[VO(PS-BBMA)](SO4) catalyzed Α-oxygenation of benzylamines to amides in solvent free condition

Article abstract of DOI:10.1016/j.catcom.2019.01.002

Polymer anchored [VO(PS-BBMA)]SO₄ and unsupported [VO(BBMA)]SO₄ catalysts were synthesized and characterized by elemental analyses, FT-IR, ESR, UV–vis diffuse reflectance spectroscopy (DRS), thermogravimetric analysis (TGA), and vibrating sample magnetometry (VSM). A protocol for the synthesis of benzamides from benzylamine has been developed using [VO(PS-BBMA]SO₄ as an effective catalyst under solvent free conditions in presence of H₂O₂. The reactions provide good yields of the corresponding amide products with substrate scope and excellent functional group compatibility. The developed catalyst can be facilely recovered, heterogeneous, compared with unsupported analogue and reused six times without significant decrease in its activity.

Full text of DOI:10.1016/j.catcom.2019.01.002

Accepted Manuscript
[VO(PS-BBMA)](SO4)
catalyzed
α-oxygenation
of
benzylamines to amides in solvent free condition
M.K. Renuka, V. Gayathri
PII:
S1566-7367(19)30002-0
DOI:
Reference:
https://doi.org/10.1016/j.catcom.2019.01.002
CATCOM 5586
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
25 October 2018
19 December 2018
1 January 2019
Please cite this article as: M.K. Renuka, V. Gayathri , [VO(PS-BBMA)](SO4) catalyzed
α-oxygenation of benzylamines to amides in solvent free condition. Catcom (2018),
https://doi.org/10.1016/j.catcom.2019.01.002
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ACCEPTED MANUSCRIPT
[VO(PS-BBMA)](SO4) catalyzed α-oxygenation of benzylamines to amides in solvent free
condition
Renuka M K and Gayathri V*
Department of Post Graduate Studies in Chemistry, Central College Campus, Bangalore
University, Dr. Ambedkar Street, Bangalore 560001, Karnataka, India.
*Corresponding author e-mail: gayathritvr@yahoo.co.in, Phone: 080-22961348
Abstract
Polymer anchored [VO(PS-BBMA]SO4 and unsupported [VO(BBMA]SO4 catalysts were
synthesized and characterized by elemental analyses, FT-IR, ESR, UV–vis diffuse reflectance
spectroscopy (DRS), thermogravimetric analysis (TGA), and vibrating sample magnetometry
(VSM). A protocol for the synthesis of benzamides from benzylamine has been developed using
[VO(PS-BBMA]SO4 as a effective catalyst under solvent free conditions in presence of H2O2.
The reactions provide good yields of the corresponding amide products with substrate scope and
excellent functional group compatibility. The developed catalyst can be facilely recovered,
heterogeneous, compared with unsupported analogue and reused six times without significant
decrease in its activity.
Keywords: α-oxygenation, benzyl amine, H2O2, benzamide, [VO(BBMA]SO4, [VO(PS-
BBMA]SO4
1. Introduction
Oxidation of amines to amides is an important reaction because amides are valuable
synthetic intermediates in chemistry as well as in biology. Amides are widely used in
the synthesis of peptide and proteins, intensifiers of perfume, anti-block reagents, color
pigments for inks, detergents and lubricants [1, 2]. Methacrylamide and caprolactum are two
amide group-containing monomeric compounds of great industrial relevance in the
preparation of polymers [3]. Compounds of amides are known to have excellent
anthropod-controlling activity and applications in treatment of HIV disease, cardiovascular
disease, hypertension and high blood pressure [4] and also found in polymers, fine chemicals,
pharmaceuticals, and natural products [5]. The traditional synthetic approach used for amide
formation involves the reaction of an amine with activated carboxylic acid derivatives [6] or
coupling with carboxylic acids mediated by coupling reagents [7, 8], which suffers from several
common drawbacks such as the use of highly hazardous reagents, harsh reaction conditions,
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toxicity issues, poor atom-efficiency, and generation of wastes that not only reduce process
efficiency but also pose environmental problems. This necessitates the development of efficient
methods for amide bond formation that circumvents these problems. To address these challenges
in amide synthesis, a plethora of novel amide formation reactions have been developed [9, 10],
such as catalytic acylation of amines with carboxylic acid [11, 12], dehydrogenative amidation of
alcohol [13-15], amino carbonylation of haloarenes [16-18], hydroamination of alkynes [19, 20],
transamidation of primary amides [21-24], and oxidative amidation of aldehydes [25-29]. Direct
oxygenation of primary amines to primary amide by molecular oxygen was reported in presence
of water and ruthenium hydroxide supported on alumina catalyst [30]. Tanaka et al. [31] reported
amine group was protected with tert.-butoxycarbonylgroup (Boc) and then oxygenated using in
situ generated RuO4 followed by deprotection of the Boc group. There are only a few reports in
the literature regarding the oxidation of amines [32].
Amines are desirable starting materials for the synthesis of amides because they are readily
available and inexpensive. The oxidation of benzylamine is reported over [VO(PS-BBMA]SO4
catalyst with oxidant. Vanadium complexes play a very important role in catalytic organic
synthesis because hydroxyl radicals are very efficiently generated from H2O2 under the action of
vanadium catalysts [33]. Benzylamine can be converted into benzamide using this catalyst
(Scheme 2). A remarkable feature is that the process was carried out in the absence of base,
solvent or any other external additives which makes the process environmental benign.
2. Experimental
2.1. Materials and methods
Chloromethylated polystyrene divinyl benzene (PS) was obtained as a gift from THERMAX Ltd.
India. The VOSO4, o-phenylenediamine, iminodiacetic acid, benzylamine, substituted benzyl
amines, H2O2 were purchased from Merck and used without further purification. Solvents were
used after purification by standard methods.
Elemental analyses were carried out using an Elementar Vario Micro Cube CHNS analyzer.
Ultraviolet-visible light (UV-vis) reflectance spectra of the solid samples were recorded on a
Shimadzu UV-Vis-NIR model-3101P spectrophotometer, with reference to non-absorbing
BaSO4 as a standard. IR spectra were recorded in KBr on Shimadzu 8,400 s spectrometer in the
range 4000-400 cm-1. Thermogravimetric analyses was carried out with a TA instrument, SDT
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analyser model Q600. ESR measurements of solid sample at room temperature in a standard X
band JES - FA200 spectrometer. The vanadium content was determined by using atomic
absorption spectrometer, after destruction of the complexes with concentrated H2SO4/H2O2
mixture by using Avanta Pm, GF 3000. Molar conductivity measurements were carried out by
using a WTW conductivity meter. LCMS measurements were recorded on a LC-MSD-Trap-
XCT-Plus. Magnetic moment of the complex was measured by a VSM model 7404 at IIT
Madras. Nuclear magnetic resonance spectra were recorded on 400 MHz Bruker Avance
spectrometer using TMS as standard in CDCl3 solvent. Spin multiplicities are described as s
(singlet), d (doublet), t (triplet), q (quartet), m (multiplet), dd (doublet of doublet) and br s (broad
singlet).
2.2. Preparation of unsupported vanadium complex [VO(BBMA)]SO4
The unsupported complex [VO(BBMA)]SO4 was synthesized by mixing aqueous solution of
VOSO4.5H2O (1.5 g, 10 mmol) and BBMA in methanol 1:1 molar ratio (1.1 g, 10 mmol) and
stirred for 24 h. The grey colored compound was filtered, Soxhlet extracted with ethanol and
dried in the oven for 12 h. Yield: 2.1 g; 75 %.
2.3. Preparation of polymer supported vanadium complex [VO(PS-BBMA)]SO4
Bis(benzimidazol-2-ylmethyl)amine (BBMA) was synthesized according to the procedure
described in the literature [34]. Chloromethylated polystyrene (1.0 g, Cl content 6 mmol) was
swelled in DMF for 2 h, and then a solution of BBMA (0.29 g, 0.78 mmol) in DMF (10 mL) was
added, followed by triethylamine (4 mL) and ethyl acetate (30 mL). The mixture was heated at
100 °C for 48 h. After cooling, it was filtered, Soxhlet extracted with ethanol, and dried. 1.0 g of
functionalized polymer (PS-BBMA) was added to water (10 mL) and kept for 1 h. A solution of
VOSO4.XH2O (0.23 g; 1 mmol) in water (10 mL) was added to it and the mixture was stirred at
RT for 24 h. The resultant grey coloured beads were filtered off and washed by Soxhlet
extraction and dried at 100 °C (Scheme 1).
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Scheme 1. Synthesis of [VO(PS-BBMA)]SO4
2.4. General protocol for synthesis of amides
Benzylamine (0.214 g, 2.00 mmol), [VO(PS-BBMA]SO4 (80 mg, 0.08 mmol) and H2O2 (4
mmol) was heated at 80 °C for 10 h in a round bottom flask under nitrogen atmosphere. The
reaction was monitored by TLC, and after complete consumption of benzylamine, the reaction
mixture was cooled to room temperature, filtered and concentrated in vacuo. The residue was
purified by column chromatography (ethyl acetate: hexane = 1:8) to afford benzamide. The
product was analyzed by 1H NMR spectroscopy.
3. Results and discussion
3.1. Characterization of [VO(PS-BBMA)]SO4 and [VO(BBMA)]SO4
3.1.1. CHN, molar conductance and AAS
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Polymer beads were functionalized with BBP, followed by the reaction with VOSO4.H2O to
yield the complex [VO(PS-BBMA)]SO4. Elemental analysis showed that the functionalized BBP
has 5.1 % nitrogen content, which indicated that the successful functionalization of the polymer.
Metal estimation by AAS indicated 5.1 and 10.8 % of V in the polymer supported and
unsupported vanadium catalysts respectively (Table 1). The molar conductance value of
-1
unsupported complex was 69 ohm cm2 mol-1 in DMF its indicated 1:1 electrolytic nature.
Table 1
Elemental analyses of PS, BBMA, PS-BBMA, [VO(PS-BBMA)](SO4) and [VO(BBMA)](SO4)
CHN analyses (%)
Compound
BBMA
C
N
23.1
-
H
6.8
2.2
4.1
8.3
2.9
(3.2)
V
70.1
97.8
90.7
75.1
41.1
-
-
PS
PS-BBMA
5.2
7.0
14.9
-
[VO(PS-BBMA)](SO4)
[VO(BBMA)](SO4)*
5.1
10.8
(11.6)
(43.7) (15.9)
*Calculated values are in parentheses
3.1.2. FT-IR analyses
The IR spectrum of the PS exhibited peaks at 1,267 and 829 cm-1 due to νCH2-Cl and νC–Cl
(Table 2, Fig. S1). On functionalization, these bands decreased in intensity, indicating the
bonding of BBMA onto the polymer support. The free BBMA exhibited a band around 3056 and
3010 cm-1 corresponding to ArN–H and aliphatic N-H group (Fig. S2) whereas later peak was
absent in the functionalized polymer (Fig. S3), as the bonding of the ligand to the polymer beads
is accomplished through the aliphatic nitrogen. The functionalized beads exhibited a band at
-1
1580 cm-1 due to νC=N, which was shifted to 1508 cm in the anchored beads, indicating that
the nitrogen atoms of the pyridine and benzimidazole moieties of the ligand are coordinated to
vanadium (Fig. S4). The unsupported complex exhibited bands at 3202, 3121, 1628 cm-1 due to
νArN–H, νN–H and νC=N respectively (Fig. S5). Both supported and unsupported complexes
-1
exhibited bands in the range 1100-1200 and 600-630 cm due to ν3 and ν4 of sulphate outside the
coordination sphere [35].
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Table 2
FTIR spectral data of PS, BBMA, PS-BBMA, [VO(PS-BBMA)](SO4) and [VO(BBMA)](SO4)
-1
IR spectral data ν(cm )
Compound
BBMA
CH2-Cl C-Cl ArN-H N-H C=N ν3(SO4) ν4(SO4)
-
-
3056 3010 1583
-
-
-
-
-
PS
1267 (s) 829 (s)
1267(w) 829(w) 2924
-
-
-
-
-
PS-BBMA
1580
-
[VO(PS-BBMA)](SO4)
[VO(BBMA)](SO4)
-
-
-
-
2923
1508 1200
620
612
3202 3121 1628 1168
3.1.3. Electronic spectral studies
The electronic spectra of [VO(PS-BBMA)](SO4) (Fig. 1) and [VO(BBMA)](SO4) (Fig. S6)
complexes exhibited strong absorption bands in the region 206 to 300 nm due to π → π* or n →
π* transitions of BBMA and PS. The electronic spectra of both the complexes showed bands
2
2
2
around 448, 530, 700 nm and 425, 540 and 745 nm , which are assigned to B1g → Eg, B1g →
2
2
2B2g and B1g → A1g transitions [36] of [VO(PS-BBMA)](SO4) and [VO(BBMA)](SO4)
respectively having V(IV) with d1 configuration in a distorted tetrahedral geometry respectively.
530
700
448
249
200
300
400
500
600
700
800
Wavelength (nm)
Fig. 1. Electronic spectrum of [VO(PS-BBMA)](SO4)
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3.1.4. TGA studies
Thermogram of [VO(BBMA)](SO4) (Fig. S7) displayed first weight loss in the range 170-223 °C
which corresponds to the loss of sulphate. Further weight loss (60 %) was observed between
223 and 465 °C due to the loss of BBMA. The thermal decomposition of [VO(PS-
BBMA)](SO4) proceeded in two stages (Fig. S8), weight loss of 10 % from 200-240 °C, which
could be attributed to sulphate group, 22 % weight loss in the range 240-325 °C due to loss of
BBMA, whilst above 325 °C due to decomposition of polymer occurred. In conclusion, the
polymer supported oxovanadium complex is more stable compared to its homogeneous
analogue.
3.1.5. EPR and magnetic moment measurements
The X-band EPR spectra of [VO(BBMA)](SO4) (Fig. S9) and [VO(PS-BBMA)](SO4) (Fig. 2)
recorded in the solid state at room temperature. The spectrum showed two types of resonance
components, one set due to the parallel peaks and the other set due to perpendicular peaks which
indicates axially symmetric anisotropy with sixteen line hyperfine splitting, characteristic of
interaction between electron and vanadium nuclear spin.
Room temperature magnetic moments of [VO(BBMA)](SO4) and [VO(PS-BBMA)](SO4) were
measured using a vibrating sample magnetometer. They exhibited a value of ~1.74 BM, which
was very consistent with the expected spin-only magnetic moment S = ½, d1 vanadium(IV)
system.
1000
500
0
-500
-1000
10000
20000
30000
40000
50000
60000
Magnetic field (mT)
Fig. 2. X-band ESR spectrum of [VO(PS-BBMA)](SO4)
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3.1.6. LCMS Studies
For [VO(BBMA)](SO4), the molecular ion peak was found at m/z 343.2 with 4.2 retention time
due to the cationic ion [VO(BBMA)]2+ (Fig. S10).
Based on the above results, tetrahedral geometry was suggested for both polymer anchored and
unsupported V(IV) complexes.
3.2. Catalytic activity
Initially, benzylamine oxidation was chosen as model substrate to optimize the reaction
conditions with [VO(PS-BBMA]SO4 catalyst. Some screening results are summarized in Table
3. The reaction proceeded successfully in solvent free conditions. Hence, temperature, substrate
to oxidant ratio and catalyst concentration was varied.
The reaction proceeded well at 70 °C with the yield of 90 % (Table 3) and at 80 °C, the reaction
reached the highest yield of 92 % and with further increase in temperature, the conversion
slightly decreased (entry 3). Therefore, the temperature of 80 °C was selected for further
exploration. Further, oxidants screening showed that H2O2 as oxidant in the reaction gave 92 %
yield (Table 3, entry 2) while using oxidants O2 and TBHP, the reaction gave very low yield (42
and 63 %) (Table 3, entry 9 and 8). Effect of [VO(PS-BBMA]SO4 concentration was
investigated with 1:2 molar ratio of benzylamine:H2O2, 2 mmol of the benzylamine for 10 h at
80 °C. It was found that with 80 mg (0.08 mmol) of catalyst (Table 3, entry 2) the reaction gave
92 % yield. At lower concentration (70 mg, 0.07 mmol), 72 % yield was obtained. Increase in
catalyst concentration to 90 mg (0.09 mmol), the conversion was same. Hence 80 mg of [VO(PS-
BBMA]SO4 was chosen as the optimum catalyst concentration.
When the reaction was carried out with 2 mmol of benzylamine, 80 mg of [VO(PS-BBMA]SO4
with 1:1 ratio of benzylamine and H2O2 ratio concentration for 10 h at 80 °C, the reaction was
slow with only 50 % conversion (entry 4). Further increase in the ratio to 1:2, the yield was 92 %
(entry 2). When the benzylamine:H2O2 molar ratio was increased to 1:2 and 1:3, there was no
significant change in benzylamine oxidation (entry 5). Hence, 1:2 molar ratio of
benzylamine:H2O2, 2 mmol of the benzylamine, 80 mg of [VO(PS-BBMA]SO4 for 10 h at 80 °C
was the most optimized condition for the benzylamine oxidation (entry 2).
Under optimized reaction conditions, unsupported [VO(BBMA]SO4 complex and VOSO4 salt
were tested for benzylamine oxidation. It was observed that [VO(BBMA]SO4 showed 32 % yield
of benzamide with 100 % selectivity whereas VOSO4 gave 25 % yield.
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Table 3
Optimization of the reaction conditions
Entry Temperature Benzylamine: Oxidants
Catalyst
concentration (mg) (%)
Yielda
(ºC)
70
80
90
80
80
80
80
80
80
80
H2O2
1:2
1:2
1:2
1:1
1:3
1:2
1:2
1:2
1:2
1:2
1
2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
TBHP
O2
80
90
92
85
51
92
72
92
63
42
15
80
3
80
4
80
5
80
6
70
7
90
8
80
80
9
10
H2O2
Blank reaction
a Isolated yield.
Similarly, blank reaction without catalyst gave only 15 % of product (Table 3, entry 10). By
keeping these optimized reaction conditions, various electronically and structurally diverse
benzylamines were subjected to oxidation at optimized reaction conditions and the results from
these studies are shown in Table 4. The results indicated that both electron rich and electron
deficient benzylamines could be successfully transformed into their corresponding products in
moderate to good yields. Specifically, excellent yields were obtained for 4-methyl-substituted
benzylamine (Table 4, entry 2) and 4-methoxy-substituted benzylamine (Table 4, entry 10). On
the other hand, strong electron-withdrawing groups such as Cl and NO2 induced lower reaction
efficiency. The meta substituted group on the benzylamine gave lower yields (entries 3, 5, and 8)
as compared to para substituted benzyl amines.
Table 4
Substrate scope of benzylamine surrogates
Entry
Benzylamines
Benzamides
Yieldb
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1
2
92
94
3
92
4
5
82
75
6
90
7
8
85
82
10

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9
75
96
10
General conditions: Substituted benzylamines (2 mmol), H2O2 (2 mmol), VO(PS-
BBMA)(H2O)(SO4) (80 mg, 0.08 mmol), 80 °C, 10 h. b Isolated yields.
3.3. 1H-NMR spectral data of amides
1. Benzamide
1H NMR (400 MHz, CDCl3): δ 7.75-7.74 (m, 2H), 7.49-7.19 (m, 3H), 6.00 (bs, 1H), 5.60 (bs,
1H).
2. 4-methylbenzamide
1H NMR (400 MHz, CDCl3): δ 7.72-7.70 (m, 2H), 7.26-7.24 (m, 3H), 6.02 (bs, 1H), 5.65 (bs,
1H), 2.41 (s, 3H).
3. 3-methylbenzamide
1H NMR (400 MHz, CDCl3): δ 7.65-7.57 (m, 1H), 7.34 (m, 1H), 7.33-7.26 (m, 2H), 6.07 (bs,
1H), 5.68 (bs, 1H), 2.41 (s, 3H).
4. 4-nitrobenzamide
1H NMR (400 MHz, CDCl3): δ 8.31-8.29 (m, 3H), 8.10-8.08 (m, 2H), 7.73 (bs, 1H).
5. 3-nitrobenzamide
1H NMR (400 MHz, CDCl3): δ 8.70-8.39 (m, 1H), 8.37-8.30 (m, 3H), 7.79-7.73 (m, 2H).
6. nicotinamide
1H NMR (400 MHz, DMSO): δ 9.03-8.76 (m, 1H), 8.75 (m, 1H), 8.19-8.16 (m, 1H), 7.44-7.27
(m, 1H), 6.29 (bs, 1H), 6.01 (bs, 1H).
7. 4-chlorobenzamide
1H NMR (400 MHz, CDCl3): δ 7.69-7.67 (m, 2H), 7.37-7.19 (m, 2H), 5.94-5.89 (bs, 2H).
8. 3-chlorobenzamide
1H NMR (400 MHz, DMSO): δ 7.82-7.67 (m, 1H), 7.52-7.50 (m, 1H), 7.41 (m, 1H), 7.26 (m,
1H), 6.07-5.90 (2 bs, 2H).
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9. N-phenylbenzamide
1H NMR (500 MHz, CDCl3): δ 7.84-7.79 (m, 5H), 7.52-7.25 (m, 5H), 6.50 (bs, 1H).
10. 4-methoxybenzamide
1H NMR (400 MHz, CDCl3): δ 7.74-7.72 (m, 2H), 7.24-7.22 (m, 2H), 5.92 (m, 2H), 3.73 (s,
3H).
Recycling ability
One of the main objectives of supporting a metal complex on a polymer is to enhance the
lifetime of the resulting catalyst. To verify this property in the current systems, the catalyst
[VO(PS-BBMA]SO4 was separated from the reaction mixture after each run by filtration, washed
with alcohol and dried before using it in subsequent runs. The catalyst could be recycled up to
seven times (Fig. S11) without much loss in its activity. In order to see whether the present
catalyst was functioning in a truly heterogeneous manner, the conventional hot filtration test was
carried out for all the oxidation reactions under the optimized conditions. After 1 h, the catalyst
was immediately filtered off from the hot reaction mixture in order to avoid re-adsorption of any
leached metal onto the catalyst surface. The filtrate was collected and the reaction was continued
for another 2 h, showed no further increase in conversion. Vanadium content in the hot filtrate
and recycled catalyst was determined by ICP-OES and the absence of vanadium indicated that
the reaction was truly heterogeneous in nature. % of metal present in the catalyst after
performing the reaction was determined (Fig. S12). It was found th1t up to fifth cycle the metal
remains same and after that there was slight decease in metal content.
4. Conclusion
Polymer immobilized vanadium complex [VO(PS-BBMA]SO4 has been synthesized and
characterized by physicochemical techniques. The catalytic activity of [VO(PS-BBMA]SO4 has
been examined for liquid phase oxidation of benzylamine to benzamide using H2O2 as oxidant.
Under optimum reaction conditions, catalyst showed highest benzylamine conversion (92 %) and
100 % selectivity for benzamide. Various benzamide surrogates have been synthesized in
moderate to good yields under mild and neutral reaction conditions. So this study reveals that
catalyst can be a convenient ecofriendly substitute for hazardous stoichiometric oxidants without
solvent. This catalyst can be used in oxidation of benzylamine up to seven cycles without any
loss in activity.
Acknowledgement
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Authors are thankful to Bangalore University for providing research facilities. Thermax Ltd.
India is gratefully acknowledged for providing polymer beads. Renuka M K acknowledges the
University Grants Commission (UGC), New Delhi, India, for awarding UGC-JRF fellowship.
Authors declare ‘no conflict of interest’.
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14

ACCEPTED MANUSCRIPT
[VO(PS-BBMA)](SO4) catalyzed α-oxygenation of amines to amides in solvent free
condition
Renuka M K and Gayathri V*
Department of Post Graduate Studies in Chemistry, Central College Campus, Bangalore
University, Dr. Ambedkar Street, Bangalore 560001, Karnataka, India.
*Corresponding author e-mail: gayathritvr@yahoo.co.in, Phone: 080-22961348
GRAPHICAL ABSTRACT
92 %
15

ACCEPTED MANUSCRIPT
Highlights
 Novel unsupported and polymer supported complexes were prepared and characterized
 Geometry of the complexes were confirmed by CHN, ESR, IR, Electronic and LC-M
spectrum
 Elegant method for the preparation of amides using H2O2 in solvent free conditions
 The developed catalyst was facilely recovered, truly heterogeneous and reusable
16

Figure 1

Figure 2