Solvent-free and room temperature visible light-induced C-H activation: CdS as a highly efficient photo-induced reusable nano-catalyst for the C-H functionalization cyclization of: T -amines and C-C double and triple bonds

Article abstract of DOI:10.1039/c8gc03297a

Nano-sized CdS was successfully prepared, fully characterized and applied as a highly efficient reusable photocatalyst for the synthesis of pyrrolo[3,4-c]quinolone and pyrrolo[2,1-a]isoquinoline-8-carboxylate derivatives through a condensation reaction of N,N-dimethylanilines or alkyl 2-(3,4-dihydroisoquinolin-2(1H)-yl)acetates with maleimides via a C-H activation approach under benign and eco-friendly conditions at room temperature without using any solvent and oxidant under visible light irradiation. Besides, the prepared photocatalyst has been successfully applied for the condensation reaction of N,N-dimethylanilines with alkyl but-2-ynedioates or phenyl acetylenes for the synthesis of novel 1,2-dihydroquinoline-3,4-dicarboxylate and aryl-1,2-dihydroquinoline derivatives for the first time. Using this method, all favourable products were obtained in good yields and relatively short reaction times under benign conditions with the application of visible light irradiation, a renewable energy source. The catalyst was easily recovered and reused several times without any loss of its activity.

Full text of DOI:10.1039/c8gc03297a

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Solvent-free and room temperature visible
light-induced C–H activation: CdS as a highly
efficient photo-induced reusable nano-catalyst for
the C–H functionalization cyclization of t-amines
and C–C double and triple bonds†
Cite this: Green Chem., 2018, 20,
5540
Somayeh Firoozi,
Mona Hosseini-Sarvari * and Mehdi Koohgard
Nano-sized CdS was successfully prepared, fully characterized and applied as a highly efficient reusable
photocatalyst for the synthesis of pyrrolo[3,4-c]quinolone and pyrrolo[2,1-a]isoquinoline-8-carboxylate
derivatives through a condensation reaction of N,N-dimethylanilines or alkyl 2-(3,4-dihydroisoquinolin-2
(1H)-yl)acetates with maleimides via a C–H activation approach under benign and eco-friendly conditions
at room temperature without using any solvent and oxidant under visible light irradiation. Besides, the pre-
pared photocatalyst has been successfully applied for the condensation reaction of N,N-dimethylanilines
with alkyl but-2-ynedioates or phenyl acetylenes for the synthesis of novel 1,2-dihydroquinoline-3,4-
dicarboxylate and aryl-1,2-dihydroquinoline derivatives for the first time. Using this method, all favourable
products were obtained in good yields and relatively short reaction times under benign conditions with
the application of visible light irradiation, a renewable energy source. The catalyst was easily recovered
and reused several times without any loss of its activity.
Received 21st October 2018,
Accepted 5th November 2018
DOI: 10.1039/c8gc03297a
rsc.li/greenchem
and facile procedures and catalytic systems for C–H
functionalization in organic synthesis.
Introduction
It is noteworthy that C–H bonds are one of the least reactive
Nowadays, synthesized molecules bearing amine based
bonds and their direct functionalization required pre-acti- scaffolds have attracted the attention of pharmaceutical chem-
vation steps, such as halogenation and stoichiometric metala- istry researchers because they can be applied as beneficial
tion of the starting materials. So, direct C–H bond functionali- building blocks in pharmaceuticals and modern drug design
zation has considerably attracted the interest of organic syn- approaches.4 Among them, quinolones have attracted much
thesis researchers over the past several years.1 In particular, attention because they show some important pharmaceutical
the direct functionalization at a C–H centre adjacent to a activities including analgesic, antipsychotic, antibacterial, and
heteroatom (nitrogen, oxygen and/or sulfur) has been exten- antidepressant activities.5
sively studied.2 In all cases, these synthetic routes involve a
Interestingly, pyrrolo[3,2-c]quinolines have been used for
single electron transfer (SET) step and are highly restricted to the treatment of Alzheimer’s disease (Fig. 1 compound A)6 and
the presence of both a catalyst and an oxidant. Though, in the also, heterocyclic tetrahydroquinolines show potent antibacter-
case of some organic peroxides, coupling at the C–H centre ial activity against a wide range of microorganisms (Fig. 1 com-
has been achieved without any catalyst, the need for excess pounds B–E).7 In addition, these compounds have been fre-
amounts of peroxides and the use of elevated temperatures are quently studied as antipsychotic, antidepressant, and analge-
two crucial drawbacks that limit the practicability of these sic agents (Fig. 1 compound F).5
methods.3 So, there still is a great demand for more practical
Accordingly, introducing advanced practical pathways for
the preparation of pyrrolo[3,2-c]quinolines will lead to the
enhancement of pharmaceutical chemists’ ability to synthesize
structurally-diverse derivatives of these compounds. So,
various non-photochemical8–13 and photochemical14–24
approaches have previously been reported for the synthesis of
pyrrolo[3,4-c]quinolones (Scheme 1).
Department of Chemistry, Shiraz University, Shiraz 7194684795, I.R. Iran.
E-mail: hossaini@shirazu.ac.ir
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8gc03297a
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bulky counterparts, chalcogenide semiconductor nano-
structures such as PbS, CdS, ZnS, PbSe, and CdSe have
found wide applications.26 Among them, CdS nanoparticles
(CdS NPs) are glamorous semiconductors that are frequently
used as a heterogeneous photocatalyst. In addition, they have
charming applications in photo-devices and logic circuits and
also, they have been notably applied in the transmutation
solar energy to chemical energy and solar fuels.27 These par-
ticles are efficient, practical, and cost effective candidates for
changing solar energy to chemical energy, and are granted
a place in the sun for supplying a reliable long-term solution
with potential to address the environmental and energy
issues.28–32 It is usually supposed that CdS NPs effectively
absorb solar light because of a suitable band gap and high
surface area leading to more photon absorption on their sur-
faces, which make them useful as versatile photocatalysts in
the treatment of organic wastes from water.33–39
In continuation of our recent research on the introduction
of efficient photocatalysts for the synthesis of pyrrolo[3,4-c]qui-
nolones40 and in order to improve the simplicity and catalyst
efficiency and to develop a more eco-friendly procedure, we
herein report the synthesis and characterization of nano-sized
CdS as a heterogeneous reusable and highly efficient photo-
catalyst for the synthesis of pyrrolo[3,4-c]quinolones (3a–u),
pyrrolo[2,1-a]isoquinoline-8-carboxylates (5a–i), 1,2-dihydro-
quinoline-3,4-dicarboxylates (7a–f) and aryl-1,2-dihydroquino-
lines (7g–k) via a photo-induced C–H activation approach
through the reaction between N,N-dimethylanilines (1a–d) or
alkyl 2-(3,4-dihydroisoquinolin-2(1H)-yl)acetates (4a,4b) with
Fig. 1 The chemical structures of some biologically active
pyrroloquinolones.
Scheme 1 The synthesis of pyrrolo[3,4-c]quinolone derivatives from
maleimides and N,N-dimethylanilines in previous reports.
Non-photochemical approaches are restricted to the pres-
ence of a catalyst and an oxidant at the same time and high
temperatures under either an oxygen or nitrogen
atmosphere.8–13 However, photochemical approaches use
organic colours as photocatalysts14–16 in which the challenges
created by the homogeneity of catalyst and environmental
contamination are unavoidable. Recently, chlorophyll-cata-
lysed pathways have also been reported.17 Furthermore, toxic
and expensive metal complexes such as iridium18 and ruthe-
nium19 for these kinds of C–H activations were applied.20
So, introducing more benign and recoverable catalysts with
lower toxicity to avoid environmental pollution is of great
interest.
Heterogeneous photocatalytic reactions using semi-
conductors or other irradiated solids are noteworthy catalytic
approaches. In recent decades, noticeable researches on
semiconductor nanoparticles have been done.25 Having
Scheme 2 The photo-induced C–H functionalization for the synthesis
of pyrrolo[3,4-c]quinolones, pyrrolo[2,1-a]isoquinoline-8-carboxylates,
1,2-dihydroquinoline-3,4-dicarboxylates and aryl-1,2-dihydroquinolines
in the presence of CdS NPs as a highly efficient heterogeneous reusable
unique mechanical and optical attributes compared with photocatalyst.
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maleimide, N-aryl and N-alkyl maleimides (2a–r) or a conden- pletion of the reaction, ethanol (2 ml) was added to the reac-
sation reaction between N,N-dimethylanilines (1a–c) and alkyl tion mixture and insoluble CdS NPs were completely separated
but-2-ynedioates (6a, 6b) and phenyl acetylenes (6c, 6d) at by centrifugation, washed with ethanol (1 mL, 3 times), dried
room temperature under irradiation with sunlight or a blue under reduced pressure and reused. In order to obtain pure
LED lamp without the use of any solvent and oxidant products, the solvent (ethanol) was evaporated under reduced
(Scheme 2). It is worth mentioning that, nevertheless, there is pressure and the obtained crude products were purified by
no report on the condensation of N,N-dimethylanilines and column chromatography to afford pure pyrrolo[3,4-c]quinolone
triple bonds via a C–H activation approach. This strategy is products. When both thereactants are solid, for instance in the
attractive and novel for the synthesis of 1,2-dihydroquinoline- synthesis of 3q, acetonitrile (3 mL) as a solvent was used
3,4-dicarboxylates and aryl-1,2-dihydroquinolines as new cat- instead of solvent-free conditions.
egories of quinolines. Moreover, according to a literature
survey there is no report based on using solvent-free con-
ditions for the synthesis of organic compounds in the pres-
ence of photocatalysts as visible light-induced catalysts.
General procedure for the synthesis of 1,2-dihydroquinoline-
3,4-dicarboxylates and aryl-1,2-dihydroquinolines
An open test tube equipped with a magnetic stir bar was
charged with CdS NPs (0.01 mmol, 0.014 g). Then, N,N-di-
methylaniline (1.5 mmol) and alkyl but-2-ynedioate or phenyl
acetylene (1.0 mmol) derivatives were added. The open test
tube was placed under sunlight or a blue LED and the mixture
was stirred at room temperature for 24–36 h under solvent-free
conditions. The reaction improvement was checked by thin
layer chromatography (TLC). After the completion of the reac-
tion, ethanol (2 ml) was added to the reaction mixture and in-
soluble CdS NPs were completely separated by centrifugation,
washed with ethanol (1 mL, 3 times), dried under reduced
pressure and reused. In order to obtain pure products, the
solvent (ethanol) was evaporated under reduced pressure and
the obtained raw products were purified by column chromato-
graphy to afford pure 1,2-dihydroquinoline-3,4-dicarboxylate
products.
Experimental
All required chemicals were obtained from Fluka and Merck
companies and were used without any extra purification. A
FT-IR spectrometer model Shimadzu FT-IR 8300 was applied
to FT-IR measurement using KBr pellets. The X-ray diffraction
(XRD) of all samples was performed using a Bruker AXS D8-
advance X-ray diffractometer using CuKα radiation (λ
=
1.54178 Å). The morphology of catalyst was determined using
Leica Imaging Systems, Cambridge, England model s360,
version V03.03. Scanning electron microscopy (SEM) was per-
formed using a HITACHI S-4160. A Bruker advanced DPX-250
spectrometer was applied to obtain NMR spectra in CDCl3
operating for 13C at 62.9 MHz and for 1H at 250 MHz. The
product yield was measured using a GC model Shimadzu
GC 14B.
Results and discussion
General procedure for the synthesis of CdS NPs
At first, the CdS NPs were synthesized using a facile thermal
approach and were fully characterized by XRD, FT-IR, SEM,
and TEM as well as solid UV analysis. The crystalline structure
of CdS NPs was investigated by XRD. The obtained XRD pat-
terns are shown in Fig. 2 and compared with the published
data (JCPDS 10-454). Reflections are observed at 2θ = 26.3,
30.5, 44.7 and 51.9 belonging to the (111), (200), (220) and
(311) crystallographic planes, respectively. The average crystal-
In a typical procedure, in a 500 mL round bottom flask,
N-cetyl-N,N,N-trimethylammonium bromide (CTAB) (0.4 g) and
sodium sulfide hexahydrate (Na2S·6H2O) (0.93 g, 5 mmol) were
dissolved in deionized water (300 mL). Then, cadmium nitrate
tetrahydrate (Cd(NO3)2·4H2O) (100 mL, 0.045 M) was added
dropwise and the obtained solution was stirred for 1 h at room
temperature. After this time, the resulting mixture was stirred
at reflux temperature for 36 h. Finally, the resultant yellow pre-
cipitates were collected by centrifugation at 4000 rpm for
5 min, washed with deionized water and ethanol (100 mL,
3 times) and dried at 50 °C for 24 h.
General procedure for the synthesis of pyrrolo[3,4-c]
quinolones and pyrrolo[2,1-a]isoquinoline-8-carboxylates
An open test tube equipped with a magnetic stir bar was
charged with CdS NPs (0.01 mmol, 0.014 g). Then, maleimide
(1.0 mmol) and N,N-dimethylaniline or alkyl 2-(3,4-dihydroiso-
quinolin-2(1H)-yl)acetate derivatives (1.5 mmol) were added.
The open test tube was placed under sunlight or a blue LED
and the mixture was stirred at room temperature for 12–36 h
under solvent-free conditions. The reaction improvement was
checked by thin layer chromatography (TLC). After the com- Fig. 2 The XRD patterns of freshly synthesized CdS NPs.
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NPs, and the range of size is approximately between 6 and
8 nm measuring from TEM which confirms the data from XRD
analysis.
To determine the optical properties of the synthesized CdS
NPs, UV-visible absorption measurements were accomplished
and the obtained graph is shown in Fig. 5. Clearly, the CdS
NPs could be excited in the presence of visible light. The CdS
NPs show the photo-absorption from UV light to visible light,
and the wavelength of the absorption edge is about
400–480 nm, which could explain its visible-light induced
photo-catalytic activity (Fig. 6).
Fig. 3 FT-IR spectra of CdS.
Fig. 7 shows the photoluminescence spectra of the syn-
thesized CdS nanoparticles at an excitation wavelength of
420 nm. It is seen that maximum emissions appear at a wave-
length of 624 nm with the excitation wavelength at 420 nm.
After successfully preparing CdS NPs, their application as a
heterogeneous photocatalyst was pursued in the synthesis of
pyrrolo[3,4-c]quinolone derivatives. The condensation reaction
between N-phenylmaleimide (2a) and N,N-dimethylaniline (1a)
was chosen as a model reaction for the synthesis of 5-methyl-
2-phenyl-3a,4,5,9b-tetrahydro-1H-pyrrolo[3,4-c]quinoline-1,3
(2H)-dione (3a). Various reaction conditions were optimized
and the best results were obtained with 10 mol% of CdS NPs
without any solvent under sunlight or blue light irradiation
and air at room temperature. According to Tables 1 and 2
various solvents, catalyst quantities, different wavelengths of
irradiation produced from distinct resources and different
atmospheres were tested. A batch of solvents including H2O,
line nanoparticle size of CdS was calculated using the Scherrer
equation and is equal to 7 nm (Fig. 2).
The FT-IR spectra of CdS NPs show absorption in regions
3440.8, 1620.1, 1419.5, and 1118.6 cm−1 (Fig. 3).
The SEM image of freshly synthesized CdS NPs is shown in
Fig. 4. The nanoparticles of CdS with a diameter around
20–30 nm can be observed. These particles have a spherical
morphology.
The TEM image of the synthesized CdS NPs is shown in
Fig. 5. Spherical particles could be clearly seen for the CdS
Fig. 4 SEM images of CdS.
Fig. 6 The UV-visible absorption graph of freshly synthesized CdS NPs.
Fig. 5 TEM images of CdS.
Fig. 7 The photoluminescence spectra of freshly synthesized CdS NPs.
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Table 1 The effect of various solvents and different quantities of N,N-
dimethylaniline as a precursor in the synthesis of 5-methyl-2-phenyl-
3a,4,5,9b-tetrahydro-1H-pyrrolo[3,4-c]quinoline-1,3(2H)-dione
Table 2 The optimization of the model reaction based on diverse
amounts of photocatalyst, wavelengths of light, and atmosphere (gas)
Entry
CdS amount (mol %)
Light
Atmosphere
Yield (%)
Entry
Solvent
Yield (%)
1
2
3
4
5
6
7
8
Catalyst free
5
White
White
White
White
White
White
Dark
Blue
Green
Red
White
Blue
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
O2
O2
Ar
—
1
2
3
4
5
6
7
8
H2O
EtOH
CH3CN
EtOAc
Acetone
CH2Cl2
CHCl3
THF
DMSO
DMF
Solvent-free
Solvent-free
Solvent-free
Solvent-free
50a
84a
91a
67a
74a
18a
23a
47a
89a
77a
88b
91c
95a
95d
75
10
15
20
25
10
10
10
10
10
10
10
10
10
95a
95
96
95
—
96b
33
9
9
10
11
12
13
14
15
24
96a,c
95b,c
10
11
12
13
14
d
White
Blue
Sun
—
d
Ar
Air
—
96e
The condensation reaction of N,N-dimethylaniline (1–2 eq.),
N-phenylmaleimide (0.3 mmol), CdS as nano photocatalyst (10 mol%)
and solvent (2 mL) under irradiation with a white LED 12 W lamp for
The condensation reaction of N-phenylmaleimide (0.3 mmol), N,N-
dimethylaniline (0.45 mmol) and different amounts of CdS as a nano
photocatalyst (mol %) under different LED 12 W lamps and solvent-
free conditions for 24 h at room temperature in air. a The completion
time of reaction is 18 h. b The completion time of reaction is 12 h.
c Oxygen balloon. d Argon balloon. e The completion time of reaction is
8 h.
24
h
at room temperature in air. a N,N-Dimethylaniline 1.5 eq.
(0.45 mmol). b N,N-Dimethylaniline
1
eq. (0.3 mmol). c N,N-
Dimethylaniline 1.2 eq. (0.36 mmol). d N,N-Dimethylaniline 2 eq.
(0.6 mmol).
EtOH, CH3CN, EtOAc, acetone, CHCl3, CH2Cl2, THF, DMF and
Next, in order to find the generality, different pyrrolo[3,4-c]
DMSO in the presence of CdS NPs and under white light quinolone derivatives were synthesized under the optimized
and solvent-free conditions were tested. As can be seen in conditions (Table 3). Both electron releasing groups (ERG)
Table 1, the best results have been obtained under solvent-free (Table 3, products 3b–j) and electron withdrawing groups
conditions.
(EWG) (Table 3, products 3k–m) at ortho, meta and para posi-
After finding the solvent-free conditions, diverse amounts tions on the aromatic ring in N-aryl maleimides as the precur-
of catalyst were tested. The efficiency of reaction in the pres- sor showed no impressive distinction in reaction yields. It is
ence of less than 10 mol% of CdS was negligible (Table 2, supposed that due to being away from the reaction site, aro-
entry 2) and amounts more than 10 mol% of CdS showed no matic rings have no effect on the reaction outcome. N-Aryl mal-
increase in the yield of desired product or reduction in the eimides and also N-alkyl maleimides (Table 3, products 3n–p)
reaction time (Table 2, entries 4–6). The reaction was not pro- yielded the corresponding products in excellent yields. There
ceeded without CdS NPs as a photocatalyst (Table 2, entry 1). are somehow different results for N,N-dimethylaniline com-
Effects of light irradiation and different atmospheres were also pared with N-aryl maleimide. In this case, electron donating
studied and it was observed that the reaction did not progress and electron withdrawing groups such as Br, CH3 and NO2,
in the absence of light (Table 2, entry 7). This phenomenon have different effects on the reaction. An increment in the
shows that CdS photocatalyst gets excited by light radiation. yield and a reduction in the reaction time were observed with
The yield of the reaction was decreased and the reaction time electron donating groups at aromatic ring of amines. It may
was increased with the red and green lights, while the same come from the stability of the cation radical related to the
yield and reaction time was found for white (Table 2, entries 3 presence of electron donating groups that can accelerate the
and 11) and blue (Table 2, entries 8 and 12) lights. However, radical creation process (Table 3, products 3r, 3s, 3t). These
the reaction time reduced from 18 h to 12 h with blue light substitutions can also increase the electron releasing capa-
irradiation. On the other hand, the reaction time decreased to bility of the amines. With NO2 substitution on N,N-di-methyl-
8 h with sunlight irradiation (Table 2, entry 15). The same data aniline, the reaction was not progressed (Table 3, product 3u).
were obtained for the oxygen and air atmosphere but there was It may be due to the high electron withdrawing effect of NO2
no noticeable result under an argon atmosphere. So, the air substituent causes an inhibition of amine to place its electrons
atmosphere was selected as the best condition.
in the hole and evanescence cation radicals.
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Table 3 The synthesis of pyrrolo[3,4-c]quinolone derivatives
Based on the obtained results, the best reaction conditions for
the synthesis of pyrrolo[2,1-a]isoquinoline-8-carboxylate are
highly similar to the optimized reaction conditions for the syn-
thesis of pyrrolo[3,4-c]quinolones. The best reaction conditions
were obtained with CdS (10 mol%) as a photocatalyst under
sunlight or blue LED lamp irradiation without any solvent and
oxidant at room temperature and under air. The generality of
this reaction was established using various alkyl 2-(3,4-di-
hydroisoquinolin-2(1H)-yl)acetates and maleimides and the
corresponding products were yielded in excellent amounts
(Scheme 3 and Table 4).
Scheme 3 The photo-induced C–H functionalization for the synthesis
of pyrrolo[2,1-a]isoquinoline-8-carboxylates in the presence of CdS
NPs.
Table 4 The synthesis of pyrrolo[2,1-a]isoquinoline-8-carboxylate
derivatives
Reaction conditions: N,N-Dimethylaniline (1.5 mmol), maleimide
derivatives (1.0 mmol), and CdS nanoparticles (10 mol%) under
irradiation with a blue LED 12 W lamp and solvent-free conditions at
room temperature in air.
Furthermore, the scope of this reaction was explored with
an oxidation-[3 + 2] cycloaddition-aromatization reaction for
the synthesis of aryl-1,2-dihydroquinolines. In order to
appraise this proposed implication, initially
a reaction
between ethyl 2-(3,4-dihydroisoquinolin-2(1H)-yl)acetate (4a)
and N-phenylmaleimide (2a) in the presence of CdS as a
photocatalyst was selected as a model reaction and various
reaction conditions were optimized (see the ESI† for details).
Reaction conditions: Alkyl 2-(3,4-dihydroisoquinolin-2(1H)-yl)acetates
(1.5 mmol), maleimide derivatives (1.0 mmol), and CdS nanoparticles
(10 mol%) under irradiation with a blue LED 12 W lamp and solvent-
free conditions at room temperature in air.
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These interesting results encouraged us to establish the
condensation of N,N-dimethylanilines and the triple C–C bond
as an attractive and new strategy for the synthesis of novel aryl-
1,2-dihydroquinolines. For this purpose, the optimized reac-
tion conditions for the synthesis of pyrrolo[3,4-c]quinolones
and pyrrolo[2,1-a]isoquinoline-8-carboxylates have been
applied and some new 1,2-dihydroquinoline-3,4-dicarboxylate
derivatives (7a–f) were successfully synthesized with the reac-
tion of N,N-dimethylanilines (1a–c) and alkyl but-2-ynedioates
(6a,b) (Scheme 4 and Table 5).
Scheme 5 The photo-induced C–H functionalization for the synthesis
of aryl-1,2-dihydroquinolines in the presence of CdS NPs.
Phenyl acetylenes (6c, 6d) have been applied instead of
alkyl but-2-ynedioates (6a, 6b) and all desired products (7g–l)
were successfully obtained in the presence of CdS as a photo-
catalyst by irradiating with sunlight or a blue LED lamp
without any solvent at room temperature and under air for the
first time (Scheme 5) and the obtained results are summarized
in Table 6.
Table 6 The synthesis of aryl-1,2-dihydroquinoline derivatives
The recyclability of CdS catalyst as a heterogeneous nano-
photocatalyst showed an acceptable photo-catalytic activity of
CdS, and it can be used repeatedly with no significant
reduction in the reaction yield (Fig. 8).
Reaction conditions: N,N-Dimethylaniline (1.5 mmol), phenyl
acetylene derivatives (1.0 mmol), and CdS NPs (10 mol%) under
irradiation with a blue LED 12 W lamp and solvent-free conditions at
room temperature in air.
Scheme 4 The photo-induced C–H functionalization for the synthesis
of 1,2-dihydroquinoline-3,4-dicarboxylates in the presence of CdS NPs.
Table 5 The synthesis of 1,2-dihydroquinoline-3,4-dicarboxylate
derivatives
Fig. 8 The reusability of the catalyst for a series of seven consecutive
runs of the reaction of N-phenylmaleimide (0.3 mmol), N,N-dimethyl-
aniline (0.45 mmol) and CdS as a nano-photocatalyst (10 mol%) under
LED 12
W lamp irradiation and solvent-free conditions at room
temperature.
To propose a feasible mechanism for the synthesis of the
title compounds in the presence of CdS NPs some additional
studies were performed. Therefore, for the synthesis of
5-methyl-2-phenyl-3a,4,5,9b-tetrahydro-1H-pyrrolo[3,4-c]quino-
line-1,3(2H)-dione (3a), according to the optimized conditions,
the reaction between N,N-dimethylaniline and N-phenylmaleimide
was carried out in the presence of radical and hole scavengers
to show the radical pathway of the reaction (Scheme 6).
Reaction conditions: N,N-Dimethylaniline (1.5 mmol), but-2-ynedioate
derivatives (1.0 mmol), and CdS NPs (10 mol%) under irradiation with
a
blue LED 12
W lamp and solvent-free conditions at room
temperature in air.
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Scheme 7 The proposed mechanism for the synthesis of 5-methyl-2-
phenyl-3a,4,5,9b-tetrahydro-1H-pyrrolo[3,4-c]quinoline-1,3(2H)-dione
(3a) via a C–H activation approach in the presence of CdS NPs as a
heterogeneous photocatalyst.
conduction band by the incidence of photons to CdS NPs.
Therefore, a hole is created in the valence band and an elec-
tron in the conduction band is absorbed by oxygen. Then, a
N,N-dimethylaniline radical cation (B) can be created by elec-
tron transfer from N,N-dimethylaniline (A) to the holes. Then,
the radical (C) forms via losing a proton from the N,N-di-
methylaniline radical cation (B). In the next step, this radical
(C) attacks the maleimide double bond (D) and the intermedi-
ate E will be produced. The attack on the π bond of the
benzene ring leads to intra-cyclization (F) and finally, the
corresponding product (G) can be obtained by losing an elec-
tron and proton to set up aromaticity.
Finally, the existence of cadmium in the final products
was checked with the ICP-OES method. For this purpose,
the model reaction of N,N-dimethylaniline (1a) and
N-phenylmaleimide (2a) was conducted under the optimized
conditions and after the purification of the final products (3a),
the isolated products were dissolved in HNO3. The ICP-OES
analysis result showed that Cd is not present in the analysed
sample.
Scheme 6 The mechanism studies: the condensation reaction of
N-phenylmaleimide (0.3 mmol) and N,N-dimethylaniline (0.45 mmol) in
the presence of CdS NPs (10 mol%) under irradiation of a blue LED 12 W
lamp and solvent-free conditions for 24 h at room temperature under
air. (A) In the presence of TEMPO (0.45 mmol) as a radical scavenger. (B)
In the presence of triethanolamine (0.45 mmol) as a filler of holes. (C) 1H
NMR from the reaction mixture (solubilized in CDCl3 as a NMR solvent)
and the detected H2O2 peak. (D) The starch paper study of the presence
a
of H2O2 during the expedition of the reaction: the starch test paper,
b
the starch test paper after being in the reaction mixture (solubilized in
c
EtOH (2 mL)), the starch test paper after being in a mixture of H2O2
(30% w/w, 0.1 mL) and EtOH (2 mL), d the starch test paper after being in
a mixture of H2O2 (30% w/w, 0.3 mL) and EtOH (2 mL), the starch test
e
paper after being in a mixture of H2O2 (30% w/w, 1 mL) and EtOH
(2 mL).
As shown in Scheme 6, in the presence of 2,2,6,6-tetra-
methylpiperidine 1-oxyl (TEMPO) as
(Scheme 6A) and triethanolamine (TEA) as a filler of the holes
a radical scavenger
Conclusion
(Scheme 6B) the target molecule (3a) was not formed. These In summary, nano-sized CdS was prepared as a heterogeneous
results confirmed that the reaction is likely to involve a radical photocatalyst. The actual structure of the prepared photo-
process. On the other hand, to confirm the formation of H2O2 catalyst was investigated with XRD, IR, SEM, TEM and solid
1
during the reaction, the H NMR spectrum was obtained from UV-visible spectroscopy techniques. The synthesized catalyst
the reaction mixture (Scheme 6C).
was then successfully applied for the synthesis of a wide spec-
1
The H2O2 representative peak was detected in the H NMR trum of quinolone derivatives to enhance their ecofriendly
spectrum at δ = 9.63 ppm (in CDCl3 as a NMR solvent). In aspects. In this method, N,N-dimethylanilines and/or alkyl
addition, starch test paper was applied to determine the pres- 2-(3,4-dihydroisoquinolin-2(1H)-yl)acetates were successfully
ence of peroxides (H2O2) during the reaction (Scheme 6D). The condensed with maleimides and triple C–C bonds via a C–H
color of the starch test paper changed from white to pale activation approach in the presence of CdS NPs as a photo-
yellow that confirms the presence of H2O2 in the reaction induced catalyst under visible light irradiation at room tem-
mixture.
perature without any solvent. The method takes advantage of
Our proposed mechanism is illustrated in Scheme 7. the visible-light photoredox catalytic cycle to generate pyrrolo
Firstly, electrons will be excited from the valence band to the [3,4-c]quinolones and novel 1,2-dihydroquinoline-3,4-dicarboxyl-
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ates under very mild reaction conditions (room temperature,
visible-light irradiation under air) with physical and chemical
stability, recoverability and reusability of applied photocatalyst,
high yields of products, avoidance of the toxic waste pro-
duction, ease of product isolation, application of a highly
efficient and renewable energy source, and finally good agree-
ment with some aspects of green chemistry. To the best of our
knowledge, this is the first report on the synthesis of 1,2-dihy-
droquinoline-3,4-dicarboxylate and condensation of N,N-di-
methylaniline and phenyl acetylenes for the synthesis of aryl-
1,2-dihydroquinolines through a photo-induced C–H activation
approach.
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There are no conflicts to declare.
5 H. S. Shapiro, Hexahydropyrrolo [3,4-c] quinoline com-
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The authors gratefully acknowledge the support of this work
by the Shiraz University Council. This work is financially sup-
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