Visible-light-induced photoxidation-Povarov cascade reaction: synthesis of 2-arylquinoline through alcohol andN-benzylanilines under mild conditionsviaAg/g-C3N4nanometric semiconductor catalyst

Article abstract of DOI:10.1039/d0cc00885k

With a Ag/g-C₃N₄nanometric semiconductor as the photocatalyst, 2-arylquinolines were synthesized through a photoxidation-Povarov cascade reaction ofN-benzylanilines and alcohols under visible light irradiation. Under the blue light of a 3 W LED, good yields were achieved for various substrates in oxygen at room temperature. This methodology provides a green and mild alternative for the formation of 2-arylquinoline derivatives. Remarkably, the Ag/g-C₃N₄nanocomposite can be conveniently recovered and reused several times with satisfying yields.

Full text of DOI:10.1039/d0cc00885k

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DOI: 10.1039/D0CC00885K
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Visible-light-induced photoxidation-Povarov cascade reaction:
synthesis of 2-arylquinoline through alcohol and N-benzylanilines
under mild conditions via Ag/g-C3N4 nanometric semiconductor
catalyst
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Peng Wang,a Xiaowen Wang,a Xiyu Niu,a Li Zhu,b,* and Xiaoquan Yaoa,*
www.rsc.org/
With Ag/g-C3N4 nanometric semiconductor as photocatalyst, 2-
arylquinolines were synthesized through a photoxidation-Povarov
cascade reaction of N-benzylanilines and alcohols under visible
light irradiation. Under the blue light of a 3W LED, good yields
were achieved for various substrates in oxygen at room
a) Wang, Fan & Torok's group, ref. 12
O
NH2
H
R1
+
R2
R3
+
AuCl3-CuBr, 4d;[12a] FeCl3, 110oC,1d;[12b] solid acid, MW[12c]
temperature. This methodology provides
a green and mild
b) Takasu & Wang's group, ref. 13
R3
alternative for the formation of 2-arylquinoline derivatives.
Remarkably, the Ag/g-C3N4 nanocomposite can be conveniently
recovered and reused for several times with satisfied yields.
R1
+
R3(H)
N
or
O
R2
R1
R3
H
N
Tf2NH (10 mol%), 60 oC, 24 h;[13a] I2, benzene, reflux, 0.5h[13b]
R2
The utilization of solar radiant energy to drive organic
reactions stands out as the most promising choice to meet our
demand for green synthesis due to its abundance, absence of
cost, and eco-friendly nature.[1] Recently, many photocatalytic
synthetic reactions have been reported.[2] However, most of
the examples are involved in metal complexes or dyes as
catalyst. There are limited examples with solid nanometric
semiconductor as photocatalyst,[3] in which, to the best of our
knowledge, few of examples for a cascade reaction.[4]
c) Liu's group, ref. 14
R1
N
R3
R2
+
H
(tBuO)2 , FeCl3, DCE, 12 h
d) Li's group, ref. 15
O
NH2
+
H
R1
R3CH2CH2OH
R2
+
FeCl3.6H2O(10 mol%), PTSA(20 mol%), KI(20 mol%), 140 o
C
Due to the strong localized surface plasmon resonance (LSPR)
effect, coinage metal nanoparticles were recognized as a new
form of medium that is particularly efficient in harvesting light
energy for chemical processes over a wide range of the visible
and UV regions of the solar spectrum.[5] On the other hand,
graphitic carbon nitride (g-C3N4) is a novel aggregate type
semiconductor material and works as an important catalyst for
building a new photocatalytic system.[6] The combination of Ag
NPs and g-C3N4, provides a novel strategy to enhance the
photocatalytic performance of g-C3N4.[7] In our recent research,
Ag/g-C3N4 nanocomposite has exhibited highly photocatalytic
activities toward aerobic oxidative amidation of aromatic
aldehydes under visible light irradiation.[8]
This work:
R3(H)
R1
R1
Ag/g-C3N4(20 mol%)
Blue LED, rt, O2
N
R3CH2CH2OH
R2
+
N
H
R2
Scheme 1. Selected recent Povarov reactions for the synthesis of 2-
arylquinolines.
Quinolines and their derivatives represent a wild variety of
biological activities and are valuable synthetic intermediates
for many synthetic compounds with pharmacological
properties,[9] in which 2-arylquinoline backbones are
important structure for biologically active molecules.[10]
Moreover, these compounds are also crucial intermediates for
the preparation of dyes, polymers and materials with unique
electronic and optical properties. [11]
Recently, a few elegant examples to construct quinolines
have been disclosed. Among these examples (Scheme 1),
Wang’s, Fan’s and Török’s groups demonstrated a Povarov
reaction of aniline, benzaldehyde and phenylacetylene
(Scheme 1, a), respectively.[12] Takasu’s group reported a
a. Department of Applied Chemistry, School of Material Science and Technology,
Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China.
E-mail: yaoxq@nuaa.edu.cn.
b. Department of Chemistry, School of Pharmacy, Nanjing Medical University,
Nanjing 211166, PR China. Email: zl9422@njmu.edu.cn.
†Electronic Supplementary Information (ESI) available: See DOI: 10.1039/
x0xx00000x
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Table 1. Optimization of reaction conditions. [a]
cyclization-dehydroaromatization of N-arylaldimine and alkene.
Also, with aldehyde instead, a synthetic methodology for
substituted quinolines was developed by Wang’s group
(Scheme 1, b).[13] With DTBP as oxidant, Liu and his co-workers
described an iron-catalysed annulation of N-alkyl aniline and
alkyne to construct quinoline skeleton (Scheme 1, c).[14]
Moreover, in Li’s group reported example, with FeCl3-PTSA-KI
as catalyst combination, ethanol, instead of alkene or alkyne,
participated in reacting with aromatic aldehydes and anilines
to give quinoline at high temperature (Scheme 1, d). [15]
View Article Online
DOI: 10.1039/D0CC00885K
Ag/g-C3N4, O2
N
N
CH3CH2OH, rt
H
Blue LED
OMe
OMe
1a
2a
Yield [%] [b]
Additive
Entry
1
Catalyst (mol%)
Ag/g-C3N4 (10)
g-C3N4
none
none
62
36
27
n.r.
n.r
49
38
53
78
12
9
2[c]
3
In most of examples above, complicated & non-recyclable
catalysts, high temperature or stoichiometric oxidants have to
be used frequently to promote the reaction. Thus, to develop
an environmental friendly and green methodology is still under
Ag NPs (10)
none
4
none
none
investigation.
Herein,
with
Ag/g-C3N4
nanometric
5[d]
6
Ag/g-C3N4 (10)
Ag/bulk-C3N4 (10)
Cu/g-C3N4 (10)
Ag/g-C3N4 (5)
Ag/g-C3N4 (20)
Ag/g-C3N4 (20)
Ag/g-C3N4 (20)
Ag/g-C3N4 (20)
Ag/g-C3N4 (20)
Ag/g-C3N4 (20)
none
semiconductor as photocatalyst, we hope to report a green
and mild alternative for the synthesis of 2-arylquinoline
derivatives via a photoxidation-Povarov cascade reaction of N-
benzylanilines and alcohols under visible light irradiation.
Under the blue light of a 3W LED, good yields were achieved
for various substrates at room temperature in oxygen.
Remarkably, the Ag/g-C3N4 nanocomposite can be
conveniently recovered and reused for several times with
satisfied yields.
none
7
none
8
none
9
none
10
11
12[e]
13[f]
14[g]
PTSA (20 mol%)
Aniline (20 mol%)
none
The g-C3N4 was prepared through the solvothermal process
by using melamine and cyanuric chloride as precursors in
o
acetonitrile at 180 C.[8,16] With the porous g-C3N4 as support,
Ag/g-C3N4 hybrid material was then prepared by a facile
reduction method in ethanol solution with AgNO3 and
trisodium citrate as starting materials (the detailed
preparation and characterization were provided in ESI).
For the optimization of conditions, the reaction of N-(4-
meoxy)benzyl aniline (1a) and ethanol was selected as the
porotype. Under the blue light of a 3W LED, the desired
product, 2-(4-methoxyphenyl) quinoline (2a), was isolated with
62% of yield at room temperature under oxygen atmosphere
(entry 1, Table 1).
43
23
23
none
none
[a]
Reaction conditions, unless otherwise noted: N-benzylaniline (1a, 0.10 mmol),
ethanol (2.0 mL), photocatalyst, and 3W blue LED at ambient temperature in
oxygen for 48 h. [b] Isolated Yield.. [c] g-C3N4 (20 mg). [d] Reaction in dark. [e] 15W
blue LED. [f] Reaction time: 24h. [g] Reaction in air.
g-C3N4 and Ag NPs were then utilized alone as the
photocatalyst, respectively. As seen from entries 2&3 in Table
1, the absence of either Ag NPs or g-C3N4 went against this
reaction. The combination of g-C3N4 and metal nanoparticals
seems to be essential in the reaction.
When the intensity of light was enhanced to 15W, however,
the yield of 2a was decreased to 43% (entry 12, Table 1). We
also tried to short the reaction time to 24h, but significantly
lower yield was observed (entry 13, Table 1). Moreover, if the
reaction was conducted in air, only 23% yield was obtained
(Table 1, entry 14).
With the optimal conditions in hand (entry 9, Table 1), we
scanned the scope of the synthetic methodology for quinoline
derivatives. In the Ag/g-C3N4-catalyzed, visible-light-induced
Povarov tandem reaction, various N-benzylanilines were
examined under the standard reaction with ethanol as
substrate & solvent, and the results are summarized in Table 2.
As shown in Table 2, moderate to good yields were achieved
for various substrates. It seems that the substitutes on anilines
show marked effect on the reaction. As seen from 2a-2d, the
para-substituted electron-donating groups, such as methyl and
methoxyl, were disfavoured to the reaction, and a similar
trend was also observed from the other series. Most of
Subsequently, under blue light irradiation, a blank reaction
was carried out in the absent of photocatalyst and no reaction
occurred (entry 4, Table 1). As another controlled experiment,
a reaction in dark was also examined (entry 5, Table 1). From
both of the results above, obviously, either the visible light
irradiation or photocatalyst is indispensable in the reaction.
[17]
On the other hand, bulk-C3N4 was utilized instead of g-
C3N4, which proved to be less effective in the reaction (entry 6,
Table 1). Moreover, copper, instead of silver, was supported
by g-C3N4 and used to catalyse the reaction under the same
conditions, but only 38% of 2a was isolated (entry 7, Table 1).
Different loadings of the photocatalyst were also screened
in entries 8 & 9, 20 mol% of Ag/g-C3N4 is the best of choices,
and up to 78% of 2a was achieved. Furthermore, some
additives, such as PTSA and aniline, were then added in the
reaction, respectively. The desired product declined to only 12% substrates derived from 4-cholo-aniline gave the yields around
and 9% yield, respectively (entries 10 & 11，Table 1). [18]
80% (2b, 2f, 2j, 2m and 2q); on the contrary, only 50-60% of
yields were obtained from 4-methoxy-aniline derivatives (2d,
2 | J. Name., 2012, 00, 1-3
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2h and 2o). Moreover, 2-heteroaryl-substitued quinolines
were also synthesised via the current method, and up to 85%
of yield was achieved (2l-2o & 2p-2r).
View Article Online
(a)
2a (78%)
DOI: 10.1039/D0CC00885K
Ag/g-C3N4, O2
+
N
H
N
EtOH, blue LED, rt.
OMe
OMe
1a
1a
4
4
<10%
Table 2. Scope of the photoxidation-Povarov tandem reaction.[a]
Ag/g-C3N4, O2
N
H
N
(b)
MeCN, blue LED, rt.
R1
R1
OMe
OMe
53%
Ag/g-C3N4, O2
Scheme 2. The formation of imine 4 on Ag/g-C3N4
N
N
CH3CH2OH, rt
Blue LED
Ar
H
Ar
R2
O
R2
2[b]
1
Ag/g-C3N4, O2, blue LED, rt.
R1
R1
OH
observed from 1H NMR
or Ag/g-C3N4, H2O2, rt.
H
Scheme 3. Ag/g-C3N4 catalysed oxidation of EtOH under blue light or with H2O2.
N
N
OMe
Me
2a, R1 = H, 78%
2b, R1 = Cl, 82%
2c, R1 = Me, 63%
2e, R1 = H, 73%
2f, R1 = Cl, 71%
Ag/g-C3N4 (20 mol%)
blue LED, O2, MeCN, rt
N
N
41%
H
2d, R1 = OMe, 57%
5
6
R1
R1
Scheme 4. Ag/g-C3N4 catalysed dehydroaromatization of tetrahydroquinoline.
N
N
To widen the applicability of the procedure, n-propanol,
instead of ethanol, was then tested as the substrate as well as
the solvent. As shown in Table 3, the tandem reaction
proceeded smoothly with n-propanol, giving 3a-3c with
reasonable to moderate yields.
As a kind of heterogeneous photocatalyst, the Ag/g-C3N4
nanocomposite can be easily separated from the reaction
mixture by centrifugation and reused for new reaction. Thus,
the recyclability of the nanocomposite catalyst in the reaction
of 1a & ethanol was investigated (Figure 1). As shown in Figure
1, the Ag/g-C3N4 catalyst could be recovered and reused for 3
times at least without appreciable loss in yields.
2i, R1= H, 63%
2j, R1= Cl, 83%
2k, R1=Me, 75%
2g, R1 = Me, 67%
2h, R1 = OMe, 61%
R1
R1
S
O
N
N
2l, R1 = H, 84%
2p, R1 = H, 71%
2q, R1 = Cl, 85%
2r, R1 = Me, 67%
2m, R1 = Cl, 73%
2n, R1 = Me, 66%
2o, R1 = OMe, 53%
[a] The reaction was carried out by using N-benzylanilines (1, 0.10 mmol), ethanol
(2.0 mL), photocatalyst (20 mol%), oxygen atmosphere, and 3W blue LED
irradiation at ambient temperature for 48 h. [b] Isolated yield.
Table 3. The synthesis of substituted 2-aryl-3-methyquinolines with n-propanol[a]
.
From the reaction mixture of 1a & ethanol, an imine
compound (4), was isolated as by-product (Scheme 2, a). In our
previous research on the mechanism of the Ag/g-C3N4
catalysed oxidative amidation of aldehydes under visible light,
the single-electron oxidation of THF occurred on the catalyst's
surface has proven to be the initial step for the reaction.[8] We
proposed that the imine 4 might be formed following a similar
approach. Thus, as a controlled experiment, 1a was then
treated in MeCN solution alone with Ag/g-C3N4 under blue
light irradiation in oxygen at room temperature, and 53% of 4
were isolated.
R1
R1
Ag/g-C3N4, O2, rt
N
H
N
R2
CH3CH2CH2OH
Blue LED
R2
3[b]
1
MeO
Me
N
N
3a, 39%
3b. 43%
Cl
N
On the other hand, two blank reactions without 1a were also
carried out under oxygen or with H2O2, respectively. From
crude NMR, the formation of acetaldehyde was observed in
both of the reactions (Scheme 3). Furthermore, as a model-
reaction, tetrahydroquinoline (5) was also treated by Ag/g-
C3N4 & blue light in MeCN under oxygen atmosphere, and 41%
of quinoline (6) was isolated after 24 h (Scheme 4). It
suggested that Ag/g-C3N4 might also be an effective catalyst
for the dehydroaromatization of tetrahydroquinoline
derivatives under visible light irradiation.
3c, 54%
[a]
the reaction was carried out by using N-benzylanilines (1, 0.10 mmol), n-
propanol (2.0 mL), photocatalyst (20 mol%), oxygen atmosphere, and 3W blue
LED irradiation at ambient temperature for 48 h. [b] Isolated yield.
100
90
80
70
60
50
40
30
20
10
0
Based on these results above and relative literature reports,[7,
8, 13b]
a plausible reaction mechanism was proposed in Scheme
5. Initially, under visible light irradiation, g-C3N4 can be excited
to generate conduction band electrons (e-) and valence band
holes (h+), and then, amine 1a released one electron on the
valence band holes to give N-radical cation intermediate (A).
On the other hand, the photogenerated electrons excited to
conduction band were entrapped by Ag NPs, and could reduce
Fresh
Cycle 1
Cycle 2
Cycle 3
Figure 1. The recyclability of Ag/g-C3N4 photocatalyst in the reaction of 1a & ethanol.
This journal is © The Royal Society of Chemistry 20xx
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[3, 8, 19]
D. W. C. MacMillan, Chem. Rev., 2013, 113, 5322; (c) I.
the electron acceptor O2 to superoxide radical anion.
Furthermore, with the promotion of superoxide radical anion,
the N-arylaldimine (4) was then formed.[20]
Ghosh, L. Marzo, A. Das, R. Shaikh and B. KönigVi,ewAAcrcti.clCe hOenlmine.
DOI: 10.1039/D0CC00885K
Res., 2016, 49, 1566.; (d) Q.-Q. Zhou, Y.-Q. Zou, L.-Q., Lu, and
W.-J. Xiao, Angew. Chem. Int. Ed. 2019, 58, 1586.
Recent review and examples: (a) H. Kisch, Acc. Chem. Res.
2017, 50, 1002; (b) A. Hainer, N. Marina, S. Rincon, P. Costa,
A. E. Lanterna and J. C. Scaiano, J. Am. Chem. Soc., 2019, 141,
4531; (c) Y.-Y. Liu, D. Liang, L.-Q. Lu, and W.-J. Xiao, Chem.
Commun., 2019, 55, 4853; (d) L. Li, X. Mu, W. Liu, Z. Mi and C.-
J. Li, J. Am. Chem. Soc., 2015, 137, 7576.
Subsequently, the in situ formed N-arylaldimine (4) reacted
with acetaldehyde, which is from the in situ photo-oxidation of
ethanol with Ag/g-C3N4 as catalyst, to generate the
intermediate B underwent a Povarov process.[13b] Finally, B
went through oxidative dehydroaromatization to give the final
product 2a.
3
4
5
J.-R. Chen, D.-M. Yan, Q. Wei, and W.-J. Xiao ChemPhoto-
Chem, 2017, 1, 148
(a) S. Linic, P. Christopher, and D. B. Ingram, Nat. Mater.
2011, 10, 911; (b) K. Watanabe, D. Menzel, N. Nilius, and H. J.
Freund, Chem. Rev. 2006, 106, 4301.
Recent reviews: (a) W. Ong, L. Tan, Y. H. Ng, S. Yong and S.
Chai, Chem. Rev., 2016, 116, 7159; (b) S. Cao, J. Low, J. Yu
and M. Jaroniec, Adv. Mater., 2015, 27, 2150; (c) L. Song, S.
Zhang, X. Wu, H. Tian and Q. Wei, Indust.&Eng. Chem. Res.,
2012, 51, 9510.
(a) L. Ge, C. Han, J. Liu and Y. Li, Appl. Catal. A, 2011, 409,
215; (b) X. Bai, R. Zong, C. Li, D. Liu, Y. Liu and Y. Zhu, Appl.
Catal. B, 2014, 147, 82.
O2
e-
CB
O2
Ag
hv
g-C3N4
N
H
6
1a
OMe
VB
O2
+ H2O2
OMe
N
- H
N
H
7
4
A
OMe
O
OH
Ag/g-C3N4, hv
OH
Povarov
Reaction
8
9
L. Wang, M. Yu, C. Wu, N. Deng, C. Wang and X. Yao, Adv.
Synth. Catal., 2016, 358, 2631.
Some examples: (a) G. J. Atwell, B. C. Baguley and W. A.
Denny, J. Med. Chem., 1989, 32, 396; (b) A. T. Vu, S. T. Coh
and E. S. Manas, Bioorg. Med. Chem. Lett., 2005, 15, 4520.
O
2 or H2O2
H
OH
Ag/g-C3N4, hv
O2
10 A. A. Joshi and C. L. Viswanathan, Bioorg. Med. Chem. Lett.,
2006, 16, 2613.
N
N
H
2a
OMe
OMe
B
11 Recent examples: (a) T. Rodrigues, D. Reker, J. Kunze, P.
Schneider and G. Schneider, Angew. Chem. Int. Ed., 2015, 54,
10516; (b) S. Sabatini, F. Gosetto, G. Manfroni, O. Tabarrini,
G. W. Kaatz, D. Patel and V. Cecchetti, J. Med. Chem., 2011,
54, 5722.
12 (a) F. Xiao, Y. Chen, Y. Liu and J. Wang, Tetrahedron, 2008, 64,
2755; (b) K. Cao, , F. Zhang, Y. Tu, X. Zhuo and C. Fan,
Chemistry, 2010, 15, 6332; (c) A. Kulkarni and B. Török,
Green Chem., 2010, 12, 875.
13 (a) N. Shindoh, H. Tokuyama, Y. Takemoto and K. Takasu, J.
Org. Chem., 2008, 73, 7451; (b) X. Lin, S. Cui and Y. Wang,
Tetrahedron Lett., 2006, 47, 3127; a similar example for
quinoline-2-carboxylate: (c) H. Huang, H. Jiang, K. Chen and
H. Liu, J. Org. Chem., 2009, 74, 5476.
14 P. Liu, Y. Li, H. Wang, Z. Wang and X. Hu, Tetrahedron Lett.,
2010, 53, 6654.
Scheme 5. Proposed reaction mechanism.
Conclusions
In summary, a solid nanometric semiconductor, Ag/g-C3N4,
was utilized successfully as a recyclable photocatalyst for a
photoxidation-Povarov tandem reaction of N-benzylanilines
and ethanol under visible light irradiation in oxygen. The
transformation provided an environment-friendly alternative for
the synthesis of 2-arylquinoline derivatives. Further studies to gain
more mechanistic details of this reaction and apply this strategy to
other organic transformations are ongoing in our laboratory.
15 X. Li, Q. Xing, P. Li, J. Zhao and F. Li, E. J. Org. Chem., 2017, 3,
618.
16 Y. J. Cui, Z. X. Ding, X. Z. Fu and X. C. Wang, Angew. Chem. Int.
Ed., 2012, 51, 11814.
17 S. Huang, Y. Zhao and R. Tang, Rsc. Adv., 2016, 6, 90887.
18 Obviously, aniline strongly inhibited the reaction. Moreover,
Acknowledgments
We are grateful to the financial support from National Natural
Science Foundation of China (21772091, 21472092 to XY). This
is a project funded by the Priority Academic Program
Development of Jiangsu higher education institutions.
a
three-component, one-pot
reaction of 4-methoxy-
benzaldehyde, aniline and ethanol was also carried out with
Ag/g-C3N4 as catalyst under blue light, but no product was
detected (please also see Scheme S1 in ESI).
19 (a) X. Chen, J. Zhang, X. Fu, M. Antonietti and X. Wang, J. Am.
Chem. Soc., 2009, 131, 11658; (b) X. Dai, M. Xie, S. Meng, X.
Fu and S. Chen, Appl. Cat. B, 2014, 158-159, 382.
Conflicts of interest
There are no conflicts to declare.
20 Y. Nosaka and A. Y. Nosaka, Chem. Rev. 2017, 117, 11302.
Notes and references
1
Z. Zou, J. Ye, K. Sayama and H. Arakawa, Nature, 2001, 414,
625.
Selected reviews: (a) D. Ravelli, M. Fagnoni and A. Albini,
Chem. Soc. Rev., 2013, 42, 97; (b) C. K. Prier, D. A. Rankic and
2
4 | J. Name., 2012, 00, 1-3
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Ag/g-C3N4 nanometric semiconductor catalyzed cascade reaction for the synDthOeI:s1i0s.103o9f/D0CC00885K
2-arylquinoline through alcohol and N-benzylanilines under visible light irradiation
O2
e-
CB
O2
Ag
hv
R
g-C3N4
N
H
R'
1
VB
O2
R
OH
Ag/g-C3N4, hv
R
R
- H
N
N
N
H
R'
R'
R'
2