Visible light-induced recyclable g-C3N4catalyzed thiocyanation of C(sp2)-H bonds in sustainable solvents

Article abstract of DOI:10.1039/d1gc00938a

A metal-free photocatalytic strategy for the preparation of thiocyanated heterocycles from inexpensive NH₄SCN has been developed using carbon nitride (g-C₃N₄) as a general heterogeneous catalyst in a green solvent under an

Full text of DOI:10.1039/d1gc00938a

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Visible light-induced recyclable g-C3N4 catalyzed
thiocyanation of C(sp2)–H bonds in sustainable
solvents†‡
Cite this: DOI: 10.1039/d1gc00938a
Fan-Lin Zeng,§ Hu-Lin Zhu,§ Xiao-Lan Chen, * Ling-Bo Qu and Bing Yu
*
A metal-free photocatalytic strategy for the preparation of thiocyanated heterocycles from inexpensive
NH4SCN has been developed using carbon nitride (g-C3N4) as a general heterogeneous catalyst in a
green solvent under an air atmosphere and the irradiation of a blue LED. Various thiocyanated hetero-
cycles including indolo[2,1-a]isoquinolin-6(5H)-ones, benzimidazo[2,1-a]isoquinolin-6(5H)-ones, thiofla-
vones, azaspiro[4.5]trienones and imidazo[1,2-a]pyridines were successfully synthesized in good yields
(up to 96%). Importantly, this metal-free and external-oxidant-free procedure employed dimethyl carbon-
ate as a green medium, avoiding the traditional volatile organic solvents. Moreover, the recyclable g-C3N4
catalyst could be used at least 8 times without significant loss of activities.
Received 16th March 2021,
Accepted 13th April 2021
DOI: 10.1039/d1gc00938a
rsc.li/greenchem
the same year, Hu’s group realized visible-light-induced
aerobic oxidation of various benzylic C–H bonds with g-C3N4
Introduction
In the past decades, the development of clean, economical, as a catalyst.8 Recently, our group found that g-C3N4 could be
and efficient chemical processes has received widespread an efficient heterogeneous catalyst for the hydroaminomethyl-
attention.1 Visible light, as an easily available and renewable ation or annulation reaction of quinoxalin-2(1H)-ones and
natural resource, has attracted great interest from chemists.2 N-aryl glycines under visible light.9 Despite these achieve-
In this context, many photocatalytic strategies have been devel- ments, the application of g-C3N4 as a heterogeneous photo-
oped to build valuable products in organic synthesis.3 Among catalyst for organic transformations is still in its infancy. Thus,
the reported photocatalytic systems, homogeneous photocata- the exploration of novel organic transformations catalyzed by
lysts like metal complexes and organic dyes are extensively g-C3N4 is still highly desirable.
applied.4 However, these homogeneous catalysts are difficult
Thiocyanates exhibit various biological activities such as
to recycle and reuse, which may cause environmental issues antimicrobial, enzyme inhibitory, and insecticidal in the fields
and high costs. Therefore, the development of efficient hetero- of medicine, pesticides, and materials.10 Therefore, the instal-
geneous catalysts for photocatalytic reactions is highly lation of a –SCN group into heterocycles has gained huge
significant.
attention in recent years.11 For example, Hajra’s group used an
In recent years, the application of semiconductor materials organic dye (eosin Y) as a photocatalyst to achieve the syn-
as heterogeneous photocatalysts has attracted great interest thesis of thiocyanated imidazo[1,2-a]pyridines under metal-
due to their excellent photovoltaic properties and good stabi- free conditions in 2015.12 Sarvari’s group reported the for-
lity/recyclability.5 In particular, graphitic carbon nitride mation of a C–S bond through direct thiocyanation reactions
(g-C3N4) as an inexpensive, easily available, metal-free, and catalyzed by Alizarin red S-TiO2 under visible light in 2020.13
recyclable semiconductor photocatalyst has emerged as an Although great progress has been made, some shortcomings
ideal catalyst for organic synthesis.6 For example, in 2019, still need to be solved urgently, such as non-recyclable organic
König’s group reported the g-C3N4 catalyzed bifunctionaliza- dyes and toxic organic solvents. Herein, we disclosed a general
tion of arenes/heteroarenes under blue light irradiation.7 In photocatalytic thiocyanation protocol using dimethyl carbon-
ate (DMC) as a green solvent (Scheme 1). In this study, with
the recyclable g-C3N4 as a “universal catalyst”, various thiocya-
Green Catalysis Center, College of Chemistry, Zhengzhou University, Zhengzhou
nated heterocycles including indolo[2,1-a]isoquinolin-6(5H)-
ones, benzimidazo[2,1-a]isoquinolin-6(5H)-ones, thioflavones,
450001, China. E-mail: bingyu@zzu.edu.cn
†Dedicated to the 100th anniversary of Chemistry at Nankai University.
azaspiro[4.5]trienones and imidazo[1,2-a]pyridines were suc-
‡Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1gc00938a
cessfully achieved from inexpensive NH4SCN in good yields. To
the best of our knowledge, this is the first case of the construc-
§These authors contributed equally to this work.
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Table 1 Synthesis of thiocyanated indolo/benzimidazo[2,1-a]isoquino-
lin-6(5H)-onesa
Scheme 1 Synthesis of thiocyanated heterocycles.
tion of thiocyanated indolo/benzimidazo[2,1-a]isoquinolin-6
(5H)-ones and thioflavones.
Results and discussion
a Reaction conditions: 1 (0.2 mmol), NH4SCN (0.4 mmol), g-C3N4
(10 mg) in DMC (2 mL) in air at room temperature for 12 h under the
irradiation of a 10 W blue LED. Isolated yields.
We initiated our study by establishing optimal experimental
conditions using the model reaction of 1-(2,3-diphenyl-1H-
indol-1-yl)-2-methylprop-2-en-1-one (1a) with NH4SCN in the
presence of photocatalysts under the irradiation of a blue LED
(for details, see the ESI, Table S1‡). After extensive experi-
ments, the optimal conditions were established as follows: 1a
(0.2 mmol), NH4SCN (0.4 mmol), g-C3N4 (10 mg), DMC (2 mL)
at room temperature for 12 h under an air atmosphere and the
irradiation of a blue LED.
methylthiolated alkynones 3 with NH4SCN under the standard
conditions delivered the corresponding thiocyanated thiofla-
vone products 4a–f in yields of 70–93% (Table 2).
Furthermore,
the
radical
cascade
reaction
of
N-arylpropiolamide (5a) with NH4SCN under the standard con-
ditions provided the desired 3-thiocyanated azaspiro[4.5]trien-
one 6a in 84% yield (Table 3). Further exploration demon-
strated that this strategy has a good functional group toler-
ance, giving various substituted N-arylpropiolamides 6b–h in
good yields.
Subsequently, the substrate scope of this g-C3N4 catalyzed
reaction was further explored to synthesize various
thiocyanated
indolo/benzimidazo[2,1-a]isoquinoline-6(5H)-
ones (Table 1). Firstly, the model reaction gave the desired
product 2a in a good isolated yield of 82%. Then the 1-(2,3-
diphenyl-1H-indol-1-yl)-2-methylprop-2-en-1-ones
Encouraged by these exciting results, we tried to apply this
strategy to the direct thiocyanation of heterocycles (Table 4).
1
bearing
different substituents R1 (–CH3, –OMe, –tBu, –F, –Cl, –Br, –CN
and –CF3) were tested (1b–k). Initially, the substrate bearing an
electron-withdrawing halo group (–Cl) at C4-position gave the
desired product 2b in an excellent yield of 86%. When the sub-
strates with the electron-donating groups (–CH3, –OMe and
–tBu) and electron-withdrawing groups (–F, –Cl, –Br and –CN)
were at C5-position, the target products 2c–i were obtained in
good to excellent yields. The 6-Cl, 7-CF3, and 3-Me substituted
substrates also react with NH4SCN smoothly to give the corres-
ponding products (2j–m) in 70–91% yields. Moreover, when
the benzimidazole type substrate (1n) was employed to react
with NH4SCN, the target product 2n was obtained in 81%
yield. Simultaneously, a gram-scale reaction was performed to
synthesize 2a, and a satisfactory yield of 60% was obtained
(Scheme S3‡).
Table 2 Synthesis of thiocyanated thioflavonesa
To explore the generality of this thiocyanation strategy,
other thiocyanated heterocycles were also synthesized under
the optimal reaction conditions. For example, the reaction of
a Reaction conditions: 3 (0.2 mmol), NH4SCN (0.4 mmol), g-C3N4
(10 mg) in DMC (2 mL) in air at room temperature for 12 h under the
irradiation of a 10 W blue LED. Isolated yields.
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Table 3 Synthesis of thiocyanated azaspiro[4.5]trienonesa
required.14 Meanwhile, an isolated yield of 71% was obtained
in the gram-scale synthesis of 8k (Scheme S3‡), suggesting
that the catalytic procedure is practically promising.
To explore the synthetic application of thiocyanated hetero-
cycles, the synthesized compound 8k was treated with sulfuric
acid and LiAlH4. As shown in Scheme 2, the thioamide com-
pound 8ka and the indole sulfide 8kb were obtained in good
yields.
We further carried out the recovery experiments of g-C3N4
to verify the stability and reusability of the catalytic system.
Under the standard conditions, the model reaction of 1a and
NH4SCN gave 82% of product 2a. Then the catalyst g-C3N4 was
recycled by centrifugation, and washed twice with water and
ethanol, respectively, and then dried at 80 °C. Next, the recov-
ered g-C3N4 was used in the next run. As shown in Fig. 1i, the
catalytic activity of the recovered g-C3N4 almost remained con-
stant in the 8th run. After that, powder X-ray diffraction (XRD)
was used to characterize the fresh g-C3N4 and recovered g-C3N4
after the reaction. As shown in Fig. 1ii, the structure of the
recovered g-C3N4 still did not change compared with the fresh
g-C3N4,15 which was further confirmed by the scanning elec-
tron microscopy (SEM) and the transmission electron
microscopy (TEM) images (Fig. 2). These results suggested that
a Reaction conditions: 5 (0.2 mmol), NH4SCN (0.4 mmol), g-C3N4
(10 mg) in DMC (2 mL) in air at room temperature for 12 h under the
irradiation of a 10 W blue LED. Isolated yields.
Table 4 Synthesis of thiocyanated heterocyclesa
g-C3N4 is
reaction.16
a stable heterogeneous photocatalyst in this
Subsequently, the sensitivity of this strategy was evaluated
based on the reaction conditions, which will help us to
strengthen the understanding of this new synthetic method
and its reproducibility, following the protocol reported by
Glorius et al. (Table 5).17 The results were shown in the radar
diagram, from which we could quickly assess the sensitivity of
this strategy. It can be seen from the radar diagram that the
light intensity is a key parameter affecting the strategy, while
a Reaction conditions: 7 (0.2 mmol), NH4SCN (0.4 mmol), g-C3N4
(10 mg) in DMC (2 mL) in air at room temperature for 12 h under the
irradiation of a 10 W blue LED. Isolated yields. b Reaction for 76 h.
First, the reaction of different substituted 2-aryl imidazo[1,2-a]
pyridines and NH4SCN were conducted under the standard
conditions. It was gratifying that the corresponding target pro-
ducts 8a–f were obtained in excellent yields (83–96%). Notably,
2-phenylbenzo[d]imidazo[2,1-b]thiazole could also be used in
this strategy to produce the target compound 8g in 80% yield.
Moreover, other heterocycles including 1,3,5-trimethoxyben-
zene, N,N-dimethylaniline, tetrahydroquinoline, 2-phenylin-
dole, and azaindole were all applicable in this protocol to react
with NH4SCN under the standard conditions, giving the
corresponding products 8h–l in moderate to excellent yields.
Surprisingly, thiocyanation of the popular drug cholecysto-
kinin antagonist devazepide derivative 8m could also be
Scheme 2 Synthetic transformations of 8k.
Fig. 1 (i) Recyclability of the catalyst and (ii) the XRD pattern of the
achieved in 70% yield, albeit a long reaction time was catalyst.
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added to the reaction mixtures, the thiocyanation reactions
were completely suppressed (Scheme 3a). The signal appearing
at m/z 278.1573 of the high-resolution mass spectra (HRMS)
indicated that the adduct 9a was formed, suggesting that the
SCN radical was generated in the reaction (Fig. S2‡). Moreover,
the reaction under N2 did not afford the desired product, indi-
cating that O2 played an indispensable role in this transform-
ation (Scheme 3b).
In addition, the electron paramagnetic resonance (EPR)
experiments were performed under blue light irradiation
(Fig. 3). When the radical spin trapping reagent 5,5-dimethyl-
1-pyrroline N-oxide (DMPO) was added to the NH4SCN solu-
tion under the standard conditions, a signal was observed. We
speculated that the radical might be generated under the pro-
motion of visible light. (Fig. 3, and for details, see the ESI,
Fig. S4‡).
According to the experimental results and previous reports,
a tentative mechanism was proposed for this visible-light pro-
moted reaction, which was catalyzed by g-C3N4 (Scheme 4).
First, under the irradiation of visible light, g-C3N4 absorbs
photons and generates holes in the valence band (VB) and
electrons in the conduction band (CB). Then, the holes gener-
ated by the VB (EVB = + 1.2 V vs. SCE)7 obtain the electrons
from NH4SCN (Fig. S3,‡ E1/2ox = + 0.60 V vs. SCE was measured)
via a single electron transfer (SET) to generate the SCN radical.
Simultaneously, the electrons in the CB were transferred to O2
(air) through a SET to generate O2·−. Then, the SCN radical
was added to the CvC bonds of 1a to generate the radical
intermediate 9b, which was then subjected to an intra-
Fig. 2 (a) SEM image of the fresh g-C3N4; (b) SEM image of the recov-
ered g-C3N4; (c) TEM image of the fresh g-C3N4; and (d) TEM image of
the recovered g-C3N4.
Table 5 Sensitivity assessment of this reaction
Variation of
parameters
Lower point Center point
Higher point
Concentration (c) c − 10% c
H2O level
Standard conditions c + 10% c
+H2O;
VH2O = 1% Vrxn
Air
T + 10 °C
O2 level
Temperature (T) T − 10 °C
Light intensity (I) I/20
Scale
n*25
∑Reactions
3
5
Scheme 3 Control experiments.
the other parameters have almost no effect on the reaction and
can be regarded as random errors. On the other hand, these
results also demonstrated the good tolerance of this strategy.
After that, we further explored the reaction mechanism
through control experiments, as shown in Scheme 3. When
the radical scavengers 2,2,6,6-tetramethylpiperidin-1-yl-oxy-
Fig. 3 EPR spectrum of a mixture of DMPO and NH4SCN under the
alkyl (TEMPO) or 2,6-di-tert-butyl-4-methylphenol (BHT) were irradiation of a blue LED.
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Scheme 4 Plausible mechanism.
molecular cyclization to obtain the radical intermediate 9c.
Furthermore, 9c was oxidized by O2 (air) to form the carbo-
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Conclusions
In summary, we have developed a simple, efficient, green, and
sustainable visible light-promoted heterogeneous g-C3N4 cata-
lyzed synthetic strategy to construct thiocyanated indolo/benzi-
midazo[2,1-a]isoquinolin-6(5H)-ones, thioflavones and other
heterocyclic compounds using low-cost and easily available
NH4SCN as a raw material. This general method showed the
advantages such as having a metal-free procedure and an
external-oxidant-free procedure, utilizing a recyclable catalyst
and a green solvent, having a wide applicability, being able to
be applied at room temperature, and being easy to scale up.
The application of g-C3N4 in the field of green organic syn-
thesis is under exploration in our laboratory.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We acknowledge the financial support from the Center of
Advanced Analysis
& Computational Science (Zhengzhou
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