Selective N-acetylation with concurrent S-oxidation of o-amino thiol at ambient conditions over Ce doped ZnO composite nanocrystallites

Article abstract of DOI:10.1016/j.mcat.2018.03.007

The oxidative S–S coupling of thiol to disulfide is an imperative chemical transformation in the domain of biological processes and also finds numerous chemical applications. The CeO₂ and ZnO are significant catalysts for oxidation of thiol to disulfide and N-acetylation of amines respectively. Dithiobis(phenylene)bis(benzyldeneimine) moiety containing N-acetyl and disulfide functional groups is a potential antimicrobial agent with Leishmanicidal and antihyperlipidemic activities. Herein, we report a synchronized catalytic application of Ce doped ZnO (Ce-ZnO) and CeO₂-Ce-ZnO composites for selective synthesis of Dithiobis(phenylene)bis(benzyldeneimine) from o-amino thiol. The Ce-ZnO samples were synthesized by simple co precipitation method by calcination of hydroxide precursors at 400 °C to get 0–10% Ce-ZnO nanocrystallites. The formation of CeO₂-Ce-ZnO composite material was observed beyond 1.5% Ce concentration. The synthesized materials were well characterized by IR, XRD, DRS spectroscopy and SEM-EDS analysis. The application of Ce doped ZnO as an efficient catalyst towards the selective N-acetylation and concurrent S-oxidation of o-amino thiol to afford Dithiobis(phenylene)bis(benzyldeneimine) at ambient temperature in acetonitrile was deliberated. Among all screened catalysts, the maximum selectivity was found for 7.5% Ce-ZnO as CeO₂-Ce-ZnO composite catalyst. Lewis acidic property of catalyst supported probable mechanism for achieved dual transformations. Also, the 7.5% Ce-ZnO catalyst has demonstrated a versatile S–S coupling ability for variety of thiol substrates with excellent stability.

Full text of DOI:10.1016/j.mcat.2018.03.007

Molecular Catalysis 450 (2018) 19–28
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Selective N-acetylation with concurrent S-oxidation of o-amino thiol at
ambient conditions over Ce doped ZnO composite nanocrystallites
a
b
a,⁎
Rohidas Jagtap , Sachin Sakate , Satish Pardeshi
a
Department of Chemistry, Savitribai Phule Pune University (formerly University of Pune), Ganeshkhind, Pune, 411007, India
Catalysis Division, CSIR-National Chemical Laboratory, Pune, 411008, India
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
The oxidative SeS coupling of thiol to disulfide is an imperative chemical transformation in the domain of
o-amino thiol
Ce doped ZnO nanospheres
Lewis acid sites
N-acetylation and S-oxidation
Substrate scope
biological processes and also finds numerous chemical applications. The CeO and ZnO are significant catalysts
for oxidation of thiol to disulfide and N-acetylation of amines respectively. Dithiobis(phenylene)bis(benzylde-
neimine) moiety containing N-acetyl and disulfide functional groups is a potential antimicrobial agent with
Leishmanicidal and antihyperlipidemic activities. Herein, we report a synchronized catalytic application of Ce
2
doped ZnO (Ce-ZnO) and CeO
deneimine) from o-amino thiol. The Ce-ZnO samples were synthesized by simple co precipitation method by
calcination of hydroxide precursors at 400 °C to get 0–10% Ce-ZnO nanocrystallites. The formation of CeO -Ce-
2
-Ce-ZnO composites for selective synthesis of Dithiobis(phenylene)bis(benzyl-
2
ZnO composite material was observed beyond 1.5% Ce concentration. The synthesized materials were well
characterized by IR, XRD, DRS spectroscopy and SEM-EDS analysis. The application of Ce doped ZnO as an
efficient catalyst towards the selective N-acetylation and concurrent S-oxidation of o-amino thiol to afford
Dithiobis(phenylene)bis(benzyldeneimine) at ambient temperature in acetonitrile was deliberated. Among all
screened catalysts, the maximum selectivity was found for 7.5% Ce-ZnO as CeO
Lewis acidic property of catalyst supported probable mechanism for achieved dual transformations. Also, the
.5% Ce-ZnO catalyst has demonstrated a versatile SeS coupling ability for variety of thiol substrates with
excellent stability.
2
-Ce-ZnO composite catalyst.
7
1
. Introduction
whereas Dithiobis(phenylene)bis(benzyldeneimine) [7–9] is a potential
antimicrobial agent against human pathogens. Pyritinol [10] con-
taining disulfide bridged two pyridoxine molecules is regarded as an
encephalotropic compound used for treating some neurological dis-
orders while aryl disulfide moieties exhibit significant Leishmanicidal
Activity [11]. Tetramethylthiuram disulfide (TMTD) is used as fungi-
cide, ectoparasiticide and animal repellent agrochemical [12] and is
one of the leading accelerators in rubber technology for sulfur vulca-
nization [13]. Moreover in petrochemical refining process, sweetening
of catalyst poisoning thiols is achieved by formation of low volatile
disulfides [14]. The disulfides are very crucial intermediates in the
synthesis of sulfur containing heterocycles like benzthiazoles [15],
sulfenyl indoles [16] and bis-sulfenyls [17].
The oxidative SeS coupling of thiol to disulfide is an imperative
chemical transformation in the domain of biological processes and
synthetic chemistry. In biological systems, disulphide bonds can be
formed spontaneously in vitro by the loss of electrons from two cysteine
thiols coupled with the gain of electrons by molecular oxygen. In this
process an intermediary like transition metal or flavin is required to
overcome the kinetically sluggish yet thermodynamically favorable
association of oxygen with protein thiols [1]. The formation of disulfide
bond is essential for stability or activity of natural and biologically
important folded conformations of peptides and proteins [2]. Also, the
oxidative stress at cellular level is indicated by promotion of disulfide
formation in thiol containing proteins [3]. Apart from the biological
significance, the formation of disulfide bond from thiols has numerous
chemical applications in antioxidants, pharmaceuticals, pesticides,
rubber vulcanization agents and more importantly petrochemical re-
fining process. Allyl disulfide shows antioxidant [4] and anticancer
activities [5]. Mitomycin disulfides [6] are significant antitumor agents
In cognizance of broad spectrum of applications of disulfides, the
oxidative SeS coupling of thiols has been accomplished by various
researchers deploying various catalysts, reagents and methods. The
various Iron based catalysts along with H
ferrite [18], transition metal-Schiff base complexes [19,20], ammo-
nium-tribromide functionalized magnetic Fe [21] Iron metal-
2 2
O as an oxidizer such as Ni
3
O
4
,
⁎
Corresponding author.
E-mail address: skpar@chem.unipune.ac.in (S. Pardeshi).
https://doi.org/10.1016/j.mcat.2018.03.007
Received 9 November 2017; Received in revised form 15 February 2018; Accepted 8 March 2018
2468-8231/ © 2018 Elsevier B.V. All rights reserved.

R. Jagtap et al.
Molecular Catalysis 450 (2018) 19–28
organic framework [22], Fe(HSO
to synthesize disulfides. In this context, the strong oxidants like NaOCl
4
)
3
/DMSO [23] have been employed
diacetamide (B) (Scheme 1) in a single step. The selectivity of the
catalysts was correlated with pyridine IR based Lewis acid properties of
catalysts to propose a possible mechanism. Finally, the substrate scope
and recyclability of the catalysts is also explored.
[
[
24,25], NaOCl
2 4 5 6 3 2
[26], NaIO [27], H IO [28], NaBrO [29], SOCl
30], KMnO [31,32], bipyridinium hydrobromide-perbromide [33],
4
benzyltriphenylphosphonium dichromate [34], Iron(III) perchlorate
[
[
35], Tetrabutylammonium peroxydisulfate [36], anhydrous K
3 4
PO
2
. Experimental
.1. Chemicals
3 6 2 2
NH Ce(NO ) and Zn(OAc) ·H O (Analytical Grade, Merck India
37], VO(acac) [38], Nickel peroxide [39] used in stoichiometric
2
amount are found to be effective reagents. Handful reports are there on
transition metal based catalysts like Cu [40], Ni [41] metal nano-
particles and few oxides like TiO [42], MnO [43] to accomplish the
2 2
2
(
4
)
2
oxidative SeS coupling of thiols. Most of these enlisted catalysts possess
one or more disadvantages like long reaction times, critical work-up
procedure, room temperature incompatibility, and strong basic or
acidic media and includes some relatively expensive metals [44]. In
fact, thiols are prone to give several oxidation products like sulfones,
sulfoxides and sulfonic acids along with disulfide on using strong oxi-
dizing reagents [44]. Though these ample of catalytic options are
available for oxidative SeS coupling of o-amino thiol to disulfide,
hardly few report are available for its simultaneous N-acetylation to
afford synthesis of Leishmanicidal [11] and antihyperlipidemic [45]
N,N′-(disulfanediylbis(2,1-phenylene))diacetamide. Recently, Mali [46]
and Viglianisi et al. [47] have used Cu catalyst to achieve this dual
transformation along with thioacetic acid and acetic anhydride as an
acetylating reagent respectively. In a report, El-Shaieb [24] has
achieved this synthesis using acetyl chloride, toxic pyridine base and
strong oxidant NaOCl. Thus, there is a need to develop a mild, efficient
and selective process for the transformation of o-amino thiol to med-
icinally important N,N′-(disulfanediylbis(2,1-phenylene))diacetamide
without over oxidation to oxygenated sulfur products.
Ltd.) were used as Cerium and Zinc sources respectively. Sodium hy-
droxide (NaOH), o-amino thiol was procured from Merck India
(
Analytical Grade). All solvents were used for this work were of
Analytical Grade, purchased from Thomas Baker, India.
2.2. Preparation of catalysts
The pure (undoped) and Ce doped ZnO materials were prepared by
using simple co-precipitation method. The desired volumes of 1 M zinc
acetate dihydrate and 1 M Cerric Ammonium Nitrate to get final vo-
lume 100 ml were stirred in 250 ml beaker to a homogenous solution
and 100 ml 1 M sodium hydroxide solution was added to it to get final
volume 200 ml which attended final pH value 8.5 on thorough mixing
of solutions. The white to yellowish white precipitates were filtered
through Whatmann filter paper No.42 by giving five to six washings of
distilled water. The fluppy white to pale yellow hydroxide precursors
were dried and further ground to fine powders and calcined in muffle
furnace for 6 h at 400 °C (temperature obtained from thermal studies
which are not shown here). The obtained nanocrystallites of Ce doped
ZnO with different mol % composition of Ce are shown in Table 1. The
pure ZnO was white whereas the gradual increase in yellow colour with
increase in% of Ce in material composition was observed.
Among the various important functional metal-oxides, Zinc oxide
(
ZnO) is noteworthy due to its wide band gap energy and high thermal,
chemical, mechanical stability with high exciton binding energy
48–50]. The pure and rare earth doped ZnO are important in en-
[
vironmental remediation for solar photocatalytic degradation of toxic
water pollutants [51,52]. Brahmachari et al. have recently reported
chemoselective N-acetylation of amines [53] using ZnO catalyst which
hints for further exploration of ZnO towards N-acetylation of amino
2
.3. Characterization of catalysts
The pure and Ce doped ZnO were characterized by FTIR (Shimadzu
FTIR-8400 spectrometer equipped with KBr beam splitter), X-ray dif-
fractometer (Bruker AXSD-8 Advance X-ray diffractometer with
monochromatic CuKα, radiation, λ = 1.5406 A°), Scanning Electron
Micrograph (SEM-EDS JEOL JSM-6360A) and UV–vis Diffuse
Reflectance Absorption Spectra (DRS, Shimadzu UV-3600 UV–vis-NIR
spectrophotometer equipped with an integrating sphere with BaSO as
4
the reference). Py-IR spectra were recorded on Shimadzu FTIR 8000
attached with second sampling unit (SSU) using 20 mg catalyst sample.
thiols. In current highlighting studies, CeO
porous ceria [55], gold nanoparticles supported on CeO
2
nanoparticles [54], meso-
[56] have
2
been utilized by different research groups as efficient catalysts for
oxidative disulfide synthesis from thiols. Considering catalytic abilities
of ZnO for N-acetylation of amines and CeO
disulfide, we aimed a synchronized catalytic application of Ce doped
ZnO (Ce-ZnO) and CeO -Ce-ZnO composites to synthesize N,N′-(dis-
2
for oxidation of thiols to
2
ulfanediylbis(2,1-phenylene))diacetamide in a single step.
Herein, we report synthesis of 0–10% Ce doped ZnO by simple co-
precipitation method and their thorough characterization by several
Sample was filled in a sample cup; 10 μl of pyridine were injected in N
flow.
2
2
techniques. The application of Ce-ZnO and CeO -Ce-ZnO composite
2
thiol
.4. Catalytic activity of catalysts towards selective acetylation of o-amino
nanocrystallites as a mild, selective and efficient catalyst is studied for
N-acetylation with concurrent S-oxidation of a model substrate o-amino
thiol to synthesize disulfide N,N′-(disulfanediylbis(2,1-phenylene))
The catalytic activity of the synthesized materials was studied
Scheme 1. Catalytic activity of catalysts towards selective acetylation of o-amino thiol.
20

R. Jagtap et al.
Molecular Catalysis 450 (2018) 19–28
Table 1
pure ZnO catalyst by addition of either 2N HCl or 2N NaOH to get the
desired acidic or basic pH respectively. After maintaining the desired
pH, the reaction was run as per the solvent optimization protocol. The
ambient pH condition (without adjustment) was found to be better for
conversion of the substrate in minimum time on monitoring the reac-
tion by TLC.
Different compositions to synthesize pure and Ce doped ZnO materials.
Flask No. 1 M Zn
OAc) ·2H
1 M (NH
4
)
2
Ce
1 M
KOH ml
Expected
Composition Mol %
(
2
2
O ml
(NO
3
)
6
ml
1
2
3
4
5
6
7
8
9
100
0.0
0.5
1.0
1.5
2.0
2.5
5.0
7.5
100
100
100
100
100
100
100
100
100
pure ZnO
99.5
99.0
98.0
97.0
96.0
95.0
92.5
90.0
0.5% Ce-ZnO
1.0% Ce-ZnO
1.5% Ce-ZnO
2.0% Ce-ZnO
2.5% Ce-ZnO
5.0% Ce-ZnO
7.5% Ce-ZnO
10.0% Ce-ZnO
2
.4.3. Screening of Ce doped ZnO catalysts
On optimization of Acetonitrile solvent at ambient pH conditions for
rapid reaction, we decided to screen Ce doped ZnO catalysts to focus
upon selective synthesis of moiety B. All the reaction parameters were
kept same as that of solvent optimization experiment except the cata-
lyst. All the catalysts ranging from pure ZnO, 0.5% Ce-ZnO, 1.0% Ce-
ZnO, 1.5% Ce-ZnO, 2.0% Ce-ZnO, 2.5% Ce-ZnO, 5.0% Ce-ZnO, 7.5%
Ce-ZnO and 10% Ce-ZnO were individually employed for the reaction.
After TLC monitored completion of the reaction, the samples were
prepared for selectivity determination of A and B by GC–MS analysis as
per the protocol mentioned in Sec 2.4.1. The 7.5% Ce-ZnO catalyst was
found to be superior to give higher selectivity of the disulfide B.
10.0
towards selective N-acetylation with concurrent S-oxidation to disufide
of substrate o-amino thiol using acetyl chloride (AcCl) as an acetylating
agent. The TLC monitoring of the reactions was done using Merck DC
precoated TLC plates coated by 0.25 mm Kieselgel 60 F254 and visuali-
zation was done using 254 nm UV lamp. The GC–MS of the reaction
extracts were recorded for selectivity of products using GC Shimadzu
-
74218 having HP-5 column with FID detector and mass spectrometer,
2.4.4. Optimization of amount 7.5% Ce-ZnO catalyst
QP -2010 ultra EI, Shimadzu, positive chemical ionization. The PCI ion
source was used for acquiring the mass spectra with mass range
By keeping the reaction parameters constant as per the catalyst
screening experiment for 7.5% Ce-ZnO, we altered amount of the cat-
alysts ranging from 6.25 mg (5 mol %), 9.37 mg (mol 7.5%), (10 mol
%), 15.62 mg (12.5 mol %), 18.75 mg (15 mol %) and 25.00 mg (20 mol
%) of 7.5% Ce-ZnO catalyst. The 12.50 mg (10 mol %) was found to be
decisive amount of 7.5% Ce-ZnO catalyst to give almost exclusively
single desired target compound B.
1
13
1
.5–1090 m/z, maximum scan rate of 20,000 amu/s. The H and
NMR of the pure compounds were recorded in δ ppm scale on Bruker
400 and 100 MHz) NMR spectrometer. The pH of suspension in pH
C
(
optimization experiments were recorded with EUTECH-pH510 pH
meter.
3. Results and discussion
2.4.1. Solvent optimization
Initially, we tried the conversion of o-amino thiol in to desired
3
.1. Characterization of the pure and Ce-doped ZnO
acetyl disulfide derivative using pure (undoped) ZnO as a catalyst. The
.00 mM of o-amino thiol, 5 mol % pure ZnO, 8.0 ml of different sol-
vents were stirred in 50 ml R. B. flask at ambient temperature for
0 min and 1.05 mM of AcCl was added to the resultant suspensions.
The time required for conversion of the substrate in to products was
monitored by TLC till disappearance of the o-amino thiol using 15%
ethyl acetate in hexane as a mobile phase. Acetonitrile was found to be
the better solvent for rapid conversion of substrate.
In each solvent, we observed two spots on product side demon-
strating the formation of two different organic moieties. In order to
determine selectivities of these products, volatile solvent from each
reaction mixture was evaporated after TLC monitored completion of
reaction. The resultant residues were further dissolved in ethyl acetate
and filtered to remove the suspended catalyst and washed by 10%
1
The IR spectra of the prepared pure and Ce doped ZnO are shown in
Fig. 1. The IR spectra shows the presence of single Zinc metal oxygen
2
−
1
bond stretching frequency at 415 cm for pure ZnO and 0.5, 1.0, 1.5%
Ce-ZnO. For further compositions from 2.0% Ce-ZnO to 10% Ce-ZnO,
along with the ZneO stretching band, another metal oxygen IR band for
CeeO stretching at 474 cm is emerged and became prominent as per
the increase in Ce % in the composition. These observations depicted
that up to 1.5% Ce-ZnO single phase material is formed with perfect
doping of Ce in ZnO; on further increase in percentage of Ce, the Ce
−1
2
remains out of doped lattice to give a CeO -Ce-ZnO composite material.
The additional IR bands observed at 769, 864, 1362, 1442 and 1743
are attributed to free OeCeeO stretching [57], CeO bending in acetate
species [58], CeO bending of CO
2
−
3
[59], symmetric–asymmetric
NaHCO
erated in acetylation. Finally, ethyl acetate extracts were dried over
anhydrous Na SO and the aliquots from every extracts were taken for
3
solution to get rid of any traces of unreacted AcCl, HCl gen-
2
4
GC–MS analysis for selectivity determination. In order to sepearate the
mixture of two products, all the ethyl acetated extracts were collec-
tively subjected for silica column chromatographic separation using
1
0% ethyl acetate in hexane as an eluting solvent medium. The isolated
1
products were characterized by IR, H NMR and GC–MS analysis and
found to be N-(2-mercaptophenyl) acetamide (A) and its SeS coupled
N-acetylated disulfide derivative i.e. N,N′-(disulfanediylbis(2,1-pheny-
lene))diacetamide (B) as shown in Scheme 1.
The formation of compound B was an encouraging result as it si-
multaneously attempted the dual transformations i.e. N-acetylation and
oxidative SeS coupling of o-amino thiol.
2.4.2. pH optimization studies
In terms of getting higher selectively of disulfide product B, we
decided to tune the pH conditions using acetonitrile as a suitable re-
action solvent and by keeping all other parameters constant (Sec 2.4.1).
The pH of the reaction medium was maintained prior to administrating
Fig. 1. IR spectra of pure and Ce doped ZnO.
21

R. Jagtap et al.
Molecular Catalysis 450 (2018) 19–28
Fig. 3. Bragg’s angle shift to higher 2θ for 101 plane of ZnO.
Fig. 2. XRD pattern of pure and Ce doped ZnO.
stretching and C]O stretching vibrations of acetate species [58] re-
spectively. The additional IR bands indicated the presence of free CeO
and adsorbed acetate, carbonate species on material surface.
2
The XRD spectra of synthesized pure and Ce doped ZnO are shown
in Fig. 2. The two theta (2θ) values for the 100, 002, 101, 102, 110,
103, 200, 112, 201, 004 and 202 planes are in well agreement with the
reference pattern (JCPDS No. 36-1451 for ZnO) corresponding to ZnO
Wrutzite structure of all the samples. Along with the ZnO peaks, in
2
3
1
.0% Ce-ZnO and onward compositions the prominent peak at 2θ value
2 matches with the 111 plane of CeO Cubic structure (JCPDS No. 89-
397) that reveals the presence of CeO along with Ce doped ZnO
2
2
material. This also supports the observation from IR spectra for for-
mation of Ce-ZnO up to 1.5% Ce-ZnO and CeO
materials on further % increase of Ce.
2
-Ce-ZnO composite
The shifting of Bragg’s angle towards the higher 2θ values for 101
plane of pure ZnO to 2.5% Ce-ZnO by 0.18 ascertains the presence of
doped Ce in ZnO crystal lattice. No more 2θ angle shifting beyond 2.5%
Fig. 4. UV–Vis DRS spectra of pure and Ce doped ZnO.
The FESEM images (Fig. 5) of the pure and doped materials shows
the crystallite size maintained either less or equal to 150 nm which is
attributed to lower calcination temperature (400 °C). The pure ZnO
shows perfect spherical granular morphology (image a) which is re-
tained up to 1.5% Ce-ZnO (image d).
2
Ce-ZnO indicates the further utilization of Ce for formation CeO -Ce-
ZnO composite in onward compositions, which is in correlation with
the observations of IR and XRD profiles of the materials. The observed
Bragg’s angle shift for 101 planes in doped and composite materials is
depicted in Fig. 3.
The further increase of the % of Ce results in transformation of
granular nanocrystallites into elongated granules (image e, f) which
are observed to attain the well defined needles like (flower shaped)
morphology in 5, 7.5% Ce-ZnO (image g, h). Finally nanocrystallites
needle get agglomerated into flakes like appearance in 10% Ce-ZnO
(image i).
The EDS elemental analysis shows the presence of Ce along with Zn
in doped ZnO samples. The Ce % observed in EDS is in order of gradual
increase in Ce amount in compositions. The observed Ce % for 0.5, 1.0,
Also, the lattice parameter, a, calculated for 101 plane in undoped
ZnO was 0.3250 (standard value, JCPDS 36-1451, a = 0.3249) and its
values for 0.5, 1.0, 1.5, 2.0, 2.5, 5.0, 7.5, 10% Ce-ZnO were found to be
0.3252, 0.3553, 0.3255, 03258, 0.3259, 0.3259, 0.3260, 0.3260 re-
spectively. The increase in lattice parameter, a, indicates the presence
2
+
of Ce ions into ZnO lattice by substituting the Zn
ion (ionic radius
(0.092 nm) than com-
(0.103 nm) [60]. In addition, the observed
4
+
0
.074 nm) sites by preferentially stable Ce
3
+
paratively larger Ce
consistent increase in lattice parameter, a, up to 2.5% Ce-ZnO and its
almost constant values in onward compositions illustrates the formation
of composite materials.
1
1
.5, 2.0, 2.5, 5.0, 7.5 and 10% Ce-ZnO are found to be 0.35, 0.68, 1.23,
.34, 1.4, 3.36, 6.32 and 8.9% respectively. The observed losses in Ce %
from theoretical and actual values may be attributed to loss of Ce while
washing the hydroxide precursor and heating these precursors during
calcination stages. The EDS spectra of the representative 1.0 and 5.0%
Ce-ZnO is shown in Fig. 6 whereas the EDS-element analysis for all the
materials is given in Supplementary data (S1).
The UV Visible-Diffuse Reflectance Absorption Spectra (DRS) of the
pure and Ce doped ZnO shows the phenomenal red shift in absorption
band edge wavelength with gradual increase in % of Ce in doped ma-
terials as depicted in Fig. 4. The presence of Ce makes the material more
optical and hence increases the absorption band edge wavelength from
366.18 nm for pure ZnO to 372.55 nm for 10% Ce-ZnO. The increase in
UV absorption indicates the presence of Ce in Ce-ZnO and CeO
composite materials [61].
2
-CeZnO
22

R. Jagtap et al.
Molecular Catalysis 450 (2018) 19–28
Fig. 5. FESEM images of pure ZnO (a), 0.5% (b), 1% (c), 1.5% (d), 2% (e), 2.5% (f), 5% (g), 7.5% (h), 10% (i) Ce-ZnO.
powder, IR (ATR, υ cm 1): 3294 (NeH), 1604 (C]O), 1298 (CeS),
1125 (SeS), 742 (AreH); 1H NMR (CDCl
, 400 MHz): δ ppm 1.99 (s,
H), 7.02 (td,1H, J = 7.6, 1.3 Hz), 7.43 (m, 2H), 7.94 (s, broad, 1H),
−
3
thiol
.2. Catalytic activity of catalysts towards selective acetylation of o-amino
3
3
+
3.2.1. Solvent optimization
8.35 (d, 1H, J = 8.1 Hz), M = 334 (Supplementary data S3, S5, S7).
With the prior objective of the selective N-acetylation of o-amino
thiol over 5 mol % of pure ZnO catalyst in various solvents, we tried
several solvents like water, DCM, Methanol, THF, Acetonitrile and
Toluene. Initially absolutely no change was observed (identified by
TLC) in the substrate when reaction was tried in water which may be
ascribed to very poor solubility of substrate and the hydration of active
Lewis acidic centers of the catalyst in water. Among various organic
solvents DCM, methanol, THF and toluene afforded formation of the
two products (A and B) up to 65 min whereas in acetonitrile the rapid
conversion of the o-amino thiol took place (45 min). The observed %
selectivity of desired product B was 14, 22, 15, 20 and 13% for the used
solvents DCM, methanol, THF, acetonitrile and toluene respectively.
The solvent optimization studies along with selectivity of products are
graphically represented in Fig. 7.
These outcomes revealed that, though methanol showed highest
selectivity for disulfide compound B, acetonitrile is found to better
solvent for a comparatively rapid reaction suggesting higher activity of
ZnO catalyst in acetonitrile. The solvent studies enabled us to use
acetonitrile as an appropriate solvent for a rapid conversion of the o-
amino thiol.
3
.2.2. pH optimization
In order to get the desired moiety B selectively in a time efficient
manner, we further tuned the pH of the reaction medium by addition of
either 2N HCl or 2N NaOH using pure ZnO catalyst. The acetylation was
not observed in acidic conditions (pH = 1, 4, 6) but it was took place in
alkaline conditions with comparatively higher reaction time (Fig. 8).
For ambient pH condition (pH = 7) the transformation of the substrate
took place in 45 min whereas on increase in pH increased to 8 and
1
0 pH scale the reaction time required were higher and found to be 50
and 60 min respectively. The inhibition of the reaction at acidic con-
ditions may be attributed to protonation of thiol, amine functional
groups of the substrate as well as blocking of catalyst sites by Cl . On
other hand the higher reaction time with respect to increasing alkaline
conditions is perhaps may be ascribed to the blocking of Lewis acidic
sites of the catalyst surface by OH ions from NaOH. Though, both
products (A and B) were formed at ambient pH, at alkaline pH the
formation of only compound A as observed on TLC monitoring of the
reaction. This indicates the rapid dissociation of thiol group at alkaline
conditions which adversely affect SeS coupling along with blocking of
−
−
Reaction extracts obtained in each solvent optimization studies
were mixed and separation of two products (A and B) using silica
column chromatography was achieved using 10% Ethyl acetate in
hexane as an eluent. The obtained characterization data for compounds
A and B is as below.
−
active sites of catalyst by OH ions. The absence of S-acetyl product
perhaps may be due to conversion of S-acetyl to N-acetyl derivative.
The time required for the conversion of o-amino thiol was least (45 min)
for ambient pH. Hence, it is proved that the ambient pH is the better
condition for the desired transformation in acetonitrile.
3
.2.1.1. N-(2-mercaptophenyl) acetamide (A). Yellowish powder, IR
3
.2.3. Screening of Ce-ZnO catalysts for selectivity of the products A and B
In order to achieve exclusive synthesis of disulfide B, the catalyst
−1
(
ATR, υ cm ): 3427 (SeH), 3232 (NeH), 1658 (C]O), 1301 (CeS),
1
7
34 (AreH); H NMR (CDCl
3
, 400 MHz): δ ppm 2.51 (s, 3H), 5.43 (s,
screening was done by using the whole range of the catalysts from pure
ZnO to 10% Ce-ZnO in 6.25 mg (5 mol %) amounts. It is found that the
increase of Ce % in the catalyst composition leads to decrease in % A
and promotes the formation of target compound B with lesser reaction
time. The reaction time and product selectivities in catalyst screening
broad, 1H), 6.43 (m,1H), 6.73 (m,1H), 6.99(dt,1H, J = 12.0, 6.1 Hz),
7
+
.07(m,1H); M = 149 (Supplementary data S2, S4, S6).
3
.2.1.2. N,N′-(disulfanediylbis(2,1-phenylene))diacetamide (B). White
23

R. Jagtap et al.
Molecular Catalysis 450 (2018) 19–28
Fig. 6. EDS spectra and elemental analysis of pure ZnO and 1 and 5% Ce-ZnO.
studies are given in Table 2 and graphically represented in Fig. 9.
The reaction time for conversion of the substrate in to products A
and B was found to be decreasing as the % Ce increases, this clearly
demonstrate role of the Ce in rapid SeS coupling reaction. For pure ZnO
the reaction was rapid but the product A found in major amount (80%).
Although the incorporation of the Ce gives initial increase in the re-
action time for 0.5–2.5% Ce-ZnO the results were improved for % B
which was obtained in 73% in 50 min for 5% Ce-ZnO. Further en-
couraging results were obtained for 7.5% Ce doped ZnO which gave
3.2.4. Optimization of amount the 7.5% Ce-ZnO catalysts for enhanced
selectivity of B
Finally, to improve the selectivity of disulfide product B, we decided
the catalyst amount variation by keeping the reactants amount and
acetonitrile solvent volume as per catalysts screening studies. The effect
of amounts of catalyst are illustrated in Table 3 and graphically re-
presented in Fig. 9.
As our foremost interest was to enhance the selectivity without re-
ducing reaction time the amount of catalyst was not reduced in com-
parison to previous screening studies (5.0 mol %, 6.25 mg). However it
is found that the higher amounts of 7.5% Ce-ZnO catalyst plays a vital
role to enhance the selectivity of the reaction. The initial 84% se-
lectivity for disulfide B at 5 mol % of catalyst was enhanced up to
maximum 98% at an amount of 12.50 mg (10.0 mol %) in 45 min.
Though the further increase in amount of the catalyst reduces the re-
action time up to 35 min, it decreases the selectivity of the B as 96, 93
and 90% for 15.62, 18.75 and 25.00 mg respectively as shown in
84% of disulfide B in only 45 min. For 10% Ce-ZnO the compound B
was obtained in slightly decreased in selectivity (75%) and with mar-
ginal increase in reaction time (50 min), may be due to bulky mor-
phology of the catalyst as seen in Fig. 5 (image i).
The catalyst screening demonstrated that the 7.5% Ce-ZnO catalyst
is the better choice for higher % of compound B in minimum reaction
time.
24

R. Jagtap et al.
Molecular Catalysis 450 (2018) 19–28
Fig. 7. Solvent optimization for rapid acetylation of o-amino thiol.
Fig. 9. The reaction time and selectivity of the products in catalyst screening.
Table 3
Amount of 7.5% Ce-ZnO towards enhanced selectivity of B.
Entry
Amount of catalyst (mol %)
Time in min
% A
% B
1
2
3
4
5
6
.
.
.
.
.
.
6.25 mg (5.0%)
9.37 mg (7.5%)
12.50 mg (10.0%)
15.62 mg (12.5%)
18.75 mg (15.0%)
25.00 mg (20.0%)
45
45
45
40
35
40
16
03
02
04
07
10
84
97
98
96
93
90
Fig. 8. pH optimization for acetylation of o-amino thiol.
Table 2
Screening of the catalysts for selectivity of the compound B.
Entry
Catalyst used
Reaction time in min
% A
% B
1
2
3
4
5
6
7
8
9
.
.
.
.
.
.
.
.
.
Pure ZnO
45
65
65
60
60
55
50
45
50
80
76
63
55
48
36
27
16
25
20
24
37
45
52
64
73
84
75
0.5% Ce-ZnO
1.0% Ce-ZnO
1.5% Ce-ZnO
2.0% Ce-ZnO
2.5% Ce-ZnO
5.0% Ce-ZnO
7.5% Ce-ZnO
10.0% Ce-ZnO
Fig. 10. Amount of 7.5% Ce-ZnO, reaction time and selectivity of B.
towards selectively synthesis of B, we carried out the Py-IR profiles
using normalized absorption spectra of the pure ZnO, 2.5% Ce-ZnO, 5%
Ce-ZnO, 7.5% Ce-ZnO and 10% Ce-ZnO catalysts as shown in Fig. 11.
Before measurement, the samples were activated to clean ad-
Fig. 10.
From these studies it can be concluded that the reaction conditions
to obtain almost single compound B are 1.0 mM of o-amino thiol,
sorbents like CO32 by preheating at 200 °C under a vacuum of 10 Pa
−
−3
1.1 mM of AcCl, 12.50 mg of 7.5% Ce-ZnO catalyst at ambient pH,
temperature, and within 45 min.
for 2 h. To get the unequivocal results, all samples were injected with
1
2
0 μl of the pyridine under N flow at R.T. and the normalized IR
spectra were recorded at 30 min adsorption period. Though the pure
ZnO and 2.5% Ce-ZnO were observed to have weak pyridine adsorp-
tion, the 5.0, 7.5 and 10% Ce-ZnO catalyst showed the significant
3.2.5. Pyridine-IR profile of the catalysts and proposed mechanism for the
product selectivity
In order to understand why 7.5% Ce-ZnO catalyst is highly active
25

R. Jagtap et al.
Molecular Catalysis 450 (2018) 19–28
adsorption of anhydride oxygen. Further addition of the o-amino thiol
−
will show the stronger binding of more nucleophilic eS towards
higher electropositive Ce4+ centers of the composite catalyst which is
in agreement to studies by Baiker et al. [65]. Under these circum-
stances, the amine nucleophile is expected to have comparatively more
freedom to attack on electrophillic carbonyl carbon to give N-acetyl
4
+
derivative as shown in Fig. 12. Thus, enhancement in % of Ce ions in
−
the catalytic composition offers stronger adsorption of the thiol eS
and also the residual time of thiol S on catalyst surface which promotes
−
the N-acetylation. The Cl ions from generated HCl may be playing the
−
key role to desorption of eS from eS⋯Zn/Ce binding which leads to
−
attack of one eS on another adsorbed S atom from N-acetylated
molecule. Thus the delayed desorption of sulfur from more Lewis acidic
centers (Ce4+) may be responsible for the higher selectivity of the
disulfide B obtained by concurrent oxidation of eSH to eSeSe couple.
3.2.6. Substrate scope
Encouraged by excellent SeS coupled disulfide selectivity in o-
Fig. 11. Py-IR of the catalysts for Bronsted-Lewis acidity.
amino thiol we thought to expand the scope of this 7.5% Ce-ZnO cat-
alyst for some different aromatic thiols under the optimized conditions
as shown in Table 4. The four different aromatic substrates viz. p-amino
thiol, p-chloro thiol, thiosalicylic acid and 2-thiobarbituric acid were
used in order to study the SeS coupling ability of the optimized cata-
lytic conditions.
−
1
adsorption peaks at 1543 cm
for Brönsted acidic sites (B) and
−1
1453 cm
for Lewis acidic sites (L) with a remarkable peak at 1495
represent the simultaneous presence of both Brönsted and Lewis acidity
62]. The B/L ratios calculated from peak intensities for the peaks at
[
1
−1
543:1453 cm
[62] was observed to be 0.91, 0.88 and 0.90 respec-
The oxidative SeS coupling was observed in all studied substrates
except thiosalicylic acid with above 99% conversion and exhibiting
comparatively lower conversion rate in 2-thiobarbituric acid. The op-
timized catalytic reaction conditions (Sec. 3.2.4) were also found to be
effective to afford disulfides of p-chloro thiol, 2-thiobarbituric acid and
N-acetylated disulfide of p-amino thiol with 94, 97 and 96% product
selectivity respectively. On contrary, in case of thiosalicylic acid the
exclusively observed product was S-acetyl thiosalicylic acid. The in-
hibition of oxidative SeS coupling in this substrate may be attributed to
protonation of thiol at higher acidic condition generated by strong
dissociation ability of thiosalicylic acid, which led to formation of S-
acetyl product with prolonged reaction time. This observed failure of
SeS coupling in thiosalicylic acid was in agreement with report by
Garcia et al. [40] on thiobenzoic acid.
tively. Among these ratios the 7.5% Ce-ZnO was found to have more
Lewis acidic sites and hence proves its capacity to catalyze the Lewis
acid mediated reaction. The Pyridine IR profile of the catalyst was also
significant to propose the product selectivity of the catalyst on the basis
of preferential adsorption of thiol functional group.
In the proposed mechanism, the Lewis acidic sites in pure and Ce
doped ZnO are considered to focus the selectivity of product A and B.
Also, there are equal chances of dissociation of the o-amino thiol (pKa
9.7) in polar aprotic medium i.e. acetonitrile [63]. In comparison with
Ce (electron affinity 0.55 eV) [64] the Zn is a weaker Lewis acid. In the
course of reaction using pure ZnO, chloride from AcCl will be initially
2
+
adsorbed to relatively more abundant Zn
centers which is in agree-
ment with mechanism proposed by Bramhachari et al. [53] for
Fig. 12. Possible mechanism for N-acetylation with concurrent SeS coupling.
26

R. Jagtap et al.
Molecular Catalysis 450 (2018) 19–28
Table 4
Substrate scope of 7.5% Ce-ZnO catalyst.
Entry
Substrate
Time in min
50
Conversion (%)
> 99
Product
% Selectivity
96
1
.
.
2
40
> 99
94
3
4
.
.
180
80
> 99
> 99
99
97
coupled disulfide B at ambient temperature and pH condition in acet-
onitrile. The delayed desorption of sulfur from Ce4+ centers from 7.5%
Ce-ZnO composite catalyst is responsible for the concurrent oxidation of
N-acetyl o-amino thiol to eSeSe couple. The substrate scope studies
exhibited versatile SeS coupling ability of 7.5% Ce-ZnO catalyst for
different thiols except thiosalicyic acid with retained activity up to
studied five reaction cycles.
Acknowledgements
The author RJ is thankful to Dr. Sudhir Arbuj, C-MET, Pune and Dr.
C.V. Rode, NCL, Pune for extending characterization facilities.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.mcat.2018.03.007.
References
Fig. 13. Recyclability of the catalyst.
[
[
1] C.S. Sevier, C.A. Kaiser, Nat. Rev. Mol. Cell Biol. 3 (2002) 836–847.
2] M.V. Trivedi, J.S. Laurence, T.J. Siahaan, Curr. Protein Pept. Sci. 10 (2009)
614–625.
3
.2.7. Recyclability of catalyst
In order to explore heterogeneous nature and stability of the 7.5%
[
3] R.C. Cumming, N.L. Andon, P.A. Haynes, M. Park, W.H. Fischer, D. Schubert, J.
Biol. Chem. 279 (2004) 21749–21758.
Ce-ZnO catalyst, we evaluated the recyclability using o-amino thiol as a
substrate at optimized conditions (Sec. 3.2.4). After each reaction cycle
of consecutive five experiments, the catalyst was recovered and washed
by MeOH to remove any adsorbed organics and further dried at 110 °C
prior to use for next cycle. The catalyst was found to be remained active
even after five consecutive cycles with almost similar % conversions of
the substrates as shown in Fig. 13. These results indicated that the
catalyst exhibits excellent stability without any activity lost.
[
4] L.Y. Chung, J. Med. Food 9 (2006) 205–213.
[5] A.K. Maurya, S.V. Singh, Cancer Lett. 57 (1991) 121–129.
[
6] S.H. Lee, Arch. Pharm. Res. 32 (2009) 299–315.
[
7] M.G. Bhowon, S.J. Laulloo, N. Soukhee, A. Allibacus, V. Shiboo, J. Coord. Chem. 60
(
2007) 1335–1343.
[8] W. Lu, L.C. Arraes, W.T. Ferreira, J.M. Andrieu, Nat. Med. 10 (2004) 1359–1365.
9] A.G. Lavekar, R. Kumar, A.K. Sinha, J. Mol. Catal. B: Enzym. 116 (2015) 113–123.
10] I. Hindmarch, D.M. Coleston, J.S. Kerr, Neuropsychobiology 24 (1990/91)
59–164.
[
[
1
[11] K.M. Khan, M. Taha, F. Naz, M. Khan, F. Rahim, Samreen, S. Perveen,
M.I. Choudhary, Med. Chem. 7 (2011) 704–710.
[
12] J.H. Montgomery, Agrochemicals Desk Reference, 2nd ed., Lewis Publishers, Boca
Raton, USA, 1997, pp. 412–413.
13] R. Steudel, Y. Steudel, M.W. Wong, Chem. Eur. J. 14 (2008) 919–932.
14] D.L. Holbrook, Handbook of Petroleum Refining Processes, McGraw-Hill, New
York, 1996 (Chapter 11.3).
4
. Conclusions
[
[
The pure and Ce doped nanocrystallites are synthesized by simple co
precipitation method and thoroughly characterized by various techni-
ques. The IR spectra and XRD pattern of the materials illustrated that up
to 1.5% successful doping of Ce in ZnO is obtained and further increase
in Ce % leads to formation of composite material. The DRS spectra of
the Ce doped ZnO showed the phenomenal red absorption shift on
successive increase in % of Ce in the catalysts. The FE-SEM images
demonstrated granular to needle like transformation of the nanocrys-
tallites with increased % Ce in catalyst. The 7.5% Ce-ZnO as CeO -Ce-
2
ZnO composite Lewis acid catalyst with B/L ratio 0.88 is significant for
selective N-acetylation with concurrent S oxidation to get exclusive SeS
[
15] J. Srogl, J. Hyvl, A. Revesz, D. Schroder, Chem. Commun. (2009) 3463–3465.
[16] W. Ge, Y. Wei, Green Chem. 14 (2012) 2066–2070.
[
[
17] Y. Liu, H. Wang, J. Zhang, J.-P. Wan, C. Wen, RSC Adv. 4 (2014) 19472–19475.
18] A.M. Kulkarni, U.V. Desai, K.S. Pandit, M.A. Kulkarni, P.P. Wadgaonkar, RSC Adv. 4
(
2014) 36702–36707.
[19] A. Ghorbani-Choghamarani, Z. Darvishnejad, M. Norouzi, Appl. Organomet. Chem.
9 (2015) 170–175.
20] A. Ghorbani-Choghamarani, Z. Darvishnejad, B. Tahmasbi, Inorg. Chim. Acta 435
2015) 223–231.
[21] L. Shiri, A. Ghorbani-Choghamarani, M. Kazemi, Res. Chem. Intermed. 43 (2017)
707–2724.
22] A. Dhakshinamoorthy, M. Alvaro, H. Garcia, Chem. Commun. 46 (2010)
2
[
(
2
[
27

R. Jagtap et al.
Molecular Catalysis 450 (2018) 19–28
[43] E.P. Papadopoulos, A. Jarrar, C.H. Issidoides, J. Org. Chem. 31 (1966) 615–616.
[44] S. Samanta, S. Ray, A.B. Ghosh, P. Biswas, RSC Adv. 6 (2016) 39356–39363.
[45] P. Hanumantharao, S.V. Sambasivarao, L.K. Soni, A.K. Gupta, S.G. Kaskhedikar,
Indian J. Chem. Sect. B 44B (2005) 1481–1486.
[46] S.M. Mali, R.D. Bhaisare, H.N. Gopi, J. Org. Chem. 78 (2013) 5550–5555.
[47] C. Viglianisi, P. Bonaccorsi, L. Simone, L. Nassini, S. Menichetti, Eur. J. Org. Chem.
9 (2012) 1707–1711.
6
476–6478.
[23] H. Eshghi, M. Bakavoli, H. Moradi, A. Davoodnia, Phosphorus Sulfur Silicon Relat.
Elem. 184 (2009) 3110–3118.
[24] K.M. El-Shaieb, J. Sulfur Chem. 28 (2007) 223–229.
[25] M.M. Hashemi, A. Rahimi, Z. Karimi-Jaberi, Lett. Org. Chem. 2 (2005) 485–486.
[26] R. Krishnamurthy, S. Natarajan, Synth. Commun. 25 (1995) 227–234.
[27] M. Montazerozohori, S. Joohari, B. Karami, N. Haghighat, Molecules 12 (2007)
6
94–702.
[48] H.W. Ra, K.S. Choi, J.H. Kim, Y.B. Hahn, Y.H. Im, Small 4 (2008) 1105–1109.
[49] S. Ozturk, N. Kilinc, N. Tasaltin, Z.Z. Ozturk, Thin Solid Films 520 (2011) 932–938.
[50] Y.J. Chen, C.L. Zhu, G. Xiao, Sens. Actuators B 129 (2008) 639–642.
[51] S.K. Pardeshi, A.B. Patil, Sol. Energy 82 (2008) 700–705.
[52] R.M. Jagtap, S.R. Bhor, A.B. Patil, S.K. Pardeshi, J. Nanoeng. Nanomanuf. 5 (2015)
186–191.
[
[
[
28] M. Taha, N.H. Ismail, S. Imran, A. Wadood, F. Rahim, L.M.R.A. Muqarrabin,
H.M. Zaki, N. Ahmat, A. Nasir, F. Khan, Bioorg. Chem. 68 (2016) 15–22.
29] G. Joshi, S. Bhadra, S. Ghosh, M.K. Agrawal, B. Ganguly, S. Adimurthy, P.K. Ghosh,
B.C. Ranu, Ind. Eng. Chem. Res. 49 (2010) 1236–1241.
30] H. Xiao, J. Chen, M. Liu, H. Wu, J. Ding, Phosphorus Sulfur Silicon Relat. Elem. 184
(
2009) 2553–2559.
[53] G. Brahmachari, S. Laskar, S. Sarkar, J. Chem. Res. 34 (2010) 288–295.
[54] F. Rollin-Genetet, C. Seidel, E. Artells, M. Auffan, A. Thiery, C. Vidaud, Chem. Res.
Toxicol. 28 (2015) 2304–2312.
[55] D. Chauhan, P. Kumar, C. Joshi, N. Labhsetwar, S.K. Ganguly, S.L. Jain, New J.
Chem. 39 (2015) 6193–6200.
[
[
[
31] A. Shaabani, F. Tavasoli-Rad, D.G. Lee, Synth. Commun. 35 (2005) 571–580.
32] A. Shaabani, A. Bazgir, D.G. Lee, Synth. Commun. 34 (2004) 3595–3607.
33] A.R. Joshaghani, H. Jafary, I. Mohammadpoor-Baltork, Phosphorus Sulfur Silicon
Relat. Elem. 180 (2005) 117–123.
[
[
[
34] A.R. Hajipour, S.E. Mallakpour, J. Chem. Res. (S) (2000) 32–33.
35] A. Parmar, H. Kumar, Synth. Commun. 37 (2007) 2301–2308.
36] F.-E. Chen, Y.-W. Lu, Y.-P. He, Y.-F. Luo, M.-G. Yan, Synth. Commun. 32 (2002)
[56] A. Corma, T. Rodenas, M.J. Sabater, Chem. Sci. 3 (2012) 398–404.
[57] H.B. Balkhoyor, M.M. Rahman, A.M. Asiri, RSC Adv. 6 (2016) 58236–58246.
[58] C. Abinay, M. Marikkannan, M. Manikandan, J. Mayandi, P. Suresh,
V. Shanmugaiah, C. Ekstrum, J.M. Pearce, Mater. Chem. Phys. 184 (2016) 172–182.
[59] D. Liu, Y. Lv, M. Zhang, Y. Liu, Y. Zhu, R. Zong, Y. Zhu, J. Mater. Chem. A 2 (2014)
15377–15388.
[60] S. Anandan, M. Miyauchi, Phys. Chem. Chem. Phys. 13 (2011) 14937–14945.
[61] J.K. Vaishnav, S.S. Arbuj, S.B. Rane, D.P. Amalnerkar, RSC Adv. 4 (2014)
47637–47642.
[62] H. Hu, J. Lyu, Q. Wang, Q. Zhang, J. Cen, X. Li, RSC Adv. 5 (2015) 32679–32684.
[63] J.P. Danehy, K.N. Parameswaran, J. Chem. Eng. Data 13 (1968) 386–389.
[64] J. Felton, M. Ray, C.C. Jarrold, Phys. Rev. A 89 (2014) 033407.
[65] P. Haider, A. Urakawa, E. Schmidt, A. Baiker, J. Mol. Catal. A: Chem. 305 (2009)
161–169.
3
487–3492.
37] A.V. Joshi, S. Bhusare, M. Baidossi, N. Qafisheh, Y. Sasson, Tetrahedron Lett. 46
2005) 3583–3585.
38] S. Raghavan, A. Rajender, S.C. Joseph, M.A. Rasheed, Synth. Commun. 31 (2001)
477–1480.
[
[
[
[
(
1
39] K. Nakagawa, S. Shiba, M. Horikawa, K. Sato, H. Nakamura, N. Harada, F. Harada,
Synth. Commun. 10 (1980) 305–309.
40] A. Dhakshinamoorthy, S. Navalon, D. Sempere, M. Alvaro, H. Garcia,
ChemCatChem 5 (2013) 241–246.
[
[
41] A. Saxena, A. Kumar, S. Mozumdar, J. Mol. Catal. A: Chem. 269 (2007) 35–40.
42] K. Selvam, M. Annadhasan, R. Velmurugan, M. Swaminathan, Bull. Chem. Soc. Jpn.
8
3 (2010) 831–837.
28