NH4I-promoted oxidative formation of benzothiazoles and thiazoles from arylacetic acids and phenylalanines with elemental sulfur

Article abstract of DOI:10.1039/d1ob00671a

A NH₄I/K₃PO₄-based catalytic system has been established to enable oxidative formation of thiazole compounds from arylacetic acids and phenylalanines with elemental sulfur. While the three-component reaction of anilines or β-naphthylamines with arylacetic acids and elemental sulfur affords benzo[2,1-d]thiazoles and naphtho[2,1-d]thiazoles, the annulation of phenylalanines with elemental sulfur produces 2-benzyl and 2-benzoylthiazoles. This work well complements previous three-component annulations of benzothiazoles from other coupling partners.

Full text of DOI:10.1039/d1ob00671a

Organic &
Biomolecular Chemistry
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NH4I-promoted oxidative formation of
benzothiazoles and thiazoles from arylacetic acids
and phenylalanines with elemental sulfur†
Cite this: Org. Biomol. Chem., 2021,
19, 5108
Yujia Xia, Huawen Huang, * Wei Hu and Guo-Jun Deng
*
A NH4I/K3PO4-based catalytic system has been established to enable oxidative formation of thiazole
compounds from arylacetic acids and phenylalanines with elemental sulfur. While the three-component
reaction of anilines or β-naphthylamines with arylacetic acids and elemental sulfur affords benzo[2,1-d]
thiazoles and naphtho[2,1-d]thiazoles, the annulation of phenylalanines with elemental sulfur produces
2-benzyl and 2-benzoylthiazoles. This work well complements previous three-component annulations of
benzothiazoles from other coupling partners.
Received 7th April 2021,
Accepted 12th May 2021
DOI: 10.1039/d1ob00671a
rsc.li/obc
example, aniline compounds exhibited low reactivity, even at a
very high reaction temperature (150 °C). Hence, to improve the
Introduction
Structurally privileged thiazole compounds have widely been reactivity of C–H sulfuration of anilines for thiazole annula-
exploited in pharmaceuticals,1 naturally occurring products,2 tions, we speculate that a variant of the aldehyde coupling
and functional materials.3 These practical applications have partner as a C1 source as well as a new catalytic systems may
intensively triggered the structural and synthetic development be exploited. Inspired by the multicomponent decarboxylative
of this skeleton motif. While various cyclization reactions have reaction of sulfur, carboxylic acids, and amines,9 herein, we
been discovered to construct the key thiazole ring,4 more
specifically, traditional methods to synthesize benzothiazoles
mainly depend on condensation and coupling reactions of pre-
functionalized ortho disubstituted arenes such as ortho-halo
anilines or nitroarenes (Scheme 1(a)) and 2-aminobenze-
nethiols (Scheme 1(b)).5 With user-friendly elemental sulfur as
a building block for sulfur–heterocycle formation,6 our group
and others have developed direct C–H sulfuration of
β-naphthylamines and anilines that leads to highly efficient
annulations of 2-arylbenzothiazoles and 2-arylnaphthothia-
zoles under facile transition-metal-free conditions,7 in which
benzaldehydes,7a,f methylheteroarenes,7b,c benzylamines,7e
and acetophenones7g were all effective C1 sources
(Scheme 1(c)). Our reaction with benzaldehyde was recently
used to produce stable thiazole-linked material molecules of
covalent organic frameworks (COFs), which were found to
feature high sacrificial hydrogen evolution rates from water
under photocatalysis.8 Although significant utility has been
primarily established, this method still has limitations. For
Key Laboratory for Green Organic Synthesis and Application of Hunan Province, Key
Laboratory of Environmentally Friendly Chemistry and Application of Ministry of
Education, College of Chemistry, Xiangtan University, Xiangtan 411105, China.
E-mail: gjdeng@xtu.edu.cn, hwhuang@xtu.edu.cn; Fax: (+86)0731-58292251;
Tel: (+86)0731-58298601
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1ob00671a
Scheme 1 Methods for benzothiazole formation.
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explore phenylacetic acids as the coupling partner for the elemental sulfur to form S3 radical anions (S3•−).11 Hence, a
efficient thiazo-annulation of anilines (Scheme 1(d)). Further series of inorganic bases were screened (entries 3–7). Of them,
studies reveal that the corresponding amino acids could play while NaOEt completely quenched the annulation reaction,
the same role of a C1 source, while phenylalanines undergo others including K2CO3, K3PO4, NaOH, and Cs2CO3 were all
two-component thiazo-annulation with elemental sulfur to effective and K3PO4 gave the best result (82% yield, entry 4).
produce two kinds of 2,5-disubstituted thiazole products Then, the solvent effect was explored. DMSO was found to be
(Scheme 1(e)). In 2018, Wu and coworkers developed an critical to the C–H sulfuration reaction. While PhCl enables
iodine-promoted cyclization reaction of phenylalanine with the reaction with 20% yield (entry 8), other solvents such as
Na2S as the sulfur source, affording only benzoyl thiazole pro- NMP, 1,4-dioxane, and toluene did not work at all (entries
ducts while elemental sulfur did not work in that system 10–12). As previously obtained in the case of aldehydes as the
(Scheme 1(f)).10
coupling partner, the combinational use of DMSO and PhCl
(v/v = 1 : 3) proved superior (entry 9). These results indicate
that DMSO probably plays a dual role of both a solvent and an
oxidant or an activating reagent. Unexpectedly, elemental
iodine did not work or prohibit the present annulation reac-
tion (entry 13) because the reaction in the absence of a catalyst
also afforded 3a in moderate yield (entry 14). The reaction
yield was slightly declined when reducing the catalyst loading
to 5 mol% (entry 15). Notably, the reaction yield was dramati-
cally decreased either at lowered or elevated temperature
(entries 16 and 17). Finally, oxygen in air proved to be helpful
since an inert atmosphere reduced the yield to 77% (entries 18
and 19).
Results and discussion
In related reports, sp2 C–H sulfuration with elemental sulfur
occurred normally with electron-rich arenes,7 thereby electro-
philic substitution was mechanistically proposed. To com-
mence our study, 3,4-dimethoxyaniline (1a) and phenylacetic
acid (2a) were used as model substrates to optimize the reac-
tion conditions (Table 1). In the absence of a base, the KI/NMP
system, which has been demonstrated to be effective with ben-
zaldehydes, did not work for the present cyclization (entry 1).
As a comparison, the NH4I/DMSO/PhCl-based catalytic system
gave modest yield of the desired benzothiazole product 3a
(entry 2). The addition of a base could facilitate the decarboxyl-
ation process of carboxylic acids as well as activation of
With the optimal reaction conditions in hand, the substrate
scope of the three-component annulation reaction was probed
(Scheme 2). Anilines bearing electron-donating functional
groups including methoxy and tert-butyl were smoothly
cyclized, generating the corresponding benzothiazoles in good
to excellent yields (3a–3f, 67%–91% yields). Electron-deficient
anilines only afforded the possible imine and thioamide inter-
mediate products under the present catalytic system. As
expected, β-naphthylamines as well as β-anthramine featured
high reactivity (3g–3j, 74%–97% yields), with bromo function-
ality well tolerated (3h). Quinolin-6-amine and quinolin-3-
amine were also productive, albeit in low yields. Subsequently,
with β-naphthylamine as the coupling partner the generality of
arylacetic acids was explored. Generally, substituted phenylace-
tic acids produce the target 2-arylnaphtho[2,1-d]thiazoles in
moderate to excellent yields (3m–3v, 45%–88% yields),
although their yields are to some extent lower than those of
our previous systems with aldehydes. A broad range of useful
functional groups such as nitro, methoxyl, fluoro, chloro, and
bromo are all compatible with the current system. 2-
(Naphthalen-2-yl)acetic acid worked well to afford 3w in 82%
yield. Other fused arylacetic acids with tetrahydrofuran and
the dioxolane moiety delivered the corresponding naphtho
[2,1-d]thiazoles 3x and 3y in 51% and 85% yields, respectively.
The reactivity of heteroarylacetic acids was exemplified by pyri-
dinyl and thienyl coupling partners, which furnished the
expected biheteroarene products (3z 84%, 3aa 48%). Notably,
isoxepac, a non-steroidal anti-inflammatory drug, was also an
effective substrate, generating the structurally significant
naphthothiazole-attached dibenzo[b,e]oxepin-11(6H)-one 3ab.
Amino acids are readily available biomass and have been
widely exploited as building blocks in molecule synthesis. To
our delight, 2-amino-2-phenylacetic acid (2a′) could be
Table 1 Optimization of reaction conditionsa
Entry
Catalyst (mol%)
Base
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
KI (20)
NMP
DMSO/PhCl
DMSO
DMSO
DMSO
DMSO
DMSO
PhCl
ND
55
71
82
65
—
70
20
91
—
NH4I (10)
NH4I (10)
NH4I (10)
NH4I (10)
NH4I (10)
NH4I (10)
NH4I (10)
NH4I (10)
NH4I (10)
NH4I (10)
NH4I (10)
I2 (10)
K2CO3
K3PO4
NaOH
NaOEt
Cs2CO3
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
9
DMSO/PhCl
1,4-Dioxane
NMP
10
11
12
13
14
15
16c
17d
18e
19f
—
—
Toluene
DMSO/PhCl
DMSO/PhCl
DMSO/PhCl
DMSO/PhCl
DMSO/PhCl
DMSO/PhCl
DMSO/PhCl
Trace
60
88
64
50
90
77
—
NH4I (5)
NH4I (10)
NH4I (10)
NH4I (10)
NH4I (10)
a The reactions were performed with 1a (0.2 mmol), 2a (0.2 mmol), S8
(1.0 mmol, 32 g mol−1), catalyst (mol%), and base (1.0 equiv.) in
solvent (1 mL) under air for 16 h. b Yields of isolated product 3a are
given. c 140 °C. d 100 °C. e Charged with O2 (sealed tube). f Charged
with Ar.
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Scheme 3 Reactivity of amino acids.
iodine were all accommodated with the NH4I/K3PO4 system
(Scheme 4).
To gain mechanistic insight into the sulfuration/annula-
tions with arylacetic acids, some control experiments were
carried out (Scheme 5). When the co-solvent DMSO was
removed, the reaction of β-naphthylamine 1g and phenylacetic
acid 2a afforded 3g in only 20% yield, along with the possible
imine intermediate product 7a (28%). This result suggests that
DMSO could promote both oxidative decarboxylation and
thiazo-annulation (Scheme 5a). Both 1g and 2a were recovered
in the absence of elemental sulfur, revealing oxidative de-
carboxylation enabled by elemental sulfur (Scheme 5b). The
imine 7a smoothly transformed into a naphtho[2,1-d]thiazole
product (Scheme 5c), while only a trace amount of product was
detected when N-(naphthalen-2-yl)benzothioamide was
employed (Scheme 5d). Finally, the C–H sulfuration and
thiazo-annulation of β-naphthylamine also occurred in the
absence of phenylacetic acid 2a, where DMSO probably served
Scheme 2 Substrate scope of benzothiazole formation (a yields with
the use of benzaldehyde instead of phenylacetic acid are given in
parentheses).
employed to replace phenylacetic acid in our three-component
cyclization reaction. The reaction with β-naphthylamine under
standard conditions afforded 2-phenylnaphtho[2,1-d]thiazole 3g
in 65% yield, with highly enhanced reactivity under an oxygen
atmosphere (86% yield) (Scheme 3a). Unexpectedly, when
phenylalanine (4a) was used, the β-naphthylamine component
did not participate in the reaction. Instead, two molecules of 4a
reacted with elemental sulfur to produce benzyl- and benzoyl
thiazole products 5a and 6a, respectively (Scheme 3b).
Previously, Wu and coworkers developed an iodine-promoted
cyclization reaction of phenylalanine with Na2S as the sulfur
source, affording only benzoyl thiazole products while elemen-
tal sulfur did not work in that system.10 Complementary to this
work, we modified the reaction conditions for both thiazole for-
mations. Reducing the amount of K3PO4 seemed to be the key
for the formation of the “non-oxidative” product 5a. Notably,
prolonged reaction time could lead to significant formation of
2-benzoylthiazoles. Then, the use of copper catalyst instead of
NH4I completely turned the chemoselectivity over the 2-benzoyl
product (Scheme 3c and d).
Thereafter, we explored the substrate scope of benzylthia-
zole formation by using a range of substituted phenylalanines.
Generally, the 2-benzoylthiazole products were formed in mod-
erate yields. Halogens including fluoro, chloro, bromo, and Scheme 4 Substrate scope of benzylthiazole formation.
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acid B.12 The intermediate B condenses with arylamine fol-
lowed by decarboxylation to afford imine C. Then, ortho elec-
trophilic sulfuration of C with elemental sulfur leads to inter-
mediate D, with a sequential deprotonation/nucleophilic cycli-
zation/protonation process to give dihydrobenzothiazole E.
Finally, oxidative dehydrogenative aromatization of E furnishes
benzothiazole product 3. With regard to the thiazole formation
from phenylalanines (Scheme 6b),11 the initial step should be
oxidative decarboxylation of 4, which probably produces imine
or ethanethial F. Condensation of F with another molecule of
phenylalanine gives enamine intermediate G. Again,
Willgerodt–Kindler-type sulfuration of G with elemental sulfur
happens to generate polysulfur intermediate H, with tautomer-
ism to form imine I. Subsequently, intramolecular nucleophi-
lic attack of sulfur anions on imine moieties, along with oxi-
dative decarboxylation, delivers dihydrothiazole J, with the
final dehydrogenative oxidation to form 2-benzyl-5-arylthiazole
product 5.
Conclusions
In summary, we have developed NH4I-promoted benzothiazole
formation from anilines, arylacetic acids, and elemental
sulfur. This protocol provides an alternative access to valuable
benzo- and naphtho[2,1-d]thiazole compounds from in-
expensive and bench-stable starting materials. Arylacetic acids
could also be replaced by the corresponding amino acid
biomass. Moreover, benzyl- and benzoyl thiazole products
were unexpectedly produced when phenylalanine was used,
which well complements the iodine-promoted benzoylthiazole
formation with Na2S.
Scheme 5 Control experiments.
as the C1 source to enable the formation of naphtho[2,1-d]thia-
zole 7b (Scheme 5e).
On the basis of our experimental results and literature
reports,7a,9a we proposed a possible reaction mechanism of
thiazo-annulations (Scheme 6). First, the deprotonation of phe-
nylacetic acid 2 occurs to give acetate A, which undergoes
Willgerodt–Kindler-type sulfuration to generate thioxoacetic
Experimental
General information
All reactions were carried out under an air atmosphere.
Column chromatography was performed using silica gel
(200–300 mesh) or thin layer chromatography was performed
using silica gel (GF254). 1H NMR and 13C NMR spectra were
recorded on a Bruker-AV (400, 100 and 376 MHz, respectively)
instrument internally referenced to tetramethyl silane (TMS)
or chloroform signals. Mass spectra were measured on an
Agilent 5977 GC-MS instrument (EI). High-resolution mass
spectra (HRMS) were recorded on an Agilent 6230 TOF LC/MS.
The structures of known compounds were further corroborated
1
by comparing their H NMR, 13C NMR data and MS data with
those of the literature. Melting points were measured with a
YUHUA X-5 melting point instrument and were uncorrected.
Starting materials were obtained from commercial suppliers
and used without further purification.
General procedure for the synthesis of benzothiazoles
A 10 mL reaction vessel was charged with NH4I (0.04 mmol,
Scheme 6 Possible reaction mechanism.
0.2 equiv.), K3PO4 (0.2 mmol, 1.0 equiv.), arylamine (1,
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0.2 mmol, 1.0 equiv.), aryl acetic acid (2, 0.3 mmol, 1.5 equiv.),
and sulfur (1.0 mmol, 5.0 equiv.). DMSO (0.25 mL) and chloro-
benzene (0.75 mL) were added to the sealed reaction vessel
using a syringe. The resulting solution was stirred at 120 °C for
16 h. The mixture was then allowed to cool down to room
temperature and flushed through a short column of silica gel
with ethyl acetate. After rotary evaporation, the residue was
purified by column chromatography (silica gel, petroleum
ether/EtOAc = 10 : 1 to 20 : 1) to give benzothiazoles 3.
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General procedure for the synthesis of 2,5-disubstituted
thiazoles
A 10 mL reaction vessel was charged with NH4I (0.04 mmol,
0.2 equiv.), K3PO4 (0.1 mmol, 0.5 equiv.), phenylalanine (4,
0.4 mmol, 2.0 equiv.), and sulfur (1.0 mmol, 5.0 equiv.). DMSO
(1.0 mL) was added to the sealed reaction vessel using a
syringe. The resulting solution was stirred at 120 °C for 16 h.
The mixture was then allowed to cool down to room tempera-
ture and flushed through a short column of silica gel with
ethyl acetate. After rotary evaporation, the residue was purified
by column chromatography (silica gel, petroleum ether/EtOAc
= 10 : 1 to 20 : 1) to give 2,5-disubstituted thiazoles 5.
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
Support from the National Natural Science Foundation of
China (21871226 and 22071211), the Science and Technology
Planning Project of Hunan Province (2019RS2039), the Hunan
Provincial Natural Science Foundation of China (2020JJ3032),
and the Collaborative Innovation Center of New Chemical
Technologies for Environmental Benignity and Efficient
Resource Utilization is gratefully acknowledged.
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