Visible light motivated synthesis of polyhydroquinoline derivatives using CdS nanowires

Article abstract of DOI:10.1007/s11164-016-2822-2

Abstract: In this article, we report the synthesis of one-dimensional cadmium sulphide nanowires (CdS NWs) by chemical synthesis approach. These as-synthesized materials were characterized by ultraviolet–visible spectroscopy, X-ray diffraction, scanning e

Full text of DOI:10.1007/s11164-016-2822-2

Res Chem Intermed
DOI 10.1007/s11164-016-2822-2
Visible light motivated synthesis of polyhydroquinoline
derivatives using CdS nanowires
1
1
Rajkumar R. Harale
1
•
Praveen V. Shitre
Murlidhar S. Shingare
•
1
Bhaskar R. Sathe
•
Received: 5 October 2016 / Accepted: 25 November 2016
Ó Springer Science+Business Media Dordrecht 2016
Abstract In this article, we report the synthesis of one-dimensional cadmium sulphide
nanowires (CdS NWs) by chemical synthesis approach. These as-synthesized materials
were characterized by ultraviolet–visible spectroscopy, X-ray diffraction, scanning
electron microscopy with energy dispersive spectroscopy, and transmission electron
microscopy analysis. These nanowires have an average diameter of *20 nm and length
up to several micrometres. CdS NWs are highly stability and are found to be a very
efficient recyclable/reusable (minimum four times without any significant loss) photo-
catalyst for polyhydroquinoline synthesis under visible light (Halogen lamp, 70 W) at
room temperature. This system offers a mild, efficient, and highly economical alternative
to the existing protocols in its catalytic activity under visible light irradiation
(k C 420 nm). These findings will open up new opportunities for developing low-cost
efficient photocatalyst synthesis of value added intermediates.
&
Murlidhar S. Shingare
prof_msshingare@rediffmail.com
Bhaskar R. Sathe
bhaskarsathe@gmail.com
1
Department of Chemistry, Dr. Babasaheb Ambedkar Marathwada University,
Aurangabad 431004, India
123

R. R. Harale et al.
Graphical abstract
Keywords CdS nanowires Á Photocatalyst Á Polyhydroquinoline Á Visible light Á
Green approach
Introduction
Nowadays, photocatalysis is of promising significance for the purpose of
environmental protection/remediation. However, heterogonous photocatalysis by
semiconductor materials has been gaining increasing interest due to its promising
potential towards solar energy renovation [1–5]. Conventional chemical processes
use hazardous reagents that are concerning considering the environmental crises.
Moreover, transition metal complex-based homogenous photocatalysts also ulti-
mately generate secondary toxic wastes. To overcome these issues, developing an
alternative semiconductor based heterogenous photocatalyst that is new, efficient,
stable, eco-friendly, clean, and by greener approach, as well as its sustainable are a
great challenge to the chemist.
Furthermore, the most commonly studied semiconductors, such as TiO and
2
ZnO, with band-gap 3.2 and 3.4 eV, respectively, absorb ultraviolet photons that
account for less than 5% of solar light [6]. In contrast to this, cadmium sulfide
(
CdS), a typical II–VI semiconductor, has been extensively explored and become an
alternative in the field of photocatalysis due to its having a shorter and
suitable band-gap (2.4 eV) corresponding well with the spectrum of sunlight.
Because of the larger surface area (high surface to volume ratio), high capacity for
absorbing solar light, and many active sites, CdS semiconductors offer wide
applications in H generation reactions [7], photochemical reduction of CO [8],
2
2
solar cells [9], photolytic cells [10], and also in organic transformations [11–14].
Compared to bulk CdS and its nanoparticles (NPs), one-dimensional (1D) CdS
nanowires (NWs) have recently attracted special attention due to their firm and
prolonged distance electron transportation based on its exclusive 1D geometry.
1
23

Visible light motivated synthesis of polyhydroquinoline…
However, it has extraordinary (enhanced) light absorption/interaction capacity and
scattering due to the high length-to-diameter/width ratio of CdS NWs [15–18].
Hence, it is of substantial interest to examine the potential applications in organic
transformation of 1D CdS NWs as a heterogeneous photocatalyst [19–22].
Significantly, among the N-heterocyclic intermediates, polyhydroquinoline has
attracted much attention because of its diverse pharmacological and therapeutic
properties such as vasodilation, bronchodilation, anti-mutagenic, anti-atheroscle-
rotic, anti-hypertensive, hepatoprotective, antitumor, geroprotective, and anti-
diabetic [23]. The polyhydroquinoline derivatives are found in many drugs such
as nifedipine, nitrendipine, and nimodipine, which explain the important efforts
devoted to the synthesis of these heterocycles [24]. Several methods have been
reported for the preparation of the polyhydroquinoline using various catalysts such
as molecular iodine [25], iron(III) trifluoroacetate [26], L-proline [27], ceric
ammonium nitrate [28], heteropoly acid [29], Sc (OTf) [30], baker’s yeast [31],
3
p-TSA [32], trimethylsilyl chloride (TMSCl) [33], Yb (OTf) [34], MgO, and Fe O
3 4
3
nanoparticles [35, 36]. Unfortunately, most of these methods have several
disadvantages such as the use of high temperature, expensive metal precursors,
environmentally harmful catalyst, and harsh reaction condition. Interestingly,
smaller nanoparticles with their high active surface area (larger active sites for use)
coupled with visible light as a source of energy is the best alternative for
conventional methods. Moreover, as previous literature shows, Das et al. [37] have
been reported the synthesis of the 2-substituted benzothiazoles using CdS
nanosphere in visible light. However, (from this report) we focus on the catalytic
activity of CdS nanospheres in condensation reaction for the synthesis of
benzothiazoles, which encourages us to check the catalytic activity of as-
synthesised 1D CdS nanowires as catalyst for the synthesis of potent biologically
active polyhydroquinoline in the presence of visible light as an energy source.
Synthesis of CdS bulks
Briefly, the preparation method of CdS bulks involves initially 3CdSO Á8H O
4
2
(
0.5 mmol) dissolved in 40 mL DI water mixed with thiourea (1 mmol) in the
0 mL DI water, and the pH of the solution was adjusted to 9–10 by slow addition
2
of aqueous ammonium hydroxide under stirring. This resulting mixture was stirred
further for 1 h at 70 °C without addition of any external reducing and capping
agent. A yellow precipitate was obtained, which was separated by centrifugation
and washed with ethanol and water several times. The product was then dried in a
vacuum oven at 120 °C for 3–4 h.
Synthesis of CdS nanowires (NWs)
CdS nanowires were synthesised using tetrabutyl ammonium bromide (TBAB) as a
capping agent. In brief, the above prepared aqueous solution of CdSO (50 mL) was
mixed with an aqueous solution of thiourea (20 mL) and the pH of the solution was
4
123

R. R. Harale et al.
adjusted to 9–10 by slow addition of dilute ammonium hydroxide solution. Another
0
.5 g TBAB was added, which acts as a capping agent to avoid the further
aggregation of the nanoparticles. In continuation of this, the reaction mixture was
then stirred for 1 h at 70 °C. A yellow precipitate was obtained, indicating the
formation of CdS nanowires. This yellow precipitate was separated from the
reaction mixture by centrifugation and washed several times with ethanol and water
to remove excess/unbound capping agent and other side products. The product was
then dried in a vacuum oven at 120 °C for 3–4 h. Further, the formation of CdS
nanowires along with their morphology and wire size was proven by SEM and TEM
images at different magnifications. Accordingly, Fig. 1a shows low magnification
scanning electron microscope (SEM) image of as-synthesised 1D CdS NWs, and
Fig. 1b shows low magnification TEM image with 20 nm size and insets of the
surface morphology of single CdS NWs. Fig. 2a shows high magnification CdS
NWs with uniform 20 nm size; Fig. 2b shows the TEM image taken after four
cycles. XRD analysis was further carried out to investigate the crystallinity of these
as-synthesised CdS NWs and also to evaluate their size distribution. Figure 3a
shows the XRD patterns with peaks of 2h values are assigned to (100), (002), (101),
(
102), (110), (103), (201), (203), (114), and (105) crystal planes, respectively,
corresponding to hexagonal phase crystal structure and is in good agreement with
the previous reports [38, 39]. Moreover, there are slight differences observed in the
orientation, FWHM, and intensity, which might be due to their 1D growth along the
c-axis and controlled dimensions in the nano regime. Moreover, the XRD of bulk
CdS is also shown in Fig. 3b. In addition to this, Fig. 4 shows the UV–visible
absorption spectrum of as-synthesised CdS NWs and bulk CdS, respectively. It
shows the absorption of the CdS NWs observed at 413 nm, while the bulk CdS
absorption is observed at 415 nm. The absorption edge of CdS NWs is slightly blue
shifted; this slight shifting in the absorption edge is the outcome of the quantum
confinement effect of the CdS NWs, which is slightly distinct from CdS bulk
(absorption). The CdS NWs were further characterized by the spot EDS mapping to
prove the local elemental composition (Fig. 5), and it confirms the CdS compound
Fig. 1 a Low magnification scanning electron microscope (SEM) image of as-synthesised 1D CdS NWs,
b transmission electron micrographs (TEM) of as-synthesised CdS NWs at low magnification and insets
of the surface morphology of single CdS NWs
1
23

Visible light motivated synthesis of polyhydroquinoline…
Fig. 2 a High magnification TEM image of CdS NWs having average particle diameter of the 20 nm,
and b TEM analysis after four cycles
˚
Fig. 3 a X-ray diffraction (XRD) pattern of as-synthesised CdS NWs using Cu-Ka (1.54 A) radiations.
b X-ray diffraction (XRD) pattern of bulk CdS
Fig. 4 UV–visible absorbance
spectra of as-synthesised
(
a) CdS NWs and (b) bulk CdS
123

R. R. Harale et al.
Fig. 5 EDS spectrum of CdS NWS
formation and homogenous distribution of Cd and S in CdS NWs. These results
were in good agreement with morphological studies shown in Fig. 1b. These above
as-synthesized and well-characterized catalytic CdS NWs were further tested for
photocatalytic synthesis of polyhydroquinoline derivatives.
General procedure for the photochemical synthesis
of polyhydroquinolines
A mixture of aldehyde 1a (0.002 mol), dimedon 2 (0.002 mol), ethyl acetoacetate 3
(
0.002 mol), ammonium acetate 4 (0.003 mol), and CdS NWs (8 mg) was taken in
the 50 mL double-walled quartz photoreactor with continuous flow of water to
maintain the room temperature of the reaction vessel. The reaction was exposed to
visible light (70 W, Halogen lamp with range 400–700 nm) equipped with cut off
filter (k [ 420 nm) was used as visible light source for an appropriate time under
constant stirring. The progress of the reaction was monitored by thin layer
chromatography (TLC) using ethylacetate: n-hexane (4:6) composition. After
completion of the reaction the catyalyst was recoved by the filtration and obtained
crude products were filtered and recrystalised from ethanol to afford
polyhydroquinoline.
Spectral data for the representative compound
Ethyl 4-(4-chlorophenyl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexahy-droquinoline-3-
1
arboxylate (Table 5-5e) H NMR (400 MHz, CDCl ) d ppm: 0.81 (s, 3H, –CH ),
3
3
0
.93 (s, 3H, –CH ), 1.11 (t, 3H, J = 7.3 Hz, –CH ), 2.04 (dd, 2Hn,–CH –), 2.12
3 3 2
(
dd, 2H, –CH –), 2.26 (s, 3H, –CH ), 3.95 (q, 2H, J = 7.2 Hz, –OCH –), 4.93 (s,
2 3 2
1
H, –CH–), 7.07 (d, 2H, J = 9.1 Hz, Ar–H), 7.15 (d, 2H, J = 9.2 Hz, Ar–H), 7.91
1
23

Visible light motivated synthesis of polyhydroquinoline…
1
3
(
brs, 1H,–NH); CNMR (100 MHz, CDCl ): d14.7, 18.6, 26.5, 29.5, 31.4, 36.3,
3
4
1.1, 50.5, 60.3, 75.8, 104.3, 110.2, 125.5, 129.8, 132.6, 143.2, 149.6, 165.1, 192.9.,
.
ES-MS) = 396.07 [M ? Na]
?
(
Results and discussion
Experimental
Chemicals and instruments
All reagents were purchased from Merck and Aldrich and used without any further
purification. The particle size and morphology of the synthesized catalysts were
characterized with a scanning electron microscopy (SEM, JSM-6400) transmission
˚
electron microscopy (TEM, PHILIPS, CM 200, 20–200 kv Resolution: 2.4 A). X-
ray diffraction (XRD) measurements were performed using a Bruker AXS
Company, D8 ADVANCE diffractometer (Germany). Infrared (IR) spectra were
1
recorded on a Thermo Nicolet AVATAR-370. H NMR spectra were recorded on a
Bruker DRX 400 spectrometer respectively.
To find out the better catalytic systems for the polyhydroquinolines derivatives,
optimisation of the reaction condition was carried out by taking benzaldehyde 1a
(0.002 mol), dimedon 2 (0.002 mol), ethylacetoacetate 3 (0.002 mol), and ammo-
nium acatate (0.003 mol) as a model substrates (Scheme 1).
In order to understand the nature of catalyst amount, here we have studied the
effect of CdS NWs at different loadings (amount) on model reaction.
In brief, in search of better percentage yield of polyhydroquinoline, CdS NW
photocatalyst at different amount (loadings) has been carried out at 4, 8, 10 and
1
2 mg, respectively, but 8 and 10 mg of catalyst was sufficient to offer the excellent
yield. Moreover, the same product yield (94%) has been found at different catalyst
amounts, i.e. 8 and 10 mg (Table 1, entry 1). Furthermore, a model reaction has
been performed at a minimum catalytic amount of 4 mg and maximum at 12 mg to
find the exact amount of the catalyst that resulted in 85 and 94% yield, but there was
not a considerable difference found in product yield at the highest catalyst loading,
i.e. the catalyst was unable to offer the expected yield at 12 mg catalyst. Finally,
catalytic activity of CdS NWs was compared to that of bulk CdS, CdSO , and
4
CdCl (Table 1). The application of bulk CdS (entry 2), CdsO (entry 3), and CdCl
2
2
4
(
entry 4) as a catalyst at 4, 8, 12 mg loadings, respectively, did not give
Scheme 1 Standard model reaction for the synthesis of polyhydroquinolines
123

R. R. Harale et al.
Table 1 Screening of the
catalysts with concentrations
a
Catalysts loading (mg) Yield (%)
Entry Catalyst
1
CdS NWs
4
8
1
1
4
8
1
4
8
4
8
–
85
8 (in dark)
94 No condensation
0
2
94
94
2
CdS bulk
40
45
2
47
Reaction conditions: 1a
3
4
5
CdSO
4
No condensation
No condensation
No condensation
No condensation
No condensation
(
(
0.002 mol), 2 (0.002 mol), 3
0.002 mol), 4 (0.003 mol),
stirred under visible light (70 W,
Halogen lamp) in ethanol
solvent (5 mL) for 35 min
CdCl2
No catalyst
a
Isolated yield
condensation under our experimental conditions, in ethanol as a solvent. The only
bulk CdS system that led to some conversion as 47% of 5a was produced in ethanol
with 12 mg loading (entry 2).
Herein, we applied potential, recoverable, and reusable CdS NWs as heteroge-
nous catalyst for carrying out the synthesis of polyhydroquinolines by condensation
of various aromatic aldehydes with dimedone, ethylacetoacetate, and ammonium
acetate. The experiments began with the purpose of optimizing the reaction
conditions for this multicomponent reaction by varying solvents and keeping the
energy source and reaction time constant. To determine the most effective solvent
for this condensation reaction using CdS NWs as catalyst, we screened the various
protic and aprotic solvents shown in (Table 2 entry 1–7). But the ethanol was found
to be the best solvent system for this condensation reaction with CdS NWs catalyst
(
Table 2, entry 2). At room temperature with 8 mg of the CdS NWs catalyst, the
Table 2 Screening of solvent
a
Yield (%)
Entry
Solvents
Time (min)
1
2
3
4
5
6
7
Water
35
35
35
35
35
35
35
80
94
70
60
74
89
45
Ethanol
THF
DCM
Acetonitrile
Methanol
1,4-Dioxane
Reaction conditions: 1a (0.002 mol), 2 (0.002 mol), 3 (0.002 mol), 4 (0.003 mol), CdS NWs (8 mg)
stirred under visible light (70 W, Halogen lamp) in solvent (5 mL) for 35 min
a
Isolated yield
1
23

Visible light motivated synthesis of polyhydroquinoline…
reaction appeared to proceed very well to provide the desired product in an excellent
yield (i.e. 94%) in 35 min.
Model reaction has been performed under the following different conditions:
conventional, unconventional, and photocatalytic (under visible light), which acts as
activation energy source (Table 3). In more detail, the model reaction (Scheme 1)
performed at reflux condition using CdS NWs as a catalyst (8 mg) offered 65%
yield in 70 min (Table 3, entry 1). Furthermore, the same model reaction was
carried out under ultrasound irradiation at 80 °C, which offered 80% yield of 5a in
5
0 min (Table 3, entry 2). Both conditions (conventional and ultrasonication)
applied on model reaction at room temperature for better comparison run of
photocatalyst but not expected yield observed. Finally, when the same model
reaction was stirred under visible light (70 W, Halogen lamp) at room temperature,
excellent yield (i.e. 94%) was obtained in only 35 min (Table 3, entry 3). In this
entire course of conventional and non-conventional experimental conditions, the
model reaction was found to proceed effectively under visible light within a very
short reaction time, delivering the desired product in excellent yield. Inspired by
this, it was decided to synthesize a number of derivatives following the developed
reaction condition by photochemical method.
One of the advantages of using a heterogeneous catalyst is the possibility to
recycle the catalyst, thus reducing the cost and environmental impact of the process.
To investigate this key feature, four successive condensations of 1a were performed
with the same CdS NW sample, which was recovered by the simple method
described as follows.
The reaction mixture was filtered through Whatman 40 filter paper. The insoluble
catalyst remained as residue and was collected and dried. The filtrate was extracted
with ethyl acetate. The collected CdS NWs (catalyst) were dried and put into a
round bottomed flask, and a reaction was carried out with the same amount of
starting material. No significant decrease of the catalytic activity was observed
throughout the different condensation reactions. As a result, quinoline was produced
in nearly constant yield. However, after a third cycle slight loss of the catalytic
activity was observed, which may be due to the deactivation or personal loss of the
catalyst during the course of the reaction and recovery process. At the fourth reuse
the catalyst also offered 89% in 45 min (Table 4).
Table 3 Study of catalytic activity of catalyst under conventional and unconventional conditions
o
Temp. ( C)
a
Yield (%)
Entry
1
Conditions
Catalyst
Time (min)
Conventional
CdS NWs
Reflux
RT
70
70
50
50
35
65
30
80
40
94
2
3
Ultrasound
CdS NWs
CdS NWs
80
RT
Photocatalysis
RT
Reaction conditions: 1a (0.002 mol), 2 (0.002 mol), 3 (0.002 mol), 4 (0.003 mol), CdS NWs (8 mg)
stirred in ethanol solvent (5 mL)
a
Isolated yield
123

R. R. Harale et al.
Table 4 Recycling experiment
a
Yield (%)
Entry
Cycle
Time (min)
1
2
3
4
5
Fresh
35
35
35
35
45
94
93
90
88
89
1st reuse
2nd reuse
3rd reuse
4th reuse
Reaction conditions: 1a (0.002 mol), 2 (0.002 mol), 3 (0.002 mol), 4 (0.003 mol), CdS NWs (8 mg)
stirred under visible light (70 W, Halogen lamp.) in ethanol (5 mL) for 35 min
a
Isolated yield
Table 5 Synthesis of polyhydroquinoline derivatives
a
Yield (%)
o
M.P. ( C)
Entry
Comp.
R
Time (min)
Found
Literature
1
2
3
4
5
6
7
8
9
5a
5b
5c
5d
5e
5f
2-Cl
45
40
37
30
30
35
40
35
45
30
40
35
40
50
45
90
88
90
92
92
88
85
94
89
94
84
90
80
87
89
207–209
211–213
176–178
220–223
244–246
232–234
240–242
203–205
259–262
257–260
205–207
256–257
248–250
240–241
234–236
208–210
210–212
177–178
220–222
245–246
232–234
242–244
204–205
261–262
258–259
206–207
254–255
248–249
241–242
233–234
4-OH,3-OCH
3
3-NO
3-OH
4-Cl
4-OH
4-NO
H
2
5g
5h
5i
2
4-CH
4-OCH
CH =CH–
4-Br
3
10
11
12
13
14
15
5j
3
5k
5l
2
5m
5n
5o
2-Furyl
2-Thienyl
3 2
4-N(CH )
Reaction conditions: 1a (0.002 mol), 2 (0.002 mol), 3 (0.002 mol), 4 (0.003 mol), CdS NWs (8 mg)
stirred under visible light (Halogen lamp, 70 W) in solvent (5 mL) for 35 min
a
Isolated yield
Model reaction has been performed using CdS NWs as a potential photo catalyst
in the presence of visible light (70 W, Halogen lamp) as an energy source. In this
experiment, the model reaction was found to proceed effectively within very short
1
23

Visible light motivated synthesis of polyhydroquinoline…
Table 6 Comparative study of CdS NWs (our studies) and other catalysts reported for synthesis of
polyhydroquinoline derivatives
Entry
Catalyst
NaHSO
Time (min)
Yield (%)
References
1
2
3
4
5
6
7
8
4
–SiO
2
300–480
45
75–90
92
[43]
Silica–sulfuric acid
Cellulose sulfuric acid
P-Toluene sulfonic acid
Molecular iodine
[44]
120–300
120
78–92
93
[45]
[46]
30
93
[47]
Yb (OTf)
PPh
CdS NWs
3
300
90
[48]
3
120–300
35
72–96
90–94
[49]
(This work)
reaction time delivering the desired product in excellent yield. Inspired by this, it
was decided to synthesize a number of derivatives following the developed reaction
condition with visible light as a green energy source (Table 5). With that result in
hand, we optimized the reaction parameters. We generalized the applicability of this
method for the synthesis of a series of polyhydroquinolines starting from various
aromatic aldehydes. The aldehyde with both the electron donating and electron
withdrawing substituents didn’t show any significant effect on the product yields.
Furthermore, we also tried catalytic activity for the checkout yield of heterocyclic
polyhdydroquinolines starting from 2-furyl and 2-thienyl heterocyclic aldehydes
(
Table 5, entry 14, 15). Our conclusive finding says that aldehydes with no
substituent on phenyl ring reacted smoothly and products were obtained in excellent
yield 94% (Table 5, entries 8). Furthermore, the polyhydroquinoline products were
isolated and characterised, and their melting points were found to be in good
agreement with those reported in the literature [40–42]. To show the merits of this
work over the methods reported in the literature, comparison of results obtained by
use of CdS NWs with different catalysts such as [bmim][PF ], PEG400, Bi
6
(SCH COOH) , HClO –SiO , Silica gel, Saccharomyces cerevisiae are shown in
2 3 4 2
Table 6.
The cyclocondensation reaction of the polyhydroquinoline seems to be very
feasible it may be due to the electrophilic-like character of holes generated in the
CdS NWs on irradiation of the visible light. The visible light having the capacity to
?
excite the electron from valence band to conduction band and resulting holes (h )
within the CdS NWs may responsible for the condensation of reactant and
accelerate the rate of reaction.
Conclusion
In summary, 1D CdS NWs was found to be a promising photocatalytic system to
perform the synthesis of polyhydroquinoline in the presence of visible light
(
Halogen lamp, 70 W). Some attractive features of our methodology are (1) the
123

R. R. Harale et al.
mildness of the reaction conditions as polyhydroquinoline were formed at room
temperature (2) the very low catalytic loading (8 mg) (3) the possible recyclability
of the catalyst and (4) the scope of CdS NWs photocatalytic system was also applied
in a one-pot three-component system (condensation reaction). The results obtained
in the context of this study thus compared favourably to previous reports in terms of
overall efficiency.
Acknowledgements An Emeritus Scientist Fellowship awarded to M.S.S. by the Council of Scientific
and Industrial Research, New Delhi (Project vide No. 21(0919)/12/EMR-II Dated 25-04-2013) is
gratefully acknowledged. One of the authors, R.R.H., is thankful to CSIR, New Delhi, India, for financial
assistance in the form of Senior Research Fellowship. Authors are also grateful to the Department of
Chemistry, Dr. B. A. Marathwada University, Aurangabad, for providing the laboratory facilities. We also
thank SAIF Divisions of IIT Bombay and CDRI, Lucknow, for providing analytical data.
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