
polymerization reaction of the Bunte salt forming the di-
sulfide. Considering the role of silver salts, other Lewis acids
were also investigated and the reactions could not give a better
result (Table 1, entries 21–23). Next, adjusting the reaction
time had an inconspicuous effect on this reaction (Table 1,
Scheme 2 Substrate scope of Bunte salts. Standard conditions: 1a
(0.20 mmol), 2 (0.30 mmol), Cu(OAc)2 (2.0 equiv.), AgNO3 (20 mol%),
DMF (2.0 mL), under N2 at 120 °C for 22 h, isolated yield. a Reaction at
130 °C. b Reaction at 110 °C.
Scheme 3 Substrate scope of azoles. Standard conditions:
1
(0.20 mmol), 2a (0.30 mmol), Cu(OAc)2 (2.0 equiv.), AgNO3 (20 mol%),
DMF (2.0 mL), under N2 at 120 °C for 22 h, isolated yield. a Reaction at
110 °C.
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Unfortunately, we did not observe the corresponding product
when aryl Bunte salts bearing electron-withdrawing groups
such as –Cl and –NO2 were employed in this system.
After examining the scope of Bunte salts, we further evaluated
the substrate scope of azoles (Scheme 3). Benzothiazole with 5-Cl
and 5-Br could give the desired sulfides in 80% and 62% yields,
respectively (4a and 4b). Interestingly, benzothiazole bearing the
electron-withdrawing –NO2 group could provide the desired
product in 53% yield (4c). The reactions with thiazole and 4,5-di-
methylthiazole also proceeded well, giving the products in 61%
and 64% yields, respectively (4d and 4e). Thiazoles with sensitive
functional groups, namely, –CO2CH3 and –CHO, were all compa-
tible (4f and 4g). Benzoxazoles and differently substituted ben-
zoxazoles such as 5-Cl, 5-Br, 5-Me and 6-Me were well tolerated,
affording the desired products in moderate to good yields (4h–
4l). Oxazole and imidazole were successfully transformed to
afford the target products in 44% and 30% yields, respectively
(4m and 4n). Additionally, benzoxazole could also react with
different Bunte salts to produce the target products in moderate
yields (4o–4q). The reactions of oxazole and imidazole with aryl-
substituted Bunte salts were not successful and only provided
the desired product in less than 10% yield.
Scheme 4 Scale-up reaction.
Next, we carried out a scale-up reaction of benzothiazole
(1a, 2 mmol) with sodium S-benzyl thiosulfate (2a) under the
optimal reaction conditions to demonstrate the synthetic
utility of this transformation as shown in Scheme 4. The reac-
tion afforded the corresponding product 3a in 64% yield.
To probe the mechanism of this transformation, some experi-
ments were carried out. The thiolation of benzothiazole with ben-
zenethiol and CuII thiolate was carried out and the corres-
ponding products were obtained, which proved that benzenethiol
and CuII thiolate could be involved in this reaction process
(Scheme 5a and b). The reaction of benzothiazole (1a) with tolyl
Bunte salt (2k) and benzyl Bunte salt (2b) in one pot provided 3b
in 34% yield and 3k in 17% yield (Scheme 5c). It showed that
benzyl Bunte salts could perform better in this system.
Scheme 5 Mechanistic experiments.
of Bunte salts bearing larger organic groups. Isopropyl Bunte
salt could provide the expected product with a moderate yield
(3j). Aryl Bunte salts with electron-donating groups such as
methyl and methoxyl on the benzene ring were smoothly con-
verted to the desired products in moderate yields (3k–3m).
Scheme 6 Plausible mechanism.
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On the basis of our experiments and related literature pre- 20 mol%). The tube was fitted with a rubber septum and taken
cedents, a plausible reaction mechanism for this transform- out of the glove box. Then azole (0.2 mmol) was added
ation is proposed as shown in Scheme 6. First, Bunte salt 2 through the rubber septum using a syringe under an atmo-
reacts with CuII and releases SO3 to afford Cu thiolate inter- sphere of N2. DMF (2 mL) was added to the Schlenk tube
mediate A.9h,17 The nitrogen atom of azoles 1 is coordinated through the rubber septum using a syringe. The septum was
with AgI to generate intermediate B to increase the acidity of replaced by a Teflon screwcap under N2 flow. The mixture was
C2–H.15e,16 Next, intermediate C is delivered through a con- stirred at 120 °C (preheated to 120 °C) for 22 h. After cooling,
certed metalation–deprotonation process and RSH intermedi- the mixture was diluted with ethyl acetate (10 mL) and filtered
ate D is released.7g,15b,15e RSH could react with CuII to afford through a pad of silica gel, followed by washing of the pad of
Cu thiolate intermediate A. Then a disproportionation reaction silica gel with ethyl acetate (20 mL). The organic phase was
between C and the CuII salt affords CuIII intermediate concentrated under reduced pressure. The residue was then
E.13,15e,18 Finally,
E undergoes reductive elimination to purified by flash chromatography on silica gel to provide the
produce the corresponding product 3.
corresponding product.
Conflicts of interest
Conclusions
There are no conflicts to declare.
In summary, we have achieved a direct C–H thiolation of
azoles with Bunte salts using a combination of copper and
silver salts. This protocol employs Bunte salts as sulfur
sources, avoiding the use of air-sensitive and smelly thiols, di-
sulfides and other sulfur reagents. Various substituted 2-thio-
azoles have been obtained in moderate to good yields. This
method has a broad substrate scope and possesses good func-
tional group tolerance.
Acknowledgements
Financial support from the Shandong Provincial Natural
Science Foundation, China (ZR2016JL009 and ZR2020MB006)
and the University of Jinan is greatly appreciated.
Notes and references
Experimental
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General information
All reagents were purchased and used without further purifi-
cation. DMF was distilled from CaH2 under nitrogen and
stored over 4 Å molecular sieves under nitrogen. 1H NMR
(400 MHz) and 13C NMR (101 MHz) spectra were obtained
using a Bruker spectrometer with CDCl3 as the solvent and
tetramethylsilane (TMS) as the internal standard. Chemical
shifts are reported in units (ppm) by assigning the TMS reso-
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7.26 ppm). Data are reported as follows: chemical shift, multi-
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In a glove box, a 25 mL Schlenk tube equipped with a stir bar
was charged with the Bunte salt (0.3 mmol, 1.5 equiv.),
Cu(OAc)2 (0.4 mmol, 2.0 equiv.), and AgNO3 (0.04 mmol,
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