Aminopropylated PEG as a novel, eco-friendly and biodegradable basic catalyst for bis-Michael addition to α,β-unsaturated ketones under solvent-free conditions

Article abstract of DOI:10.1002/aoc.3171

A solvent-free and highly efficient protocol has been developed for the synthesis of novel bis-Michael addition products (3a, 3b, 3c, 3d, 3e, 3f, 3g, 3h, 3i, 3j, 3k, 3l, 3m, 3n, 3o) using aminopropylated PEG-6000 (NH ₂-PEG) as a biodegradable a

Full text of DOI:10.1002/aoc.3171

Full Paper
Received: 3 April 2014
Revised: 2 May 2014
Accepted: 5 May 2014
Published online in Wiley Online Library: 24 June 2014
(wileyonlinelibrary.com) DOI 10.1002/aoc.3171
Aminopropylated PEG as a novel, eco-friendly
and biodegradable basic catalyst for bis-Michael
addition to α,β-unsaturated ketones under
solvent-free conditions
Tabassum Khan and Zeba N. Siddiqui*
A solvent-free and highly efficient protocol has been developed for the synthesis of novel bis-Michael addition products (3a–o)
using aminopropylated PEG-6000 (NH2-PEG) as a biodegradable and recyclable catalyst in excellent yields under solvent-free
conditions. Other remarkable features of this environmentally benign protocol are shorter reaction time, tolerance of a wide
range of C―H-activated acids, high yield of products, and simple experimental and work-up procedure as compared to
conventional methods. The NH2-PEG catalyst is characterized by using FT-IR, powder XRD and scanning electron microscopy–
energy dispersion X-ray spectrometric analyses. The catalyst can be recycled several times without significant loss of its catalytic
activity. Copyright © 2014 John Wiley & Sons, Ltd.
Keywords: NH2-PEG; bis-Michael addition products; solvent-free conditions
To improve the efficiency of a catalytic process, an intense
research activity has recently been devoted towards the develop-
Introduction
The Michael addition reaction is a versatile method for the forma-
tion of carbon–carbon bonds in organic chemistry. Traditionally,
the reaction is catalyzed by strong bases[1] and Lewis acids,[2]
which often leads to undesirable side products and unsatisfac-
tory product yield. However, now a considerable improvement
in terms of yield and time has been observed in the Michael
addition reaction. In this context, a number of reagents have
been developed which include silica nanoparticles,[3] basic ionic
liquid,[4] tributylphosphine,[5] poly(N-vinylimidazole),[6] st-DNA,[7]
tetraethylammonium superoxide,[8] KF/CP,[9] [C4dabco]OH ionic
liquid,[10] Rasta resin,[11] immobilized lipase on Fe3O4/ZnO core/
shell magnetic nanoparticles,[12] polyvinyl sulfonic acid,[13] silica
sulfuric acid,[14] KG-60-NEt2,[15] KF/NP,[16] NaHSO4.SiO2,[17] 1-Gd
(OTf)3,[18] Kf/basic alumina,[19] Zn-HAP[20] and CSC-Star-
SO3AlCl2.[21] Moreover, most of the reactions are referred to
mono-Michael addition reaction, and very few reagents have
been reported for the bis-Michael addition reaction to date.[3,5,22]
Thus effective and high-yielding processes are still in demand for
the development of a milder reagent for a general bis-Michael
addition of a variety of C―H-activated acids to α, β-unsaturated
ketones in a single step.
Catalytic applications of functional polymers in organic synthe-
sis have attracted much attention from researchers all over the
world.[23] To simplify the isolation of products and catalyst
recycling, immobilization of catalyst over an inert solid support
proved to be an efficient approach, leading towards green syn-
thesis.[23] Among the wide range of polymeric matrices employed
in solid-phase chemistry, polyethylene glycols have emerged as
convenient supports for the synthesis of a variety of small organic
molecules, ligands and catalysts.[24] Moreover, they are readily
functionalized, environmentally benign and inexpensive.
ment of solid supported catalysts involving easy catalyst recovery
and recycling.[25] Herein, we have synthesized aminopropylated
PEG-6000 (NH2-PEG) as a polyamine basic catalyst which is novel,
easily preparable, mild, biodegradable, recyclable and non-corro-
sive. Therefore, based on the above findings and in continuation
of our interest in the synthesis of novel catalysts and development
of efficient, economical methodologies,[26] we now wish to explore
the catalytic activity of NH2-PEG in bis-Michael addition of a variety
of C―H-activated acids to bis-α,β-unsaturated ketones under
solvent-free conditions (Scheme 2). The catalyst was recyclable
up to several runs. The structure and morphology of the catalyst
was established with the help of FT-IR, powder X-ray diffraction
(XRD), scanning electron microscopy (SEM) and energy dispersion
X-ray spectrometry (EDX).
Experimental
General Information
Melting points of all synthesized compounds were taken in a
Riechert Thermover instrument and are uncorrected. The IR spec-
tra (KBr) were recorded on PerkinElmer RXI spectrometer. 1H NMR
and 13C NMR spectra were recorded on a Bruker DRX-300 and
Bruker Avance II 400 spectrometer using tetramethylsilane
(TMS) as an internal standard and DMSO-d6/CDCl3 as solvent.
*
Correspondence to: Zeba N. Siddiqui, Department of Chemistry, Aligarh Muslim
University, Aligarh, 202002, India. E-mail: siddiqui_zeba@yahoo.co.in
Department of Chemistry, Aligarh Muslim University, Aligarh202002, India
Appl. Organometal. Chem. 2014, 28, 620–630
Copyright © 2014 John Wiley & Sons, Ltd.

Novel synthesis of bis-Michael addition products catalysed by NH2-PEG
(4× CH3), 44.8 (2× CH3′), 50.86 (2× CH2), 52.69 (2× CH3″), 99.69
(2× C3), 102.40 (2× C5), 126.21, 137.49 (Ar―C), 161.29 (2× C6),
169.25 (2× C2), 182.95 (2× C4), 191.51 (2× C1′), 192.57 (2 x C4″),
192.76 (2× C2″). ESI-MS: (m/z) 634.2 (M++1). Anal. Calcd for
C33H34O12: C, 63.65; H, 5.50; found: C, 63.68; H, 5.52.
Scheme 1. Schematic illustration of preparation of the catalyst (NH2-PEG).
3,3′-(1,3-Phenylene)bis(4-acetyl-1-(4-hydroxy-6-methyl-2-oxo-2H-pyran-3-yl)
hexane-1,5-dione) (3b)
Electrospray ionization (ESI) mass spectra were recorded on a
Thermo Finnigan LCQ Advantage max ion trap mass spectrome-
ter having an ESI source. Elemental analyses (C, H and N) were
conducted using an Elemental Vario EL III elemental analyzer
and the results were found to be in agreement with the calcu-
lated values. Chemicals were of commercial grade and used
without further purification. Homogeneity of the compounds
was checked by thin-layer chromatography (TLC) on glass plates
coated with silica gel G254 (Merck) using a chloroform–methanol
(3:1) mixture as mobile phase and visualized using iodine vapors.
X-ray diffractograms of the catalyst were recorded in the 2θ
range of 10–70° with a scan rate of 4° minÀ1 on a Rigaku Minifax
X-ray diffractometer with Ni-filtered Cu-Kα radiation at a wave-
length of 1.54060° A. SEM-EDX characterization of the catalyst
was performed on a JEOL JSM-6510 scanning electron micro-
scope equipped with energy-dispersive X-ray spectrometer
operating at 20 kV.
White solid; m.p. 121–123°C; IR (KBr) cmÀ1: 1630, 1658, 1696 (CO),
3431 (OH). H NMR (400 MHz, CDCl3): δ = 1.98 (12H, s, 4 × CH3),
2.03 (6H, s, 2 × CH3), 2.98–3.01 (4H, m, 2 × CH2), 3.45–3.50 (2H,
m, 2 × CH3′), 5.21 (2H, s, 2 × H5), 5.71 (2H, d, J = 5.4 Hz, 2 × CH3″),
7.08–7.12 (3H, m, C6H4), 7.24 (1H, s, C6H4). 13C NMR (100 MHz,
CDCl3): δ = 16.17 (2 × CH3), 19.75 (4 × CH3), 43.7 (2 × CH3′), 49.10
1
Preparation of Catalyst
Aminopropylated PEG was prepared by refluxing PEG (10 g) with
3-aminopropyltrimethoxysilane (3.3 mmol) in dry toluene for 18
h. The solid material was then filtered off and washed with hot
toluene and then dried in oven at 110°C overnight to give
surface-bound amine.
Figure 1. FT-IR spectra of (a) NH2-PEG, (b) PEG and (c) APTMS.
General Procedure for the Bis-Michael Addition of Varieties
of C―H-Activated Acids to Bis-α,β-Unsaturated Ketones un-
der Solvent-Free Conditions
Bis-α, β-unsaturated ketones (1a–b) (10 mmol), acyclic and cyclic
C―H-activated acids (2a–m) (20 mmol) and aminopropylated
PEG (NH2-PEG) catalyst (1.00 g) were mixed in a 100 ml
round-bottom flask for the specified time (Table 3) at 70°C. Af-
ter completion of the reaction (monitored by TLC), the reac-
tion mixture was cooled to room temperature and H2O (50
ml) added and shaken for 3 min to dissolve the NH2-PEG.
The crude product (insoluble in water) was filtered and recrys-
tallized from ethyl alcohol (30 ml) to afford the pure product
(3a–o). In order to recover the catalyst, the filtrate was
evaporated under reduced pressure and Et2O (50 ml) added.
The solid, as obtained, was filtered, washed with Et2O (20 ml × 2)
and reused after drying.
Spectral Data of Compounds
3,3′-(1,4-Phenylene)bis(4-acetyl-1-(4-hydroxy-6-methyl-2-oxo-2H-pyran-3-yl)
hexane-1,5-dione) (3a)
Yellow solid; m.p. 125–127°C; IR (KBr) cmÀ1: 1627, 1654, 1686 (CO),
3428 (OH). 1H NMR (400 MHz, CDCl3): δ = 1.86 (12H, s, 4 × CH3), 2.11
(6H, s, 2 × CH3), 2.88–2.90 (4H, m, 2 × CH2), 3.55–3.62 (2H, m,
2 × CH3′), 5.25 (2H, s, 2 × H5), 5.74 (2H, d, J = 5.9 Hz, 2 × CH3″), 7.58
(4H, s, C6H4). 13C NMR (100 MHz, CDCl3): δ = 14.48 (2 × CH3), 20.75
Figure 2. Powder XRD of (a) PEG, (b) fresh NH2-PEG and (c) NH2-PEG
after eight runs.
Appl. Organometal. Chem. 2014, 28, 620–630
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

T. Khan and Z. N. Siddiqui
4 × OCH2), 5.71 (2H, d, J = 5.4 Hz, 2 × CH3″),
6.01 (2H, s, 2 × H5), 7.58 (1H, s, C6H4). 13C NMR
(100 MHz, CDCl3): δ = 13.59 (4 × CH3), 19.10
(2 × CH3), 42.10 (2 × CH3′), 48.21 (2 × CH2),
57.12 (2 × CH4″), 61.30 (4 × OCH2), 99.19
(2 × C3), 101.01 (2 × C5), 128.80, 132.60
(Ar―C), 162.30 (2 × C6), 167.22 (2 × C2),
169.90 (4 × CO), 180.01 (2 × C4), 189.31
(2 × C1′). ESI-MS: (m/z) 754.2 (M++1). Anal.
Calcd for C38H42O16: C, 60.47; H, 5.60; found:
C, 60.50; H, 5.57.
Diethyl-3,3′-(1,4-phenylene)bis(2-acetyl-5-(4-hydroxy-
6-methyl-2-oxo-2H-pyran-3-yl)-5-oxopentanoate) (3e)
White solid; m.p. 205–207°C; IR (KBr) cmÀ1
:
1629, 1657, 1699 (CO), 1711 (COO), 3413 (OH).
1H NMR (400 MHz, CDCl3): 1.27–1.30 (6H, m,
Figure 3. EDX analysis of the catalyst (NH2-PEG).
2 × CH3), 2311 (6H, s, 2 × CH3), 2.49 (6H, s, 2 × CH3), 3.09–3.11 (4H,
m, 2 × CH2), 3.85–3.89 (2H, m, 2 × CH3′), 4.19–4.21 (4H, m,
2 × OCH2), 5.80 (2H, d, J = 5.2 Hz, 2 × CH3″), 6.18 (2H, s, 2 × H5),
7.11 (1H, s, C6H4). 13C NMR (100 MHz, CDCl3): δ = 15.03 (2 × CH3),
19.90 (2× CH3), 21.41 (2× CH3), 44.11 (2× CH3′), 53.21 (2× CH2),
54.18 (2× CH3″), 61.32 (2× OCH2), 98.23 (2× C3), 101.25 (2× C5),
129.23, 135.01 (Ar―C), 161.20 (2× C6), 166.32 (2× C2), 171.09
(2× COO), 185.21 (2× C4), 191.21 (2× C1′), 194.11 (2× C2″). ESI-MS:
(m/z) 694.1 (M++1). Anal. Calcd for C36H38O14: C, 62.24; H, 5.51;
found: C, 62.27; H, 5.48.
Diethyl-3,3′-(1,4-phenylene)bis(2-cyano-5-(4-hydroxy-6-methyl-2-oxo-2H-
pyran-3-yl)-5-oxopentanoate) (3f)
Green solid; m.p. 257–259°C; IR (KBr) cmÀ1: 1630, 1659 (CO), 1713
(COO), 2214 (CN), 3476 (OH). 1H NMR (400 MHz, DMSO-d6):
δ = 1.29–1.32 (6H, m, 2 × CH3), 2.51 (6H, s, 2 × CH3), 3.34–3.36
(4H, m, 2 × CH2), 3.62–3.65 (2H, m, 2 × CH3′), 4.21–4.25 (4H, m,
2 × OCH2), 5.67 (2H, d, J = 5.0 Hz, 2 × CH2″), 6.10 (2H, s, 2 × H5),
7.22 (1H, s, C6H4). 13C NMR (100 MHz, DMSO-d6): δ = 14.10
(2 × CH3), 20.90 (2 × CH3), 45.01 (2 × CH3′), 51.13 (2 × CH2), 58.11
(2 × CH2″), 60.80 (2 × OCH2), 101.31 (2 × C3), 105.10 (2 × C5),
116.21 (2 × CN), 126.80, 134.10 (Ar―C), 165.10 (2 × C6), 166.21
(2 × C2), 168.50 (2 × COO), 183.11 (2 × C4), 198.11 (2 × C1′). ESI-
MS: (m/z) 658.2 (M++1). Anal. Calcd for C35H34N2O11: C, 63.82; H,
5.20; N, 4.25; found: C, 63.80; H, 5.17; N, 4.22.
Figure 4. SEM images of fresh catalyst (NH2-PEG) at different magnifications.
(2 × CH2), 53.10 (2 × CH3″), 99.71 (2 × C3), 101.81 (2 × C5), 123.30,
128.1, 135.72, 139.34 (Ar―C), 162.01 (2 × C6), 168.11 (2 × C2),
180.21 (2 × C4), 190.11 (2 × C1′), 191.31 (2 × C4″), 191.52 (2 × C2″).
ESI-MS: (m/z) 634.1 (M++1). Anal. Calcd for C33H34O12: C, 63.65;
H, 5.50; found: C, 63.62; H, 5.47.
2,2′-(1,4-Phenylene)bis(3-(4-hydroxy-6-methyl-2-oxo-2H-pyran-3-yl)-3-oxopr-
opane-1,1-diyl)dimalononitrile (3c)
Dimethyl-3,3′-(1,4-phenylene)bis(2-acetyl-5-(4-hydroxy-6-methyl-2-oxo-2H-
pyran-3-yl)-5-oxopentanoate) (3g)
Shining red solid; m.p. 131–133°C; IR (KBr) cmÀ1: 1642, 1722 (CO),
2214 (CN), 3416 (OH). H NMR (400 MHz, CDCl3): δ = 2.04 (6H, s,
White solid; m.p. 245°C; IR (KBr) cmÀ1: 1627, 1659, 1691 (CO),
1714 (COO), 3484 (OH). H NMR (400 MHz, CDCl3): δ = 2.01 (6H,
1
1
2 × CH3), 2.93-2.95 (4H, m, 2 × CH2), 3.71–3.74 (2H, m, 2 × CH3′),
5.31 (2H, s, 2 × H5), 5.71 (2H, d, J = 5.0 Hz, 2 × CH2″), 7.56 (1H, s,
C6H4). 13C NMR (100 MHz, CDCl3): δ = 20.90 (2 × CH3), 40.11
(2 × CH3′), 49.40 (2 × CH2), 53.30 (2 × CH2″), 99.70 (2 × C3), 101.51
(2 × C5), 113.01 (2 × CN), 125.80, 138.61 (Ar―C), 161.20 (2 × C6),
168.21 (2 × C2), 181.20 (2 × C4), 190.20 (2 × C1′). ESI-MS: (m/z)
566.1 (M++1). Anal. Calcd for C30H22N4O8: C, 63.60; H, 3.91; N,
9.89; found: C, 63.57; H, 3.89; N, 9.86.
s, 2 × CH3), 2.21 (6H, s, 2 × CH3), 3.12–3.15 (4H, m, 2 × CH2), 3.31
(6H, s, 2 × OCH3), 3.46–3.48 (2H, m, 2 × CH3′), 5.71 (2H, d, J = 5.3
Hz, 2 × CH3″), 5.98 (2H, s, 2 × H5), 7.28 (4H, s, C6H4). 13C NMR
(100 MHz, CDCl3): δ = 19.10 (2 × CH3), 22.02 (2 × CH3), 43.27
(2 × CH3′), 47.01 (2 × CH2), 50.19 (2 × OCH3), 61.29 (2 × CH3″),
100.21 (2 × C3), 103.15 (2 × C5), 125.08, 131.11 (Ar―C), 162.01
(2 × C6), 166.23 (2 × C2), 171.01 (2 × C4″), 183.20 (2 × C4), 193.12
(2 × C1′), 198.11 (2 × C2″). ESI-MS: (m/z) 666.1 (M++1). Anal. Calcd
for C34H34O14: C, 61.25; H, 5.14; found: C, 61.21; H, 5.11.
Tetraethyl-2,2′-(1,4-phenylene)bis(3-(4-hydroxy-6-methyl-2-oxo-2H-pyran-3-yl)-
3-oxopropane-1,1-diyl)dimalonate (3d)
3,3′-(1,4-Phenylene)bis(5-(4-hydroxy-6-methyl-2-oxo-2H-pyran-3-yl)-5-oxo-2-
phenylpentanoic acid) (3h)
Dark-red crystals; m.p. > 280°C; IR (KBr) cmÀ1: 1623, 1654 (CO),
1710 (COO), 3374 (OH).1H NMR (400 MHz, CDCl3): δ = 1.27–1.29
(12H, m, 4 × CH3), 2.11 (6H, s, 2 × CH3), 2.86–2.90 (4H, m,
2 × CH2), 3.10–3.15 (2H, m, 2 × CH3′), 4.11–4.15 (8H, m,
Reddish-brown solid; m.p. > 300°C; IR (KBr) cmÀ1: 1654, 1698 (CO),
1
1714 (COO), 3456 (OH). H NMR (400 MHz, CDCl3): δ = 1.91 (6H, s,
2 × CH3), 3.11–3.14 (4H, m, 2 × CH2), 3.70–3.72 (2H, m, 2× CH3′),
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Appl. Organometal. Chem. 2014, 28, 620–630

Novel synthesis of bis-Michael addition products catalysed by NH2-PEG
2,2′-(1,4-Phenylenebis(3-(4-hydroxy-6-methyl-2-oxo-2H-pyran-3-yl)-3-oxop-
ropane-1,1-diyl))bis(5,5-dimethylcyclohexane-1,3-dione)) (3k)
5.74 (2H, d, J= 5.6 Hz, 2 × CH3″), 6.94 (2H, s, 2 × H5), 7.13–7.94 (14H,
m, C6H4, Ar―H), 10.01 (2H, s, 2 × COOH). 13C NMR (100 MHz, CDCl3):
δ = 22.10 (2 × CH3), 43.12 (2 × CH3′), 53.61 (2 × CH2), 59.16 (2 × CH2″),
100.21 (2 × C3), 103.15 (2 × C5), 125.80, 128.19, 135.11, 139.31,
140.61 (Ar―C), 164.13 (2 × C6), 170.23 (2 × C2), 179.30 (2 × C1″),
181.21 (2 × C4), 198.11 (2 × C1′). ESI-MS: (m/z) 706.2 (M++1). Anal.
Calcd for C40H34O12: C, 67.98; H, 4.84; found: C, 67.96; H, 4.86.
White powder; m.p. > 300°C; IR (KBr) cmÀ1: 1625, 1658, 1698 (CO),
3377 (OH). 1H NMR (400 MHz, DMSO-d6): 1.01 (12H, s, 4 × CH3), 2.09
(6H, s, 2 × CH3), 2.62–2.65 (8H, m, 4 × CH2), 3.03–3.06 (4H, m,
2 × CH2), 3.35–3.38 (2H, m, 2 × CH3′), 5.42 (2H, d, J = 5.9 Hz,
2 × CH2″), 6.01 (2H, s, 2 × H5), 7.31 (4H, s, C6H4). 13C NMR (100
MHz, DMSO-d6): δ = 22.19 (4× CH3), 25.16 (2× CH3), 30.40
(2× C5″), 40.11 (2× CH3′), 46.61 (2× CH2), 52.11 (2× C4″, C6″),
71.11 (2× CH2″), 99.71 (2× C3), 101.15 (2× C5), 131.11, 138.25
(Ar―C), 164.23 (2× C6), 169.10 (2× C2), 182.01 (2× C4), 193.15
(2× C1′), 197.31 (2× C1″, C3″). ESI-MS: (m/z) 714.1 (M++1). Anal.
Calcd for C40H42O12: C, 67.21; H, 5.92; found: C, 67.17; H, 5.94.
5,5′-(1,4-Phenylenebis(3-(4-hydroxy-6-methyl-2-oxo-2H-pyran-3-yl)-3-
oxopropane-1,1-diyl))bis(1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione) (3i)
Light-yellow solid; m.p. > 300°C; IR (KBr) cmÀ1: 1658, 1682, 1700
(CO), 3416 (OH). 1H NMR (400 MHz, DMSO-d6): δ = 2.51 (6H, s,
2 × CH3), 3.02 (12H, s, 4 × CH3), 3.21–3.23 (4H, m, 2 × CH2), 3.40–
3.44 (2H, m, 2 × CH3′), 5.72 (2H, d, J = 5.8 Hz, 2 × CH5″), 6.12 (2H,
s, 2 × H5), 7.23 (4H, s, C6H4). 13C NMR (100 MHz, DMSO-d6):
δ = 23.01 (2 × CH3), 27.41 (4 × CH3), 37.13 (2 × CH3′), 45.16
(2 × CH2), 50.21 (2 × CH5″), 98.91 (2 × C3), 100.01 (2 × C5), 129.11,
132.71, (Ar―C), 153.17 (2 × C2″), 163.11 (2 × C6), 165.21 (2 × C2),
170.13 (2 × C4″, C6″), 183.14 (2 × C4), 197.01 (2 × C1′). ESI-MS: (m/
z) 746.2 (M++1). Anal. Calcd for C36H34N4O14: C, 57.90; H, 4.59;
N, 7.50; found: C, 57.87; H, 4.57; N, 7.47.
2,2′-(1,4-Phenylenebis(3-(4-hydroxy-6-methyl-2-oxo-2H-pyran-3-yl)-3-oxopr-
opane-1,1-diyl))bis(1H-indene-1,3(2H)-dione) (3l)
Red powder; m.p. 256°C; IR (KBr) cmÀ1: 1658, 1702 (CO), 3420 (OH).
1H NMR (400 MHz, DMSO-d6): δ = 2.11 (6H, s, 2 × CH3), 3.28–3.31
(4H, m, 2 × CH2), 4.06–4.08 (2H, m, 2 × CH3′), 5.87 (2H, d, J = 5.4
Hz, 2 × CH2″), 5.99 (2H, s, 2 × H5), 7.27 (4H, s, C6H4), 7.98–8.05 (8H,
m, Ar―H). 13C NMR (100 MHz, DMSO-d6): δ = 21.21 (2× CH3),
36.11 (2× CH3′), 47.16 (2× CH2), 70.21 (2× CH2″), 99.11 (2× C3),
105.05 (2× C5), 124.71, 127.93, 129.01, 133.14, 135.11 (Ar―C),
161.01 (2× C6), 162.10 (2× C2), 181.21 (2× C4), 199.31 (2× C1″,
C3″), 200.11 (2× C1′). ESI-MS: (m/z) 726.4 (M++1). Anal. calcd for
C42H30O12: C, 67.98; H, 4.84; found: C, 67.95; H, 4.82.
5,5′-(1,3-Phenylenebis(3-(4-hydroxy-6-methyl-2-oxo-2H-pyran-3-yl)-3-oxopro-
pane-1,1-diyl))bis(1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione) (3j)
Yellow solid; m.p. > 300°C; IR (KBr) cmÀ1: 1659, 1684, 1703 (CO), 3426
1
(OH). H NMR (400 MHz, DMSO-d6): δ =2.02 (6H, s, 2×CH3), 2.91
(12H, s, 4×CH3), 3.33–3.36 (4H, m, 2 × CH2), 3.44–3.48 (2H, m,
2×CH3′), 5.73 (2H, d, J= 5.6 Hz, 2 × CH5″), 6.09 (2H, s, 2 × H5), 7.21–
7.48 (3H, m, C6H4), 7.61 (1H, s, C6H4). 13C NMR (100 MHz, DMSO-d6):
δ =21.23 (2×CH3), 29.40 (4 × CH3), 38.32 (2 × CH3′), 48.16 (2 × CH2),
50.23 (2 × CH5″), 98.11 (2 × C3), 101.51 (2 × C5), 123.31, 128.11,
133.21, 136.31, 139.01 (Ar―C), 150.71 (2 × C2″), 162.01 (2 × C6),
164.25 (2 × C2), 169.31 (2 × C4″, C6″), 181.21 (2 × C4), 195.25
(2 × C1′). ESI-MS: (m/z) 746.1 (M++1). Anal. Calcd for C36H34N4O14:
C, 57.90; H, 4.59; N, 7.50; found: C, 57.92; H, 4.56; N, 7.52.
3,3′-(1,4-Phenylenebis(3-(4-hydroxy-6-methyl-2-oxo-2H-pyran-3-yl)-3-
oxopropane-1,1-diyl))bis(6-methyl-2H-pyran-2,4(3H)-dione) (3m)
Light-yellow powder; m.p. > 300°C; IR (KBr) cmÀ1: 1653, 1685, 1698
(CO), 3429 (OH). 1H NMR (400 MHz, DMSO-d6): δ =1.94 (6H, s,
2×CH3), 2.43 (6H, s, 2 × CH3), 3.03–3.05 (4H, m, 2 × CH2), 3.23–3.26
(2H, m, 2 × CH3′), 5.40 (2H, d, J=5.5 Hz, 2×CH4″), 6.02 (2H, s,
2×H5), 7.27–7.81 (14H, m, C6H4, Ar―H). 13C NMR (100 MHz, DMSO-
d6): δ =15.21 (2×CH3), 19.19 (2 × CH3), 39.01 (2 × CH3′), 44.70
(2 × CH2), 51.01 (2 × CH4″),
100.10 (2 × C3), 103.02
(2 × C5), 125.81, 128.90,
130.16, 135.06, 138.90
(Ar―C), 153.13 (2 × C3″),
159.13 (2 × C6), 161.21
(2 × C5), 171.91 (2 × C5″),
183.01 (2 × C4), 192.21
(2 × C1′). ESI-MS: (m/z)
782.2 (M++1). Anal. Calcd
for C44H38N4O10: C, 67.51;
H, 4.89; N, 7.15; found: C,
67.48; H, 4.85; N, 7.12.
3,3′-(1,4-Phenylenebis(3-(4-hy-
droxy-6-methyl-2-oxo-2H-py-
ran-3-yl)-3-oxopropane-1,1-
diyl))bis(6-methyl-2H-pyran-2,4
(3H)-dione) (3n)
White powder; m.p.
>
:
300°C; IR (KBr) cmÀ1
1595, 1654, 1686 (CO),
3464 (OH). 1H NMR (400
MHz, DMSO-d6): δ = 2.22
(6H, s, 2 × CH3), 2.36 (6H,
s, 2 × CH3), 3.13–3.15 (4H,
Scheme 2. NH2-PEG-catalyzed bis-Michael addition to bis-α,β-unsaturated ketones.
Appl. Organometal. Chem. 2014, 28, 620–630
Copyright © 2014 John Wiley & Sons, Ltd.
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T. Khan and Z. N. Siddiqui
m, 2 × CH2), 3.85–3.89 (2H, m, 2 × CH3′), 5.69 (2H, d, J = 5.6 Hz,
2 × CH3″), 6.09 (2H, s, 2 × H5″), 6.21 (2H, s, 2 × H5), 7.35 (4H, s,
C6H4). 13C NMR (100 MHz, DMSO-d6): δ = 18.01 (2 × CH3), 20.21
(2 × CH3), 40.01 (2 × CH3′), 50.11 (2 × CH2), 63.12 (2 × CH3″), 99.30
(2 × C3), 103.01 (2 × C5), 108.10 (2 × C5″), 128.11, 136.02 (Ar―C),
159.01 (2 × C6), 164.21 (2 × C2), 169.11 (2 × C2″), 170.01 (2 × C6″),
182.12 (2 × C4), 187.23 (2 × C4″), 193.01 (2 × C1′). ESI-MS: (m/z)
686.3 (M++1). Anal. Calcd for C36H30O14: C, 62.97; H, 4.40; found:
C, 62.95; H, 4.37.
164.59 (2 × C2), 167.80 (2 × C2″), 180.65 (2 × C4), 187.17 (2 × C4″),
192.01 (2 × C1′). ESI-MS: (m/z) 758.1 (M++1). Anal. Calcd for
C42H30O14: C, 66.49; H, 3.98; found: C, 66.44; H, 4.94.
4-Acetyl-1,3-diphenylhexane-1,5-dione (3p)
White powder; m.p. 144–146°C (146–147°C); IR (KBr) cmÀ1: 1657, 1676
(CO). 1H NMR (400 MHz, CDCl3): δ =1.80 (3H, s, CH3), 2.18 (3H, s, CH3),
3.10–3.19 (1H, dd), 3.25–3.30 (1H, dd), 4.18–4.26 (1H, m), 4.33–4.35
(1H, d, J= 10.6 Hz), 7.10–7.15 (1H, m), 7.23–7.26 (4H, m), 7.36–7.40
(2H, m), 7.45–7.50 (1H,m), 7.77–7.80 (2H, m). ESI-MS: (m/z) 309.1
(M++1). Anal. Calcd for C20H20O3: C, 77.89; H, 6.53; found: C, 77.86; H,
6.55.
3,3′-(1,4-Phenylenebis(3-(4-hydroxy-6-methyl-2-oxo-2H-pyran-3-yl)-3-oxopr-
opane-1,1-diyl))bis(chroman-2,4-dione) (3o)
Shining white powder; m.p. 265°C; IR (KBr) cmÀ1: 1611, 1658,
Ethyl-(2-acetyl-5-oxo-3,5-diphenyl)pentanoate (3q)
1
1694 (CO), 3476 (OH). H NMR (400 MHz, CDCl3): δ = 2.13 (6H, s,
2 × CH3), 2.51–2.53 (4H, m, 2 × CH2), 4.21–4.25 (2H, m, 2 × CH3′),
5.77 (2H, d, J = 5.4 Hz, 2 × CH3″), 6.32 (2H, s, 2 × H5), 7.15–7.81
(12H, m, C6H4, Ar―H). 13C NMR (100 MHz, CDCl3): δ = 19.33
(2 × CH3), 39.01 (2 × CH3′), 46.51 (2 × CH2), 61.20 (2 × CH3″), 89.11
(2 × C3), 103.18 (2 × C5), 115.31 (2 × C10″), 119.91, 125. 41,
128.11, 133.21, 136.02 (Ar―C), 152.53 (2 × C9″), 160.01 (2 × C6),
White solid; m.p. 115–117°C (lit. 116–118°C). IR (KBr) cmÀ1: 1659, 1670
(CO), 1704 (COO). 1H NMR (400 MHz, CDCl3): δ = 1.01 (t, 3H), 2.05 (s, 3H),
3.35–3.54 (m, 2H), 3.93 (2H), 4.02–4.05 (m, 1H), 4.12–4.17 (m, 1H), 7.14–
7.17 (m, 1H), 7.19–7.23 (m, 4H), 7.37–7.41 (m, 2H), 7.52–7.56 (m, 1H),
7.85–7.92 (m, 2H). ESI-MS: (m/z) 339.0 (M++1). Anal. Calcd for
C
21H22O4: C, 74.53; H, 6.55; found: C, 74.50; H, 6.52.
Table 1. Effect of various solvents versus the solvent-free on the model reactiona
Entry
Solvent
Temperature
Timeb
Yield (%)c
1
Ethanold
Ethanol
Reflux
Reflux
15 h
3.1 h
2.8 h
6 h
Trace
65
2
3
Acetic acid
Methanol
Reflux
59
4
Reflux
53
5
Isopropanol
Water
Reflux
7.4 h
9.9 h
13 h
11 h
15 h
16 h
16 h
24 h
24 h
24 h
4.7 h
4.3 h
4.1 h
7 h
50
6
Reflux
48
7
Dichloroethane
Acetonitrile
Dichloromethane
THF
Reflux
34
8
Reflux
28
9
Reflux
23
10
11
12
13
14
15
16
17
18
19
20
21
22
23
Reflux
22
DMF
Reflux
20
Dioxane
Reflux
—
Toluene
Reflux
—
Hexane
Reflux
—
PEG-200
Reflux
Trace
29
PEG-400
Reflux
PEG-600
Reflux
32
Ethylene glycol
Glycerol
Reflux
30
Reflux
5.3 h
1.4 h
16 min
3 min
3 min
32
Solvent-free (grinding)
Solvent-free
Solvent-free
Solvent-free
Room Temperature
50°C
21
76
70°C
95
80°C
95
aReaction conditions: bis-α, β-unsaturated ketone, (1a, 1.00 mmol), acetyl acetone (2a, 2.00 mmol), catalyst (0.1 g).
bReaction progress monitored by TLC.
cIsolated yields.
dNo catalyst.
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Novel synthesis of bis-Michael addition products catalysed by NH2-PEG
3-Hydroxy-5,5-dimethyl-2-(3-oxo-1,3-diphenylpropyl)cyclohex-2-en-one (3r)
vibrations and C―O―C symmetric stretching of PEG moiety,
respectively. Thus the FT-IR spectrum showed successful grafting
of aminosilane species on the surface of PEG.
White powder; m.p. 234°C; IR (KBr) cmÀ1: 1680, 1595 (CO), 3430
(OH).1H NMR (400 MHz, DMSO-d6): δ = 0.98 (6H, s, CH3), 2.00
(2H, s, CH2), 2.25 (2H, s, CH2), 3.55 (1H, dd, CH2), 3.84 (1H, dd,
CH2), 4.75 (1H, t, CH), 7.06 (1H, t, Ar―H), 7.17 (2H, t, Ar―H),
7.28 (2H, d, J = 7.5 Hz, Ar―H), 7.53 (2H, t, Ar―H), 7.63 (1H, t,
Ar―H), 7.93 (2H, d, J = 7.4 Hz, Ar―H), 10.44 (1H, br. s, OH). 13C
NMR (100 MHz, DMSO-d6): δ = 28.8, 32.4, 35.6, 41.5, 115.7 (2 × C),
124.5, 127.3 (2 × C), 127.6 (2 × C), 127.8 (2 × C), 128.9 (2 × C),
133.5, 136.4, 145.9, 198.6 (2 × C). ESI-MS: (m/z) 349.0 (M++1). Anal.
Calcd for C23H24O3: C, 79.28; H, 6.94; found: C, 79.30; H, 6.92.
Powder XRD analysis of the catalyst
The structure of the catalyst (NH2-PEG) was identified by powder
XRD (Fig. 2). The XRD pattern showed characteristic diffraction
peaks for the PEG moiety which indicated that the modification
with aminopropyl groups did not result in changes to the
structural properties of the PEG matrix.
EDX analysis of the catalyst
EDX analysis (Fig. 3) of the catalyst showed the presence of C, O, N and
Si elements, indicating the formation of the NH2-PEG catalytic system.
Results and Discussion
SEM analysis of the catalyst
Catalyst Preparation and Characterization
SEM micrographs (Fig. 4) showed the smooth, compact and
uniform surface of the catalyst.
The strategy applied for the preparation of NH2-PEG catalyst is
outlined in Scheme 1.
FT-IR spectrum of catalyst
Optimization of Reaction Conditions and Catalytic Activity
The FT-IR spectrum shows the characteristic bands belonging to
the silanol groups at 3786 cmÀ1 (Fig. 1). A broad absorption band
in the range 3350–3500 cmÀ1 was due to NH2 stretching band
overlapping with that of O―H stretching vibration.[27] The strong
band at 2888 cmÀ1 was assigned to asymmetric stretching vibra-
tions of CH2 groups of n-propylamine and PEG moieties. The
N―H bending vibrations and NH2 symmetric bending vibrations
The catalytic activity of aminopropylated PEG was explored for
Michael addition of varieties of C―H-activated acids to bis-α,β-
unsaturated ketones under solvent-free conditions (Scheme 2).
Effect of Different Solvents and Temperature
In order to optimize reaction conditions, the reaction was studied
by employing both solvents and solvent-free conditions at
altered temperature with the expectation to maximize product
were present at 695 and 1567 cm
.
À1 [28] Peaks in the range 1344–
1468, 1111 cmÀ1 were due to (C―H) alkane scissoring, bending
Table 2. Comparison of catalytic activity of different catalysts on the model reactiona
Entry
Catalyst
Timeb
Yield (%)c
1
NH2-PEG
NaOH
3 min
45 min
60 min
1.4 h
1.4 h
2.1 h
4.5 h
4.4 h
4.9 h
5.1 h
3.5 h
4.2 h
5.3 h
5.7 h
95
2
32
3
KOH
43
4
K2CO3
Na2CO3
Et3N
55
5
46
6
42
7
Pyridine
Piperidine
Urea
22
8
38
9
31
10
11
12
13
14
Thiourea
P2O5
28
33
L-proline
ZnO
Trace
Trace
Trace
MgO
aReaction conditions: bis-α,β-unsaturated ketone (1a, 1.00 mmol), acetyl acetone (2a, 2.00 mmol), catalyst (0.1 g), solvent-free conditions, T = 70°C.
bReaction progress monitored by TLC.
cIsolated yields.
Appl. Organometal. Chem. 2014, 28, 620–630
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T. Khan and Z. N. Siddiqui
desired product. Thus our study revealed that solvent-free con-
ditions were best for NH2-PEG-catalyzed bis-Michael addition to
bis-α,β-unsaturated ketones in terms of reduced reaction time
and enhanced yield of the products (Scheme 2). The highest
catalytic activity under solvent-free conditions may be due to
the good dispersion of active reagent sites, which facilitates better
contact between reactant molecules and the catalyst.[29]
Moreover, due to absence of any solvent (as medium), there is
no dilution effect and the heat needed for energy of activation
is directly available to the reactant molecules, leading to high con-
versions in a minimum time period.
Effect of Catalysts
A comparative study was also carried out using other catalysts for
obtaining the best yield of 3a under solvent-free conditions at
70°C. It afforded lower to moderate yields of the product with
longer reaction time. When the model reaction was carried out
in the presence of NH2-PEG (0.1 g) under solvent-free conditions
at 70°C, product 3a was obtained in excellent yield (95%) in a
short span of time (3 min) (Table 2, entry1).
Figure 5. Effect of catalyst loading on the model reaction.
Effect of Catalyst Loading
Variation of the catalyst loading also had a profound effect on the
catalytic activity (Fig. 5). When the reaction was carried out using
0.02 g, 0.04 g, 0.06 g and 0.08 g of the catalysts, the rate of reac-
tion progressed steadily with lower to moderate yields. However,
the best performance was observed with 0.1 g NH2-PEG under
solvent-free conditions at 70°C. Further increase in the catalyst
loading had no significant enhancement in the yield of the
desired product and the rate of addition reaction.
Figure 6. Recycling data of NH2-PEG for model reaction.
Recycling Study of the Catalyst
yield in a minimum reaction time period (Table 1). For this
purpose, reaction between bis-α,β-unsaturated ketone (1a)
(1.00 mmol) and acetyl acetone (2a) (2.00 mmol) for the synthesis
of adduct (3a) was selected as a model reaction in the presence of
different protic, aprotic, non-polar solvents, and the results are
summarized in Table 1. It was observed that when the model
reaction was carried out in ethanol in the absence of any catalyst
under reflux conditions, a trace amount of the product was
obtained after 15 h (Table 1, entry 1) and indicated the need for
a catalyst. Then, the reaction was conducted in the presence of
NH2-PEG under the same conditions as above and yielded
product (65%) in 3.1 h. In other polar protic solvents such as acetic
acid, methanol, isopropanol and water, a moderate yield of the
desired product (3a) was obtained after a longer time period
(Table 1, entries 3–6), whereas in polar aprotic solvents such as
dichloroethane, acetonitrile, dichloromethane, THF and DMF,
a lower yield of the product (3a) was obtained (Table 1, entries
7–11). In non-polar solvents such as dioxane, toluene and hexane,
the reaction did not take place (entries 12–14), whereas in envi-
ronmentally benign solvents such as PEG-200, PEG-400, PEG-
600, ethylene glycol and glycerol the reaction gave unsatisfactory
results in terms of time and yield of the product (entries 15–19).
The reaction performance was drastically enhanced under
solvent-free conditions at 70°C and gave 3a in excellent yield
(95%) within a few minutes (3 min) (Table 1, entries 20–23).
Therefore, 70°C was chosen as the optimum temperature in
further investigations. Further increase in temperature to 80°C
did not show any significant enhancement in the yield of the
The reusability of the catalyst was examined using the model re-
action of bis-α,β-unsaturated ketone (1a) (1.00 mmol) and acetyl
acetone (2a) (2.00 mmol) under solvent-free conditions (Fig. 6).
After completion of the reaction, the reaction mixture was cooled
to room temperature and H2O (5 ml) added with shaking for 3
min to dissolve the NH2-PEG. The crude product (insoluble in wa-
ter) was filtered and recrystallized from ethyl alcohol (3 ml) to
Figure 7. SEM images of recovered catalyst (NH2-PEG) at different
magnifications.
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Novel synthesis of bis-Michael addition products catalysed by NH2-PEG
afford pure products (3a–o). In order to recover the catalyst, the
filtrate was evaporated under reduced pressure and Et2O (5 ml)
added. The solid, as obtained, was filtered, washed with Et2O (2
ml × 2) and dried in an oven at 50°C for 30 min. The recovered
catalyst was employed for subsequent cycles (eight runs)
adopting the identical protocol (Fig. 6). The yields obtained in
nine, ten runs were 85% in 7 min and 77% in 9 min, respectively.
Further, the yield decreased substantially after the tenth cycle
(68% in 9 min) onwards.
A similar pattern of powder XRD (Fig. 2c) and SEM-EDX analy-
sis (Figs. 7a, b and Fig. 8) was observed for the recovered catalyst,
with some low-intensity and extra peaks which may be due to
the catalyst deactivation or some change in morphology of the
catalyst after eight runs.
Encouraged by the remarkable results, we investigated the
scope and generality of this protocol by the reaction of bis-α,β-
unsaturated ketones (1a–b) with different acyclic (2a–g) and cy-
clic activated C―H acids (2h–m) under solvent-free conditions
in the presence of NH2-PEG as catalyst. All the reactions
proceeded smoothly and were completed
within 3–7 min to afford the products (3a–o)
in excellent yield (95–90%) (Tables 3 and 4).
To further investigate scope of the reaction
for construction of mono-Michael addition
products, the reaction of α,β-unsaturated ke-
tone (1c) was conducted with acetyl acetone
(2a), ethyl acetoacetate (2d) and dimedone
(2i) as C―H-activated acids by adopting an
identical protocol. The reactions went on well,
affording the products (3p–r) in very good
yields (95–92%) in a short span of time (3–6
min) (Table 5).
The structure of compounds 3a–o was de-
duced on the basis of their spectral analysis.
The IR spectrum of the newly synthesized com-
pound 3a showed a broad band for OH groups
at 3428 cmÀ1 and strong bands at 1686, 1654
and 1627cmÀ1 for carbonyl groups. 1H NMR
Figure 8. EDX analysis of the recovered catalyst (NH2-PEG).
Table 3. Bis-Michael addition of acyclic activated C―H acids to bis-α,β-unsaturated ketones under solvent-free conditionsa
Product
Dialdehyde
Activated C―H
Timeb (min)
Yieldc (%)
acids
3a
3b
3c
3d
3e
1a
1b
1a
1a
1a
R = R′ = COCH3
R = R′ = COCH3
R = R′ = CN
R = R′ = COOEt
R = COOEt
3
4
3
6
5
95
95
95
93
90
R′ = COCH3
R = COOEt
3f
1a
1a
1a
4
6
7
94
93
92
R′ = CN
3g
3h
R = COOCH3
R′ = COCH3
R = COOH
R′ = CH2Ph
aReaction conditions: bis-α,β-unsaturated ketones (1a–b, 10 mmol), acyclic activated C―H acids (2a–g, 20 mmol), catalyst (1.00 g), solvent-free
conditions, T = 70°C.
bReaction progress monitored by TLC.
cIsolated yields.
Appl. Organometal. Chem. 2014, 28, 620–630
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T. Khan and Z. N. Siddiqui
Table 4. Bis-Michael addition of cyclic activated C―H acids to bis-α,β-unsaturated ketones under solvent-free conditionsa
Product
Dialdehyde
Activated C―H acids
Timeb (min)
Yieldc (%)
3i
1a
1b
1a
1a
1a
1a
1a
2h
2h
2i
3
3
6
7
7
4
3
95
95
94
90
93
94
95
3j
3k
3l
2j
3m
3n
3o
2k
2l
2m
aReaction conditions: bis-α,β-unsaturated ketones (1a–b, 10 mmol), cyclic activated C―H acids (2h–m, 20 mmol), catalyst (1.00 g), solvent-free
conditions, T = 70°C.
bReaction progress monitored by TLC.
cIsolated yields.
spectroscopy exhibited sharp singlets at δ 1.86 and 2.11 for the
methyl groups of acetyl acetone and lactone moieties, respectively.
Two multiplets were displayed at δ 2.88–2.90 and 3.55–3.62 for
COCH2 and CH protons. Two more singlets integrating for two pro-
tons each at δ 5.25 and 5.74 were assigned to H5 protons of lac-
tone moieties and methine protons. Four aromatic protons
appeared as a singlet at δ 7.58. The 13C NMR spectrum showed ar-
omatic carbons which appeared in the range δ 126.21–137.49,
whereas signals at δ 169.25, 192.57 and 192.76 were due to car-
bonyl groups of lactone and acetyl acetone moieties,
respectively. Another signal at δ 182.95 was assigned to the C-4
carbon of the lactone moiety. Further confirmation for 3a was
provided by the ESI mass spectrum, which showed the molecular
ion peak as base peak at m/z 634.2 (M++1). A plausible mechanism
for the bis-Michael addition is depicted in Scheme 3.
Conclusion
In summary, we have synthesized NH2-PEG as a biodegradable,
highly cost-effective and environmentally benign polymer-based
novel solid basic catalyst for the synthesis of bis-Michael addition
products in excellent yields under solvent-free conditions. Mild
reaction conditions, economic feasibility, environmental benig-
nity, practicability, minimum reaction time period, easy work-up
Table 5. Mono-Michael addition of activated C―H acids to α,β-unsaturated ketone under solvent-free conditionsa
Product
Activated C―H acids
Time,b min (lit.)
Yield,c % (lit.)
3pd
3qd
3rd
R = COCH3
R = COOEt
2i
3 (2 h)
6 (2 h)
6 (5 h)
95 (60)
92 (53)
94 (89)
aReaction conditions: α,β-unsaturated ketone (1c, 10 mmol), activated C―H acids (2a, 2d, 2i, 10 mmol), catalyst (1.00 g), solvent-free conditions,
T = 70°C.
bReaction progress monitored by TLC.
cIsolated yields.
dThe products were confirmed by melting points and compared with reported literature.[30,31]
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Novel synthesis of bis-Michael addition products catalysed by NH2-PEG
Scheme 3. Plausible mechanism for the bis-Michael addition.
[5] D.-Z. Xu, M.-Z. Zhan, Y. Huang, Tetrahedron 2014, 70, 176.
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tive. These features will enable this method to find extensive
applications in the field of organic synthesis.
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Acknowledgments
The authors are thankful to the SAP scheme (DRS-I) from the
University Grants Commission and DST (FIST & PURSE), New
Delhi. The authors are also thankful to the Centre of Nanotech-
nology, Department of Applied Physics, University Sophisticated
Instrument Facility (USIF), AMU, Aligarh, for providing the powder
X-ray diffractometer, SEM-EDX facilities, and to SAIF, CDRI, Luck-
now, Punjab University, Chandigarh, for providing spectral data.
One of the authors (T.K.) would like to acknowledge the Univer-
sity Grant Commission (UGC), New Delhi, for the financial assis-
tance in the form of a Senior Research Fellowship (F. 17-43/08
(SA-I)).
[10] S. Keithellakpam, W. S. Laitonjam, Chin. Chem. Lett. 2014,
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