Liebeskind-Srogl-type cross-coupling reaction of azole-2-thiones with triarylbismuthines: Synthesis of 2-arylazoles

Article abstract of DOI:10.1016/j.tetlet.2020.152152

Liebeskind-Srogl-type C(HetAr)–C(Ar) bond formation using trivalent organobismuth compounds as a new class of arylating reagents is described. The reaction of benzazole-2-thiones with triarylbismuthines in the presence of 10 mol% Pd(dba)₂ and 2.0 equiv. Cu(OAc)₂ at 80 °C affords 2-arylbenzothiazoles, benzoxazoles, and N-methyl benzimidazole in moderate-to-high yield. The reaction is sensitive to the electronic nature of triarylbismuthines: compounds bearing an electron-withdrawing group on the phenyl ring showed higher reactivity than those having an electron-donating group.

Full text of DOI:10.1016/j.tetlet.2020.152152

Tetrahedron Letters xxx (xxxx) xxx
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Tetrahedron Letters
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Liebeskind-Srogl-type cross-coupling reaction of azole-2-thiones with
triarylbismuthines: Synthesis of 2-arylazoles
⇑
Yuki Murata, Aki Terazoe, Misato Kiba, Yuki Kitamura, Mio Matsumura, Shuji Yasuike
School of Pharmaceutical Sciences, Aichi Gakuin University, 1-100 Kusumoto-cho, Chikusa-ku, Nagoya 464-8650, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Liebeskind-Srogl-type C(HetAr)–C(Ar) bond formation using trivalent organobismuth compounds as a
new class of arylating reagents is described. The reaction of benzazole-2-thiones with triarylbismuthines
in the presence of 10 mol% Pd(dba)2 and 2.0 equiv. Cu(OAc)2 at 80 °C affords 2-arylbenzothiazoles, ben-
zoxazoles, and N-methyl benzimidazole in moderate-to-high yield. The reaction is sensitive to the elec-
tronic nature of triarylbismuthines: compounds bearing an electron-withdrawing group on the phenyl
ring showed higher reactivity than those having an electron-donating group.
Received 12 April 2020
Revised 10 June 2020
Accepted 15 June 2020
Available online xxxx
Keywords:
Bismuth
Ó 2020 Elsevier Ltd. All rights reserved.
Cross-coupling reaction
Palladium catalyst
Thiazole
Triarylbismuthine
Introduction
Muthusubramanian et. al. developed the reaction of benzothia-
zole-2-thione with arylboron pinacol esters, for the synthesis of
Liebeskind-Srogl cross-coupling is
a
well-known reaction,
2-arylbenzothiazoles [10]. The reaction was catalyzed by a Pd
(OAc)2/Ph3P catalytic system and mediated by 2.0 equiv. CuTC
and 2.0 equiv. CsF. As described above, arylboronic acids, arylboron
pinacol esters, arylstannanes, and arylsiloxanes have been used as
aryl group donors in the Liebeskind-Srogl-type cross-coupling
reactions for achieving C(Het)–C(Ar) bond formation. With regard
to the development of a new arylating method, there is a continu-
ous demand. Organobismuth compounds are mostly nontoxic,
environmentally benign, and potential synthetic reagent candi-
dates [11–13]. Trivalent organobismuth compounds such as tri-
arylbismuth (Ar3Bi) have attracted considerable attention in
recent years, particularly because of their use in the Pd-catalyzed
C(Ar)–C(Ar or sp2) bond formation reactions with aryl or vinyl
halides [14–16]. For example, arylation of heterocycles has been
reported via the cross-coupling reaction of Ar3Bi with aryl halides
and aryl triflates containing aromatic heterocyclic rings, such as
thiophene, furan, benzofuran, pyridine, pyridazine, pyrimidine,
and pyrazine [17–25]. However, the synthesis of 2-arylbenzothia-
zoles and benzoxazoles using this method has not been reported
so far, primarily because of the low availability of the correspond-
ing heteroaryl halides and triflates. The only known synthetic
method is limited to Pd-catalyzed C–H arylation of benzazoles
with Ar3Bi, as reported by Nagarkar et. al. [26]. Moreover, Liebe-
skind-Srogl-type cross-coupling reactions of benzazole-2-thione
with Ar3Bi using the Pd catalyst and the Cu reagent for the
syntheses of 2-arylbenzazoles have not hitherto been reported.
widely used for carbon(C)–carbon(C) bond formation via desulfur-
ization. In 2000, Liebeskind and Srogl reported the pioneering
cross-coupling reaction of thioesters with boronic acids in the
presence of 1.0 mol% tris(dibenzylideneacetone)dipalladium
[Pd2(dba)3] and 1.6 equiv. Cu-thiophene-2-carboxylate (CuTC) to
form ketones [1]. Since then many such cross-coupling reactions
of thiole esters, (hetero) aryl thioethers, cyclic thioamides, and
thioalkynes with boronic acids, using a Pd catalyst and stoichio-
metric amount of Cu-carboxylate, have been extensively investi-
gated [2,3]. The concept was applied further to successfully
achieve desulfitative C(Het)–C(Ar) bond formation between vari-
ous heterocyclic thioamides and aryl donors such as arylboronic
acids [4–10]. For instance, in 2004, Kappe et. al. developed the
microwave-assisted Liebeskind-Srogl-type cross-coupling reaction
of cyclic thioamides with arylboronic acids, in the presence of
5.0 mol% tetrakis(triphenylphosphine)palladium [Pd(PPh3)4] and
3.0 equiv. CuTC for the preparation of 2-aryl-1,4-dihydropyrimidi-
nes [4]. They further reported that arylstannanes and arylsiloxanes
can also be utilized, instead of boron reagents, as aryl donors,
under similar reaction conditions [7]. In fact, in 2010, Suzenet et.
al. carried out the reaction of 2-thiouracil with arylstannanes using
5.0 mol% Pd(PPh3)4 and 2.2 equiv. CuBrꢀMe2S without using
microwave irradiation to give 2-aryluracil [8]. In 2015,
⇑
Corresponding author.
E-mail address: s-yasuik@dpc.agu.ac.jp (S. Yasuike).
https://doi.org/10.1016/j.tetlet.2020.152152
0040-4039/Ó 2020 Elsevier Ltd. All rights reserved.
Please cite this article as: Y. Murata, A. Terazoe, M. Kiba et al., Liebeskind-Srogl-type cross-coupling reaction of azole-2-thiones with triarylbismuthines:
Synthesis of 2-arylazoles, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.152152

2
Y. Murata et al. / Tetrahedron Letters xxx (xxxx) xxx
Recently, we developed the Cu-catalyzed S-arylation of benza-
selectively proceed with either C–C or C–S bond formation,
depending upon the transition metal used along with the bismuth
reagent.
zole-2-thione using Ar3Bi as the aryl donor to synthesize 2-(aryl-
sulfenyl)azoles [27]. Inspired by the aforementioned reports and
following our continuing efforts for investigating the coupling
reaction of heteroazole-2-thione with Ar3Bi [26], we present in this
paper a Pd-catalyzed Cu-mediated desulfitative C(HetAr)–C(Ar)
bond formation reaction using azole-2-thione and Ar3Bi. This reac-
tion does not require an additive to activate the bismuth reagent
and is a novel method applicable to the synthesis of 2-arylbenza-
zoles, which have been rarely used in the previously known Liebe-
Result and discussion
Initially, we focused on identifying the optimum experimental
conditions for the synthesis of 2-phenylbenzothiazole (8) by cou-
pling benzothiazole-2-thione (1a) with organobismuth or
organoantimony compounds (2a-7). The results, which include
screening for suitable Pd catalysts, Cu reagents, solvents, and aryl
donors, are summarized in Table 1. First, we performed reactions
of benzothiazole-2-thione (1a) (0.5 mmol) with Ph3Bi (2a)
(0.5 mmol) using several available Pd catalysts (0.05 mmol) along
with Cu(OAc)2 (1.0 mmol) in DMF at 80 °C under argon atmosphere
(entries 1–6). Pd(dba)2 was found to be most efficient in catalyzing
the reaction, considering factors such as expected coupling product
(8) yield (86%), reaction time (3 hr), and by-product yield [biphenyl
(9) 9%, respectively]. It was also observed that coupling between 1a
and 2a did not occur when the reaction was carried out in absence
of any Pd catalyst (entry 7). Next, various Cu reagents were
skind-Srogl reaction. The reactions shown in Scheme
1 can
Scheme 1. C–S and C–C bond formation of benzazole-2-thione using organobis-
muth reagent.
Table 1
Pd-catalyzed reaction of organobismuth and antimony compounds with benzothiazole-2-thione.a
.
Entry
1
Change from standard conditions
Time (hr)
3
Yield (%)b
8
9
None
86 (80)c
9
Palladium catalyst instead of Pd(dba)2
2
3
4
5
6
7
Pd(PPh3)4
Pd(OAc)2
PdCl2
PdCl2(MeCN)2
5.0% Pd-C
Without Pd cat.
24
24
24
24
24
24
47
64
22
52
8
36
23
13
20
5
—
—
Copper reagent instead of Cu(OAc)2
8
9
CuCl2
CuO
CuOAc
CuTC
CuCl
Cu2O
Without Cu reagent
Solvent instead of DMF
NMP
DMSO
Toluene
1,4-Dioxane
MeCN
1,2-DCE
EtOH
24
24
24
3
3
24
24
9
2
9
3
19
8
4
10
11
12
13
14
39
44
16
—
3
3
6
15
16
17
18
19
20
21
3
3
3
3
24
24
24
83
73
79
79
16
—
21
26
19
19
28
19
13
16
Arylating reagent instead of Ph3Bi (2a)
22
23
24
25
26
27
28
29
30
31
32
33
Ph3BiCl2 (3)d
24
24
24
24
24
24
6
35
64
—
—
—
54
44
42
76
25
29
23
24
16
11
—
Ph3Bi(OAc)2 (4)d
Ph3Sb (5)d
Ph3SbCl2 (6)d
Ph3Sb(OAc)2 (7)d
At 60 °C instead of 80 °C
Under O2 instead of Ar
Under air instead of Ar
Pd(dba)2 (5.0 mol%)
Cu(OAc)2 (30 mol%)
1a (0.75 mmol), 2a (0.25 mmol)e
1a (0.75 mmol), 2a (0.25 mmol), Cs2CO3 (1.0 mmol)e
—
20
26
26
21
29
16
9
6
24
24
24
24
a
b
c
Conditions: 1a (0.5 mmol). 2a (0.5 mmol), Pd cat. (0.05 mmol), Cu reagent (1.0 mmol).
GC yield using dibenzyl as internal standard. The yield 100% corresponds to the formation of 0.5 mmol of 8, 0.75 mmol of 9.
Isolated yield.
0.5 mmol of triphenylpnictogen reagent 3–7.
The yield 100% corresponds to the formation of 0.75 mmol of 8.
d
e
Please cite this article as: Y. Murata, A. Terazoe, M. Kiba et al., Liebeskind-Srogl-type cross-coupling reaction of azole-2-thiones with triarylbismuthines:
Synthesis of 2-arylazoles, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.152152

Y. Murata et al. / Tetrahedron Letters xxx (xxxx) xxx
3
Table 2
screened (entries 1, 8–13), keeping the Pd catalyst and other
Substrate scope reaction of triarylbismuthines with azole-2-thiones.a,b
parameters of standard conditions (entry 1) unchanged. Among
all the Cu reagents, Cu(OAc)2 was found to be the most effective
one. CuTC is reported to be a widely used copper reagent in Liebe-
skind-Srogl reactions [4–7,10]. However, we observed that Cu
(OAc)2, which is more easily available, showed much better results
in case of the reactions, we carried out, using bismuth reagent. The
results of solvent screening showed that the reaction between 1a
and 2a, using Pd(dba)2 and Cu(OAc)2, proceeded effectively in
DMF (86%), NMP (83%), toluene (79%), 1,4-dioxane (79%), and
DMSO (73%) (entries 1, 15–18), whereas MeCN (16%), and EtOH
(16%) exhibited inferior results (entries 1, 19–21).
Next, coupling of 1a was performed with various aryl donors
(2a, 3–7), containing bismuth and antimony, in DMF at 80 °C, using
Pd(dba)2 catalyzed and Cu(OAc)2 mediated conditions (entries 1,
22–26). The reaction proceeded smoothly with the trivalent
organobismuth compound (2a) (entry 1), whereas the use of pen-
tavalent Ph3BiCl2 (3) and Ph3Bi(OAc)2 (4) not only decreased the
yield of 8 but also necessitated extended reaction times (entries
22–23). Moreover, all antimony reagents such as Ph3Sb (5), Ph3-
SbCl2 (6), and Ph3Sb(OAc)2 (7) failed to undergo coupling with
1a. These results indicate that the reactivity of triphenylpnictogen
reagents are considerably affected by the constitutive metal (Bi, Sb)
as well as their valencies [(III), (V)].
We also investigated the effect of reducing reaction tempera-
ture by performing the coupling reaction between 1a and 2a at
60 °C and keeping all other parameters of standard conditions
unchanged (entry 27). In this case, although 8 was formed in 54%
yield, reaction time increase to 24 hr. A significant decrease in
the yield of product 8 and increase in that of byproduct 9 were
observed when coupling was carried out under O2 and air instead
of Ar (entries 28, 29). Decreasing the catalyst [Pd(dba)2] loading
from 10 to 5.0 mol% resulted in decreased yield of 8 and extended
the reaction time (entry 30). Similar observations were also made
when the catalytic amount of Cu(OAc)2 (30 mol%) was used instead
of 2.0 equiv., the standard amount (entry 31).
After screening various reagents and changing reaction condi-
tions, we observed that the best result was obtained when 1a
was treated with Ph3Bi (2a) in the presence of Pd(dba)2 (10 mol
%) and Cu(OAc)2 (2.0 equiv.) in DMF at 80 °C under argon atmo-
sphere. As Ph3Bi (2a) has three phenyl groups, the coupling reac-
tion was performed using a 3: 1 molar ratio of benzothiazole-2-
thione (1a: 0.75 mmol) and Ph3Bi (2a: 0.25 mmol). However, the
yield was low (29%), indicating that only one of the three phenyl
groups on bismuth is involved in C(HetAr)–C(Ar) bond formation
under the present reaction conditions (entry 32). In Pd-catalyzed
cross-coupling reactions using triarylbismuthine, bases are widely
used for the activation of bismuth reagents [17–25]. However,
when we performed coupling of 1a and 2a using Cs2CO3 as a base,
we did not observe any yield improvement (entry 33).
In order to investigate the efficiency and substrate scope of the
Liebeskind-Srogl-type reaction described in this work, we per-
formed reactions of various azole-2-thiones (1a-l) (0.5 mmol) with
Ar3Bi (2a-k) (0.5 mmol) under the previously optimized conditions
(Table 2). Reactions of 1a with various Ar3Bi (2b-g) proceeded suc-
cessfully to achieve C(HetAr)–C(Ar) bond formation. However, the
yield of the coupling products was observed to be dependent on
the electronic nature of the substituents on the phenyl rings of Ar3-
Bi. For instance, Ar3Bi bearing a methyl group, halogens, and elec-
tron-withdrawing groups such as ethoxycarbonyl and
trifluoromethyl on the phenyl rings produced coupling products
(11–15) in good-to-high yield. In contrast, Ar3Bi, containing strong
electron-donating groups, such as methoxy, produced coupling
product 10 in low yield. Sterically hindered ortho-substituted Ar3-
Bi (2 h-j) was coupled with 1a, affording the respective coupling
products (16–18, respectively) with relatively lower yield, except
a1a-l (0.5 mmol), 2a-k (0.5 mmol), Pd(dba)2 (0.05 mmol), Cu(OAc)2 (1.0 mmol).
bIsolated yield based on one aryl group in triarylbismuthines.
for the ortho-methyl derivative (16). The reaction of 1a with 2k
having a thienyl group generated 19 in lower yield. Next, the reac-
tion of various benzothiazole-2-thiones (1b-d), bearing electron-
donating or electron-withdrawing groups at the 5-position, was
performed with 2a, affording the corresponding coupling products
(20–22) in high-to-excellent yield. Additionally, the reaction of
various other benzoxazole-2-thiones (1e-h) with 2a generated
the corresponding oxazoles (23–26, respectively) without any dif-
ficulty. Similarly, the reaction of thiazolopyridine-2-thione (1i) and
2a afforded 27, but in moderate yield. Moreover, the reaction of N-
methylbenzimidazole-2-thione (1j) with 2a produced 28 in 51%
yield. On the other hand, the reaction using a benzimidazole-2-
thione (1k) instead of 1j yielded a complex mixture, and the
excepted coupling product (29) was not obtained. Monocyclic thi-
azole-2-thione (1l) reacted with 2a to afford the desired product 30
in 56% yield.
At present, the reaction mechanism of the C(HetAr)–C(Ar) bond
formation, reported in this paper, is unclear. We presume that the
Please cite this article as: Y. Murata, A. Terazoe, M. Kiba et al., Liebeskind-Srogl-type cross-coupling reaction of azole-2-thiones with triarylbismuthines:
Synthesis of 2-arylazoles, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.152152

4
Y. Murata et al. / Tetrahedron Letters xxx (xxxx) xxx
zothiazole, benzoxazole, and N-methylbenzimidazole. Detailed
mechanistic studies of the above-mentioned arylation and the
reaction of triarylbismuthines (Ar3Bi) with other coupling partners
are currently under progress.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgments
This research was supported by a research grant from Nagai
Memorial Research Scholarship from the Pharmaceutical Society
of Japan (Y. K.) and JSPS KAKENHI (Grant Number JP19K07005)
(S. Y.). The authors also thank Institute of Pharmaceutical Life
Sciences, Aichi Gakuin University for generous financial support.
Scheme 2. Possible mechanism.
Appendix A. Supplementary data
mechanism would be similar to that of Liebeskind-Srogl cross-cou-
pling between cyclic thioamides and arylboronic acids, as proposed
by Kappe et. al. [4,5]. A possible mechanism for the present cou-
pling reaction, considering the reports by Muthusubramanian
[10], is shown in Scheme 2. The initial step is proposed to be the
reaction of azole-2-thione with Cu(OAc)2 to form a complex (A)
via thione-thiol tautomerization [28]. Subsequently, oxidative
addition of the Ar(Het)–S bond of A occurs onto the Pd(0) species
followed by addition of another equivalent of a Cu(OAc)2 cofactor
to form an intermediate (B). Then, transmetallation of B with Ar3-
Bi generates Ar(Het)-Pd-Ar (D) accompanied by elimination of Ph2-
Bi(OAc) and Cu2S via another intermediate (C). In C, the
coordination between the bismuth and oxygen of the acetoxy moi-
ety assists the transmetallation. It is known that bismuth shows
affinity for oxygen [29], and several reactions based on the bis-
muth-oxygen affinity have been developed to date [12]. Therefore,
an activator of the bismuth reagent, such as a base, which is widely
used in cross-coupling reactions, is considered unnecessary in this
case. Finally, complex D undergoes reductive elimination to give
the coupling product and regenerates the Pd (0) species. On the
other hand, intermediate C and Ar2Bi(OAc), expected to be pro-
duced in this process, have not be confirmed or isolated at this
point.
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2020.152152.
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Conclusion
We have reported triarylbismuthines as a new class of arylating
agents for the Liebeskind-Srogl-type cross-coupling reactions of
azole derivatives. Trivalent organobismuth compounds with vari-
ous electron-donating and electron-withdrawing functional groups
afforded the corresponding 2-arylazoles in moderate-to-high yield
through Pd-catalyzed and Cu-mediated desulfitative C(HetAr)–C
(Ar) bond formation under mild conditions. The characteristics of
this reaction are that no additives other than Pd and Cu reagents
are required to activate triarylbismuthines, and aryl groups can
be introduced at the 2-position of various benzazole such as ben-
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Please cite this article as: Y. Murata, A. Terazoe, M. Kiba et al., Liebeskind-Srogl-type cross-coupling reaction of azole-2-thiones with triarylbismuthines:
Synthesis of 2-arylazoles, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.152152