Polymer supported Zn-salen complexes: An effective one-pot oxidative esterification of aldehydes to carboxylic esters

Article abstract of DOI:10.1016/j.molliq.2017.07.105

Polymer-supported Zn-salen (PS-Zn-salen) complexes were synthesized, characterized and used as a catalyst for one-pot oxidative esterification of aldehydes with alcohols. The PS-Zn-salen heterogeneous catalyst exhibited a high-performance for the oxidative esterification of aldehydes to the corresponding methyl/ethyl esters using hydrogen peroxide as a green oxidant. Due to the synergistic effect of polymer support, the heterogeneous catalyst presented superior catalytic activity and afford 100% conversion of 3,4,5-trimethoxybenzaldehyde and 4-chlorobenzaldehyde to corresponding esters under optimized conditions. The different alcohol substrates study (viz. methyl alcohol, ethyl alcohol, allyl alcohol and benzyl alcohol) showed reasonable selectivity for esters, indicating the scope of the catalysts. Significantly, the synthesized complexes possess good hydrophobic/heterogeneous properties, which permit facile reclamation of the catalyst by using mere filtration. Moreover, the effect of counter anions of complex also studied which indicated that there is no appreciable influence on the conversion of product. The catalyst was reused up to 5^th successive run with the average conversion of ester 87.4%. Mechanistic studies have recognized that this “one pot” direct oxidative esterification proceeds through acid formation, proven by a GC–MS. The catalyst is also found to be very stable, up to 280?°C, confirmed by the thermo gravimetric study.

Full text of DOI:10.1016/j.molliq.2017.07.105

Accepted Manuscript
Polymer supported Zn-salen complexes: An effective one-pot
oxidative esterification of aldehydes to carboxylic esters
Kamlesh Rudreshwar Balinge, Avinash Ganesh Khiratkar,
Pundlik Rambhau Bhagat
PII:
DOI:
Reference:
S0167-7322(17)33348-2
doi: 10.1016/j.molliq.2017.07.105
MOLLIQ 7684
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
27 May 2017
25 June 2017
7 July 2017
Please cite this article as: Kamlesh Rudreshwar Balinge, Avinash Ganesh Khiratkar,
Pundlik Rambhau Bhagat , Polymer supported Zn-salen complexes: An effective one-pot
oxidative esterification of aldehydes to carboxylic esters, Journal of Molecular Liquids
(2017), doi: 10.1016/j.molliq.2017.07.105
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ACCEPTED MANUSCRIPT
Polymer supported Zn-salen complexes: An effective one-
pot oxidative esterification of aldehydes to carboxylic esters
Kamlesh Rudreshwar Balinge, Avinash Ganesh Khiratkar, Pundlik Rambhau Bhagat∗
Department of Chemistry, School of Advanced Sciences,
VIT University, Vellore 632 014, India.
∗Corresponding author. Tel.: +91 9047289073; Fax: +91 416 2240411.
E-mail address: drprbhagat111@gmail.com (P.R. Bhagat).
Abstract
Polymer-supported Zn-salen (PS-Zn-salen) complexes were synthesized, characterized
and used as a catalyst for one-pot oxidative esterification of aldehydes with alcohols. The PS-Zn-
salen heterogeneous catalyst exhibited a high-performance for the oxidative esterification of
aldehydes to the corresponding methyl/ethyl esters using hydrogen peroxide as a green
oxidant. Due to the synergistic effect of polymer support, the heterogeneous catalyst presented
superior catalytic activity and afford 100% conversion of 3,4,5-trimethoxybenzaldehyde and
4-chlorobenzaldehyde to corresponding esters under optimized conditions. The different alcohol
substrates study (viz. methyl alcohol, ethyl alcohol, allyl alcohol and benzyl alcohol) showed
reasonable selectivity for esters, indicating the scope of the catalysts. Significantly, the
synthesized complexes possess good hydrophobic/heterogeneous properties, which permit facile
reclamation of the catalyst by using mere filtration. Moreover, the effect of counter anions of
complex also studied which indicated that there is no appreciable influence on the conversion of
product. The catalyst was reused up to 5th successive run with the average conversion of ester
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87.4%. Mechanistic studies have recognized that this “one pot” direct oxidative esterification
proceeds through acid formation, proven by a GC-MS. The catalyst is also found to be very
stable, up to 280 °C, confirmed by the thermo gravimetric study.
Keyword: PS-Zn-salen; Oxidative esterification; One pot reaction; Methyl benzoates; Ethyl
benzoates.
1.
Introduction
Carboxylic esters are the most essential and abundant functional groups in nature but also
serve effectively as versatile building blocks for the synthesis of natural products, polymeric
materials and fine chemicals, etc. [1] Esters are valuable reagent and they have extensive
industrial applications such as in the flavoring agent, diluents and solvent extractants [2]. The
traditional esterification process based on nucleophilic substitution of carboxylic acid derivatives
such as carboxylic halides, anhydrides and activated esters with alcohols [3]. While benzyl esters
are normally prepared by way of nucleophilic displacement of a carboxylate ion on benzyl
chloride [4]. However, a number of procedures have been developed, the investigation for the
new, facile, economical, and environmentally friendly processes that avoid the use of a large
excess of chemicals and expensive activators [5]. For the one-pot oxidative esterification
reactions, various metal catalysts were used such as Palladium [6-8], Vanadium [9, 10], Copper
[11, 12], Iron [13], Ruthenium [14, 15], Calcium, Magnesium [16], Cerium and Lithium [17]. A
number of metal free catalysts like N-heterocyclic carbene (NHC) [18-20], metal based M-NHCs
complex [21-23] and Ionic liquid as a catalyst [24]. Nanoparticle such as gold nanoparticles [25],
Gold-Nickel oxide nanoparticle [26], Silica-encapsulated iron oxide nanoparticles [27],
Graphene oxide nanoparticles [28], Cesium salt with Ni substituted [2, 29]. Reagents such as
oxone [30], Sodium bromide [31], Iodine [32-34], ZnBr2 [35], KI-TBHP [36], Indium(III)Triflate
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[37], TsNBr2 [38], Imidazolium perrhenate [39], functionalized pyrrolizidines [40] and N-iodo
saccharine [41] were also reported. Despite exhaustive efforts into the oxidative esterification of
aldehydes, the development of a more effective, mild and eco-friendly method still remains a
challenge. So far, these methods suffer from few substrate scopes, use of stoichiometric
quantities of harmful and risky heavy metal oxidants [42], dry reaction conditions, longer
reaction time, inferior yields as well as low reaction efficacy. The improvement of a one-pot
oxidative esterification of aldehydes under heterogeneous catalytic conditions that reduce
perilous wastes is extremely required for both commercial and environmental points of view.
Amongst the reported schemes heterogeneous catalysts are favored due to their simplistic
reclamation, less contagion of the product and effective reusability. On the contrary, most of the
reported heterogeneous catalysts like Pd/styrene-divinyl benzene copolymer catalyst [43],
Titanium superoxide [1], phosphotungstate anchored to
MCM-41 and ZrO2 [44] are based on
the expensive metals and involve the monotonous multi-step synthetic processes. Applications of
polymer-supported reagents in organic synthesis have grown up over the years due to the
convenience in handling, easy workup procedures, and reusability of the reagents. It is well
known that hydrophilic catalysts suffer deactivation by water due to capillary condensation and
slow water desorption. However, the problem may be avoided by the use of a hydrophobic
catalyst [43]. Additionally, there are many reports involving the application of hydrophobic
styrene-divinyl benzene copolymer (SDB) used in the reaction involving water as bye-product
[45-49].
Previously, we have tested a novel method for the preparation of (PS-Zn-salen) complex
and its catalytic activities toward the synthesis of hydantoins, thiohydantoins and Schiff bases.
To the best of our knowledge, PS-Zn-salen complexes catalyzed the one-pot esterification of
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aldehydes with alcohols under heterogeneous conditions have not explored. In this context, we
wish to report oxidative esterification of aldehydes catalyzed by PS-Zn-salen complexes using
H2O2 oxidant.
2.
2.1. Materials
All chemicals were used reagent grade with the utmost purity available. ZnCl2,
Experimental
LiN(CF3SO2)2, LiClO4, 4-Vinylbenzyl chloride (used by washing with dil. NaOH), DMSO-d6,
CDCl3, NaBF4, KPF6, were purchased from Sigma Aldrich (India), ethylene diamine,
Azobisisobutyronitrile (AIBN), 4-hydroxybenzaldehyde, formaldehyde and other aldehydes,
H2O2 were acquired from SDFCL and Avra Synthesis Pvt. Ltd., ethanol from Changshu
Yangyuan chemical China, chloroform, acetonitrile, benzimidazole, THF procured from
Spectrochem Pvt. Ltd. and used as received unless otherwise it is stated.
2.2. Characterization techniques
FT-IR spectra were obtained from IR affinity 1 Shimadzu FT-IR spectrophotometer using
KBr pellet method. NMR spectra were recorded in DMSO-d6 and CDCl3 on Bruker
spectrometer operating at 400 MHz and chemical shifts are given in ppm downfield from TMS
(δ = 0.00). Surface morphology and the elemental composition of the complex were investigated
by scanning electron microscope (SEM) along with energy dispersive X-ray (EDX) spectroscopy
(Carl Zeiss EVO/18SH, UK). An accelerating voltage of 10 kV was applied to obtain SEM
images. Thermal stability of PSBIL was confirmed using TG/DTA thermoanalyser SII, 7200
(Seiko, Japan).
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2.3.
Synthesis and characterization of PS-Zn-salen complexes.
2.3.1. Synthesis of polyvinyl benzyl chloride (PVBC) 1a, poly 1-(4-sec-butyl)benzyl)-1H-
benzo[d]imidazole 1b and 5-(chloromethyl)-2-hydroxybenzaldehyde 1c.
The compounds 1a, 1b were synthesized according to literature [50] while 1c was
obtained as the Supplementary Information of literature [51] and the characterization data of the
corresponding compounds match with literature report.
(yield 70 %) 1a 1H NMR (400 MHz, CDCl3) δ= 7.08 (Ar-H, br), 6.5 (Ar-H, br), 4.56 (CH2-Cl,
br), 1.71 (br), 1.60 (br), 1.42 (br), 0.95 (br). 13C NMR (100 MHz, CDCl3) 145.56, 134.97,
128.57, 46.37, 40.36, 31.60, 25.28. FT-IR (KBr) in cm-1 : 3024, 2922, 2848, 1610, 1510, 1442,
1221, 1265, 1109, 1018, 912, 823, 707, 669, 559.
(yield: 74 %) 1b 1H NMR (400 MHz, CDCl3) δ = 7.99 (NCHN, br), 7.7 (Bim-H, br), 7.19 (Bim-
H, br), 6.6 (Ar-H, br), 6.17 (Ar-H, br), 4.95 (Bim-CH2-Ar, br), 1.84 (br), 1.48 (br), 1.25 (br),
0.84 (br). 13C NMR (100MHz, CDCl3) 143.85, 143.09, 133.95, 128.03, 127.27, 122.97, 122.35,
120.37, 110.06, 67.99, 48.33, 29.71, 25.62. FT-IR (KBr) in cm-1 : 2918, 1492, 1363, 1330, 1259,
1197, 1178, 844, 817, 738, 634, 574.
(Yield: 58 %) 1c 1H NMR (400 MHz, CDCl3) δ = 11.08 (s, Ph-CHO, 1H) 9.90 (s, Ph-OH, 1H),
7.6-7.55 (m, CH arom, 2H), 7.02-7.00 (d, J = 8.89Hz, CH arom, 1H), 4.6 (s, 2H, -CH2-).13C
NMR (100 MHz, CDCl3) 196.19, 161.64, 137.64, 133.64, 118.33, 45.24. FT-IR (KBr) in cm-
1 : 3203, 3243, 2875, 1647, 1579, 1481, 1379, 1259, 1186, 1145, 1080, 991, 876, 766, 717, 667,
572, 497.
2.3.2. Synthesis of 1-(4-(sec-butyl)benzyl)-3-(3-formyl-4-hydroxybenzyl)-1H-benzo[d]imidazol-
3-ium chloride (IL).1d
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The mixture of poly 1-(4-sec-butyl)benzyl)-1H-benzo[d]imidazole (20 mmol) and
5-(chloromethyl)-2-hydroxybenz-aldehyde (20 mmol) charged to a 100 mL round bottom flask
in acetonitrile medium. The reaction mixture was kept at room temperature with controlled
stirring for 24 h. The precipitated compound was then washed with diethyl ether to obtained
white powder of 1d. Yield: 68 %.
1H NMR (400 MHz, DMSO-d6) δ = 10.25 (CH=O, s), 8.4 (C-NH-C, s), 8.1-7.5 (Bim H, br), 7.2-
6.2 (Ar, br), 5.8, (CH2, br), 5.5 (CH2, br), 2, 1.23, 1.09, 0.85 (aliphatic proton). FT-IR (KBr) in
cm-1 : 3396, 2926, 1614, 1556, 1487, 1442, 1371, 1282, 1249, 1188, 744, 632.
2.3.3. Synthesis of 3,3'-((((1E,1'E)-(ethane-1,2- dylbis(azanylylidene)) bis(methanylylidene))
bis(4-hydroxy-3,1-phenylene))bis(methylene))bis(1-(4-(sec-butyl)benzyl)-1H-benzo[d]imidazol-
3-ium) chloride ie. [PS-salen ligand].1e
The salen ligand was prepared by the stirring solution of 1, 2-diethyl amine (7.5 mmol) in
ethanol and IL (1d) (15 mmol) for 30 min at room temperature followed by refluxing for 2 h.
The ligand was isolated and purified by repeated washing with ethanol to remove unreacted
amine. The product was then dried under vacuum to acquire the yellow powder of salen
ligand 1e. Yield: 80 %.
1H NMR (400 MHz, DMSO-d6) δ = 13.18 (OH, br), 10.26 (CH=O, s), 8.5 (C-NH-C, s), 8.4-7.5
(Bim H, br), 7.2-6.8 (Ar, br), 5.6, (CH2, br), 5.5 (CH2, br), 3.5, 2, 1.23, 1.09, 0.85 (aliphatic
proton). FT-IR (KBr) in cm-1 : 3354, 3053, 3024, 2968, 2920, 2850, 1631, 1558, 1494, 1423,
1369, 1334, 1284, 1188, 742, 634, 574.
2.3.4. Synthesis of Zn-salen complex (PS-Zn-salen complex).1f
The PS-Zn-salen complex was successfully synthesized and characterized by the altering method
from reported in the literature [52], salen ligand 1e (1 mmol) in 10 mL ethanol and ZnCl2 (1
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mmol) was added to a 100 mL and the mixture was stirred for 1h at room temperature. The
mixture was then refluxed at 75-80 °C for 2 h. The product was easily separated after the reaction
°
by recurrent washing with ethanol and dried in oven at 80 C for 2 h, get yellow fine powder of
PS-Zn-salen complex 1f (Scheme 1). Yield: 83 %.
2.3.5. Synthesis of catalysts 1g-1j.
The syntheses of catalysts 1g-1j were carried out according to the literature [53]. The
PS-Zn-salen complex (1 equivalent) was suspended in distilled water and 2.2 equivalent aqueous
solutions of NaBF4 or KPF6 or LiN(CF3SO2)2 or LiClO4 were added drop wise with constant
stirring for 15 min at room temp. The reaction mixture was allowed to stir for 5 h, after which the
solids 1g, 1h, 1i and 1j respectively were collected by filtration and washed with distilled water
repeatedly over filter paper, till negative silver nitrate test. The negative silver nitrate test ensures
the complete conversion of chloride to tetrafluoro borate, hexafluorophosphate, bis (tri-
fluoromethane) sulphonimide and perchlorate. The resulted catalysts were finally dried in an
oven at 80 °C for 2 h (Scheme 2).
2.4. General procedure for one pot oxidative esterification of aldehydes to corresponding
methyl esters with PS-Zn-salen complex (1f) and H2O2.
The reaction of 3,4,5-trimethoxybenzaldehyde (0.01 mol) with H2O2 (0.04 mol) and
methanol was carried out in 100 mL round bottom flask. The reaction mixture was reflux for
24 h. After completion of the reaction, the reaction mixture pour in distilled water; the aqueous
solution was the extracted with ethyl acetate (3×20 mL) and brine solution to get clear ethyl
acetate layer. The organic layer was dried with anhydrous sodium sulfate. It was filtered and
vaporized to dryness at reduced pressure to obtained corresponding methyl etster.
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2a) methyl 3,4,5-trimethoxybenzoate (Yield = 100%); GC-MS m/z: calcd. for C11H14O5:
1
13
226.0841, found 226.2370. H NMR (400 MHz, CDCl3) δ = 7.31 (s, 2H), 3.92 (s, 12H). C
NMR (100 MHz, CDCl3) 191.08, 166.73, 152.94, 142.19, 125.14, 106.83, 60.91, 56.29, 56.23,
52.22. FT-IR (KBr) in cm-1 : 2949, 2839, 1712, 1587, 1504, 1409, 1327, 1224, 1176, 1124, 989,
864, 761, 732, 534, 435.
2b) methyl 4-methoxybenzoate (Yield = 70%); GC-MS m/z: calcd. for C9H10O3: 166.0630,
1
found 166.2266. H NMR (400 MHz, CDCl3) δ = 6.80 (d, 4H), 3.91 (s, 3H), 3.88 (s, 3H). 13C
NMR (100 MHz, CDCl3) 167.18, 163.41, 153.69, 149.64, 132.14, 122.50, 115.01, 114.38, 55.61,
51.97. FT-IR (KBr) in cm-1 : 2951, 1720, 1600, 1435, 1273, 1176, 1109, 1026, 964, 823, 707,
686, 545.
2c) methyl benzoate (Yield = 92%); GC-MS m/z: calcd. for C8H8O2: 136.0524, found 136.2046.
1H NMR (400 MHz, CDCl3) δ = 8.07-8.59 (d, J = 7.42 Hz, 2H), 7.92-7.87 (t, J = 7.86 Hz, 1H),
13
7.48-7.44 (t, J = 7.85 Hz, 2H), 3.91 (s, 3H). C NMR (100 MHz, CDCl3) 171.09, 167.21,
133.72, 132.93, 130.20, 129.58, 128.37, 52.12. FT-IR (KBr) in cm-1 : 2953, 2835, 1691, 1602,
1506, 1436, 1290, 1213, 1166, 1101, 1028, 825, 771, 731, 609, 514.
2d) methyl 4-chlorobenzoate (Yield = 94%); GC-MS m/z: calcd. for C8H7ClO2: 170.0135, found
1
170.1256. H NMR (400 MHz, CDCl3) δ = 7.97-7.95 (d, J = 8.72 Hz, 2H), 7.41-7.39 (d, J = 8.28
Hz, 2H), 3.90 (s, 3H). 13C NMR (100 MHz, CDCl3) 165.1, 131.56, 128.91, 128.73, 128.60,
128.39, 52.29.
2e) methyl 2-nitrobenzoate (Yield = 75%); GC-MS m/z: calcd. for C8H7NO4: 181.0375, found
181.1664. 1H NMR (400 MHz, CDCl3) δ = 8.15-8.12 (d, J = 7.82 Hz, 1H), 7.9-7.86 (t, J = 8.72,
13
3h), 3.94 (s, 3H), C NMR (100 MHz, CDCl3) 165.95, 148.50, 134.08, 131.78, 129.95, 128.79,
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124.35, 53.30. FT-IR (KBr) in cm-1 : 2954, 1730, 1525, 1438, 1290, 1255, 1126, 1072, 954, 856,
786, 734, 696, 640.
2f) methyl 4-fluorobenzoate (Yield = 47%); GC-MS m/z: calcd. for C8H7FO2: 154.0430, found
154.1607.1H NMR (400 MHz, CDCl3) δ = 8.00-7.96 (q, J = 5.67 Hz, 2H), 7.05-6.97 (m, 2H),
3.84 (s, 3H), 13C NMR (100 MHz, CDCl3) 167.03, 166.26, 132.17, 132.07, 126.38, 115.04,
115.20, 52.21. FT-IR (KBr) in cm-1 : 2954, 1720, 1600, 1508, 1436, 1273, 1230, 1153, 1111,
1012, 964, 850, 765, 686, 605, 505.
2g) methyl 4-methylbenzoate (Yield = 91%); GC-MS m/z: calcd. for C9H10O2: 150.0681, found
150.1919.1H NMR (400 MHz, CDCl3) δ = 7.96-7.94 (d, J = 8.06 Hz, 2H), 7.26-7.24 (d, J = 8.73
Hz, 2H), 9.62 (s, 3H), 2.4 (s, 3H). 13C NMR (100 MHz, CDCl3) 167.45, 143.66, 129.62, 129.10,
126.93, 52.03, 21.65. FT-IR (KBr) in cm-1 : 2951, 1718, 1610, 1514, 1435, 1276, 1176, 1107,
1020, 964, 839, 754, 690, 605, 468.
2h) methyl 3-hydroxy-4-methoxybenzoate (Yield = 89%); GC-MS m/z: calcd. for C9H10O4:
182.0579, found 182.2102. 1H NMR (400 MHz, CDCl3) δ = 7.53 (m, 1H), 7.19, (s, 1H), 6.8-6.7
13
(d, J = 8.39, 1H), 5.56 (s, 1H), 3.88 (s, 3H), 3.86 (s, 3H). C NMR (100 MHz, CDCl3) 166.83,
150.38, 145.25, 123.44, 122.80, 115.59, 109.83, 56.04, 51.97. FT-IR (KBr) in cm-1 : 3410, 2953,
2845, 1699, 1612, 1510, 1436, 1532, 1276, 1211, 1126, 1020, 985, 889, 628, 532, 447.
2i) methyl 4-hydroxybenzoate (Yield = 67%); GC-MS m/z: calcd. for C8H8O3: 152.0473, found
152.2112. 1H NMR (400 MHz, CDCl3) δ = 7.38 (d, J = 8.66 Hz, 2H), 7.10 (d, J = 8.72 Hz, 2H),
13
5.9 (s, 1H), 3.91 (s, 3H). C NMR (100 MHz, CDCl3) 171.27, 167.16, 154.94, 136.62, 120.86,
56.82. FT-IR (KBr) in cm-1 :3336, 3132, 3028, 2954, 2709, 1678, 1606, 1510, 1456, 1357, 1284,
1188, 1093, 829, 758, 698, 617, 522, 482.
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2j) methyl 4-bromobenzoate (Yield = 100%); GC-MS m/z: calcd. for C8H7BrO2: 213.9629,
found 214.0043. 1H NMR (400 MHz, CDCl3) δ = 7.91-7.89 (d, J = 8.70 Hz, 2H), 7.59-7.57 (d, J
13
= 8.74 Hz, 2H), 3.92 (s, 3H). C NMR (100 MHz, CDCl3) 166.42, 131.72, 131.12, 129.08,
128.69, 52.32. FT-IR (KBr) in cm-1 : 3091, 2949, 2848, 2555, 1710, 1587, 1435, 1273, 1172,
1109, 1006, 954, 848, 754, 682, 547, 470.
2k) ethyl 3,4,5-trimethoxybenzoate (Yield = 83%); GC-MS m/z: calcd. for C12H16O5: 240.0998,
found 240.1572. 1H NMR (400 MHz, CDCl3) δ = 7.3 (s, 2H), 4.41-4.36 (q, J = 5.8 Hz, 2H), 3.9
13
(s, 9H), 1.43-1.39 (t, J = 7.19 Hz, 2H). C NMR (100 MHz, CDCl3) 166.30, 152.90, 142.08,
125.53, 106.76, 61.17, 60.92, 56.24, 14.40. FT-IR (KBr) in cm-1 : 2972, 2941, 2839, 1710, 1587,
1458, 1413, 1328, 1220, 1222, 999, 948, 864, 763, 727, 626, 536.
2l) ethyl 4-methoxybenzoate (Yield = 72%); GC-MS m/z: calcd. for C10H12O3: 180.0786, found
180.1891.1H NMR (400 MHz, CDCl3) δ = 7.9 (d, J = 8.74 Hz, 2H), 7.4 (d, J = 8.64 Hz, 2H), 4.4
(q, J = 5.67 Hz, 2H), 9.9 (s, 3H), 1.4 (t, J = 7.41 Hz, 3H). 13C NMR (100 MHz, CDCl3) 166.86,
153.63, 149.65, 116.08, 114.87, 60.90, 55.83, 14.35. FT-IR (KBr) in cm-1 : 2939, 2835, 1680,
1600, 1506, 1440, 1367, 1213, 1101, 1028, 825, 771, 731, 605, 514.
2m) ethyl benzoate (Yield = 83%); GC-MS m/z: calcd. for C9H10O2: 150.0681, found 150.1821.
1H NMR (400 MHz, CDCl3) δ = 8.15-8.01 (m, 2H), 7.65-7.46 (m, 2H), 4.43-4.38 (q, J = 5.9 Hz,
2H), 1.44-1.40 (t, J = 7.38 Hz, 3H). 13C NMR (100 MHz, CDCl3) 166.85, 132.86, 130.20,
129.55, 128.33, 61.05, 14.31. FT-IR (KBr) in cm-1 : 3064, 2983, 1695, 1600, 1450, 1367, 1273,
1105, 1026, 935, 850, 707, 646, 532.
2n) ethyl 4-chlorobenzoate (Yield = 90%); GC-MS m/z: calcd. for C9H9ClO2: 184.0291, found
184.1560. 1H NMR (400 MHz, CDCl3) δ = 7.92-7.89 (d, J = 9.01 Hz, 2H), 7.34-7.32 (d, J = 9.01,
2H), 4.33-4.27 (q, J = 5.7 Hz, 2H), 1.34-1.30 (t, J = 7.37 Hz, 3H). 13C NMR (100 MHz, CDCl3)
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165.79, 139.25, 131.55, 130.95, 128.66, 61.22, 14.29. FT-IR (KBr) in cm-1 : 2983, 1716, 1593,
1487, 1002, 1269, 1770, 1089, 1014, 848, 758, 684, 526, 474.
2o) ethyl 2-nitrobenzoate (Yield = 66%); GC-MS m/z: calcd. for C9H9NO4: 195.0532, found
1
195.0810. H NMR (400 MHz, CDCl3) δ = 7.98-7.62 (m, 4H), 4.9 (q, J = 5.6, 2H), 1.37 (t, J =
7.42, 3H). 13C NMR (100 MHz, CDCl3) 165.5, 134.41, 133.3, 131.8, 129.5, 124.01, 102.7, 62.9,
13.7. FT-IR (KBr) in cm-1 : 2980, 2899, 1728, 1525, 1348, 1290, 1255, 1128, 1070, 1010, 958,
856, 785, 734, 638.
2p) ethyl 4-fluorobenzoate (Yield = 84%); GC-MS m/z: calcd. for C9H9FO2: 168.0587, found
168.1471. 1H NMR (400 MHz, CDCl3) δ = 7.93-7.91 (d, J = 7.64 Hz, 2H), 7.60-7.58 (d, J = 7.68
13
Hz, 2H), 4.41-4.36 (q, J = 5.67 Hz, 2H), 1.29-1.26 (t, J = 7.38 Hz, 3H). C NMR (100 MHz,
CDCl3) 169.01, 166.31, 131.87, 126.38, 115.20, 60.45, 14.19.
2q) ethyl 4-methylbenzoate (Yield = 86%); GC-MS m/z: calcd. for C10H12O2: 164.0837, found
164.1788. 1H NMR (400 MHz, CDCl3) δ = 7.94-792 (d, J = 7.46 Hz, 2H), 7.21-7.19 (d, J = 8.24
Hz, 2H), 4.31-4.26 (q, J = 5.67 Hz, 2H), 2.36 (s, 3H), 1.33-1.29 (t, 7.58 Hz, 3H). 13C NMR (100
MHz, CDCl3) 165.99, 143.10, 129.32, 129.08, 125.99, 61.11, 14.35.
2r) ethyl 3-hydroxy-4-methoxybenzoate (Yield = 83%); GC-MS m/z: calcd. for C10H12O4:
196.0736, found 196.1943. 1H NMR (400 MHz, CDCl3) δ = 7.65 (d, J = 8.51 Hz, 1H), 6.90 (d, J
= 8.9 Hz, 1H), 6.7 (s, 1H), 5.6 (s, 1H), 4.98-4.33 (q, J = 5.56 Hz, 2H), 3.97 (s, 3H), 1.41-1.38 (t,
J = 7.58 Hz, 3H). 13C NMR (100 MHz, CDCl3) 167.10, 153.10, 145.66, 123.39, 123.00, 116.10,
122.17, 61.33, 13.99.
2s) ethyl 4-hydroxybenzoate (Yield = 29%); GC-MS m/z: calcd. for C9H10O3: 166.0630, found
1
166.1979. H NMR (400 MHz, CDCl3) δ = 7.4 (d, J = 8.01 Hz, 2H), 7.21 (d, J = 8.12 Hz, 2H),
11

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4.51 (q, J = 5.24 Hz, 2H), 1.40 (t, 14.64 Hz, 3H). 13C NMR (100 MHz, CDCl3) 170.19, 167.366,
155.19, 136.13, 120.96, 61.47, 14.56.
2t) ethyl 4-bromobenzoate (Yield = 79%); GC-MS m/z: calcd. for C9H9BrO2: 227.9786, found
228.0552. 1H NMR (400 MHz, CDCl3) δ = 7.93-7.91 (d, J = 7.53 Hz, 2H), 7.60-7.58 (d, J = 7.53
Hz, 2H), 4.41-4.36 (q, J = 5.68 Hz, 2H), 1.36-1.32 (t, J = 7.6 Hz, 3H). 13C NMR (100 MHz,
CDCl3) 165.95. 131.66, 130.99, 129.37, 127.92, 61.27, 14.28.
2u) allyl 4-chlorobenzoate (Yield = 56%); GC-MS m/z: calcd. for C10H9ClO2: 196.0291, found
196.0551. 1H NMR (400 MHz, CDCl3) δ = 7.90 (d, J = 8.2 Hz, 2H), 7.61 (d, J = 8.3 Hz, 2H), 6.0
(m, 1H), 5.42 (dd, 2H), 4.8 (d, J = 7.4 Hz, 2H). 13C NMR (100 MHz, CDCl3) 165.5, 138.8,
132.8, 130.1, 128.5, 128.5, 118.1, 167.1.
2v) benzyl 4-chlorobenzoate (Yield = 15%); GC-MS m/z: calcd. for C14H11ClO2: 246.0448,
1
found 245.9993. H NMR (400 MHz, CDCl3) δ = 8.09 (d, J = 8.3 Hz, 2H), 7.52 (d, J = 8.1 Hz,
13
2H), 7.5 (m, 5H), 5.31 (s, 2H). C NMR (100 MHz, CDCl3) 166.8, 135.9,131, 129.5, 127.9,
128.1, 128.4, 128.9, 66.5.
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Scheme 1. Preparation of polymer supported Zinc-centered salen complex (PS-Zn-salen)
complex (1f).
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Scheme 2. Preparation of catalyst 1g, 1h, 1i and 1j.
3.
Results and discussion
3.1. Catalyst characterization
3.1.1. Fourier transform infrared (FT-IR) spectroscopy
The FT-IR spectra of the PS-Zn-salen complex (Fig. 1) to define the coordination sites
that may be involved in chelation. The most significant IR spectral bands in the ligand for (O-H)
appeared at 3396 cm−1. Correspondingly, stretching bands observed at 1631 cm−1 for (C=N) in
the ligand. The formation of Schiff base Zn(II)-complex is evidenced by the disappearance
of (O-H) stretching band in salen ligand. Certain evidence of the bonding is also shown by the
appearance of the new band in the region (532 cm−1 and 416 cm−1) in the spectrum of the
complex due to (Zn-O) and (Zn-N) stretching vibrations. For the Schiff base, the (C=N-)
stretching of salen ligand is observed as a strong band at 1631 cm-1 is slightly shifted to the lower
14

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region at 1622 cm-1 which revealed that azomethine nitrogen is involved in coordination to the
metal [54, 55].
532
1622
416
3396
PS-Zn-salen
PS-salen ligand
1631
4000 3500 3000 2500 2000 1500 1000
-1
500
Wavenumber (cm
)
Fig. 1. The FT-IR spectrum of PS-salen ligand (1e) and PS-Zn-salne complex (1f).
3.1.2. Scanning electron microscopy (SEM) images and Energy Dispersive X-Ray Analysis
(EDX)
The surface morphology and elemental analysis of polymer supported Zn-salen complex
was characterized by SEM/EDX (Fig. 2). The scanning electron micrograph (SEM) images were
reported for PS-salen ligand (A) and PS-Zn-salen complex (C) which clearly indicate the
formation of the desired complex. The surface of PS-Zn-salen complex expose micron sized
particles deposited over the surface (Fig. 2C). The comparative spectrum of energy-dispersive X-
ray spectroscopy, ligand (B) and a metal complex (D) support the presence of Zn in the metal
complex (D).
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Eleme Weight
A
C
B
nt
C K
%
78.32
8.24
6.94
6.50
100
N K
O K
Cl K
Totals
Eleme Weight
D
nt
C K
%
65.82
7.76
7.39
12.74
6.29
100
N K
O K
Cl K
Zn K
Totals
Fig. 2. SEM/EDX of PS-salen ligand (A/B) and PS-Zn-salen complex (1f) (C/D).
3.1.3. Thermal analysis
Thermal stability of the PS-Zn-salen complex (1f) was scrutinized using TGA under a
nitrogen atmosphere with a heating rate of 10 °C min-1 over a temperature range of 30-800 °C
(Fig. 3). There is slight weight loss (4.250%) below 120 °C indicates the presence of a physically
adsorbed solvent. The weight loss (36.48%) detected in the region 280-550 °C due to a salen
group of the complex begins to decompose. Above 550 °C the complex exhibits final weight loss
(13.86%) begins to crumble due to poly 1-(4-sec-butyl)benzyl)-1H-benzo[d]imidazole in the
polymer supported Zn-salen complex. From TGA analysis it is observed that the weight loss is
°
negligible up to 280 C and found to be thermally stable, therefore it is concluded that PS-Zn-
salen (1f) catalyst is appropriate for high-temperature reaction.
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110
100
90
(1f)
4.250 %
36.48 %
0.08228 mg
0.7062 mg
80
70
13.86 %
0.2683 mg
60
Residue:
50
45.44 %
0.8798 mg
40
0
100 200 300 400 500 600 700 800
Temperature (C)
Fig. 3. Thermogravimetric Analysis (TGA) thermogram of PS-Zn-salen complex (1f)
3.2. Catalytic activity of PS-Zn-salen complexes (1f), (1g), (1h), (1i), (1j).
The catalytic activity of synthesized polymer supported Zn-salen complexes was tested
for one pot oxidative esterification reaction. Typically, the model reaction was performed by
3,4,5-trimethoxy benzaldehyde with methanol using H2O2 (30 %) as an oxidant in the presence
of catalyst 1f for 24h. The results of optimization experiments are summarized in Table 1. In the
absence of a catalyst under similar conditions, 50% conversion was observed (Table 1, entry
1). Also, only 56% conversion was found when polymer supported salen ligand 1e was used as a
catalyst under similar reaction conditions (Table 1, entry 2).
In order to optimize dose, varying the amount of catalyst, 1f was demonstrated by model
reaction at same reaction conditions (Table 1, entries 3-5). The reaction of 3,4,5-trimethoxy
benzaldehyde with H2O2 (30 %) as an oxidant in the presence of 10 mg catalyst 1f was found to
be the optimum for this organic transformation (Table 1, entry 4). Further increase of the amount
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of catalyst, there is no effect on the conversion of 3,4,5-trimethoxy benzaldehyde (Table 1,
entries 5, 6).
Next, the effect of temperature on the esterification reaction was studied. We
have performed the reactions by changing the temperature from room temperature to 80 °C
(Table 1, entries 7-12). The reaction of 3,4,5-trimethoxy benzaldehyde in the presence of an
optimum amount of catalyst was very slow at room temperature (Table 1, entry 7); while
70 °C was found to be the optimum temperature for this model reaction (Table 1, entry 11).
Further increase of temperature to 80 °C, there is slight decrease in the yield of the ester under
similar reaction conditions (Table 1, entry 12).
Moreover, to find the effect of time on the esterification was proved by changing the
duration of the reactions (Table 1, entries 13-17). The reaction was found to be very slow in 8 h
and afforded poor yield (Table 1, entry 13). While in longer reaction time 24 h was found to be
the optimum time for 100% conversion of the desired product (Table 1, entry 17). Finally, with
optimum reaction conditions (10 mg cat, 70 °C, and 24h), we tested the comparative catalytic
activity of the other complexes 1g, 1h, 1i and 1j (Table 1, entries 18-21). The result was
displayed admirable activity to get desired product 98 and 94% yield, using 1g and 1h
respectively (Table 1, entries 18 and 19). Also, the complexes 1i and 1j also showed satisfactory
catalytic activity with 80 and 81% yield. The comparative result revealed that 1f is the best
choice of catalyst.
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Table 1. Optimization of the amount of catalyst, temperature and time in one pot oxidative
esterification reaction.
Entry
Catalyst
Catalyst
amount
(mg)
Solvent
Temperature
(°C)
Time
(hour)
Yield(%)b
1
2
00
10
5
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
70
70
70
70
70
70
RT
40
50
60
70
80
70
70
24
24
24
24
24
24
24
24
24
24
24
24
8
50
56
1f
1ea
1f
1f
1f
1f
1f
1f
1f
1f
1f
1f
1f
1f
3
85
4
10
15
20
10
10
10
10
10
10
10
10
100
100
98
5
6
7
58
8
62
9
67
10
11
12
13
14
87
100
98
84
12
90
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15
16
17
18
19
20
21
10
10
10
10
10
10
10
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
70
70
70
70
70
70
70
16
20
24
24
24
24
24
94
96
1f
1f
1f
1g
1h
1i
100
98
94
80
81
1j
a1e = PS-salen ligand.
b Yield = GC-MS conversion.
3.3. Effect of different aldehyde and alcohol substrates.
With the above-optimized conditions in hand, the possibility of the oxidative
esterification of the catalyst 1f was tested with varying aromatic aldehydes and alcohols
respectively. The results were listed in Table 2. Overall, the reaction was benefited by electron
donating as well as electron withdrawing groups attached to the aromatic ring. All the substrates
were easily oxidized to give the desired product with good to excellent yields. The aromatic
aldehyde without any substituent displayed excellent reactivity with both methanol and ethanol
to get methyl benzoate or ethyl benzoate with 92 and 83% conversion respectively (Table 2,
entries 3, 13). The aromatic aldehyde with electron donating substituents like OMe, Me groups
reacted efficiently with both methanol and ethanol, under the optimized conditions to afford
good to excellent yields (Table 2, entries 1-2, 7, 11-12 and 17). Aromatic aldehydes bearing
electron withdrawing groups at a para position like Cl, Br were found to be more reactive with
methanol as well as ethanol to get desired products with appreciable yields (Table 2, entries 4, 10
and 14, 20). In the case of 4-Fluorobenzaldehyde was somewhat reactive with methanol (Table
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2, entry 6) but with ethanol showed commendable yield (Table 2, entry 16). In contrast,
4-hydroxybenzaldehyde with methanol exhibited satisfactory yield of 83% but less reactive with
ethanol to offer 29% yield (Table 2, entries 9, 19). When aromatic aldehyde with good electron
withdrawing group like 2-nitrobenzaldehyde undergo esterification with both methanol and
ethanol, resulted in good yields of 75% and 66% respectively (Table 2, entries 5 and 15). In the
case of 3-hydroxy-4-methoxybenzaldehyde reaction undergoes smoothly with methanol (Table
2, entry 8) as well as ethanol (Table 2, entry 8) to get preferred product 89 and 83%. Moreover,
the reactivity of 3,4,5-trimethoxybenzaldehyde was tested with aliphatic allyl alcohol and
aromatic benzyl alcohol. The result revealed that benzyl alcohol with aromatic aldehyde gave
less yield 15% (Table 2, entry 22) compared to allyl alcohol 56% under optimized conditions
(Table 2, entry 21). Therefore, it was found that oxidative esterification by PS-Zn-salen complex
is better than or comparable with the literature reports (Table 3).
Table 2. Catalytic performance in one pot oxidative esterification reactions with various
aromatic aldehydes with alcoholsa.
Entry
1
Aldehyde
Alcohol
Product
Yield (%)a
100
Methanol
2a
2b
2
Methanol
70
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3
4
Methanol
92
94
2c
Methanol
2d
5
6
Methanol
75
47
2e
Methanol
2f
7
8
Methanol
91
89
2g
Methanol
2h
9
Methanol
67
2i
10
Methanol
100
2j
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11
12
Ethanol
83
72
2k
Ethanol
2l
13
14
Ethanol
83
90
2m
Ethanol
2n
15
16
Ethanol
66
84
2o
Ethanol
2p
17
18
Ethanol
86
83
2q
Ethanol
2r
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19
20
21
Ethanol
29
79
56
2s
Ethanol
2t
Allyl alcohol
2u
22
Benzyl alcohol
15
2v
aYield = GC-MS convention.
3.4. Control experiments and mechanistic investigation
Mechanically, it seems plausible that benzaldehyde is oxidized with PS-Zn-salen
complex-H2O2 to the corresponding acid, which is esterified immediately with alcohol. However,
opposing to this, it has been demonstrated in many cases that there is the formation of acetal as
an intermediate [10]. To clarify, a set of the experiment were carried out (A) 3,4,5-
trimethoxybenzaldehyde and H2O2 as an oxidant used without methanol and set (B) with
methanol. Both sets of reaction were carried out at optimum reaction condition for 12h and the
reaction mixture collected periodically and analyzed by GC-MS, results are given in supporting
information (Fig. S16). In set (A) there is the detection of 3,4,5-trimethoxybenzoic acid (GC-MS
Fig. S16A). But, in the set (B) we found the formation of methyl 3 4 5-trimethoxybenzoate along
with 3,4,5-trimethoxybenzoic acid (GC-MS Fig. S16B). The study confirmed that the oxidative
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esterification of benzaldehyde in an alcoholic medium does not proceed through an acetal
intermediate as reported [10] but through insitu formation of benzoic acid [29].
Generally, the oxidation reactions with transition metal atoms, that H2O2 first binds to the
metal center (I) and then transfers an oxygen atom to the substrate. Therefore, the activation of
metal center results via the generation of the active species which may be bridging peroxo
species (II). In the current case, it may be feasible that the beginning of the reaction PS-Zn-salen
reacts with H2O2 to produce an active metal-peroxo intermediate which may be the active
intermediate for the oxidative esterification of aldehydes to ester. This mechanism has been
confirmed by Singh et al. for oxidative esterification of aldehydes using CsPW11Ni catalyst via
metal-peroxo intermediate [29]. In the present case also similar mechanism can be expected
(Scheme 3).
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Scheme 3. Probable mechanism of oxidative esterification of aldehydes catalyzed by
PS-Zn-salen complex (1f).
Table 3. Comparison of PS-Zn-salen complex (1f) with other catalysts in one pot oxidative
esterification of aldehydes to corresponding esters.
Entry
Catalyst
Catalyst
dosage
Time
(h)
Temp
(°C)
Highest
Yield
(%)b
Ref.
NHC-Fe
1.
2.
3.
4.
5.
20 mol%
0.5 mL
5 mol%
8 mol%
100 mg
24
24
6
90
25
60
80
80
97a
90a
93
[21]
[24]
[28]
[39]
[44]
BmimBF4
Co3O4/rGO nanocomposite
Imidazolium perrhenate
24
6
89
Phosphotungstates anchor to
MCM-41 and ZrO2
98
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PS-Zn-salen complex (1f)
6.
10 mg
24
70
100
Present
work
a = Isolated yield.
b = Yields determined by GC-MS.
3.4. Recycling and regeneration of the catalyst
In the present case, PS-Zn-salen complex behaves as a heterogeneous catalyst. After
completion of the reaction, the resulting mixture was cool to room temperature, and the insoluble
catalyst was separated by simple filtration. The separated catalyst was washed with ethanol,
dried in hot air oven at 80 °C for 2h and used for the next run. The organic products were
extracted with ethyl acetate and the conversion determines by GC-MS. The catalytic activity of
the regenerated catalyst was estimated under the optimized conditions.
It was observed that PS-Zn-salen complex showed appreciable efficiency with 98% and
94% conversion of ester after 1st and 2nd cycle respectively (Fig. 4). However the conversion of
ester has dropped to 74% up to 5th cycle compare to fresh catalyst. The average conversion for
ester was found to be 87.4%. The stability of PS-Zn-salen complex was confirmed by using FT-
IR and showed that the structure of the catalyst stayed intact during the esterification even after
5th recycle (Fig. 5).
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% conversion
98
100
94
89
82
80
74
60
40
20
0
1
2
3
4
5
Recycle time
Fig. 4. Recyclability of PS-Zn-salen catalyst (1f) recovers up to 5th run.
(B)
(A)
PS-Zn-salen after use
PS-Zn-salen before use
4000 3500 3000 2500 2000 1500 1000
500
-1
Wavenumber (cm )
Fig. 5. FT-IR spectra of the PS-Zn-salen (1f) (A) before reaction (B) after the reaction.
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3.
Conclusion
In conclusion, we have successfully synthesized a new polymer supported Zn-salen
complexes. The organic metal complex was clearly confirmed by FT-IR, EDX and SEM. The
heterogeneous catalyst showed high thermal stability up to 280 °C and exhibited excellent
catalytic activity in the one pot oxidative esterification reaction of aldehydes. The mechanism of
oxidative esterification of aldehyde proceeded through acid formation which was confirmed
GC-MS. The catalyst was reused up to 5th successive run with the average conversion of ester
87.4%. The stability of the recycle catalyst was further confirmed by FT-IR analysis, which
revealed that the PS-Zn-salen complex catalyzes esterification reaction without deterioration of
the ordered structure. Also, the effect of counter anions of complex also studied which showed
that there is no noticeable influence on the conversion of product. The high conversion of ester
(up to 100%), with used of low catalyst amount, easy work up and environmentally benign
protocol over the other reported procedure. Other studies of its applicability in synthetic
renovations are currently on-going in our laboratory.
Acknowledgements
The authors express their deep gratitude to RGEMS for financial support and SIF DST-
VIT-FIST for NMR, GC-MS, FT-IR and SEM/EDX VIT University, Vellore for providing
essential facility to carry this article to this level. We are also thankful to the Dean, School of
Advanced Sciences, special thanks to Mr. Murphy M and all the Scholars of our Research
Laboratory, “Smart Materials Laboratory for Bio-sensing & Catalysis” VIT University Vellore,
who have encouraged the authors during this period.
29