Thiol-yne click on nano-starch: An expedient approach for grafting of oxo-vanadium Schiff base catalyst and its use in the oxidation of alcohols

Article abstract of DOI:10.1016/j.apcata.2013.08.007

The metal-free stoichiometric click reaction between thiol-functionalized nanocrystalline starch and yne-functional oxo-vanadium Schiff base has been demonstrated to be a simple, quantitative, mild and efficient approach for the development of heterogenized homogeneous complex via covalent bonding. The developed catalyst has found to be highly efficient and recyclable for the oxidation of various alcohols to corresponding carbonyl compounds with t-BuOOH under mild reaction conditions. Importantly, no leaching of metal or ligand is observed during the course of the reaction.

Full text of DOI:10.1016/j.apcata.2013.08.007

Accepted Manuscript
Title: Thiol-yne Click on Nano-Starch: An Expedient
Approach for Grafting of Oxo-Vanadium Schiff Base Catalyst
and its use in the Oxidation of Alcohols
Author: Sanny Verma Jean Le Bras Suman L. Jain Jacques
Muzart<ce:footnote id="fn1"><ce:note-para
id="npar0010">1Fax: 03 26 91 31 66/+33 3 26 91 31
66</ce:note-para></ce:footnote>
PII:
S0926-860X(13)00491-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2013.08.007
APCATA 14388
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Applied Catalysis A: General
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Revised date:
Accepted date:
21-4-2013
31-7-2013
8-8-2013
Please cite this article as: S. Verma, J. Le Bras, S.L. Jain, J. Muzart, Thiol-yne Click
on Nano-Starch: An Expedient Approach for Grafting of Oxo-Vanadium Schiff Base
Catalyst and its use in the Oxidation of Alcohols, Applied Catalysis A, General (2013),
http://dx.doi.org/10.1016/j.apcata.2013.08.007
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Thiol-yne Click on Nano-Starch: An Expedient Approach for Grafting of Oxo-Vanadium
Schiff Base Catalyst and its use in the Oxidation of Alcohols
Sanny Vermaa, Jean Le Brasb, Suman L. Jain,a* Jacques Muzartb
aChemical Sciences Division, CSIR-Indian Institute of Petroleum
Mohkampur, Dehradun-248005; Fax: (+): 0091-135-2660202
E-mail: suman@iip.res.in
bCNRS - Université de Reims Champagne-Ardenne, Institut de Chimie, Moléculaire de Reims,
UMR 6229,; Fax : 03 26 91 31 66 +33 3 26 91 31 66;
e-mail: jacques.muzart@univ-reims.frc
Abstract
The metal-free stoichiometric click reaction between thiol-functionalized nanocrystalline starch
and yne-functional oxo-vanadium Schiff base has been demonstrated to be a simple, quantitative,
mild and efficient approach for the development of heterogenized homogeneous complex via
covalent bonding. The developed catalyst has found to be highly efficient and recyclable for the
oxidation of various alcohols to corresponding carbonyl compounds with t-BuOOH under mild
reaction conditions. Importantly, no leaching of metal or ligand is observed during the course of
the reaction.
Keywords: click chemistry, thiol-yne coupling, vanadium, oxidation, heterogeneous catalyst
1.0 Introduction
In the recent decades, increasing environmental concerns have led interest towards the potential
uses of renewable resources [1-3] for developing more efficient and atom economic technologies
in chemical industry [4-5]. Starch [6], the major energy reserve of higher plants, is a cheapest,
biodegradable and natural biopolymer produced in large amounts from several renewable plant
1
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sources. There is currently a considerable interest in processing of native starch grannules to
starch nanocrystals to be used as fillers in polymeric matrixes as promising reinforcing agent [7].
Owing to the presence of plenty of hydroxyl groups, the surface chemical modifications of the
starch nanocrystals may constitute a promising approach to broaden their applications as
supports for anchoring of other functionalities [8-9]. In this context, grafting of polymeric
materials to nanocrystalline starch to form nanocomposite materials has been well documented in
the literature [10-11]. However the uses of chemically modified starch nanocrystals for grafting
of homogeneous metal complexes are remained unexplored. Our group has recently reported the
grafting of metal complexes to chemically modified expanded corn starch for developing
inexpensive and environmentally benign synthetic methodologies [12-13]. Herein, we targeted to
use nanocrystalline starch as support for grafting of oxo-vanadium Schiff base for the oxidation
of alcohols. The ‘Click’ reaction [14] concept pioneered by Sharpless et al in which two
complementary functional groups that reacts together to give a quantitative yielding reaction, has
been established as a powerful and versatile technique in the polymer and material synthesis
domains. These reactions offer a number of advantages such as quantitative yield, mild
experimental conditions, tolerance towards other functional groups and can be applied for a wide
range of applications. Among these reactions, Cu(I) catalyzed Huisgen 1,3-dipolar cycloaddition
[15-16] of azide and terminal alkyne functionalities to form 1,2,3-triazoles has been extensively
used for a number of potential applications [17-18] including polymer synthesis, biocomposites,
drug delivery systems, pharmaceuticals and catalysis. However, one of the most obvious
disadvantages is the requirement of a copper catalyst, which sometimes causes the formation of
by-products along with the desired products. Therefore, the alternative metal-free click reactions
[19] such as thiol-ene [20-21], radical mediated thiol-yne [22-23] and reversible Diels–Alder
2
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conjugations [24-26] are gaining considerable interest in recent decades. In the present paper, we
report an efficient radical mediated thiol-yne click reaction for grafting of homogeneous oxo-
vanadium Schiff base catalyst [27] to the chemically modified starch nanocrystalline support
(Figure 1). The prepared catalyst has been used for the oxidation of various alcohols by t-
BuOOH to the corresponding carbonyl compounds.
2.0 Experimental Section
2.1 Techniques used
Vibrational spectra of the starch supported catalyst and its precursors were recorded using a
Thermo Scientific Nicolet 8700 Research FT-IR spectrophotometer with a 4 cm-1 resolution
using their KBr pellets. Thermal decomposition of developed catalyst was probed using a
Diamond TG-DTA analyzer (Perkin-Elmer). All samples were analyzed in the temperature range
of 30 to 650 °C under nitrogen flow. The morphology and structural features of the
nanocrystalline starch and nano-starch immobilized oxo-vanadium Schiff base catalyst were
characterized by field emission scanning electron microscopy (FESEM, FEI Quanta 200F).
Loading of vanadium in the nanostarch-bound oxovanadium Schiff base was determined using
an inductively coupled plasma atomic emission spectrometer (ICP-AES, PS-3000 UV, Leeman
Labs). 10 mg of each sample was dissolved in 1 mL of concentrated HNO3, heated and diluted to
10 mL. The resulting solutions were filtered and subjected to analysis for V content by ICP-AES
analysis. Loading of vanadium in the prepared catalyst was found to be 4.42mmol g-1. Further,
nanostructural features of the prepared catalyst were examined by transmission electron
1
13
microscope (TEM, JEOL 3010) at an accelerating voltage of 300 kV. The H and C NMR
spectra were recorded on a Bruker Avance 500 Spectrometer in CDCl3 with CHCl3 (7.27 ppm for
3
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1H, 77 ppm for 13C) as a standard and the chemical shifts are expressed in δ parts per million
relative to tetramethylsilane (TMS) as the internal standard.
2.2 Synthesis of nano-crystalline starch 1: Starch nanocrystals from native corn starch granules
were synthesized by following the literature procedure [10]. A suspension of native corn starch (1
g) in distilled water (20 mL) was heated at 90 °C for 1 h with continuous stirring which
subsequently followed by the drop wise addition of ethanol (20 mL) under continuous stirring at
room temperature for one hour . The suspension was washed by successive centrifugations with
ethanol to remove the water. The resulting starch nanocrystalline material was dried at 50 °C and
analyzed by SEM. The particle size of the corn starch nanocrystals was under the range of 50 to
100 nm.
2.3 Chemical modification of 1 by 3-mercaptopropyltrimethoxysilane: The chemical
modification of corn starch nanocrystals, was carried out by using the 3-
mercaptopropyltrimethoxysilane (MPTMS) as a grafting agent. In a typical experiment, 1 g of
starch nanocrystals were mixed with 20 mL of toluene and 5.0
g
of 3-
mercaptopropyltrimethoxysilane (MPTMS). The reaction mixture was refluxed for 24 h with the
continuous stirring under the nitrogen atmosphere. The modified starch nanocrystals were
recovered by filtration and subjected to a Soxhlet extraction with dichloromethane for 24 h. The
obtained material was dried under vacuum for 6 h.
2.4 Synthesis of propargylated oxo–vanadium Schiff base 8: Oxo–vanadium(IV) tridentate Schiff
base [27] 7 (2.3 g, 5.0 mmol) was added in the stirred mixture of Na2CO3 (40 mmol, 4.02 g) in
DMF (30 mL). Propargyl bromide (80 wt % in toluene, 12 mL, 10 mmol) was added drop wise
slowly and the resulting mixture was heated at 80 °C for 8 h. The solvent was removed under
reduced pressure and the residue was dissolved in dichloromethane (30 mL). The organic layer
4
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was washed with brine solution (20 mL x 3) and water. The organic layer was dried over
anhydrous MgSO4 and then, concentrated under reduced pressure. The dark brown solid thus
obtained was dried under vacuum for 4 h. IR (cm-1): 3301.7, 2922.7, 2855.0, 1604.2, 1225.4,
1022.9, 776, 898.32, and 639.1. Analytical calculation ESMS (M+ ): 488.
2.5 Grafting of 8 to thiol-functionalized nanocrystalline starch 3 by using thiol-yne coupling: In
a typical experiment, 3 (1 equiv., 10-2 mmol), 8 (10 equiv., 10-1 mmol), and a benzophenone (1
equiv., 10-2 mmol) in 1 mL of CH2Cl2 were introduced in a Pyrex tube and irradiated at 365 nm
at RT for 4 h. Then, polymer was washed several times with methanol and centrifuged or
decanted repeatedly homogeneous catalyst 8 and dried under vacuum at 80 °C. IR (cm-1):
3301.7, 2922.7, 2855.0, 1604.2, 1225.4, 1022.9, 889 and 975.1. The loading of the catalyst to
nano-starch support was calculated by ICP-AES analysis and was found to be 4.42 mmol/g.
2.6 Oxidation of alcohols 10 to carbonyl compounds 11: A 25 mL round bottom flask was
charged with alcohol (1.0 mmol), catalyst (0.02 equv.), MeCN (5 mL), TBHP (1.5 M solution in
dodecane; 1.5 eq) and then stirred at 65 °C for 1 h. According to Table 2, all reactions require 1
h. At the end of the reaction, the catalyst was filtered at room temperature over membrane filter
paper and reused for subsequent runs. The filtrate so obtained was concentrated under reduced
pressure and the residue was dissolved in dichloromethane (20 mL). The organic layer was
washed with brine solution (10 mL x 3). The organic layer was dried over anhydrous MgSO4 and
then, concentrated under reduced pressure. Product was purified by column chromatography
over silica gel. The isolated yield of the obtained product was found to be 94 % and the
conversion was determined by the GC-MS analysis. The purity of the products was determined
by comparing the 1H NMR spectra of the products with those of authentic samples.
5
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3.0 Result and discussion
3.1 Synthesis and characterization of the catalyst 9
The applications of native starch materials as a catalyst support are limited due to their naturally
low surface areas (< 1 m2g-1) and pore volumes (< 0.1 cm3g-1). Here, we have synthesized starch
nanocrystals by heating the native starch in water followed by drop wise addition of ethanol to
obtain starch nanocrystals with remarkably high surface areas (BET surface area 110 m2g-1).
After this, the nanocrystalline starch is subjected to chemical surface modification so that it can
be used for grafting of oxo-vanadium Schiff base via thiol-yne coupling (Scheme 1). The
chemically modified starch nanocrystals were treated with terminal yne-functionalized oxo-
vanadium Schiff base in the presence of benzophenone as radical initiator at λ=365 nm. The
synthesized nanostarch grafted oxo-vanadium Schiff base was characterized with various
techniques. The chemical changes occurred on the surface of nanostarch material during the
immobilization were revealed by FTIR. Their vibrational assignments are shown in Table 1. An
intense and broad peak appeared at 3394 cm-1 in the spectrum of nanostarch attributed to the
stretching mode of O–H bonds, revealing the presence of many hydroxyl groups on the surface
of nanostarch, which are major anchoring sites for MPTMS (3-mercaptopropyltrimethoxysilane).
The FTIR spectrum of the nanostarch immobilized oxo-vanadium Schiff base show a strong peak
at 1609 cm-1 corresponding to the C=N stretching and a peak at 953 cm-1 represents V=O of the
Schiff base. These vibrational bands at 1609 and 953 cm-1 confirm the successful grafting of the
oxo-vanadium Schiff base onto MPTMS coated nanostarch by covalent interaction. The loading
of Schiff base on the nanostarch support (4.42 mmol g-1) was estimated by the ICP-AES
(Inductively coupled plasma atomic emission spectroscopy).
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Fig. 2 illustrates TGA patterns for the nanostarch immobilized oxo-vanadium Schiff base and
starch nanocrystals. Starch [28-29] nanocrystals show 26.21 % weight loss near 100-150 °C,
evidently owing to the evaporation of water molecules which are held in the material. The
second significant weight loss about 70% is observed in the range of 300-350 °C, due to the
thermal decomposition of oxygen carrying functionalities. The MPTMS grafted nanostarch
immobilized oxo-vanadium Schiff base shows a small weight loss around 100 °C owing to the
loss of adsorbed water molecules. The second major mass loss starts at 250 °C owing to
decomposition of MPTMS moieties. The thermogram of the nanostarch immobilized oxo-
vanadium Schiff base catalyst illustrates an exothermic major weight loss over a wide range of
temperature (180-650 °C), indicating the slow decomposition of the Schiff base complex.
Fig. 3 illustrates UV-Vis spectra for nanostarch and nanostarch immobilized oxo-vanadium
Schiff base. The UV–Vis spectrum of supported oxo-vanadium Schiff base exhibits one broad
band between 301 and 360 nm that may be assigned either to a ligand-to-metal charge-transfer
(CT) band or to a π → π* transition corresponding to the azomethine chromophore. Other
weaker bands appearing at 201-230 nm and at 235 nm can be attributed to intra ligand
transitions. A relatively very weak, broad band appeared in the range of 500-600 nm was due to
characteristic d–d transitions of (VO)2+ in the visible region. The CT and d–d transitions confirm
the presence of anchored vanadium in the support matrix [30-31].
The transmission electron microscopy (TEM) image shown in Fig. 4 reveals the nanoscopic
features of the nanostarch immobilized oxo-vanadium Schiff base, is seen in Fig. 4, as indicated
by arrows.
7
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3.2 Catalytic activity of the catalyst 9
The development of synthetic methodologies for the oxidation of alcohols to carbonyl
compounds using environmentally benign oxidants and heterogenized homogeneous catalysts is
desirable both from environmental and economical viewpoints [32-37]. In this context a number
of heterogenized homogeneous [38-39] complexes using organic and inorganic support materials
are reported but to the best of our knowledge there is no report on the use of nanostarch
supported oxo-vanadium Schiff base as catalyst. The catalytic efficiency of 9 was studied for the
oxidation of primary and secondary alcohols to the corresponding aldehydes and ketones using t-
BuOOH as oxidant (Scheme 2).
A stirred mixture of benzhydrol, 9 (2 mol %) and t-BuOOH (1.5 M in dodecane; 1.5 equv.) in
acetonitrile was heated at 65 °C for 1 h. After completion of the reaction, the catalyst was
recovered by filtration and the filtrate so obtained was subjected to usual work-up to yield
benzophenone in 94% yield. The filtrate was also analyzed by ICP-AES analysis, no metal or
ligand was detected, indicating that no leaching had occurred during this course. Furthermore, to
check the leaching of metal or catalyst, we stirred the heterogenized complex in acetonitrile at 65
oC for 3 h. The catalyst was separated out by filtering off the hot reaction mixture and the filtrate
so obtained was charged with fresh substrate and oxidant under the described experimental
conditions. In this case, no oxidation was observed even after prolonged reaction time (2 h),
indicating that there was no leaching and the reaction was truly heterogeneous in nature.
To explore the scope of the developed protocol, the oxidation of a variety of alcohols including
diols and α-ketols was performed under described experimental conditions. The results of these
experiments are summarized in Table 2. Primary aliphatic and benzylic alcohols afforded the
corresponding aldehydes selectively without any evidence for the formation of acids. Among the
8
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secondary alcohols studied, those having aromatic substituents (Table 2, entries 1, 3) were found
to be more reactive than alicyclic alcohols (Table 2, entries 14-15).
The exact mechanism of the reaction is not clear at this stage. To understand the mechanism of
the reaction, starch-immobilized oxo-vanadium complexes suspended in DMF was added
tBuOOH and stirred the suspension for 30 min and then characterized by XPS (Supporting
Figure 1). As indicated in the XPS, a single sharp band at binding energy 516.5 eV characteristic
to V+5 evidenced the oxidation of V (IV) to V (V) by forming the peroxo complex in presence of
TBHP. Oxidation for V(IV) to V(V) and formation of peroxo complex in polymer supported oxo-
vanadium in the presence of TBHP is well reported in the literature [27].
We next examined the recyclability of the nanostarch grafted oxo-vanadium Schiff base
catalyst by using benzhydrol as a representative substrate (Table 3). In all cases, the catalyst was
readily recovered from the reaction mixture by simple filtration, washed with dichloromethane,
dried and subjected to the subsequent run (6 cycles). During these experiments, the yield and
reaction time remained almost the same, establishing the recyclability and reusability of the
catalyst.
4. Conclusion
In summary, the present paper described the first successful application of the thiol-yne
coupling “click chemistry” for grafting of oxo-vanadium Schiff base onto nanostarch support.
The unique features of the thio-yne coupling strategy presented here for the functionalization of
the nanostarch support, i.e. its simplicity in use, versatility, metal-free nature and high efficiency
with respect to overall catalyst loading, appear to be attractive for the preparation of immobilized
catalysts. The nanostarch supported oxo-vanadium Schiff base could be applied as catalyst
without observable metal or ligand leaching and loss of activity to the oxidation of alcohols with
9
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t-BuOOH to give the corresponding aldehydes or ketones in excellent yields. To the best of our
knowledge, this is the first report on the use of thio-yne click chemistry for the immobilization of
metal complex. We believe that this report will open several new avenues for developing
efficient and economical heterogeneous catalysts for synthetically important organic
transformations.
5.0 Acknowledgments
We are thankful to Director IIP, for his kind permission to publish these results. SV
acknowledges French Embassy in India for his fellowship.
Supplementary Material
The copies of spectra of 1H and 13C NMR are given in Supplementary materials file.
6.0 References
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Scheme 1: Synthetic route for the preparation of 9
OH
OH
H3CO OCH3
O
O
Toulene, Reflux
24 hrs
Si
SH
SN
Si
SH
SN
+
H3CO
O
OH
3
1
2
OH
O
V
O
N
OH
O
N
O
V
VO(acac)2
CH Cl
OH
acetone
1 hr
OHC
HO
OH
+
H2N
OH
O
HO
O
N
2
2
48 hrs
OH
4
5
7
6
Br
SN
O
NaH,DMF
+
3
O
800C,12hrs
O
O
SN
O
O
O
Si
O
O
N
Si
benzophenone
hv
365 nm
O
V
V
N
O
O
O
S
S
O
8
O
O
O
V
O
N
9
13
Page 13 of 24

Figure 1: Thiol-yne coupling
Figure 2: TGA patterns of (a) Starch nanoparticles (b) nano-starch immobilized oxo-vanadium Schiff
base catalyst 9
100
(b)
80
(a)
60
40
20
100
200
300
400
500
600
Temperature,(oC)
14
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Figure 3:UV-Vis patterns of (a) Starch nanoparticles (b) nano-starch immobilized oxo-vanadium
Schiff base catalyst 9
0.45
0.40
0.35
0.30
(b)
0.25
0.20
(a)
0.15
0.10
200
300
400
500
600
700
800
wave length/nm
15
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Figure 4: HRTEM of nano-starch immobilized oxo-vanadium Schiff base catalyst 9
16
Page 16 of 24

Figure 5: XPS spectra of peroxo-vanadium complex 10
17
Page 17 of 24

Table 1: Infrared vibration frequencies of starch nanocrystals 1, 7 and starch-grafted complex 9
with their vibrational assignments.
Starch nano
Particles
(1)
Oxo-vanadium
Vanadium complex
grafted on starch nano
particle (9)
Assignments
O-H stretch
Schiff base
(8)
-
3394.7
3376.6
-
C-H alkyne stretch
C-H asym. Stretch
3283.7
2922.7
-
2931.7
2929.7
(CH2)
C-H scissor stretch
(CH2)
1413.3
1431.3
-
C
C
-
-
-
2117.8
1604.2
1225.4
-
C=N
C-N
1609.4
1232.8
C-O
1020.3
1022.9
-
1040.6
1081.0
953.8
Si-O-C
V=O
-
-
975.1
18
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Table 2: Oxidation of alcohols, diols and α-hydroxyketonesa
Entry
1
Substrate
Product
Yield (%)b
OH
CH
O
C
94
10c
CHO
CH2OH
2
3
4
92
92
OH
O
CHO
CH2OH
92
92
90
Cl
CH2OH
Cl
CHO
5
6
7
CH3
CH3
CH2OH
CHO
OCH3
OCH3
O
O
O
90
92
O
OH
O
O
O
8
9
OH
CHO
O
O
CHO
CH2OH
90
OH
OH
CHO
10
CH2OH
85
92
NO2
OH
NO2
11d
O
O
OH
19
Page 19 of 24

12d
13
OH
O
CH3
CH3
92
OH
H3C
O
O
H3C
O
OMe
OMe
92
90
90
OH
MeO
O
MeO
OH
14
15
O
OH
O
aReaction conditions: Substrate (1 mmol), catalyst (5 mol %), tBuOOH (1.5 equv.), at 65 oC for 1 h.; bIsolated
t
yields. cBlank experiment in the absence of the catalyst. dReaction with 3 equv. of BuOOH
20
Page 20 of 24

Table 3: Recycling of the catalysta
Run
1
2
3
1
4
1
5
6
Time (h)
1
1
1
1
Yield
(%)
94
94
93 93
94
94
aReaction conditions: Substrate (1 mmol), catalyst (5 mol %), t-BuOOH (1.5 equv.), at 65 oC for 1 h
21
Page 21 of 24

Graphical Abstract
Thiol-yne Click on Nano-Starch: An Expedient Approach for Grafting of Oxo-Vanadium
Schiff Base Catalyst and its use in the Oxidation of Alcohols
S
S
S
H
+
O
OH
R
H
R
H
22
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Research Highlights
 Grafting of oxo-vanadium complex on nanocrystalline starch by click method
 Novel metal free approach for the heterogenization of homogeneous catalyst
 Utilization of renewable resources as catalyst support
 Development of enviro-economic methodology for oxidation of alcohols
 First successful application of Thiol-yne coupling in the area of catalysis.
23
Page 23 of 24

R1
R1
R2
O
O
OH
9 (2 mol%)
R2
O
OH
1.5 M t-BuOOH in decane (1.5 equiv.)
OH
MeCN, 65 °C
R
R
n
n
n=1,2
n=1,2
R1 = alkyl, aryl; R2 = aryl, COPh, alkyl
Scheme 2: Oxidation of alcohols
Page 24 of 24