Photocatalytic synthesis of unsymmetrical thiourea derivativesviavisible-light irradiation using nitrogen-doped ZnO nanorods

Article abstract of DOI:10.1039/d0nj02197k

An efficient, mild, and environmentally friendly route has been developed for the synthesis of unsymmetrical thiourea derivatives in moderate yields by the reaction of tertiary aromatic and aliphatic amines with phenyl-iso-thiocyanate in the presence of N-ZnO as a photocatalyst under visible-light irradiation. This method provides a pathway to activate the tertiary aromatic and aliphatic aminesviaC-N bond cleavage.

Full text of DOI:10.1039/d0nj02197k

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Photocatalytic synthesis of unsymmetrical thiourea derivatives via
visible-light irradiation using nitrogen-doped ZnO nanorods
a
a
a
Mehdi Koohgard , Abdollah Masoudi Sarvestani , and Mona Hosseini-Sarvari*
Received 00th January 20xx,
Accepted 00th January 20xx
An efficient, mild, and environmentally friendly route has been developed for the synthesis of unsymmetrical thiourea
derivatives in moderate yields from the reaction of tertiary aromatic and aliphatic amines with phenyl-iso-thiocyanate in the
presence of N-ZnO as a photocatalyst under visible-light irradiation. This method provides a pathway to actives the tertiary
aromatic and aliphatic amines via C-N bond cleavage.
DOI: 10.1039/x0xx00000x
Photocatalytic processes in ZnO happen with the absorption of light
irradiation with enough energy (hν) to be the same to or greater than
the ZnO bandgap energy (hν ≥ Eg). Photon absorption causes
Introduction
Sunlight is an unparalleled natural resource. It is a cheap, non-
polluting, numerous, and unlimited renewable source of clean
energy. Since society has become increasingly aware of the harmful
strike of human industry on the surroundings, the booming of ways
to effectively harness the energy of solar irradiation has appeared as
one of the main scientific challenges of the twenty-first century. The
junction between solar energy and environmental sustainability,
however, is a much older idea that dates to the turn of the last
century. The observation that light alone could effect unique
chemical changes in organic compounds led early twentieth-century
photochemists to recognize that the sun might represent a stable
source of clean chemical potential. One basic barrier that has finite
the use of photochemical reactions on industrial scales is the
weakness of most organic molecules to absorb visible wavelengths
of light. Normally, the ultraviolet (UV) wavelengths commonly
needed to excite conventional organic compounds during the
photochemical process. As UV irradiation makes divergent pathways
that resulted in unwanted products as well as UV is not abundant in
the solar spectrum, the design and fabrication of photocatalysts with
highly effective solar energy utilization have attracted considerable
-
excitation and transfer of electrons (e ) from the valence band (VB)
-
+
to the conduction band (e CB), which leads to holes (h ) generation in
+
the valence band (h VB). The second steps are separation and
-
+
migration of charge-carriers (e and h ) to the ZnO surface. The
extremely reactive electrons and holes at the surface of ZnO
photocatalyst tend to carry out reduction and oxidation reactions to
generate reactive radical species (A , B ) The photocatalytic
process continues with the participation of these reactive radical
species to operate the chemical reactions over the oxidation and
1
-3
17-19
reduction.
A
CB
e-
A
B
E
g
hν ≥ Eg
h+
VB
4
-6
attention recently.
Among different kinds of photocatalysts,
, ZnO, etc. as photocatalysts, have
semiconductor solids such as TiO
2
7
, 8
been extensively studied. Nowadays in the field of semiconductor
photocatalyst materials, ZnO has appeared as the prominent
candidate and also as an effective and promising candidate in green
photocatalysis processes because of its excellent properties,
including low cost, high redox potential, nontoxicity, and
environmentally friendly features.9-11 ZnO has a good Electrical and
optical properties, such as a direct band-gap of 3.28 eV and a large
exciton binding energy of 60 meV at room temperature and the n-
type ZnO semiconductor has higher electron mobility, high
B
Figure 1 The Mechanism of ZnO photocatalysis pathways
Because of the broadband structure of ZnO, (UV) light should be
applied as the photoinitiation source. This is the important limiting
factor in the usage of ZnO photocatalysts because the solar spectrum
makes only a small fraction of UV light (~4%). Accordingly, ZnO
photocatalysts show poor efficiency under solar irradiation. To usage
solar irradiation, it is needed to change the electronic property of
ZnO. Several strategies have been engaged to better the
1
2-16
breakdown voltages, and higher breakdown field strength.
The
Mechanism of ZnO photocatalysis pathways is shown in Figure 1.
20-22
photocatalytic efficiency of ZnO under visible light.
a. Department of Chemistry, Faculty of Science, Shiraz University, Shiraz,
194684795 I.R. Iran. E-mail: hossaini@shirazu.ac.ir
Electronic Supplementary Information (ESI) available: [experimental section and
spectroscopy data of all compounds are included]. See DOI: 10.1039/x0xx00000x
Doping with metals and nonmetals is being extensively considered to
develop the absorption wavelength range of ZnO via modifying the
electronic bandgap in the visible-light region.
7
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Exclusively, because of the commonly smaller atomic size of
nonmetal elements, nonmetal doping into ZnO lattice is usually much
easier to be performed experimentally than metal doping and also
nonmetal elements as a doping agent are cheaper and available than
metal elements. In contrast, metal doping is more likely to induce the
formation of vacancies or defect states that can serve as the
recombination center for photogenerated electrons and holes.
Previous works
View Article Online
DOI: 10.1039/D0NJ02197K
CS
2, NaOH
S
NC
H
N
S
H
H
+
R2 R2
2O,
N
+
reflux
Dil.
HCl
R1 R1
R2 NH2

R1
S
R2
R1 NC
R2 NH2
CS2, EtOH
r.t.
Solid state
r.t.
+
+
+
N
N
SCN
H2N
CS
ZnO/ Al2O3
CS2
CS2
R
NH2
2
MW,
1
00 °C, 2h
H2O
3h
C,
NH2
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Till now, among all non-metal dopants, nitrogen has attracted the
most attention. This is due to its advantages, including the similarity
of nitrogen and oxygen ionic radii, N 2p and O 2p energy states, high
solubility, and also low formation energy. Hybridization of N 2p and
O 2p states raises the valence band upper edge and thus, bandgap
narrowing occurs.
neat
N
H
H
+
2 O
reflux,
+
2 ph NH2
°
0
6
8h
NH2
NH2
+
O
Cl
+
S
This work
So far, visible light photocatalysis processes of N-ZnO such as water
treatment and air purification are extensively investigated for the
removal of pollutants. However, no usage for this photocatalyst in
visible-light-driven organic synthesis, selective oxidation, and
reduction process has been reported in recent years.
R2
N
R2
N
NCS N-ZnO nanorods (0.003 g)
H
N
+
R1
R1
R2
EtOH ( 4 mL)
CFL 15 W
4 h, air
S
1
R = Ar, CH2CH3
2
2
Thiourea comprising compounds have been offered to have a
plethora of biological virtues such as molecules anti-bacterial, anti-
fungal, anti-inflammatory and anti-cancer.
applied as supramolecular assemblies, and anion sensing.
other hand, the field of organocatalysts has been prevailed by the
cinchona-thiourea based catalysts for a variety of organocatalytic
transformations. These inherent qualities of thioureas, in
combination with the widespread research on their application in the
synthesis of diverse heterocyclic compounds, make them prominent
R = CH3, CH2CH3
2
3-26
Moreover, they are
2
7-29
On the
c)
30
ZnO
N-ZnO
Scheme 1 Synthesis of thiourea
3
1-36
building blocks for synthetic chemists.
consequently, a great deal
of effort has been focused on the development of newer synthetic
strategies for this important building block. Up to now, one of the
most common methods for the synthesis of thiourea has been the
c)
d)
a)
b)
e)
3
7-45
reaction of primary or secondary amines with isothiocyanates.
These reported methods have their disadvantages including the use
of toxic reagents (thiophosgene & hydrogen sulfide), harsh reaction
conditions, and/or are limited to the synthesis of symmetrical
thiourea derivatives. So far, no procedure has been reported to use
inactive tertiary amines for the synthesis of thiourea. In this work,
the tertiary amines as a precursor were activated in an optical
pathway, and then by a nucleophilic attack to the highly electrophilic
Figure 2 (a) Undoped ZnO, (b) 0.30%, (c) 0.32%, (d) 0.38%, (e) 0.45% N-
ZnO NRs
carbon of the isothiocyanate resulted in the formation of To evaluate the nitrogen content, each of the synthesized catalysts
unsymmetrical thiourea (Scheme 1).
was analyzed by CHNS analysis. The data obtained by CHNS analysis
of nano N-ZnO powder are given in Table 1. To describe the
reproducibility of the CHNS analysis for the percentages of N
incorporated in ZnO, this analysis was replicated three times.
Results and Discussion
The first step in this study was the selection of a method for the
preparation of ZnO NRs. The ZnO NRs were achieved by a
Table 1. CHNS analysis of nano N-ZnO NRs
4
6
hydrothermal method. After that, nitrogen-doped ZnO NRs were
prepared by heating mixture a ZnO NRs and urea as nitrogen source
Catalyst ZnO/urea g ( weight ratio) Percentage % of nitrogen
◦
47
at 600 C for 4 h. The pink-colored product was then obtained and
denoted as N-ZnO NRs. As can be seen, the color of the ZnO changed
after the doping process, and this change in color confirms the
doping process. Corresponding colors of undoped ZnO and nitrogen-
doped ZnO are presented in (Figure 2). We also decided to evaluate
the effect of the weight of urea as a source of nitrogen on visible-
light responsive of N-ZnO NRs. Therefore, we prepared a series of
catalysts by heating 0.5 g ZnO with different amounts of urea (2, 4
and 8g). The color change was observed from white to pink every
time after the doping process (Figure 2)
0.5/1
0.5/2
0.30
0.32
0.38
0.45
0
.5/4
0.5/8
2
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The UV–vis diffuse reflectance spectra of five samples are displayed
4.5
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View Article Online
DOI: 10.1039/D0NJ02197K
in Figure 3. All prepared N-ZnO NRs could be excited by visible light,
whereas undoped ZnO exhibited the absorption at the ultraviolet
light range. As can be seen, from figure 3 three types of N-ZnO NRs
3
2
1
0
.5
3
(
0.30%, 0.32%, 0.38%), does not show any significant difference in
the absorption intensity in the visible region. However, the catalyst
containing 0.45% doped nitrogen has the highest absorption
intensity in the visible region. So, this N-ZnO NRs were chosen for
more characterizations.
.5
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undoped ZnO
(0.30%) N-ZnO
1
.5
1
0
1
2
3
4
(0.32%) N-ZnO
hv=(ev)
(0.38%) N-ZnO
Figure 4 The plot for the bandgap calculation of 0.45% N-ZnO NRs.
4
91nm (0.45%) N-ZnO
XRD
0.5
0
The diffractogram of the pure ZnO and 0.45% N-ZnO NRs are shown
in Figure 5. It is observed that ZnO is having the wurtzite phase
(JCPDF: 891397) in both samples. No significant peak for nitrogen is
observed in N-ZnO NRs which suggests the possibility of introduction
of nitrogen into the ZnO lattice without affecting the ZnO crystal
structure. Hence the lattice parameter (a=3.15 Ǻ, c=5.05 Ǻ)
2
10
1210
Wavelenght (nm)
Figure 3 UV–vis diffuse reflection absorption spectra of five type synthesized
samples.
decreases
in
comparison
to
the
pristine
ZnO
The band gaps can be determined by UV–vis diffuse reflectance
spectroscopy.48 Figure 3 shows the UV–vis diffuse reflectance
spectra of the pure and 0.45% N-ZnO NRs. The pure ZnO had no
absorption above the wavelength of 400 nm. But, the nitrogen-
doped ZnO showed the absorption in the visible region, indicating
that the bandgap was greatly narrowed owing to the impurity N 2p
states isolated above the valence-band of ZnO.49
(a=3.17 Ǻ, c=5.08 Ǻ). Oxygen and nitrogen have similar atomic radii
and the introduction of nitrogen into the ZnO lattice does not make
too much difference in the XRD pattern. However, it has been
reported that the bond length of Zn–N is smaller than Zn–O in O–Zn–
N after N doping in the ZnO lattice which is the main reason for the
shifting of XRD peak (higher 휃 values) and lattice parameter in N-
ZnO.55 The particle size of N-ZnO as evaluated by the Scherrer
formula is 48.63 nm while that of the pure ZnO is 49.5 nm. The
reduction of particle size may be attributed to the incorporation of
nitrogen in ZnO.
The optical bandgap of the semiconductors can be calculated by
using the absorption spectrum and the following equation to draw
the Tauc plot. 50, 51
Eq.1: (αhν)1/n = A(hν - Eg)
a)
h: Planck's constant, ν: frequency of vibration, α: absorption
(extinction ) coefficient, Eg: bandgap, A: proportionality constant, n:
the value of the exponent n denotes the nature of the transition. In
this study, this method was applied to estimate the band gap energy
value of the photocatalyst which obtained from UV–Vis spectra of
the corresponding semiconductors52, 53. Fig. 4 depicts the plot of
band-gap energy for 0.45% N-ZnO NRs, obtained by Tauc's equation
(
1). 54
Eq.2: [ahv]1/2 = A(hv-Eg)
The calculated band-gap energy found to be 2.5 eV for 0.45% N-ZnO
NRs. Noticeably, by treatment of nitrogen with ZnO, the band-gap of
0
.45% N-ZnO was significantly reduced.
This journal is © The Royal Society of Chemistry 20xx
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Figure 7 SEM (a and b), and TEM (c,d) images of 0.45% N-ZnO NRs.
b)
View Article Online
(
101)
DOI: 10.1039/D0NJ02197K
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Figure 5 a) The XRD pattern of the pure ZnO and 0.45% N-ZnO NRs,b) Peak
shift and peak broadening are depicted through the enlarged view of (101)
peaks for undoped and 0.45% N-ZnO NRs
FT-IR
-1
The FT-IR Spectrum of the catalyst in the region of 400-4000 cm is
5
6
-1
shown in Figure. 6. There is a broad band at 3410 cm indicates
the presence of –OH groups of absorbed water during the
preparation process. The band arising from the bonding between
ZnO (473 cm , 532 cm ) is clearly represented. FT-IR spectrum
reveals the presence of stretching vibrational bond C–O (1650 cm-1),
-
1
-1
-
1
C–H (1381 cm ).
Table 2 Results of BET surface area measurements for 0.45% N-ZnO
1
00
NRs
80
BJH adsorption summary
2
Surface area
Pore volume
Pore Diameter Dv(d)
9.554 m /g
0.072 cc/g
1.29 nm
6
4
0
0
BET summary
Surface Area
7.823 m²/g
Undoped ZnO
.45% N-ZnO NRs
600 2800
a
b
20
0
0
Synthesis of thiourea derivatives using N-ZnO NRs as visible light
photocatalyst
3
2000
1200
400
-1
Wavelength (cm )
Figure 6 FT-IR spectrum of undoped ZnO (a) and 0.45% N-ZnO NRs
b).
In our effort to exploit the photocatalytic activity of N-ZnO NRs,
herein we report the simple, and efficient synthesis of thiourea
derivatives at room temperature from N,N-dimethylaniline and
pheny-iso-thiocyanate under visible light irradiation. Various
parameters including the solvent, light, atmosphere, and the kind of
catalyst were investigated for the reaction of N,N-dimethyl-p-
toluidine 1a, and phenyl-iso-thiocyanate 2a as the model substrates
(
SEM and TEM
SEM and TEM of the synthesized 0.45% N-ZnO NRs are
represented in Figure 7. The SEM images of catalyst show that the
surface morphologies are in the form of a nano-rod, with a wide
range of sizes. The size is approximately 46.8 nm in TEM, and also
after the doping process, despite the changes occurring at the
surface, the nano-rod is confirmed. The TEM image of the catalyst
shows that the catalyst has a nano-rod-shaped morphology.
(
Scheme 2).
visible light
N-ZnO NRs
NCS
S
N
+
N
N
solvent
open to air, 24 h
H
1a
2a
3a
BET
Scheme 2 A model reaction for the synthesis of thiourea 3a.
The data obtained by BET measurements of nano 0.45% N-ZnO NRs
have been given in Table 2. The BET surface area was found to be
At first, different types of solvents were examined. As shown in Table
3, among all tested solvents, EtOH gave the best result in terms of
the yield (entry 1), as ethanol is a polar protic solvent and according
to the proposed mechanism (Figure 8), we hypothesize intermediate
B (iminium ion) is solvated by ethanol so makes B more stable,
7
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.823 m²/g, whereas the BJH adsorption surface area of pores was
.554 m²/g. The single point total pore volume was found to be 0.072
cc/g and pore diameter was 1.29 nm.
a)
b)
4
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consequently results in an acceleration in the reaction process. So,
was chosen EtOH as a standard solvent to optimize the other
reaction parameters.
Table 5 The optimization of light in the synthesis of
View Article Online
thiourea.
Entry
DOI: 10.1039/D0NJ02197K
Light
Yield of 3a (%)
1
2
3
4
CFL 15W White
83
51
Table 3 Optimization of solvent.
Entry
Solvent (2mL)
Yield of 3a (%)
Blue LED 12w
Green LED 12w
Red LED 12w
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
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1
2
3
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1
2
3
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8
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EtOH
DMSO
CH CN
3
81
61
51
49
48
43
39
28
10
0
DMF
CHCl
18
3
2 2
CH Cl
Acetone
EtOAc
THF
8
1
0
None
Reaction conditions: (0.5 mmol) of 2a and (1.5 mmol) of 1a in the presence
of 0.005 g N-ZnO NRs (0.45%) at room temperature, 24 h, and CFL White
5
6
Dark
0
(15 W) under air.
Sunlight
61
In the following, the optimization of the catalyst was evaluated.
Consequently, 0.003 g of the catalyst was found to be the best
amount. The results are summarized in Table 4. By using undoped
ZnO, the desired product was generated in only a 32% yield (entry 5).
Reaction conditions: (0.5 mmol) of 2a and (1.5 mmol) of 1a, in the
presence of N-ZnO NRs (0.45%, 0.003 g) in EtOH (2mL), 24 h, in
-
2
air. CFL 15W White (0.9 W cm , 400-700 nm), Blue LED 12w (0.7
-2
-2
W cm , 450-460 nm), Green LED 12w (0.7 W cm , 535-545 nm),
-2
Blue LED 12w (0.7 W cm , 640-650 nm).
Next, the effect of light was investigated. According to data
Table 4. Optimization of catalyst.
According to observations (Table 6), the desired product was found
in lower yield under argon atmosphere rather than the air
Entry
%)
Catalyst (g)
Yield of 3a
(
1
2
3
4
5
6
7
8
9
1
(0.45%) N-ZnO (0.005g)
(0.38%) N-ZnO (0.005g)
(0.32%) N-ZnO (0.005g)
(0.30%) N-ZnO (0.005g)
ZnO (0.005g)
(0.45%) N-ZnO (0.010g)
(0.45%) N-ZnO (0.005g)
(0.45%) N-ZnO (0.003g)
(0.45%) N-ZnO (0.001g)
None
81
72
71
63
32
82
81
83
59
atmosphere. So, this indicates that the presence of O
for the reaction.
2
is necessary
Table 6. The optimization of the atmosphere.
Entry
1
Atmosphere
Yield of 3a (%)
O
2
Air
83
2
3
O
2
balloon
83
29
0
trace
Reaction conditions: (0.5 mmol) of 2a and (1.5 mmol) of 1a in EtOH (2 mL),
under CFL White (15 W), 24 h, in air.
Argone
represented in Table 5, CFL white 15 W power was shown the best
results. The reaction afforded no product in a dark condition which
this result might support the photo-induced electron transfer
pathway. Also, under natural sunlight, the desired product was
obtained in good yield (61%) (entry 6). This observation shows that The N-ZnO NRs photocatalyst could be also separated and recovered
the N-ZnO NRs can efficiently absorb solar irradiation to handle this conveniently by centrifugation from the reaction mixture and reused
organic synthesis. As the reaction was performed in the balcony of in the next runs. Accordingly, the catalyst was recycled and reused
Shiraz University (December. 18-19, 2019, 29°59′ north latitude and effectively four times without a significant decrease in photocatalytic
52°58′ east longitude, 1500 m above the sea level), we supposed the activities. Besides, the XRD pattern and FT-IR spectrum of the
light intensity and light angle of sunlight was not suitable enough to recovered catalyst after the last cycle were compared to the fresh
Reaction conditions: (0.5 mmol) of 2a and (1.5 mmol) of 1a, in the
presence of N-ZnO NRs (0.45% , 0.003 g) in EtOH (2ml), under CFL white
1
5w, 24 h, under open air condition.
accelerate reaction as much as LED lamps.
catalyst and revealed no significantchanges (Figure 8).
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2020, 00, 1-3 | 5
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6
a) 100
Table 7. Scope of thiourea derivatives.
80
80
View Article Online
DOI: 10.1039/D0NJ02197K
7
8
7
6
visible light
NCS
S
N
N-ZnO NRs
+
N
N
50
0
solvent
H
open to air, 24 h
1a
2a
3a
Entry
1a
2a
Product
0
1
2
3
4
5
6
7
8
9
0
1
2
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4
5
6
7
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9
0
1
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0
1
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7
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9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
Run
NCS
N
S
N
1
8
00
0
1
b)
N
H
3
a, 83%
60
N
S
N
4
0
NCS
NCS
fresh N-ZnO NRs
N-ZnO after runs
2
3
20
0
N
H
3
b, 73%
3
600
2800
2000
1200
400
-1
N
Br
Wavenumber (cm )
S
c)1000
Br
N
N
H
fresh N-ZnO NRs
N-ZnO after runs
8
6
4
2
00
00
00
00
0
3
c, 68%
S
NCS
N
4
N
N
H
3
d, 70%
H
N
20
40
Position 2￿
60
80
N
NCS
NCS
N
5
Figure. 8 (a) Recyclability of recovered N-ZnO NRs for thiourea synthesis,
b) FT-IR spectrum, (c) XRD pattern of the N-ZnO NRs after four-cycle.
S
(
3
e, 72%
Finally to show the scope and generality of the N-ZnO NRs, a variety
of structurally diverse of tertiary aromatic and aliphatic amines were
employed as the reaction substrates. The results are summarized in
Table 7. The results indicated that either tertiary aromatic or aliphatic
amines with phenyl-iso-thiocyanate were transformed to the
corresponding thiourea in moderate yields.
H
N
6
N
N
S
3f, 63%
Reaction conditions: (0.5 mmol) of 2a and (1.5 mmol) of 1a, in the
presence of 0.45% N-ZnO NRs (0.003 g) in EtOH (2mL), under CFL white
The proposed mechanism is shown in Figure 9. Under visible light
irradiation, electron and hole pairs are generated between N 2p
states and conduction band of Zn 3d. The excited electrons e CB in the
conduction band will move to the surface and further transfer to
surface-adsorb oxygen-producing O
electron transfer between N,N-dimethyl-para-toluidine 1a and hole
affords an amine radical cation A. O
hydrogen atom from A to form HO
1
5w, 24 h, in air.
-
.-
2
superoxide anion. A single
.-
2
superoxide anion abstracts a
and iminium ion B. Because
•–
2
intermediate B is unstable, it is easily hydrolyzed to give a secondary
amine C in O and water. Then the secondary amine C, nucleophilic
attacks phenyl-iso-thiocyanate 2a to afford target product 3a.
2
6
| J. Name., 2020, 00, 1-3
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O
N
standard
condition
O2 (air)
N
NCS
View Article Online
+
DOI: 10.1039/D0NJ02197K
O2
HO2
+
product 3a
(1)
was not observed
N
1
a, 1.5 mmol
2a, 0.5 mmol
NCS
TEMPO(2 equiv)
OH
e-
A
Zn 3d
standard
condition product 3a
was not observed
N
B
+
+
(2)
N
3.28 ev
2.50 ev
HO
H2O, O2
N 2p
1a, 1.5 mmol
2a, 0.5 mmol
TEA
OH
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
h+
O 2p
NH
N
NCS
S
+
no cat.
(3)
N
H
N
N
H
N
C
dark condition
A
N
S
4 1.5 mmol
2a, 0.5 mmol
NCS
C
5(97%)
5a
no cat.
S
N
6a
+
N
H
(4)
(5)
white CFL 15 w
N
H
4, 1.5 mmol
2a, 0.5 mmol
NCS
5(95%)
H
N
N
N-ZnO
+
S
S
N
H
White CFL 15 W
N
N
4
, 1.5 mmol
2a, 0.5 mmol
H
5(93%)
N-ZnO
EtOH, White CFL 15 W
H
N
(6)
N
N
N
H
S
6, 0.5 mmol
7(33%)
Scheme 3 Control experiments.
7
a
Conclusions
Figure 9 Proposed mechanism of synthesis of thiourea
In conclusion, we report the preparation of visible-light to activate N-
doped ZnO nanostructure using the combustion reaction method
To clarify this proposed mechanism of the reaction we set a series of then we have shown that the nitrogen as a non-metal dopant has a
control experiments (Scheme 3). When the radical-trapping agent positive effect on the N-ZnO as a visible-light-driven photocatalyst.
TEMPO (2 equiv.) was added to reaction mixtures of 1a and 2a under For the first time, we successfully introduced a new procedure for
the relevant standard conditions for the formation of 3a, product 7a the synthesis of unsymmetrical thiourea derivatives through a
was not observed in the presence of TEMPO (Scheme 3, eq. 1). Under reaction between tertiary aliphatic and aromatic amines and phenyl-
the optimized conditions, we also investigated the effect of iso-thiocyanate as precursors over N-ZnO as an activated
triethanolamine (TEA) as a scavenger to trap the holes. According to photocatalyst in the visible-light region. The method proceeds via
observation, we found that the reaction afforded no desired product straightforward formation of N-C bond through the nucleophilic
in the presence of TEA (scheme 3, eq. 2). These results suggested that attack of amine to highly electrophilic carbon of phenyl-iso-
our reaction is likely to involve a radical process.
thiocyanate to generate thiourea in EtOH as a green solvent.
To prove the nucleophilic attacks between secondary amine C and
phenyl-iso-thiocyanate, we surveyed three conditions in a model Acknowledgments
reaction using N-methyl aniline 4 and phenyl-iso-thiocyanate 2a
We gratefully thank the support of this work by the Shiraz University
(
Scheme 3, eq. 3-5). According to this, we found that in each every
Research Council.
three conditions the same product 5 was formed. To prove the
1
structure of desired product 5, we characterized the product with H-
NMR analysis (See SI).
Notes and references
To confirm that secondary amine C, is produced in an optical
pathway, we synthesized N-methyl aniline
dimethylaniline 6, in the presence (0.45%) N-ZnO in EtOH, under
visible-light irradiation (Scheme 3, eq. 6). To prove the structure of
desired product 7, we characterized the product with H-NMR
1
2
3
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New Journal of Chemistry
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View Article Online
DOI: 10.1039/D0NJ02197K
Photocatalytic synthesis of unsymmetrical thiourea derivatives via
visible-light irradiation using nitrogen-doped ZnO nanorods
a
a
a
Mehdi Koohgard , Abdollah Masoudi Sarvestani , and Mona Hosseini-Sarvari*
1
1
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N-ZnO as a photocatalyst under visible-light irradiation promoted for environmentally friendly route for
the synthesis of unsymmetrical thiourea derivatives