Visible-light-induced regioselective cross-dehydrogenative coupling of 2-isothiocyanatonaphthalenes with amines using molecular oxygen

Article abstract of DOI:10.1007/s11426-020-9811-6

An efficient and eco-friendly protocol for the construction of naphtho[2,1-d]thiazol-2-amines through visible-light photoredoxcatalyzed C(sp²)-H/S-H cross-dehydrogenative coupling reactions between 2-isothiocyanatonaphthalenes and amines was established. In this reaction, the new C-N and C-S bonds are formed simultaneously in a single step. This new method provides a straightforward approach for constructing valuable sulfur-containing compounds.[Figure not available: see fulltext.].

Full text of DOI:10.1007/s11426-020-9811-6

SCIENCE CHINA
Chemistry
•ARTICLES•
https://doi.org/10.1007/s11426-020-9811-6
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Visible-light-induced regioselective cross-dehydrogenative coupling
of 2-isothiocyanatonaphthalenes with amines using molecular
oxygen
Ziyu Gan1,2, Guoqing Li2, Xiaobo Yang3, Qiuli Yan1, Guiyun Xu1, Gaoyang Li4,
Yuan-Ye Jiang2* & Daoshan Yang1,2*
1Key Laboratory of Optic-electric Sensing and Analytical Chemistry for Life Science, Ministry of Education, Shandong Key
Laboratory of Biochemical Analysis; College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology,
Qingdao 266042, China;
2School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, China;
3Institute of Catalysis for Energy and Environment, College of Chemistry & Chemical Engineering, Shenyang Normal University,
Shenyang 110034, China;
4Qingdao West Coast New Area Poli Junior Middle School, Qingdao 266409, China
Received May 13, 2020; accepted July 2, 2020; published online August 3, 2020
An efficient and eco-friendly protocol for the construction of naphtho[2,1-d]thiazol-2-amines through visible-light photoredox-
catalyzed C(sp2)–H/S–H cross-dehydrogenative coupling reactions between 2-isothiocyanatonaphthalenes and amines was es-
tablished. In this reaction, the new C–N and C–S bonds are formed simultaneously in a single step. This new method provides a
straightforward approach for constructing valuable sulfur-containing compounds.
visible light, cross-dehydrogenative coupling, C–S bond, synthetic methods, metal-free
Citation:
Gan Z, Li G, Yang X, Yan Q, Xu G, Li G, Jiang YY, Yang D. Visible-light-induced regioselective cross-dehydrogenative coupling of 2-
isothiocyanatonaphthalenes with amines using molecular oxygen. Sci China Chem, 2020, 63, https://doi.org/10.1007/s11426-020-9811-6
1 Introduction
halides with disulfides or thiols [4]. From an organic
synthesis perspective, sulfenylation reaction using cross-
dehydrogenative coupling (CDC) method is one of the most
efficient and straightforward strategies for the formation of
C–S bonds due to its high atom economy and low number of
synthetic steps [5]. However, a detailed literature search re-
vealed that this synthetic strategy for the C–S bond formation
is limited when compared to the forming reaction of C–N, C–
O or C–C bonds [6]. This might be due to the facile over-
oxidation of sulfur-containing compounds under oxidative
conditions. Therefore, it is highly desirable to develop more
economical, efficient, and practical CDC strategies for con-
structing C–S bonds under mild conditions [7].
Organosulfur compounds pervasively exist in bioactive
natural products, pharmaceuticals, organic photoelectric
materials and flavor compounds [1]. In 2011, sulfur-
containing drugs accounted for 20% of the top 200 retail
drugs in the USA [2]. Thus, designing efficient and en-
vironmentally friendly approaches for the formation of C–S
bonds remains a fundamentally important goal for the syn-
thetic community [3]. The classical approaches for the
synthesis of C–S bonds are the transition-metal-catalyzed
cross-coupling of aryl halides, arylboronic acids or pseudo-
2-Aminobenzothiazole and its derivatives are important
*Corresponding authors (email: yuanyejiang@qfnu.edu.cn;
yangdaoshan@tsinghua.org.cn)
© Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020
chem.scichina.com link.springer.com

2
Gan et al. Sci China Chem
sulfur-containing structural motifs in many pharmaceu-
ticals and agrochemicals and possess excellent biological
and medicinal activities, including antitumor, antic-
onvulsant, anti-inflammatory, anti-infective, HIV-1 pro-
tease inhibition, neuroprotective and antimicrobia [8].
Consequently, the development of efficient and green
synthetic strategies to access 2-aminobenzothiazole and its
derivatives remains one of the most attractive research
areas in organic and pharmaceutical chemistry. Generally,
the 2-aminobenzothiazoles forming pathway involves the
following methods: (1) transition-metal-catalyzed direct
oxidative coupling of benzothiazoles with amines [9]; (2)
base or transition-metal promoted coupling of 2-halo-
benzothiazoles with amines [10]; (3) transition-metal-
catalyzed cascade condensation and cyclization of 2-ha-
loanilines with isothiocyanates [11]; and (4) cyclization of
N-aryl thioureas through transition-metal-catalyzed in-
tramolecular C–S bond formation [12]. Despite successes
with this approach, these methods have certain drawbacks,
including unavailable precursors, toxic metal salt catalysts,
and harsh reaction conditions. In 2017, Lei and co-workers
[13] demonstrated an environmentally friendly electro-
chemical reaction protocol for the synthesis of 2-amino-
benzothiazoles through the direct coupling of aryl
isothiocyanates with aliphatic amines. In 2017, Fan and
Zhang et al. [14] also developed an elegant approach to 2-
aminobenzothiazoles via iodine-catalyzed cascade reac-
tions of isothiocyanatobenzenes with primary or secondary
amines. However, these elegant reactions still have several
drawbacks that limit potential applications: (1) the amines
mainly focused on secondary aliphatic amines. Primary
aliphatic amines and aryl amines were not compatible in
Lei’s work; (2) toxic chlorobenzene used as the solvent;
and (3) high reaction temperature. Therefore, the devel-
opment of a facile and novel method that can complement
existing synthetic methods while meeting requirements of
sustainable and green chemistry remains an ongoing
challenge.
Scheme 1 Recent strategies for the synthesis of 2-aminobenzothiazoles
(color online).
2 Experimental
2.1 General information
All reagents and solvents were obtained from commercial
suppliers and used without further purification. The photo-
catalysts were purchased from Sigma Aldrich (USA). Flash
chromatography was performed on silica gel (200–300
1
mesh). H and 13C NMR data were recorded at 500 and
125 MHz on a BRUKER 500 spectrometer (Germany).
Chemical shifts (δ) are expressed in parts per million (ppm),
coupling constants (J) are in Hz. Proton and carbon magnetic
resonance spectra (1H NMR and 13C NMR) were recorded
using tetramethylsilane (TMS) as the internal standard in
DMSO-d6 or in CDCl3. Mass analyses and high resolution
mass spectrometry (HRMS) were obtained by electrospray
ionization (ESI) on a time of flight (TOF) mass analyzer. All
diffraction data were obtained on a Bruker Smart Apex CCD
diffractometer equipped with graphite-monochromated Mo
Kα radiation. UV-visible spectroscopy of reaction solution
was recorded on a PERSEE TU-1901 UV-visible spectro-
photometer (China). The fluorescence emission intensity of
reaction solution was recorded on a F-4600 spectro-
fluorimeter. The reactor was 3.0 cm from 12 W Blue LED.
Visible light photoredox catalysis is a versatile, powerful
and environmentally friendly synthetic tool and has attracted
extensive attention in the field of synthetic chemistry [15].
Visible-light induced oxidation is an ideal choice for C–H/S–
H cross-dehydrogenative coupling (CDC) sulfenylation re-
actions [16]. However, studies on the synthesis of 2-amino-
benzothiazoles based on light-induced transformation have
not been reported. As part of our continuing studies of
photochemical reactions in green organic synthesis and
photochemical reactions [17], herein, we report an efficient
and simple visible-light-induced Eosin Y-catalyzed method
for the synthesis of 2-aminobenzothiazoles through the direct
coupling of 2-isothiocyanatonaphthalenes and amines using
molecular oxygen as the green oxidant at room temperature
(Scheme 1(c)).
2.2 General procedure for the synthesis of 3 or 4
A 25 mL Schlenk tube equipped with a magnetic stirring bar
was charged with 1 (0.2 mmol), 2 (0.3 mmol), and Eosin Y
(1 mol%). The tube was evacuated twice and backfilled with
oxygen, and 2 mL dimethyl sulfoxide (DMSO) was added to
the tube under oxygen atmosphere. The tube was sealed with
an oxygen balloon and then the mixture was allowed to stir at

3
Gan et al. Sci China Chem
Table 1 Optimization of the reaction conditionsa)
room temperature with the irradiation of a 12 W blue LED
for 24 h. After completion of the reaction, the solvent was
removed with the aid of a rotary evaporator. The residue was
purified by column chromatography on silica gel using the
petroleum ether/ethyl acetate as eluent to provide the desired
products 3 or 4.
3 Results and discussion
We initially chose 2-isothiocyanatonaphthalene (1a) and
piperidine (2a) as model substrates to investigate the optimal
reaction conditions, including the solvents and photo-
catalysts, under an oxygen atmosphere and visible light ir-
radiation using a 12 W blue LED. As shown in Table 1, six
solvents (CH3CN, H2O, toluene, 1,4-dioxane, EtOH, and
DMSO) were screened using Eosin Y (A) as the photo-
catalyst at room temperature, and DMSO afforded the
highest yield (92%) (entries 1–6, Table 1). Subsequently,
seven photoredox catalysts were tested in DMSO. Among
these photocatalysts investigated, Eosin Y was proven to be
the most effective for obtaining the desired product 2-(pi-
peridin-1-yl)naphtho[2,1-d]thiazole (3a) in 92% yield (en-
tries 6–12, Table 1). Notably, none of the desired oxidative
cyclization product 3a was detected under a nitrogen atmo-
sphere, and instead a 92% yield of nucleophilic addition
Entry
1
Photoredox catalyst
Solvent
CH3CN
H2O
Yield (%)b)
A
A
51
42
67
56
47
92
64
37
41
73
43
0
0c)
53d)
0
0e)
0f)
2
3
A
Toluene
1,4-Dioxane
EtOH
4
A
5
A
6
A
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
7
B
product
N-(naphthalen-2-yl)piperidine-1-carbothioamide
8
C
was obtained (entry 13, Table 1). In addition, when the re-
action was performed under air atmosphere, it gave a lower
yield (entry 14, Table 1). Furthermore, none of the desired
product 3a was detected in the absence of a photoredox
catalyst (entry 15, Table 1). In addition, control experiments
showed that no oxidative cyclization conversion could be
induced by increasing the reaction temperature in the ab-
sence of light irradiation (entries 16 and 17, Table 1).
After establishing the optimized reaction conditions, we
then evaluated the substrate scope of this reaction using
different 2-isothiocyanatonaphthalene derivatives and ali-
phatic amines (Scheme 2). To our delight, a variety of ali-
phatic amines reacted smoothly with 2-isothiocyan-
atonaphthalenes, affording the corresponding naphtho[2,1-d]
thiazol-2-amines in good to excellent yields. Notably, pri-
mary aliphatic amines showed good reactivity in this pho-
tocatalytic system, while these primary amines were not well
tolerated in the electrocatalytic system (3g, 3i, 3j, 3r, and 3t).
Dicyclohexylamine which has large steric bulk also partici-
pated well in the reaction, giving a high yield (3f and 3q).
The bioactive amines methyl alaninate and cis-2-Boc-hex-
ahydropyrrolo[3,4-c]pyrrole showed good reactivity afford-
ing the corresponding products in good yield, so this method
provides a new strategy for constructing bioactive hetero-
cycles (3k and 3l). 2-(Thiophen-2-yl)ethan-1-amine was
tolerated in this transformation, affording the desired pro-
9
D
10
11
12
13
14
15
16
17
E
F
G
A
A
None
A
A
a) Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), photocatalyst
(1.0 mmol%), solvent (2 mL), temperature (rt, ∼25 °C), time (24 h). b)
Isolated yield. c) Under a nitrogen atmosphere (extrusion of air). d) In air.
e) At 40 °C, no light. f) At 60 °C, no light.
ducts in 95% and 77% yield (3j and 3r), respectively. Next,
aryl amines were screened to further demonstrate the sub-
strate scope in this photochemical process (Scheme 3). We
were pleased to find that both primary and secondary aro-
matic amines successfully afforded the corresponding cy-
clization products with high to excellent yields (4a–4l). The
electron-effect of the substituted groups in aryl amines in-
cluding electron-rich, -deficient, and -neutral groups did not
display evidently difference of reactivity. To our delight, 3-
isothiocyanatoisoquinoline was also compatible under the
standard conditions and the desired products were generated

4
Gan et al. Sci China Chem
Scheme 2 Substrate scope of 2-isothiocyanatonaphthalenes with aliphatic
amines. Reaction conditions: under an oxygen atmosphere, 2-iso-
thiocyanatonaphthalenes 1 (0.2 mmol), aliphatic amines 2 (0.3 mmol),
DMSO (2.0 mL), 12 W blue LED, temperature (rt, ∼25 °C), reaction time
(24 h). Isolated yield.
Scheme 3 Substrate scope of 2-isothiocyanatonaphthalenes with aryl
amines. Reaction conditions: under an oxygen atmosphere, 2-iso-
thiocyanatonaphthalenes 1 (0.2 mmol), aryl amines 2 (0.3 mmol), DMSO
(2.0 mL),12 W blue LED, temperature (rt, ∼25 °C), reaction time (24 h).
Isolated yield.
in good yields (4p and 4q). Finally, diphenylamine was
investigated under the standard conditions. Unfortunately,
no oxidative cyclization product was observed (4r), and only
a trace amount of the thiourea was produced. The visible-
light promoted regioselective C(sp2)–H/S–H cross-
dehydrogenative reactions could tolerate some functional
groups such as methyl, trifluoromethyl, ether, ester, C–Cl
bond, and C–Br bond, leaving ample room for further
modification.
To gain mechanistic insights into this novel oxidative cy-
clization pathway, some preliminary control experiments
were performed (Scheme 4). When the direct coupling of 2-
isothiocyanatonaphthalene (1a) with piperidine (2a) was
performed in the absence of Eosin Y, the nucleophilic ad-
dition product 3a′ was obtained in 97% yield (Scheme 4(a)).
In addition, the treatment of 3a′ under the standard condi-
tions afforded 3a in 96% yield, suggesting that 3a′ may be an
intermediate in the photocatalytic transformation (Scheme 4
(b)). Furthermore, the treatment of 2-isothiocyanatonaphth-
alene (1a) with piperidine (2a) under a nitrogen atmosphere
was examined. As expected, only a trace amount of the de-
sired product 3a was observed along with the nucleophilic
addition product 3a′ (92% yield), which reveals that oxygen
is crucial for this reaction (Scheme 4(c)). It should also be
noted that only nucleophilic addition product 3a′ was ob-
tained in the absence of light (Scheme 4(d)). Finally, the
treatment of 1-isothiocyanatonaphthalene (1f) with piper-
idine (2a) under the standard conditions gave a messy thin
layer chromatography (TLC), and only a trace amount of 1f
was recovered (Scheme 4(f)).
In order to obtain more information about the mechanism,
Stern-Volmer fluorescence quenching experiments of Eosin
Y with N-(naphthalen-2-yl)piperidine-1-carbothioamide (3a′)
were performed. As shown in Figure 1, the 572 nm fluor-
escence launched by Eosin Y was observed when it was
excited at 510 nm, and the addition of 3a′ dramatically de-

5
Gan et al. Sci China Chem
Figure 1 (a) Quenching of the Eosin Y fuorescence emission in the
presence of 3a′. (b) Stern-Volmer plots (color online).
4(e)) [18] were investigated using DFT calculations to ex-
plore the origin of the selectivity (Figure 2) (see Supporting
Information for computational details). The hydrogen ab-
straction of 3a′′ by excited Eosin Y and oxygen to generate
the radical 6′ is exergonic by 4.0 kcal/mol. From 6′, α-cy-
clization can proceed via TS1 to afford the intermediate 7′,
which cause an energy increase of only 3.3 kcal/mol. Then
electron transfer occurs to give the cation 8′, from which
facile deprotonation by a hydroperoxide anion can proceed
via TS2 to generate the product 3b with an overall energy
barrier of 11.7 kcal/mol. The aromatization makes the de-
protonation highly exergonic by about 60 kcal/mol. By
contrast, β-cyclization via TS3 affords the less stable radical
intermediate 9′ and the subsquent electron transfer makes the
overall pathway endergonic by 28.4 kcal/mol, indicating that
this pathway is less feasible than α-cyclization. The calcu-
lated different thermodynamics is easily understandable ac-
cording to resonance structures of the radical/cationic
intermediates (Figure S7, Supporting Information online).
On the other hand, the oxidative desulfurization of 3a′′ is less
Scheme 4 Control experiments (color online).
creased the fluorescence intensity. In addition, the non-linear
Stern-Volmer fluorescence quenching plots presumably in-
dicated a single electron transfer between Eosin Y’s excited
state and 3a′.
The cyclization reaction in our proposed mechanism and
the potential competitive oxidative desulfurization (Scheme
Figure 2 DFT-computed free-energy profile for the most favorable pathway (color online).

6
Gan et al. Sci China Chem
favorable than the α-cyclization by 7.8 kcal/mol but the
oxidative desulphurisation of the N-phenylthiourea deriva-
tive is more favorable than the cyclization pathway by 1.6
kcal/mol (Figure S8). These results are consistent with the
different selectivity observed in the aerobic transformations
of isothiocyanatobenzene and 2-isothiocyanatonaphthalene
(Scheme 4(e)) [18]. We found that the elementary energy
barriers of the oxidative desulfurization and the deprotona-
tion of the cationic cyclization intermediates are similar in
both reactions. By contrast, the relative energies of the ra-
dical/cationic intermediates in the cyclization pathway differ
significantly, indicating that the stability of the radical/ca-
tionic intermediates is the key factor of controlling the se-
lectivity.
and amines leading to naphtho[2,1-d]thiazol-2-amines. The
corresponding oxidative cyclization products were obtained
in good to excellent yield. The developed method can offer
the following advantages: (1) diverse amines were tolerated;
(2) easy workup procedure; (3) low-toxic and inexpensive
DMSO as the solvent; (4) oxygen as the green oxidant; (5) at
room temperature. The advantages of this developed method
meet the requirements of green and sustainable synthetic
chemistry and provide a straightforward approach to con-
struct valuable sulfur-containing compounds.
Acknowledgements This work was supported by the National Natural
Science Foundation of China (21302110, 21702119), the Natural Science
Foundation of Shandong Province (ZR2016JL012, ZR2017QB001), the
Scientific Research Foundation of Qingdao University of Science and
Technology, the Natural Science Foundation of Liaoning Province
(20180550882) and the Program for Creative Talents in University of
Liaoning Province.
Based on these preliminary experimental results, a rea-
sonable mechanism was proposed, shown in Scheme 5. First,
the photocatalyst Eosin Y was excited by visible light irra-
diation, leading to the excited species Eosin Y*. Then Eosin
Y* reacted with the intermediate 3a′′, gengerated in situ
from addition of 1 and 2, to form the radical cation 5 and the
Eosin Y•− radical anion. The oxidation of Eosin Y•− by
Conflict of interest The authors declare no conflict of interest.
Supporting information The supporting information is available online at
http://chem.scichina.com and http://link.springer.com/journal/11426. The
supporting materials are published as submitted, without typesetting or
editing. The responsibility for scientific accuracy and content remains en-
tirely with the authors.
oxygen afforded the ground state Eosin Y and O2•−. Subse-
•−
quently, the radical cation 5 was deprotonated by O2
,
leading to the thiyl radical 6. The thiyl radical 6 then un-
derwent regioselective intramolecular radical addition to
generate the carbon radical 7, which could be further trans-
formed into the intermediate 8 through single electron
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