AIBN-Promoted Synthesis of Bibenzo[ b][1,4]thiazines by the Condensation of 2,2′-Dithiodianiline with Methyl Aryl Ketones

Article abstract of DOI:10.1021/acs.orglett.8b01238

A series of bibenzo[b][1,4]thiazines with various functional groups has been synthesized by a free-radical condensation reaction. Bibenzo[b][1,4]thiazines were obtained in moderate to good yield (up to 85%) through a one-step reaction of readily available 2,2′-dithiodianiline and methyl aryl ketones with AIBN as radical initiator in HOAc. Bibenzo[b][1,4]thiazines exhibit diversiform solid-state packing.

Full text of DOI:10.1021/acs.orglett.8b01238

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
AIBN-Promoted Synthesis of Bibenzo[b][1,4]thiazines by the
Condensation of 2,2′-Dithiodianiline with Methyl Aryl Ketones
Xianqiang Huang,† Nianxin Rong,† Pengfei Li,*,‡ Guodong Shen,† Qiang Li,† Nana Xin,†
Chuansheng Cui,† Jichui Cui,† Bingchuan Yang,† Dacheng Li,† Changqiu Zhao,† Jianmin Dou,*,†
and Bo Wang*,‡
†Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, Institution of Functional Organic
Molecules and Materials, School of Chemistry & Chemical Engineering, Liaocheng University, Liaocheng 252059, P. R. China
‡Advanced Research Institute of Multidisciplinary Science, Key Laboratory of Cluster Science Ministry of Education, Beijing Key
Laboratory of Photoelectronic/Electrophotonic, School of Chemistry & Chemical Engineering, Beijing Institute of Technology,
Beijing 100081, P. R. China
S
* Supporting Information
ABSTRACT: A series of bibenzo[b][1,4]thiazines with various functional groups has been synthesized by a free-radical
condensation reaction. Bibenzo[b][1,4]thiazines were obtained in moderate to good yield (up to 85%) through a one-step
reaction of readily available 2,2′-dithiodianiline and methyl aryl ketones with AIBN as radical initiator in HOAc.
Bibenzo[b][1,4]thiazines exhibit diversiform solid-state packing.
enzothiazines are a class of heterocyclic compounds
containing both nitrogen and sulfur in the six-membered
systems have also been used to the visual detection of
peroxides and proton switched “OR” logic gate.10
B
thiazine ring, including benzo[b][1,2]thiazines,1 benzo[b][1,3]-
thiazines,2 and benzo[b][1,4]thiazines3 (Scheme 1). With their
The earliest synthesis of benzo[b][1,4]thiazines was reported
by Unger in the late 19th century.11 Later on, it was found that
the actual structure is bibenzo[b][1,4]thiazine due to the high
instability of 2H-benzo[b][1,4]thiazine.12 In light of the
potential biological and dye chemistry applications, several
synthesis methods of bibenzo[b][1,4]thiazine have been
developed, including oxidative coupling of 3-phenyl-2H-1,4-
benzothiazine with highly explosive picric acid under reflux,10,13
anodic oxidation with low yield,14 and the condensation of 2-
aminothiophenol with 3-phenyl-5(4H)-isoxazolone,15 α,α-
dibromoacetophenone,16 or 1,2-diaroylacetylene with silica-
supported perchloric acid.17 To the best of our knowledge, the
synthesize of bibenzo[b][1,4]thiazine derivatives with readily
available precursors is seldom reported. Herein, we report a
serendipitously discovered method by an AIBN-promoted
radical coupling of 2,2′-dithiodianiline with methyl aryl ketones.
The unexpected reaction was discovered while we were
attempting the synthesis of a series of 2,2′-dithiodianiline-based
Schiff base ligands. When 2,2′-dithiodianiline (1a, 1 equiv) was
condensed with acetophenone (2a, 2.1 equiv) in HOAc (2 mL)
at 80 °C, the target Schiff base was not obtained. Instead,
Scheme 1. Structure of Benzo[b]thiazines and Related
Compounds
unique structure, benzo[b][1,4]thiazine derivatives have
stimulated considerable interest in recent years due to their
remarkable biological and pharmaceutical activities, such as
antimalarial activity,4 antimicrobial agents,5 nervous system
depressants,6 anticancer properties,7 and antagonists.8 Addi-
tionally, benzo[b][1,4]thiazine is the major structural compo-
nents of trichochrome C (Scheme 1) and more complex
pheomelain pigments found in red hairs and feathers.9
Recently, benzo[b][1,4]thiazines containing chromogenic
Received: April 18, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01238
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
bibenzothiazine 3a was isolated in 8% yield with most of the
starting materials recovered (Table 1, entry 1). Inspired by a
sensitive, and the yield is nearly the same under dark (Table 1,
entry 13). Prolonging the reaction time to 28 h did not improve
the yield (Table 1, entry 14). Ultimately, the optimized reaction
conditions is 1a (1 equiv), 2a (2.1 equiv) and AIBN (20 mol
%) in the HOAc (2 mL) at 80 °C for 24 h.
a b
,
Table 1. Optimization of the Reaction Conditions
With the optimized reaction conditions in hand, the scope of
this reaction was next explored, and the results are summarized
in Scheme 2. First, various functional groups (−CH3, −F, −Cl,
Scheme 2. Exploration of Substrate Scope of Methyl Aryl
Ketones
b
entry
catalyst (mol %)
time (h)
temp (°C)
yield (%)
a,b
1
24
24
24
24
24
24
24
24
24
24
24
18
24
28
80
80
80
80
80
80
80
80
80
40
100
80
80
80
8
42
23
30
7
2
AIBN (5)
TBHP (5)
DDQ (5)
I2 (5)
3
4
5
c
6
AIBN (10)
AIBN (10)
AIBN (20)
AIBN (30)
AIBN (20)
AIBN (20)
AIBN (20)
AIBN (20)
AIBN (20)
61
7
58
84
81
57
76
80
8
9
10
11
12
13
14
d
82
84
a
Reaction condition: 1a (0.25 mmol), 2a (0.525 mmol), solvent (2
b
c
mL), 80 °C, 24 h. Isolated yield based on 1a. The product is 2,2′-
dithiodianilinium trifluoroacetate using CF3CO2H as solvent. In the
d
dark.
recently reported radical synthesis of benzo[b][1,4]thiazine3g
and a widely used phenylthio radical precursor diphenyl
disulfide/azobisisobutyronitrile (AIBN) system,18 we hypothe-
sized that this reaction might be promoted by radical initiators.
Luckily, a simple addition of 5 mol % of AIBN as radical
initiator improves the yield of 3a to 42% while other conditions
remain unchanged (Table 1, entry 2).
a
Reaction conditions: 1a (0.25 mmol), 2 (0.525 mmol), AIBN (20
b
mol %), HOAc (2 mL) at 80 °C for 24 h. Isolated yields.
−Br, −I) on the para-position of the benzene ring were
examined. p-Methyl- or methoxy-substituted phenyl rings lead
to the desired products (Scheme 2, 3b and 3c) in similar yields
as for 3a. The yields of 3d−g with halogen on the para-position
of acetophenones are slightly increased from 73 to 81% by
when the substituents are changed from fluoro-, chloro-,
bromo-, to iodo- (Scheme 2, 3d−g). In addition, sterically
hindered 2-methoxyacetophenone gives a lower yield compared
to its meta- or para- counterparts 3j and 3c. Acetophenones
with a halogen on the ortho- or meta-position are also
successfully converted to the corresponding bibenzothiazines
(Scheme 2, 3i, 3k, and 3l). Furthermore, the dimethoxy-
substituted acetophenone also yields the corresponding
product 3m in moderate yield (62%). Finally, 2-acetylth-
iophene23 is chosen as a representative of heterocyclic ketones
to further expand the substrate scope, and it worked smoothly
under the standard conditions (Scheme 2, 3n, 57% yield).
The scope of this reaction was further explored by
substituted dithiodianiline24 with a different substituted
acetophenone (Scheme 3). Dithiodianiline with the electron-
withdrawing Cl− group or electron-donating Me− and MeO−
groups could successfully condensate with acetophenones
bearing para-substituted Me−, MeO−, Cl−, Br−, and I−.
However, Cl-substituted dithiodianiline gives a slightly lower
yield compared to its nonchloride counterparts.
Since a radical initiator could dramatically increase the yield,
we screened a series of other radical initiators, such as tert-butyl
hydroperoxide (TBHP),19 2,3-dichloro-5,6-dicyano-1,4-benzo-
quinone (DDQ),20 and I2.21 It turns out that I2 barely promotes
this reaction, and TBHP and DDQ are less effective than AIBN
(Table 1, entries 3−5). Metal ions have been shown to
influence the synthesis of bibenzo[b][1,4]thiazine pigments in
vitro and in vivo.8b We study the effect of inorganic copper salts
as potential co-catalysts. However, in the presence of copper
salts with a different counteranions, this reaction was almost
completely inhibited (Table S1, entries 1−3). With AIBN as
the best radical promoter, a variety of solvents such as
chlorobenzene, ethanol, toluene, and p-xylene were further
screened, and no reaction took place in these solvent systems
(Table S1, entries 4−8). When CF3CO2H was employed as
solvent, only 2,2′-dithiodianilinium trifluoroacetate was ob-
tained in moderate yield (Table 1, entry 6, see the Supporting
Information (SI) for its crystal structure22). Further optimiza-
tion was carried out with the use of various amounts of AIBN
(Table 1, entries 1 and 7−9), and we were glad to find that
increasing the amount of AIBN from 5 to 20 mol % almost
doubled the yield of 3a (84% based on 1a); however, the yield
of 3a is not further increased when the amount of AIBN is over
20 mol %. Next, we evaluated a range of different temperatures
(Table 1, entries 8, 10, and 11) and found that 80 °C was the
optimum temperature. In addition, this reaction is not light
Furthermore, we explored the gram-scale synthesis of
compound 3a. Gram-scale synthesis was successfully achieved
(1.84 g of 3a) with compromised yield (82%). Additionally,
B
DOI: 10.1021/acs.orglett.8b01238
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
S···S interaction (3.527 Å) and 4-fold weak S···π interaction
(3.486 Å). These chains are parallel to the a-axis and held
together by C−H···N hydrogen bonding (2.718 Å), forming a
herringbone structure. Compound 3b forms a 4-fold C−H···S
interaction (2.918 Å) with the surrounding molecules.
Compound 3d also forms a one-dimensional chain with a
different molecular arrangement by 4-fold C−H···F hydrogen
bonding (2.592 Å), and C−H···N bonds (2.692 Å) are formed
between chains. For compounds 3c and 3h, the methoxyl group
makes the solid-state assembly much more complex with
multiple C−H···O, C−H···π, and π···π interactions. In
compound 3h, there is a 2D molecule layer form in the bc
plane by 4-fold C−H···O (2.629 Å), C−H···π (2.864 Å), and
double π···π (3.344 Å) between adjacent molecules. As in
compound 3n, an alternative brickwall layer is formed by the
π···π interaction (3.302 Å) view along the c-axis, and different
layers are packed together by C−H···N interactions (2.681 Å).
To gain insight into the mechanism of this reaction, three
control experiments were designed and monitored by gas
chromatography/mass spectrometer (GC−MS). First, 2,2,6,6-
tetramethylpiperidin-1-oxy radical (TEMPO) was added to the
reaction mixture to probe the radical nature of this reaction. As
AIBN generates a large amount of isobutyronitrile radical
during the reaction, we first chose the initial reaction conditions
without AIBN as our model system. Without the addition of
TEMPO, we observed imine 5, benzothiazole 7a and 7b,26 and
2H-1,4-benzothiazine 10 along with the starting materials
(Figures S54−S58). Upon addition of 4.0 equiv of TEMPO,
trapping product 11 was captured and intermediate 10 was
oxidized by TEMPO to give compound 12 (Figures S59−S61).
When the amount of TEMPO was increased to 10 equiv, the
reaction was completely inhibited without detection of 10.
Second, oxygen plays an important role in the coupling of
10.10,12 When the reaction mixture was thoroughly degassed by
freeze−pump−thaw cycles, the reaction was stopped at
intermediate 10. Intermediate 10 can be quantitatively
converted to 3a under our standard reaction conditions.
Interestingly, the amount of oxygen needed by the dehydrogen-
ation process could be provided by the oxygen dissolved in the
solvent, which prevents overoxidation.27 Finally, deuterated
acetophenone was further employed to probe the reaction
mechanism; we could observe the formation of deuterated 7a,
7b, and 10, which provides additional evidence for paths A and
B (Figures S62−S67).
Although the detailed mechanism of this condensation
remains to be elucidated, a tentative mechanism is proposed
(Scheme 4). Dithiodianiline 1a could homolytic fission under
heat to generate thiyl radical 4, which was reversibly
condensated with ketone 2a to afford imine 5. The thiyl
radical in imine 5 could attack imine carbon through path A to
give benzothiaole 7a and 7b in a trace amount. In path B, after
tautomerization of imine 5, the addition of thiyl radical to
enamine could give intermediate 9, which lost a hydrogen atom
with the help of isobutyronitrile radical to generate key
intermediate 10. Finally, intermediate 10 could homocoupled
to bibenzo[b][1,4]thiazine 3a.10
In summary, we have developed an AIBN-mediated cascade
condensation and radical-coupling strategy for the preparation
of various 3,3′-bisarylbibenzo[b][1,4]thiazines involving multi-
ple bond formation. This process could be easily scaled to
gram-scale and is practical for the preparation of potential
biologically and pharmaceutically useful organic compounds.
Bisarylbibenzo[b][1,4]thiazine derivatives exhibit various solid-
Scheme 3. Exploration of Substrate Scope of
Dithiodianiline
a,b
a
Reaction conditions: 1 (0.25 mmol), 2 (0.525 mmol), AIBN (20 mol
b
%), HOAc (2 mL) at 80 °C for 24 h. Isolated yields.
halogenated arenes are important organic intermediates for the
synthesis of useful compounds through the transition-metal-
catalyzed cross-coupling reactions.25 As an example, compound
3g is coupled with phenylboronic acid by Suzuki−Miyaura
reaction in 91% yield (SI).
The structures of bibenzothiazines were unambiguously
1
confirmed by H NMR, 13C NMR, HRMS, and single-crystal
X-ray diffraction analysis. All synthesized bibenzothiazines are
mesomers with one set of proton or carbon signals appearing in
1
their H NMR or 13C NMR. Protons attached to the newly
formed C−C bond are singlets at around 4.0 ppm. As for
compounds 3a−d, 3h, and 3n, single crystals were grown and
exhibit a similar core structure22 (Figure 1 and SI). Their
Figure 1. Crystal structure and solid-state packing of compound 3a:
(a) top view; (b) side view; (c) sulfur involved noncovalent interaction
are shown in green lines.
single-crystal structures also show central symmetry with a
staggered central carbon−carbon single bond, which agrees
with their 1H NMR and 13C NMR analysis. Two benzothiazine
units are linked by the newly formed C(2)−C(2′) single bond.
The C(2)−C(2′) single bond in the fluoride-substituted
compound 3d is 1.569 Å, which the longest compared with
the electron-donating substituted compounds, such as Me−
and MeO− (1.532−1.544 Å). The angle of S(1)−C(2)−C(3)
in the dihydrothiazine ring is quite similar, ranging from 106.35
to 108.16°. Because of the central symmetry in bibenzothia-
zines, the dihedral angle of S(1)−C(2)−C(2′)−S(1′) is 180° in
all of the crystal structures, which avoids steric hindrance
between two benzothiazines (Figure 1). Interestingly, the solid-
state packing is very sensitive to the substituents. In compound
3a, a one-dimensional molecular chain is formed by a double
C
DOI: 10.1021/acs.orglett.8b01238
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(2) Thakur, D. K.; Vankar, Y. D. Synthesis 1983, 1983, 223.
Scheme 4. Plausible Mechanism for the Formation of 3a
(3) For the recent synthesis of 1,4-benzothiazines: (a) Zhao, Y.; Wu,
Y.; Jia, J.; Zhang, D.; Ma, C. J. Org. Chem. 2012, 77, 8501.
(b) Viglianisi, C.; Bonaccorsi, P.; Simone, L.; Nassini, L.; Menichetti,
S. Eur.. J. Org. Chem. 2012, 2012, 1707. (c) Viglianisi, C.; Simone, L.;
Menichetti, S. Adv. Synth. Catal. 2012, 354, 77. (d) Zhao, Y.; Dai, Q.;
Chen, Z.; Zhang, Q.; Bai, Y.; Ma, C. ACS Comb. Sci. 2013, 15, 130.
(e) Selvi, T.; Srinivasan, K. J. Org. Chem. 2014, 79, 3653. (f) Qiu, J.-
W.; Hu, B.-L.; Zhang, X.-G.; Tang, R.-Y.; Zhong, P.; Li, J.-H. Org.
Biomol. Chem. 2015, 13, 3122. (g) Lin, Y.; Lu, G.; Wang, R.; Yi, W.
Org. Lett. 2016, 18, 6424.
(4) Barazarte, A.; Lobo, G.; Gamboa, N.; Rodrigues, J. R.; Capparelli,
M. V.; Alvarez-Larena, A.; Lopez, S. E.; Charris, J. E. Eur. J. Med. Chem.
2009, 44, 1303.
(5) (a) Sabatini, S.; Kaatz, G. W.; Rossolini, G. M.; Brandini, D.;
Fravolini, A. J. Med. Chem. 2008, 51, 4321. (b) Mitra, S.; Mukherjee,
S.; Hajra, A. RSC Adv. 2016, 6, 201.
(6) Rai, A.; Singh, A. K.; Raj, V.; Saha, S. Mini-Rev. Med. Chem. 2018,
18, 42.
(7) Goffin, E.; Drapier, T.; Larsen, A. P.; Geubelle, P.; Ptak, C. P.;
Laulumaa, S.; Rovinskaja, K.; Gilissen, J.; Tullio, P. d.; Olsen, L.;
Frydenvang, K.; Pirotte, B.; Hanson, J.; Oswald, R. E.; Kastrup, J. S.;
Francotte, P. J. Med. Chem. 2018, 61, 251.
state assemblies, which were affected by subtle functional group
difference.
(8) (a) Watanabe, Y.; Osanai, K.; Nishi, T.; Miyawaki, N.; Shii, D.;
Honda, T.; Shibano, T. Bioorg. Med. Chem. Lett. 1996, 6, 1923.
(b) Koidl, B.; Miyawaki, N.; Tritthart, H. A. Eur. J. Pharmacol. 1997,
322, 243. (c) Prasad, C. D.; Verma, A.; Sattar, M.; Kumar, S. RSC Adv.
2015, 5, 75881.
(9) (a) Thomson, R. H. Angew. Chem., Int. Ed. Engl. 1974, 13, 305.
(b) Tesema, Y. T.; Pham, D. M.; Franz, K. J. Inorg. Chem. 2008, 47,
1087. (c) Leone, L.; Crescenzi, O.; Napolitano, A.; Barone, V.;
d’Ischia, M. Eur. J. Org. Chem. 2012, 2012, 5136. (d) Napolitano, A.;
Panzella, L.; Leone, L.; d’Ischia, M. Acc. Chem. Res. 2013, 46, 519.
(10) Leone, L.; Crescenzi, O.; Amorati, R.; Valgimigli, L.;
Napolitano, A.; Barone, V.; d’Ischia, M. Org. Lett. 2013, 15, 4944.
(11) Unger, O. Ber. Dtsch. Chem. Ges. 1897, 30, 607.
(12) MacKenzie, N. E.; Thomson, R. H.; Greenhalgh, C. W. J. Chem.
Soc., Perkin Trans. 1 1980, 1, 2923.
(13) (a) Santacroce, C.; Sica, D.; Nicolaus, R. A. Gazz. Chim. Ital.
1968, 98, 85. (b) Prota, G.; Giordano, F.; Mazzarella, L.; Santacroce,
C.; Sica, D. J. Chem. Soc. C 1971, 15, 2610.
(14) Shaaban, M. R.; Inagi, S.; Fuchigamia, T. Electrochim. Acta 2009,
54, 2635.
(15) Rao, M. H.; Reddy, A. P. R.; Veeranagaiah, V. Synth. Commun.
1991, 21, 1715.
(16) Prakash, O.; Sharma, N.; Pannu, K. Synth. Commun. 2007, 37,
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b01238.
■
S
Full experimental procedures, characterization data, and
NMR spectra (PDF)
Accession Codes
CCDC 1540959−1540965 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: lipengfei@bit.edu.cn.
*E-mail: jmdou@lcu.edu.cn.
*E-mail: bowang@bit.edu.cn
1995.
ORCID
(17) Ansari, M. I.; Shankar, R.; Hussain, M. K.; Kant, R.; Maulik, P.
R.; Kumar, K. R.; Hajela, K. J. Heterocycl. Chem. 2011, 48, 1336.
(18) (a) Feldman, K. S.; Romanelli, A. L.; Ruckle, R. E., Jr.; Miller, R.
F. J. Am. Chem. Soc. 1988, 110, 3300. (b) Teng, F.; Yu, J.-T.; Yang, H.;
Jiang, Y.; Cheng, J. Chem. Commun. 2014, 50, 12139.
(19) (a) Sheares, V. V.; Wu, L.; Li, Y.; Emmick, T. K. J. Polym. Sci.,
Part A: Polym. Chem. 2000, 38, 4070. (b) O’Brien, A. G.; Maruyama,
A.; Inokuma, Y.; Fujita, M.; Baran, P. S.; Blackmond, D. G. Angew.
Chem., Int. Ed. 2014, 53, 11868.
(20) (a) Walker, D.; Hiebert, J. D. Chem. Rev. 1967, 67, 153.
(b) Huang, Z.; Jin, L.; Feng, Y.; Peng, P.; Yi, H.; Lei, A. Angew. Chem.,
Int. Ed. 2013, 52, 7151.
(21) (a) Nobuta, T.; Hirashima, S.; Tada, N.; Miura, T.; Itoh, A. Org.
Lett. 2011, 13, 2576. (b) Kariya, A.; Yamaguchi, T.; Nobuta, T.; Tada,
N.; Miura, T.; Itoh, A. RSC Adv. 2014, 4, 13191.
Pengfei Li: 0000-0001-7945-4993
Guodong Shen: 0000-0002-0631-1913
Nana Xin: 0000-0001-5025-2589
Changqiu Zhao: 0000-0002-9016-8151
Bo Wang: 0000-0001-9092-3252
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(81601549, 21502084, 21671093, 21471018, 21625102,
21401094, and 21402079) and the Program for Scientific
Research Innovation Team in Colleges and Universities of
Shandong Province.
(22) For the crystal structure of a similar compound 2,2′-
dithiodianilinium chloride, see: Bouchareb, H.; Boudraa, M.;
Bouacida, S.; Merazig, H. Acta Crystallogr., Sect. E: Struct. Rep. Online
2013, E69, o1078. For the crystal structure of compound 3a with
different lattice parameters, see ref 17.
REFERENCES
(1) Wen, J.; Tiwari, D. P.; Bolm, C. Org. Lett. 2017, 19, 1706.
■
D
DOI: 10.1021/acs.orglett.8b01238
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(23) Some heterocycles are incompatible with our reaction
conditions, such as 2-acetylfuran and 2-acetylpyridine, which give a
complex mixture.
(24) For the synthesis of substituted dithiodianilines: (a) Hyvl, J.;
Srogl, J. Eur. J. Org. Chem. 2010, 2010, 2849. (b) Lumb, I.; Hundal, M.
S.; Hundal, G. Inorg. Chem. 2014, 53, 7770.
(25) For recent reviews: (a) Zhao, K.; Shen, L.; Shen, Z.-L.; Loh, T.-
P. Chem. Soc. Rev. 2017, 46, 586. (b) Xia, Y.; Qiu, D.; Wang, J. Chem.
Rev. 2017, 117, 13810.
(26) Benzothiazole formation was observed with aryl ketones and 2-
aminobenzenethiols under different reaction conditions. Liao, Y.; Qi,
H.; Chen, S.; Jiang, P.; Zhou, W.; Deng, G.-J. Org. Lett. 2012, 14, 6004.
(27) Wu, X.; Deng, Z.; Yan, J.; Zhang, Z.; Zhang, F.; Zhang, Z. Ind.
Eng. Chem. Res. 2014, 53, 9932.
E
DOI: 10.1021/acs.orglett.8b01238
Org. Lett. XXXX, XXX, XXX−XXX