One-pot synthesis of aniline N-alkylation from benzyl alcohol over Cu-Fe catalyst

Article abstract of DOI:10.1016/j.apcata.2020.117519

Cu-Fe catalysts prepared from layered double hydroxide (LDH) precursors were applied for the straightforward N-alkylation of aniline with benzyl alcohol in base- and oxidant-free conditions. Among the bimetallic Cu-M catalysts prepared from LDH precursors

Full text of DOI:10.1016/j.apcata.2020.117519

Journal Pre-proof
Cu (II) Schiff base complex grafted guar gum: Catalyst for benzophenone
derivatives synthesis
Krishna, Shweta Kumari, Deepak Yadav, Sunil K. Sharma
PII:
S0926-860X(20)30112-5
DOI:
https://doi.org/10.1016/j.apcata.2020.117519
APCATA 117519
Reference:
To appear in:
Applied Catalysis A, General
Received Date:
Revised Date:
Accepted Date:
19 December 2019
10 March 2020
12 March 2020
Please cite this article as: Krishna, Kumari S, Yadav D, Sharma SK, Cu (II) Schiff base
complex grafted guar gum: Catalyst for benzophenone derivatives synthesis, Applied
Catalysis A, General (2020), doi: https://doi.org/10.1016/j.apcata.2020.117519
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Cu (II) Schiff base complex grafted guar gum: Catalyst for benzophenone
derivatives synthesis
Krishna, Shweta Kumari, Deepak Yadav, ,Sunil K. Sharmaa*
a†
a†
a,b
a
Department of Chemistry, University of Delhi, Delhi-110007, India
Department of Chemistry, Central University of Haryana, Mahendergarh-123031, India
b
Email: sk.sharma90@gmail.com; Phone number: +91-11-2766-6646
yadav.krishna.sep@gmail.com, shwetasingh.ism@gmail.com, deepakrao2510@gmail.com
†Both authors contributed equally to this manuscript
Gr4aphical abstract
1

2

Highlights

Cu-Fe catalyst prepared from LDH structure showed a significant promoting effect
for one-pot synthesis of aniline N-alkylation without additives.


In-situ FT-IR results confirmed that the synergistic Cu and Fe accelerated the reaction.
Cu-Fe catalyst exhibited good recyclability which is potential for industrial application.
Abstract
In this study Guar gum based Cu(II) Schiff's base complex (GG-Cu) has been synthesized and
characterized by FT-IR, PXRD, UV-Visible, TGA, XPS, FESEM, TEM, EDAX, solid-state NMR,
Elemental mapping, CHNS and AAS analysis. This moiety has been found to be an efficient
heterogeneous catalyst for selective oxidation reactions. Fifteen model reactions have been carried
to establish the catalytic behavior of GG-Cu, and five of these yield novel products. The ease of
separation of catalyst from the reaction mixture simply by filtration is an added advantage;
furthermore the catalyst can be reused up to five times without significant loss of catalytic activity.
The overall concept of developing newer, efficient and environmental benign catalysts with ease
of separation and recycling ability has been successfully demonstrated. All of the isolated products
were fully characterized on the basis of their physical and spectral data.
Keywords: Guar gum, Benzophenone derivatives, Cu complex
1
. INTRODUCTION
The use of catalysts in organic synthesis is becoming increasingly common to improve the overall
conversion, product purity, and save energy. However the catalyst’s toxicity, difficulty in
separation from the reaction mixture, and lack of reusability are some of the challenges that still
need to be addressed. Designing polymer based material for catalytic activity in organic synthesis
3

has attracted great interest in recent years.[1-2] The use of naturally occurring polymers over the
synthetic one holds promise due to their abundant availability, low cost and non-toxic nature.[3-5]
Guar-gum (GG) is a naturally occurring, edible biodegradable and nonionic galactomannan
polymer extracted from the seeds of Cyamopsis tetragonolobus.[6] It has rod like polymeric
structure wherein galactose side chains are linked on the mannose backbone with an average
molecular ratio of 1:2. The presence of large number of hydroxyl groups not only confers
hydrophilicity but also facilitate it to be readily functionalized with various functional groups,
surfactants, gelling agents, and dyes, so as to allow the resultant products to be explored for various
pharmaceutical/biomedical/industrial applications.[7-16] GG is insoluble in most of the organic
solvents and thus may be used to develop heterogeneous catalysts for organic reactions. Despite
being economical, eco-friendly and easily available, GG has not been fully explored for the
development of heterogeneous catalysts in organic synthesis beside the huge potential in this area.
Some studies have demonstrated that nanocomposites of GG act as efficient catalystin most
of these reports, GG has been described as an excellent supporting and stabilizing material.[17-22]
Our objective is to modify the structure of guar gum using a simple procedure in order to develop
new, efficient and environmental benign heterogenous catalysts. Guar-gum based copper
nanoparticulates have been used as a catalyst for the Huisgen [3+2] cycloaddition reaction of azides
and alkynes.[23] Palladium nanoparticles decorated on gaur gum based hybrid material are known
to be used for electrocatalytic hydrazine determination.[24] Sivasankar et al. reported
Fe
3
O
4
-guar-gum nanocomposites for the reduction of p-nitroaniline.[25]
Our focus is on benzophenone motifs as these are of considerable interest to chemists due
to their importance as building blocks in the synthesis of natural products and pharmacological
compounds.[26] Furthermore, benzophenone derivatives are known to induce apoptosis in
cancerous cells by generating reactive oxygen species and therefore, have immense potential for
4

the development of new chemotherapeutic agents.[27] Benzophenone derivatives have been
synthesized in numerous ways, one of the most effective and significant method is the oxidation of
benzylic methylenes.[28] A variety of catalysts have been used to accomplish such oxidation
reaction e.g. Ag/SiO
2
-CeO , Cu/ SBA-15 N-doped graphene, Rh, Bi, Au, Re, PdO/SBA-15,
2
CuNi/Co composites.[29-42] However, most of these methods suffer from drawbacks such as lack
of reusability and selectivity, cumbersome synthesis of the catalyst and lower yield of the products.
In order to address these issues, the development of new effective, low-cost and reusable catalysts
for the oxidation of benzylic methylene is highly desirable and one of the
major goals of this study.[43]
The catalytically active complexes are known to be either immobilized or grafted on a
support harvesting advantages of both homogeneous and heterogeneous catalysis, such as high
activity and selectivity together with ease of separation and reusability of the catalyst.[44-47]
Polymers are known to have high affinity for complexes. They can bind to metal complexes
through covalent bonding due to the presence of highly reactive groups such as hydroxyl,
carboxyl, amino etc.[2,48] Larionov et al. reported the immobilization of an iridium complex on
polystyrene as a catalyst for the Friedel-Crafts alkylation.[49] Chitosan anchored copper(II) Schiff
base complexes are reported as heterogeneous catalysts for N-arylation of amines.[50]
Chromium(VI) grafted on mesoporous polyaniline is reported as a reusable heterogeneous
catalyst for oxidation reactions.[51]
Herein, we report the guar-gum anchored copper(II) Schiff base complex as heterogeneous
catalyst for the synthesis of benzophenone derivatives under mild reaction conditions. To the best
of our knowledge, this is the first report of guar-gum linked Schiff base metal complex used as a
catalyst.
5

2
. RESULTS AND DISCUSSION
Guar gum in its native form has very high molecular weight and its extremely high viscosity
and gel-forming property limits its use. Therefore, it needs to be depolymerized in order to be used
further. Also, physiological properties of guar gum can be improved by the controlled partial
hydrolysis (PHGG). A partially hydrolyzed GG with average molecular weight of 31,776 g/mol
1
measured by GPC (Figure S1) and G/M value of 1:1.87 calculated by H NMR (Figure S2) was
provided by Lucid Colloids Ltd., Mumbai and used as such. GG anchored Cu(II) Schiff base
complex (GG-Cu) was synthesized by following Scheme 1. The GG was first treated with (3-
aminopropyl)trimethoxysilane (APTMS) to incorporate aminopropyl moieties on the GG surface
(APTMS-GG). This was followed by the addition of salicylaldehyde so as to allow amino groups
of APTMS moiety and salicylaldehyde to form the Schiff base (L-APTMS-GG). The resulting
compound, then complexes with Cu(II) acetate to yield GG based Cu(II) Schiff's base complex
GG-Cu (more detail of GG-Cu preparation is given in the ESI††).
6

Scheme 1. Schematic representation for the synthesis of GG-Cu.
Synthesis of the GG based copper(II) Schiff base complex and the intermediate steps were
examined by FT-IR, XRD, UV-Visible, TGA, XPS, solid-state NMR, FESEM, EDX, Elemental
mapping, TEM and AAS analysis. The structural changes during the anchoring of copper(II) Schiff
base complex on GG analyzed by FT-IR is shown in Figure 1. The FT-IR spectrum of GG exhibited
-1
a broad intense peak at 3413 cm , attributed to O-H (Str), accounting for the presence of several
hydroxyl groups which are the linking sites for APTMS.[52] In addition, strong characteristic peaks
-1
at 2905, 1076 and 1006 cm were observed due to the stretching vibration of C-H, C-O-C and C-
O groups, respectively in GG.[53] APTMS-GG in turn shows a correspondingly modified FT-IR
-1
spectrum. In particular, peaks at 1130 and 1099 cm were attributed to Si-O-Si and Si-O-C linkages
respectively.[54] The bands at 3427 and 1599 cm , due to the presence of -NH
1
groups, confirms
2
7

the successful functionalization of GG through silyl ether formation. The FT-IR spectrum of L-
-1
APTMS-GG depicted a new peak at 1629 cm due to v C=N bond, thus indicating the condensation
reaction between the carbonyl group of salicylaldehyde (L) and the amino group of GG-APTMS.
-1
Figure 1d shows the FT-IR spectrum of GG-Cu, a band at 1604 cm was observed, and blue shift
in wavenumber compared with LAPTMS-GG confirmed the coordination of copper ion to
-1
azomethine nitrogen of Schiff base. Moreover, new peaks at 675 and 560 cm were observed,
which accounts for the stretching mode of Cu-N and Cu-O bonds, respectively.[55] The above data
supports the successful formation of copper(II) Schiff base complex on GG by covalent interaction.
Figure 1. FT-IR spectra of (a) GG, (b) APTMS-GG, (c) L-APTMS-GG and (d) GG-Cu.
Figure 2 demonstrate the XRD patterns of GG, APTMS-GG, L-APTMS-GG, and GG-Cu. In the
diffraction pattern of GG, the characteristic peak appeared at 2Ѳ = 20.1o.[56] In the diffraction
spectrum of APTMS-GG the characteristic peak of GG is retained, however a new broad peak was
observed at 12.67° which corroborate with the existence of an amorphous nature.[57] No obvious
change was observed in XRD pattern of L-APTMS-GG when compared with APTMSGG.
o
Furthermore, the XRD pattern of GG-Cu exhibit a new peak at 2θ = 26.9 , 32.28° and
8

3
8.43° attributing to plane (002 , (110) and (111) of CuO, respectively.[58]
Figure 2. Powder XRD pattern of (a) GG, (b) APTMS-GG, (c) L-APTMS-GG and (d) GG-Cu.
UV-vis spectra of GG, APTMS-GG, L-APTMS-GG and GG-Cu are shown in Figure 3. The
UVvisible spectrum of GG shows an absorption peak at 262 nm, due to the presence of non-
bonding electrons on the hydroxyl group.[59] However, the UV-Vis spectra of APTMS-GG shows
a weak absorption peak at 302 nm, due to the presence of an amine group of APTMS. Figure 3c
shows a strong absorption peak at 261, 332 nm due to the n/π* transition of the imine bond in
LAPTMS-GG. The UV-Vis spectrum of the GG-Cu showed a characteristic absorption peak at
3
28 nm, due to the charge transfer between copper ion and the hydroxyl group of ligand.[60]
9

Figure 3. UV-vis spectra of (a) GG, (b) APTMS-GG, (c) L-APTMS-GG and (d) GG-Cu.
The TGA thermogram of GG and GG-Cu are presented in Figure 4. The TGA of GG showed a
o
1
0% weight loss in the range of 50-150 C, due to the loss of adsorbed water molecules. The second
weight loss around 63% at 200-420 C may be due to the polymer decomposition.[61] However, in
o
o
GG-Cu, 5% weight loss is observed in the range 30-120 C, due to the evaporation of water and
o
second loss approximately of 36% was observed at 340 C, due to the degradation of secondary
alcohol in guar gum.[62] After this it degrades slowly and a total weight loss of 59 % was observed
o
o
until 600 C. Thus, it may be concluded that GG-Cu thermally stable up to 600 C. Quantitative
evaluation of grafting of Cu(II) complex on guar gum using TGA data suggest
that approx. 25% guar gum is grafted.[63]
1
0

Figure 4. TGA thermogram of (a) GG and (b) GG-Cu.
For the chemical state variation and the surface functionalization in the GG-Cu, X-ray
photoelectron (XPS) analysis was carried out Figure 5. Figure 5a shows the presence of Cu, C, O,
Si and N species in GG-Cu. The XPS spectrum of the Cu 2p core level spectrum in Figure 5b
exhibits two bands located at the binding energies of 951.5 and 931.8 eV which correspond to the
binding energy of Cu 2p1/2 and Cu 2p3/2. Meanwhile, a Cu 2p1/2 and Cu 2p3/2 satellite peaks
were observed at 959.7 and 941.2 eV, respectively. The deconvoluted C 1s spectrum of the
GG-Cu is shown in Figure 5c, it exhibits the typical peaks at 286.4, 284.2, 283.0, 282.3 and
2
81.6 eV that can be assigned to carbon atoms in C-OH, C=N C–O, C-C and C-H, respectively.
Correspondingly, the O 1s peak was observed at energy position 529.4 eV in Figure 5d. The fitted
peak values for O 1s were 530.7, 529.5 and 528.0 eV which corresponds to the, O-H, C-O and Cu-
O, respectively. The N 1s XPS spectrum (Figure 5e) shows a peak at 397.3 which
confirms the presence of N=C.[64-68] In Figure 5f a band at 100.1 eV attributed to Si-O emerged,
which indicate the silylation of the reaction surface. The XPS results further supported the
formation of Cu(II) complex grafted on the GG.
1
1

Figure 5. X-ray photoelectron (a) survey spectra of GG-Cu, (b) Cu 2p, (c) deconvoluted C 1s, (d)
deconvoluted O 1s, (e) N 1s and (f) Si 2p.
A deep understanding of the structure and chemical composition of GG-Cu, was analyzed by solid-
1
3
state NMR spectroscopy Figure 6. The C ss nmr spectrum have four set of signals. First region
at 171 to 165 ppm due to aromatic carbons attached to hydroxyl group of salicaldehyde, second
region at 132 to 103 ppm due to aromatic carbons of salicaldehyde and anomeric carbons for
galactose and mannose of guar gum. Third region at 80 to 60 for guar gum and fourth region come
at 18 to 10 ppm for APTMS group.[69-70] The Si ss nmr spectrum of GG-Cu also consist a set of
29
signals at -50 to -65 ppm region due to silicon attached with -OMe, -CH
and GG environment.[71]
2
The ss NMR spectra results supported the presence of aromatic as well APTMS and guar gum
region in GG-Cu which confirm the formation of Cu(II) complex grafted on the
GG.
1
2

13
29
Figure 6. C and Si solid-state NMR spectra of GG-Cu.
The surface morphology of GG, APTMS-GG, L-APTMS-GG, and GG-Cu are described with the
help of FESEM and TEM images. The FESEM images of GG in Figure 7(i)a. show the
macromolecule with rough surface topology. FESEM images of APTMS-GG, L-APTMS-GG
displays nanostructural features which demonstrated the chemical modification of GG through
APTMS and salicylaldehyde in Figure 7(i)b and 7(i)c, respectively. On the other hand, FESEM
images of GG-Cu (Figure 7(i)d) exhibits different morphologies having twisted nanostructural
irregular-shape and these may serve as an ideal platform for reactant to accomplish the desired
product. The TEM image shown in Figure 7(ii) reveals the nanoscopic features of the GG-Cu with
a uniform distribution of copper(II) Schiff base complex without aggregation throughout the
GG. The EDAX analysis provides detailed chemical analysis on the GG, APTMS-GG, LAPTMS-
GG and GG-Cu. The EDAX analysis of GG-Cu in Figure 7(iii)d depicts the change in elemental
composition with the introduction of copper, nitrogen and silicon, which are core elements of the
complex apart from carbon and oxygen. Further, EDAX elemental mapping was achieved to
understand the distribution of the Cu-Schiff base complex on GG. The elemental mapping
indicated the uniform distribution of Cu, N and Si over the GG surface 7(iv). Along with the EDAX
analysis, the composition of carbon, nitrogen and hydrogen in the GG-Cu complex determined by
the CHNS analysis was found to be 26.92 %, 4.16 % and 2.46 %,
1
3

1
95 respectively. Subsequently, the quantitative estimation of the copper content in GG-Cu was done 196
-1
after digestion using AAS and the corresponding copper loading was found to be 1.48 mg L .
1
97
1
98
1
4

Figure 7. (i) FESEM of (a) GG, (b) APTMS-GG, (c) L-APTMS-GG, and (d) GG-Cu; (ii) TEM
images of GG-Cu; (iii) EDAX spectra of (a) GG, (b) APTMS-GG, (c) L-APTMS-GG, and (d) GG-
Cu and (iv) elemental mapping of GG-Cu.
The catalytic efficiency of GG-Cu was evaluated in the synthesis of benzophenone derivatives
via an oxidation reaction as shown in Scheme 2 (synthetic details given in the ESI††).
Scheme 2. Synthesis of benzophenone derivatives via catalytic oxidation.
In order to optimize the reaction conditions for the synthesis of benzophenone derivatives, various
parameters such as the catalyst amount, oxidant, reaction time, temperature, and solvent were
thoroughly investigated and summarized in the Table 1. Diarylmethane was chosen as a model
1
5

reactant for the oxidation reaction, in the control reaction carried out in the absence of a catalyst at
reflux, no noticeable reaction was observed (Table 1 entry 1). Even in the presence of
GG in water under reflux for 24 h no product formation was observed (Table1, entry 2). However,
in the presence of GG-Cu catalyst and in different solvents, e.g. water, toluene, pyridine, ethanol,
DMF and acetonitrile, the product was found to be formed (Table 1 entry 3-8). Acetonitrile was
observed to be the solvent of choice for the oxidation reaction. However, oxidant also plays an
important role in affecting the reaction to a large extent. In order to study the effect of oxidant, the
reaction was carried out in the presence of different oxidants such as TBHP, m-CPBA, O
2
, H
2
O
2
(Table 1, entry 8-11). The optimum yield of 90% was observed while using TBHP as an oxidant.
Under these conditions 5 mg of the catalyst was found to be sufficient for the oxidation reaction
of diarylmethane. A further decrease in the amount of catalyst (2 mg) was found to significantly
o
hamper the yield (Table 1, entry 13). Therefore, the use of 5 mg of the GG-Cu catalyst at 70 C in
the presence of solvent acetonitrile and TBHP as an oxidant was found to be the optimized reaction
condition to afford benzophenone (2a) in excellent yield in a short reaction time (Table 1, entry
1
4). The generality of the present method was established by using different types of diarylmethane
bearing electron withdrawing groups, tosylated group and thiophene moiety to afford the
corresponding benzophenone derivatives with excellent yields as shown in Figure 8. A few
sulfones were also included in the study as they too constitute an important functional group due
to their presence in biologically active compounds, e.g. anticancer, antibacterial, antiglaucoma
agents.[72-73] It may be noted that the benzylic oxidation proceeded exclusively at the methylene
group and methyl groups remains unaffected. Amongst the reactions tried, five newer products (2j,
1
2
k, 2l, 2m and 2n) were formed. All of the known and new products are characterized by IR, H
1
6

& 1
3C NMR and Mass spectroscopy. Table 1. Optimization of reaction conditions for the
oxidation of diarylmethane.a
o
b
Entry
Catalyst (mg)
Solvent
Oxidant
Temp ( C)
Time (h)
Yield of 2a (%)
1
2
------
Water
Water
TBHP
TBHP
Reflux
Reflux
24
24
----
----
GG (20)
3
4
5
6
7
8
9
GG-Cu (20)
GG-Cu (10)
GG-Cu (10)
GG-Cu (10)
GG-Cu (10)
GG-Cu (10)
GG-Cu (10)
GG-Cu (10)
GG-Cu (10)
GG-Cu (5)
GG-Cu (2)
GG-Cu (5)
Water
Toluene
Pyridine
Ethanol
DMF
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
m-CPBA
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
70
24
24
12
12
12
12
12
12
12
12
12
08
20
35
48
38
44
90
55
37
60
90
65
89
1
1
1
1
1
0
1
2
3
4
O
2
H
2O
2
TBHP
TBHP
TBHP
70
70
a
b
Reaction condition: Diarylmethane (1mmol) TBHP (10 mol %).
isolated yield after workup.
Furthermore, comparison of the catalytic efficiency of the GG-Cu with previously reported
catalysts, for the preparation of benzophenone derivatives, in terms of the catalyst amount and
reaction conditions, is summarized in Table 2.
1
7

2
2
43
44
Figure 8. Synthesized benzophenone derivatives via catalytic oxidation.
The catalyst was evaluated for its recyclability up to six times. It is apparent from
Figure S3 that 246 the GG-Cu catalyst could be reused up to five cycles. However,
gradual decline in the catalytic
2
45
1
8

rd
2
2
47
48
activity was noticed after 3 cycle. The XRD pattern, FTIR and SEM images also showed that
there was no change in the phase and surface morphology in recycled GG-Cu (Figure S4). AAS
2
49 of the used catalyst was found to be 1.39 mg/L which was almost similar to the amount of Cu in 250
fresh catalyst. Hence, the catalyst has shown its proficiency for various industrial applications.
2
51 Table 2. Comparative catalytic performance of the GG-Cu with other previously reported 252
catalysts in oxidation reactions.
2
53
Entry Catalyst
Amount
catalyst (mg)
of Oxidant Time (h)/ Solvent/Condition TON
yield (%)
TOF
Refs.
1
CrO
3
5.9
H
5
IO
6
1/99
3
CH CN/RT
4
9.87
49.87 74
o
2
3
4
5
6
FeCl
3
.2H
2
O
2.7
3.1
9.5
9.9
30
TBHP
24/91
12/85
24/81
8/98
Pyridine/82 C
4
1
7
1
1
5.55
6.90
.95
9.56
9.66
1.89
1.40
0.33
2.44
4.91
75
76
77
78
79
o
Fe(III)/THA
CuI
H
O
2
2
O
2
CH
3
CN/60 C
o
CH
3
CN /100 C
o
Cu(OAc)
2
.H
2
O
TBHP
TBHP
H
H
2
2
O/80 C
O/RT
[CuI(aas-
TPB)]n
GG-Cu
4/99
o
7
5.0
TBHP
10/90
CH
3
CN /70 C
This work
1
15.77 11.57
2
2
54
55
Based on the literature, a plausible mechanism for the reaction is shown in Scheme 3.77,80 It is
2
2
2
2
2
2
56
57
58
59
60
61
speculated that first a homolytic cleavage of TBHP occurs in the presence of heat that produce
the alkoxy (A) and hydroxyl radicals (equation i) which subsequently reacts with TBHP and
form radical B. On the other hand GG-Cu catalyst which binds with the reactant gets activated
by the free radical A and removal of acetic acid. In the next step Cu(II) bind with free radical B
and produces a highly reactive Cu(III)-peroxide species (2). The peroxide species, then quickly
rearranges via a radical pathway with an electron transfer to the catalyst, producing the Cu(II) 262
organoperoxides (3). Homolytic dissociation of these organoperoxide by removal of radical
hydrogen of the remaining benzylic proton (4) generates the product.
2
63
1
9

Scheme 3. Plausible mechanism for benzophenone synthesis using the GG-Cu catalyst.
3
. CONCLUSIONS
We have successfully used a carbohydrate based natural polymer obtained from guar gum (GG) to
develop a heterogeneous catalyst for organic reactions. The low cost, nontoxicity, presence of large
number of hydroxyl groups, and limited solubility of guar gum in organic solvents are the
favourable factors to employ this natural product for the synthesis of heterogenous catalyst.
2
0

Grafting of Cu(II) Schiff base complex on GG via covalent interaction afforded a heterogeneous
catalyst which was characterized by FT-IR, PXRD, UV-Visible, TGA, XPS, solid-state NMR,
FESEM, TEM, EDAX, Elemental mapping, CHNS, AAS and elemental analysis. The catalyst
reported is convenient to use due to its ease of separation from the reaction mixture and is stable in
ambient conditions. Furthermore it was found to be highly efficient, selective for the oxidation of
methylene group using TBHP as an oxidant. The reusability of the catalyst upto five cycles without
any significant loss of activity is an added advantage and allows the catalyst to be considered
superior to other commonly available catalysts.
CRediT author statement
Wahyu S. Putro: Investigation, Data curation, Writing- Original Draft Preparation
Takayoshi Hara: Formal analysis
Nobuyuki Ichikuni: Formal analysis
Shogo Shimazu: Conceptualization, Supervision, Writing - Review & Editing, Funding
acquisition and Project administration
CONFLICT OF INTEREST
There is no conflict of interest to declare.
Declaration of interests
2
1

The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
ACKNOWLEDGEMENTS
Financial support from the Department of Science and Technology (DST), Government of India is
gratefully acknowledged. We are thankful to Lucid Colloids Ltd., Mumbai for providing guar gum
and also thankful to the SAIF IIT Kanpur, USIC, University of Delhi for providing assistance in
the analysis of the samples. SK acknowledges the receipt of DSK postdoctoral fellowship.
†
†Electronic supplementary information (ESI) available: Experimental section, reusability and
spectral data of GG-Cu after 5th cycle and spectroscopic results of benzophenone derivatives.
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