Copper(II)-Catalyzed C-S Cross-Coupling with Thiazolidine-2-thiones and Boronic Acids: Synthesis of Azole Sulfides

Article abstract of DOI:10.1002/ejoc.201600160

The copper(II)-catalyzed C-S cross coupling of thiazolidine-2-thiones with organoboronic acids towards the synthesis of azole sulfides was investigated. This mild and base-free protocol has the advantages of short reaction times and excellent yields with no need for an ancillary ligand.

Full text of DOI:10.1002/ejoc.201600160

DOI: 10.1002/ejoc.201600160
Communication
Cross Coupling
Copper(II)-Catalyzed C–S Cross-Coupling with Thiazolidine-
2-thiones and Boronic Acids: Synthesis of Azole Sulfides
Jayabalan Shanmugapriya,[a] Kandasamy Rajaguru,[b] Shanmugam Muthusubramanian,*[b]
and Nattamai Bhuvanesh[c]
Abstract: The copper(II)-catalyzed C–S cross coupling of thia- free protocol has the advantages of short reaction times and
zolidine-2-thiones with organoboronic acids towards the syn-
thesis of azole sulfides was investigated. This mild and base-
excellent yields with no need for an ancillary ligand.
used for the construction of C–S bonds to yield organosulfur
compounds that find applications in the biological, pharmaceu-
Introduction
In recent years, transition-metal-catalyzed carbon–heteroatom tical, and materials fields.[2,3] Azole sulfide motifs, in particular,
cross couplings have emerged as successful organic synthetic can be found in a vast number of pharmaceutically active mol-
strategies.[1] Transition-metal catalysts have been extensively ecules (Figure 1).[4] 2-Thio-substituted 1,3-benzothiazoles can
Figure 1. Various pharmaceutically active benzo-fused thiazole sulfides.
be used as vulcanization catalysts and corrosion inhibitors and
[a] Department of Chemistry, Thiagarajar College of Engineering,
as reagents for metal-catalyzed cross coupling reactions.[1a–1c,5]
Madurai 625015, India
[b] Department of Organic Chemistry, School of Chemistry,
Aryl sulfides can act as anti-inflammatory agents, and they
Madurai Kamaraj University,
Madurai 625021, India
E-mail: muthumanian2001@yahoo.com
are also used for the treatment of Parkinson's disease, Alz-
heimer's disease, and obstructive pulmonary diseases.[6] In the
past two decades, numerous advancements have been made in
C–S bond-formation reactions by using several excellent metal
catalysts such as Fe,[7] Pd,[8] Cu,[9] and Ni,[10] and this has re-
sulted in the generation of various functional molecules.[11] Sev-
eral attempts at C–S bond formation through the coupling of
thiols or disulfides with aryl halides have been made.[12] How-
http://mkuniversity.org/direct/schools/departments/chemistry/
muthusubramanian.php
[c] X-ray Diffraction Laboratory, Department of Chemistry, Texas A & M
University,
College Station, Texas 77842, USA
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejoc.201600160.
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Communication
Scheme 1. Strategy for the synthesis of azole sulfides.
ever, very few reports are available for the synthesis of hetero- yield of the reaction increased to 89 % by using Cu(acac)2 at a
aryl sulfides through the copper-catalyzed C–S cross coupling higher loading (20 mol-%) without the addition of an ancillary
of thiols, thioamides, and diaryl disulfides with organoboronic ligand (Table 1, entry 9). However, lowering the reaction tem-
acids.[13–16] Savarin et al. described the CuI–carboxylate-cata- perature led to poor yields and longer reaction times were re-
lyzed base-free synthesis of thioethers by reaction of boronic quired (Table 1, entries 13 and 14). No substantial increase in
acids with aryl, heteroaryl, and alkyl N-thioimides.[14] Taniguchi the yield of anticipated product 3a was noticed upon testing
and Li also reported[15] the copper(I)-catalyzed C–S cross cou- other metal acetylacetonate catalytic systems, including
pling of diaryl disulfides with boronic acids to give aryl and Fe(acac)2, Fe(acac)3, Ni(acac)2, and Pd(acac)2 (Table 1, entries 9–
heteroaryl sulfides. Xu et al. reported[16] the copper(II)-catalyzed 11). Among the numerous solvents screened, 1,2-dichloro-
S-arylation of thiols by oxidative coupling of thiols with boronic ethane was ideal.
acids at room temperature under basic conditions with the use
of an ancillary ligand. Chan–Lam-type cross coupling is emerg-
Table 1. Optimization of the reaction conditions.[a]
ing as an alternative to traditional cross coupling reactions.[17]
Notably, the reported routes for the construction of heteroaryl-
fused azole sulfides require tedious reaction conditions and
long reaction times; hence, to overcome these limitations, the
present investigation was undertaken.
Recently, we reported an expedient approach for the synthe-
Entry Catalyst
(mol-%)
Ligand Solvent
Time Temp. Yield[b]
sis of fused thiazoles from organoborons and thiazolidine-2-
thiones under a desulfitative coupling strategy.[18] In continua-
tion of our enduring curiosity in the development of oxidative
C–S cross coupling reactions under mild conditions, we report
the synthesis of azole sulfides by direct C–S bond formation
between aryl/vinyl boronic acids and thiazolidine-2-thiones in
the presence of copper(II) acetylacetonate [Cu(acac)2] under
base-free and ligand-free condition (Scheme 1). The thiazol-
idine-2-thiones were first prepared from readily available 2-
haloanilines and potassium ethyl xanthate.[19]
[h]
[°C]
[%]
1
CuBr2 (10)
L1
L2
L2
L1
L3
L4
L1
L4
–
dioxane
dioxane
DCE
dioxane
THF
dioxane
DCE
DCE
8
8
16
5
5
5
100
100
80
80
80
80
80
80
80
80
80
80
50
r.t.
80
80
80
100
22
26
12
46
37
62
71
85
89
83
72
80
43
12
–
2
CuBr2 (10)
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Cu(NO3)2·3H2O (10)
CuSO4·5H2O (5)
CuSO4·5H2O (5)
CuSO4·5H2O (5)
Cu(OAc)2 (10)
Cu(OAc)2 (10)
Cu(acac)2 (20)
Cu(acac)2 (20)
Cu(acac)2 (20)
Cu(acac)2 (20)
Cu(acac)2 (20)
Cu(acac)2 (20)
Ni(acac)2 (20)
Fe(acac)2 (20)
Fe(acac)3 (20)
Pd(acac)2 (5)
2
2
DCE
0.5
0.5
0.5
0.5
12
24
8
8
24
16
–
–
dioxane
DMF
–
THF
–
DCE
Results and Discussion
–
DCE
To start, we inspected the C–S cross coupling of thiazolo[5,4-
b]pyridine-2(1H)-thione (1a, 1.0 equiv.) with a stoichiometric
amount of (3-methylphenyl)boronic acid (2a, 1.5 equiv.) in the
presence of a stoichiometric amount of cupric bromide (10 mol-
%) and 2,2′-bipyridine (10 mol-%) as a ligand in dioxane under
aerobic conditions at 100 °C. After 8 h, 2-thio-substituted thiaz-
ole 3c was obtained (22 %; Table 1, entry 1). The conversion
was not effective even with 1,10-phenanthroline as the ligand
(26 %; Table 1, entry 2). Other copper(II) salts and ligand sys-
tems were also examined, but the desired product was ob-
tained in low yields (Table 1, entries 3–6). An appreciable yield
was obtained with the combination of Cu(OAc)2 (10 mol-%) and
2,2′-bipyridine (10 mol-%) at 80 °C in 1,2-dichloroethane (DCE)
–
DCE
–
DCE
–
–
–
–
DCE
–
dioxane
[a] Reaction conditions: Thiazolidine-2-thione 1 (0.10 mmol), R–B(OH)2
(0.15 mmol), catalyst (5–20 mol-%), and ligand (5–20 mol-%) in solvent (5 mL)
heated for the specified time open to air. [b] Isolated yield.
2
Under the optimized conditions, we inspected the scope of
as the solvent, and full conversion was accomplished in 2 h this C–S coupling reaction in terms of the arylboronic acid to
under aerobic conditions (Table 1, entry 7). Upon using the obtain products 3, which were obtained in good yields, as de-
Cu(OAc)2 (10 mol-%)/acetylacetonate (10 mol-%) system, the picted in Table 2. Generally, reactions with arylboronic acids
yield increased to 85 % (Table 1, entry 8). To our delight, the having both electron-withdrawing and electron-donating
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© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
groups in the para, meta, or ortho position gave products 3a– (Table 2). It was ultimately found that electronic effects – either
h in excellent yields (74–89 %). In addition, the reaction was in the thione component or in the arylboronic acid – did not
investigated with heteroarylboronic acids, and products 3i–m affect the course of the reaction.
were afforded in decent yields (71–81 %). The coupling of thi-
azolo[5,4-b]pyridine-2(1H)-thione and (E)-ꢀ-styrylboronic acid
was also successfully achieved to give 3t in good yield (80 %).
The scope of the reaction was extended to simple aryl-, hete-
roaryl-, and fused heteroarylthiazolidine-2-thiones, and corre-
sponding cross-coupled azole sulfides 3n–u were obtained
However, the transformation was not fruitful with some func-
tionalized or sterically hindering arylboronic acids or alkyl-
boronic acids under the optimized conditions. All the synthe-
sized compounds were characterized by NMR spectroscopy,
and the structures of 3k[20] and 3p[20] were further confirmed
by single-crystal X-ray analysis (Figure 2).
Table 2. Synthesis of azole sulfides 3a–u.[a]
Figure 2. X-ray analysis of 3k (top) and 3p (bottom). Thermal ellipsoids are
shown at the 50 % probability level.
It is believed that the thioamide system is readily oxidized
to give disulfide 4 stimulated by the CuII reagent in the pres-
ence of air.[14] The arylboronic acid may then react with CuII
acetylacetonate through a transmetalation step to form CuII in-
termediate 5, which can react with disulfide 4 to give 3
(Scheme 2, path A). As an alternative possible mechanism, di-
sulfide 4 and intermediate 5 may undergo transmetalation with
the arylboronic acid to generate intermediate 6, which ulti-
mately delivers aryl sulfide 3 (Scheme 2, path B).
Scheme 2. Plausible mechanism.
The reaction of (4-methoxyphenyl)boronic acid with a 1,2-
diaryl disulfide under the optimized conditions resulted in thi-
azole sulfide 3a in good yield (Scheme 3); this result ensured
that the reaction proceeds through the formation of disulfide
intermediate 4.
[a] Reaction conditions: Thiazolidine-2-thione 1 (0.10 mmol), R–B(OH)2
(0.15 mmol) and Cu(acac)2 (20 mol-%) in dichloromethane (5 mL) heated at
80 °C for 30 min open to air.
2
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Communication
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Scheme 3. Reaction of a 1,2-diaryl disulfide with (4-methoxyphenyl)boronic
acid.
Conclusions
A simple and efficient protocol for the synthesis of a library
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Experimental Section
General Remarks: 1H NMR and 13C NMR spectra were recorded
with a 300 MHz spectrometer in CDCl3 by using tetramethylsilane
as an internal standard. Chemical shifts are reported in parts per
million (δ), coupling constants (J values) are reported in Hertz (Hz),
and spin multiplicities are indicated by the following symbols: s
(singlet), d (doublet), t (triplet), p (pentet), m (multiplet). 13C NMR
spectra were routinely run with broadband decoupling. Aluminum
plates precoated with silica gel (Merck) were used for TLC analysis
with a mixture of petroleum ether (60–80 °C) and ethyl acetate as
the eluent. Elemental analyses were performed with a Perkin–Elmer
2400 Series II Elemental CHNS analyzer. Mass spectra were recorded
with a LCQ Fleet spectrometer, Thermo Fisher Instruments Ltd., USA.
Electrospray ionization mass spectrometry (ESI-MS) analysis was
performed in the positive ion and negative ion modes with a liquid
chromatography ion trap.
[4]
Typical Procedure for the Synthesis of Fused Azole Sulfides 3:
R–B(OH)2 2 (0.15 mmol) and Cu(acac)2 (0.02 mmol, 20 mol-%) were
added to a stirred solution of thiazolidine-2-thione 1 (0.10 mmol)
in DCE (5 mL), and the mixture was heated at 80 °C for 0.5 h in
open air. Completion of the reaction was monitored by TLC. After
cooling to room temperature, the mixture was filtered through Cel-
ite. Then, the filtrate was washed with a saturated solution of NH4Cl
(3 × 10 mL) and extracted with ethyl acetate (2 × 20 mL). The com-
bined organic layer was dried with anhydrous Na2SO4 and concen-
trated in vacuo. The residue was then purified by column chroma-
tography on silica gel (hexanes/ethyl acetate, 4.5:0.5) to yield azole
sulfide 3.
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Keywords: Synthetic methods · Cross coupling · Copper ·
Sulfur heterocycles · Nitrogen heterocycles · Boronic acids
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Published Online: April 5, 2016
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© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim