An ionic liquid gel: A heterogeneous catalyst for Erlenmeyer-Plochl and Henry reactions

Article abstract of DOI:10.1039/c8nj00367j

An ionic liquid gel has been prepared by entrapping 1-butyl-3-methylimidazolium hydroxide ([Bmim]OH) in an aqueous agar gel. The ionic liquid gel has been characterized by Fourier transform infrared (FT-IR), Fourier transform Raman (FT-Raman) spectroscopy, scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and energy dispersive X-ray analysis (EDX). The ionic liquid gel has been successfully employed as a heterogeneous catalyst in the Erlenmeyer-Plochl reaction involving aldehydes, hippuric acid and acetic anhydride as well as in the Henry reaction between aldehydes and nitromethane in ethanol at room temperature. The heterogeneity of the ionic liquid gel has been confirmed by conducting hot filtration tests and leaching studies. Additionally, the ionic liquid gel could be easily recovered by simple filtration and reused five times without significant loss in catalytic activity.

Full text of DOI:10.1039/c8nj00367j

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The role of atropisomers on the photo-reactivity and fatigue of
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DOI: 10.1039/C8NJ00367J
Ionic liquid gel: A heterogeneous catalyst for ErlenmeyerꢀPlochl and Henry reaction
Megha Jagadalea, Altafhusen Naikwadea, Rajashri Salunkhea, Mohan Rajmaneb, Gajanan
Rashinkara*
aDepartment of Chemistry, Shivaji University, Kolhapur, 416004, M.S., India
bSadguru Gadge Maharaj College, Karad, 415110, M.S., India
Eꢀmail: gsr_chem@unishivaji.ac.in
∗Corresponding author. Tel.: +91 231 260 9169; fax: +91 231 2692333.
Abstract
Ionic liquid gel has been prepared by entrapping 1ꢀbutylꢀ3ꢀmethylimidazolium
hydroxide ([Bmim]OH) in an aqueous agar gel. The ionic liquid gel has been characterized by
Fourier transform infrared (FTꢀIR), Fourier transform Raman (FTꢀRaman) spectroscopy,
scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and energy dispersive
Xꢀray analysis (EDX). The ionic liquid gel has been successfully employed as a heterogeneous
catalyst in the ErlenmeyerꢀPlochl reaction involving aldehydes, hippuric acid and acetic
anhydride as well as in the Henry reaction between aldehydes and nitromethane in ethanol at a
room temperature. The heterogeneity of ionic liquid gel has been confirmed by using hot
filtration test and leaching studies. Additionally, the ionic liquid gel could be easily recovered by
simple filtration and reused for five times without significant loss in catalytic activity.
Keywords
1ꢀButylꢀ3ꢀmethylimidazolium hydroxide; agar; Ionic liquid gel; Erlenmeyerꢀ
Plochl reaction; Henry reaction
1

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Introduction
As green chemistry is becoming a central issue in the 21st century, there has been a great
deal of attention in redesigning synthetic processes in order to avoid use of harmful substances
and generation of toxic waste.1 An intriguing line of development in this regard is being fueled
by a paradigm shift from the use of volatile organic solvents to an alternative reaction media in
synthetic chemistry.2 In this context, ionic liquids (ILs) have emerged as promising alternative
reaction medium in recent years.2a,3 The tempting properties of ILs such as diminutive vapor
pressure, high thermal and electrical conductivity, large electrochemical window, low
nucleophilicity and ability to provide nonꢀcoordinating environment have triggered enormous
interest in these notorious solvents.4 A unique attribute of ILs is their modularity which allows
fine tuning of their physicochemical properties by alteration in cation–anion combinations
making them process compatible.5 However, despite these advantages, a series of drawbacks
such as their high cost as compared to traditional solvents and reusability that requires energy
consuming distillation have been major obstacles in implementing ILs at commercial levels. In
addition, the processes applying ILs cannot be performed in continuous flow fixed bed reactors.6
The recognition of these limitations has led to the flourishing activity of heterogenization of ILs.
Several elegant strategies such as supported ionic liquid phase (SILP) catalysis have been
developed for this purpose.7 Nevertheless, there is plenty of room to explore new fundamental
approaches in addition to currently employed strategies. From this perspective, we envisioned
that the entrapment of ILs in the matrix of agar can provide a highly attractive strategy to
circumvent drawbacks associated with ILs. Agar is a biopolymer composed of linear chains of
repeating agarobiose units alternating between 3ꢀlinked–βꢀDꢀgalactopyransoyl (G) and 4ꢀlinked
3,6ꢀanhydroꢀαꢀLꢀgalactopyransoyl (LA) units. Our interest in the use of agar for entrapment of
2

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ILs stems from its easy bioavailability, potent biocompatibility, cost effectiveness and good
immobilization efficiency.8 We hypothesized that the resulting heterogeneous class of IL
referred as a ionic liquid gel can allow to conglomerate the properties of ILs with that of
biopolymer support thereby facilitating significant enhancement in the catalytic activity.
Moreover, we sought that the intact network of ionic liquid gel would allow facile separation and
easy reuse thereby minimizing the energy consumption.
The ErlenmeyerꢀPlochl reaction is one of the most widely employed methods for
synthesis of azlactones which are privileged scaffolds that possess antiꢀcancer,9aꢀb antiꢀ
inflammatory,9c antiꢀHIV,9d sedative,9e cardiotonic9f and antiꢀdiabetic activities.9g Notably,
azlactones are also used as intermediates in the synthesis of organic compounds comprising
thiamine,10a polyfunctional molecules,10b amino acids10c and peptides.10d In the view of their
dynamic properties, large number of methods have been reported for synthesis of azlactones. A
most common process for the synthesis of azlactones includes the ErlenmeyerꢀPlochl reaction
between aryl aldehydes and hippuric acid in the presence of acetic anhydride. A variety of
catalytic systems have been reported for synthesis of azlactones using ErlenmeyerꢀPlochl
reaction.11
[Bmim]3PW12O40/[Bmim]4W10O32,12c
In
addition,
ILs
such
as
[Bnmim]H2PO4,12a
[Et3NH][HSO4],12b
[(C14H24N4)22W10O32].[Bmim]NO3,12d
[C6(MIm)2)2W10O32].2H2O,12e [Bmim]OH11e and Zwitterium imidazolium salt12f have been
reported to promote the efficiency of azlactones synthesis.
The Henry reaction is a classical carbonꢀcarbon bond forming reaction13 used for the
construction of βꢀnitro alcohols from nitroalkane and aldehydes. This reaction has received
overwhelming synthetic popularity as produced βꢀnitro alcohols offer considerable potential in
the synthesis of natural products and pharmaceuticals.14 In addition, βꢀnitro alcohols are used as
3

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an intermediates in synthesis of alkaloids,15 amino sugar16a and antibiotics.16b Owing to the
significant importance of βꢀnitro alcohols, development of novel protocols for the Henry reaction
is a vibrant area of research in organic synthesis. Plethora of catalysts17 including ILs such as
[C4dabco]OH,18a [P4444][Im],18b [SFHEA][HSO4]SO3H,18c [18ꢀcꢀ6k][OH],18d methoxy
propylamine acetate,18e [EMIM]Cl,18f [TMG][Lac]18g and [Bmim]OH17e have been reported to
catalyze the Henry reaction between nitroalkane and aromatic aldehydes.
An insight into various catalytic systems reported for the ErlenmeyerꢀPlochl and Henry
reactions reveal a number of drawbacks such as long reaction time, harsh reaction conditions and
difficulty in recovery and reusability of the catalyst. Moreover, most of the IL mediated
ErlenmeyerꢀPlochl and Henry reactions involve high catalyst loading and high energetic input or
use of additional solvent for recycling. These concerns necessitate need to develop a facile
protocol for the ErlenmeyerꢀPlochl and Henry reaction using a robust heterogeneous catalyst.
Considering aforementioned aspects and in continuation of our work related to green
chemistry,19 we report herein preparation of ionic liquid gel by entrapping [Bmim]OH in agar
and its application as heterogeneous catalyst in the synthesis of azlactones by the Erlenmeyerꢀ
Plochl reaction and βꢀnitro alcohols by the Henry reaction.
Experimental
Infrared spectra were recorded on the PerkinꢀElmer FTꢀIR spectrometer. FTꢀRaman
spectroscopy was done using a Bruker FT‐Raman (MultiRAM) spectrometer. Thermal
gravimetric analysis (TGA) curves were obtained using the instrument TA SDT Q600 in the
presence of static air at a linear heating rate of 10 °C minꢀ1 from 25 °C to 1000 °C. SEM was
recorded using JSMꢀ6701F, Japan. Elemental composition of materials was analyzed by energyꢀ
dispersive Xꢀray spectra (EDS) attached to the field emission scanning electron microscope (FEꢀ
4

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SEM, Model Hitachi S 4800, Japan). 1H NMR and 13C NMR spectra were recorded on a Bruker
1
13
AC (300 MHz for H NMR and 75 MHz for C NMR) spectrometer using CDCl3 as a solvent
and tetramethylsilane (TMS) as an internal standard. Chemical shifts (δ) are expressed in parts
per million (ppm) values and coupling constants are expressed in hertz (Hz). Mass spectra were
recorded on a Shimadzu QP2010 GCMS and all the chemicals including aromatic aldehyde,
nitromethane, acetic anhydride and hippuric acid (Spectrochem) were used without further
purification. Melting points were determined in an open capillary and are uncorrected. All
reactions were carried out under air atmosphere in dried glasswares.
Preparation of [Bmim]OH
[Bmim]OH was prepared from [Bmim]Br by using ion exchange column.
Amberlite®IRAꢀ400 hydroxide resin was prepared in deionized water and poured into a glass
column. [Bmim]Br (15g, 0.5M) was dissolved in 100 mL deionized water and resultant solution
was loaded to the column. The column was then eluted with deionized water as eluent with the
controlled flow rate of 0.7 mL/min. The evaporation of solvent from the eluted fractions under
vacuo afforded [Bmim]OH as the viscous liquid. FTꢀIR (KBr): υ = 3406, 2966, 1644, 1403,
1268, 1168, 1007, 979, 831, 620 cmꢀ1; 1H NMR (300 MHz, CDCl3): δ 0.49 (t, 3H, J = 7.5 Hz, ꢀ
CH3), 0.88ꢀ0.95 (m, 2H, ꢀCH2), 1.45 (t, 2H, J = 7.5 Hz, ꢀCH2), 3.48 (bs, 1H, ꢀOH), 3.65 (s, 3H,
NꢀCH3), 3.91 (t, 2H, J = 6.9 Hz, NꢀCH2), 7.21ꢀ7.33 (m, 2H, C4H, C5H of Imi), 9.37 (s, 1H, ꢀC2H
of Imi); 13C NMR (75 MHz, CDCl3): 13.5 (ꢀCH3), 19.5 (ꢀCH2), 32.0 (ꢀCH2), 36.6 (ꢀNꢀCH3),
49.6 (ꢀNꢀCH2), 121.9 (C4 of Imi), 123.6 (C5 of Imi), 135.8 (C2 of Imi) ppm.
5

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Preparation of [Bmim]OH@agar (1)
To a boiling mixture of agarꢀagar (10 g) in water (60 mL), 2.8 % w/v of [Bmim]OH was
added. The resultant mixture was boiled with stirring for few minutes and cooled in ice bath to
get the desired ionic liquid gel, [Bmim]OH@agar (1). FTꢀIR (KBr, thin film): υ = 3401, 1634,
1373, 1260, 1175, 1070, 932, 890, 621 cmꢀ1; FTꢀRaman (cm−1): 3665, 3470, 3408, 3343, 3265,
3246, 3224, 3187, 3119, 2990, 2968, 2955, 2939, 2850, 2828; Elemental analysis observed: % C
72.79, % N 1.96, % O 25.25; Loading: 0.51 mmol of functional group gꢀ1 of agar.
General procedure for synthesis of azlactones (5)
A mixture of aromatic aldehyde (1 mmol), hippuric acid (1 mmol), acetic anhydride (1.2
mmol) and [Bmim]OH@agar (1) (100 mg) in ethanol (5 mL) was stirred at ambient temperature.
After completion of a reaction as monitored by TLC, the reaction mixture was filtered to get the
crude residue of azlactone which was purified by column chromatography over silica gel.
General procedure for synthesis of βꢀnitro alcohols (8)
A mixture of aromatic aldehyde (1 mmol), nitromethane (1 mmol) and [Bmim]OH@agar
(1) (150 mg) in ethanol (5 mL) was stirred at ambient temperature. After completion of a
reaction as monitored by TLC, the reaction mixture was filtered to get crude residue of βꢀnitro
alcohol which was purified by column chromatography over silica gel.
Spectral data
4-[(4-Nitrophenyl)methylene]-2-phenyloxazol-5-one (Table 3, product entry 5a)20a: MP 239 °C
(lit.,238 °C)20b; IR (KBr, thin film): υ = 2923, 2845, 1804, 1652, 1600, 1514, 1341, 1167, 1106,
1
853, 760, 689 cmꢀ1; H NMR (300 MHz, CDCl3): δ 7.20 (s, 1H, ꢀCꢀH), 7.52 (dd, 2H, J = 4.4,
10.4 Hz, ArꢀH), 7.70 (m, 1H, ArꢀH), 8.25 (dd, 2H, J = 1.2, 8.5 Hz, ArꢀH), 8.34 (m, 2H, ArꢀH),
8.40 (m, 2H, ArꢀH); 13C NMR (75 MHz, CDCl3): 112.6 (CꢀH), 121.7 (ArꢀC), 127.5 (ArꢀC),
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128.7 (ArꢀC), 129.5 (ArꢀC), 130.2 (ArꢀC), 131.3 (ArꢀC), 131.7 (CꢀN), 141.1 (ArꢀC), 147.8 (Arꢀ
C), 160.4 (C=N), 165.9 (C=O); MS (EI): m/z 294 (M+).
4-[(3-Nitrophenyl)methylene]-2-phenyl-oxazol-5-one (Table 3, product entry 5b)20a: MP 197 °C
(lit., 195 °C)20b; IR (KBr): υ = 3093, 1792, 1762, 1658, 1531, 1491, 1325, 1168, 1095, 991, 885,
777 cmꢀ1; 1H NMR (300 MHz, CDCl3): δ 7.26 (s, 1H, ꢀCꢀH), 7.59ꢀ7.62 (m, 2H, ArꢀH), 7.65ꢀ7.68
(m, 2H, ArꢀH), 8.20ꢀ8.23 (m, 2H, ArꢀH), 8.32ꢀ8.35 (m, 1H, ArꢀH), 8.39 (d, 1H, J = 7.6 Hz, Arꢀ
H), 9.17 (t, 1H, J = 1.7 Hz, ArꢀH); 13C NMR (75 MHz, CDCl3): 112.2 (CꢀH), 120.5 (ArꢀC),
121.6 (ArꢀC), 128.7 (ArꢀC), 129.4 (ArꢀC), 129.7 (ArꢀC), 130.1 (ArꢀC), 131.4 (CꢀN), 131.7 (Arꢀ
C), 132.7 (ArꢀC), 135.9 (ArꢀC), 148.5 (ArꢀC), 160.4 (C=N), 165.8 (C=O); MS (EI): m/z 294
(M)+.
4-(4-Fluorobenzylidene)-2-phenyloxazol-5-one (Table 3, product entry 5c)21: MP 172 °C (lit.,
170 °C)20b; IR (KBr): υ = 2951, 1798, 1774, 1664, 1598, 1500, 1327, 1296, 1158, 1096, 837 cmꢀ
1; 1H NMR (300 MHz, CDCl3): δ 8.20ꢀ8.25 (m, 2H, ArꢀH), 8.16 (dd, 2H, J = 1.4, 8.5 Hz, ArꢀH),
7.60 (t, 1H, J = 7.5 Hz, ArꢀH), 7.49 (t, 2H, J = 7.5 Hz, ArꢀH), 7.20 (s, 1H, CꢀH), 7.16 (t, 2H, J =
8.6 Hz, ArꢀH); 13C NMR (75 MHz, CDCl3): 110.4 (CꢀH), 116.1(ArꢀC), 128.6 (ArꢀC), 129,0 (Arꢀ
C), 130.4 (ArꢀC), 131.1 (CꢀN), 133.2 (ArꢀC), 134.5 (ArꢀC), 134.9 (ArꢀC), 163.1 (ArꢀC), 163.5
(C=N), 167.6 (C=O); MS (EI): m/z 267 (M+).
4-(4-Benzylidene)-2-phenyloxazol-5-one (Table 3, product entry 5d)20a: MP 168 °C (lit.,170
1
°C)20b; IR (KBr): υ = 3052, 1805, 1756, 1720, 1640, 1290, 987, 860, 767 cmꢀ1; H NMR (300
MHz, CDCl3): δ 8.27ꢀ8.30 (m, 2H, ArꢀH), 8.10ꢀ8.13 (m, 2H, ArꢀH), 7.59 (t, 1H, J = 7.5 Hz, Arꢀ
13
H), 7.51 (t, 2H, J = 7.6 Hz, ArꢀH), 7.39ꢀ7.48 (m, 3H, ArꢀH), 7.20 (s, 1H, CꢀH); C NMR (75
MHz, CDCl3): 111.3 (CꢀH), 124.5 (ArꢀC), 128.3 (ArꢀC), 128.7 (ArꢀC), 128.9 (ArꢀC), 131.2 (Cꢀ
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N), 132.1 (ArꢀC), 132.7 (ArꢀC), 133.3 (ArꢀC), 133.6 (ArꢀC), 163.7 (C=N), 167.5 (C=O); MS
(EI): m/z 249 (M+).
4-(2-Hydroxybenzylidene)-2-phenyloxazol-5-one (Table 3, product entry 5e)22: MP 106 °C
(lit.,108ꢀ111 °C)22; IR (KBr): υ = 3411, 2924, 1791, 1644, 1446, 1415, 1328, 1295, 1152, 1096,
1
983, 859, 696 cmꢀ1; H NMR (300 MHz, CDCl3): δ 8.89 (s, 1H, ꢀOH), 7.95 (d, 2H, J = 6.9 Hz,
13
ArꢀH), 7.46ꢀ7.62 (m, 6H, ArꢀH), 7.35ꢀ7.39 (m, 2H, ArꢀH, ꢀCH); C NMR (75 MHz, CDCl3):
116.4 (CꢀH), 119.9 (ArꢀC), 123.4 (ArꢀC), 124.2 (ArꢀC), 125.2 (ArꢀC), 127.1 (ArꢀC), 127.9 (Arꢀ
C), 128.9 (ArꢀC), 129.7 (ArꢀC), 132.5 (CꢀN), 133.5 (ArꢀC), 149.9 (ArꢀC), 159.0 (C=N), 166.1
(C=O); MS (EI): m/z 265 (M+).
4-[(4-Hydroxyphenyl)methylene]-2-phenyloxazol-5-one (Table 3, product entry 5f)23a: MP 180
°C (lit., 182 °C)23b; IR (KBr): υ = 3465, 1812, 1635, 1599, 1130, 1061, 978, 724 cmꢀ1; 1H NMR
(300 MHz, DMSO): δ 10.36 (s, 1H, ꢀOH), 8.18ꢀ8.89 (m, 4H, ArꢀH), 7.64ꢀ8.11 (m, 5H, ArꢀH),
7.42 (s, 1H, CH); 13C NMR (75 MHz, CDCl3): 112.7 (CꢀH), 115.3 (ArꢀC), 127.5 (ArꢀC), 127.9
(ArꢀC), 128.7 (ArꢀC), 129.4 (ArꢀC), 129.7 (ArꢀC), 131.4 (ArꢀC), 131.9 (CꢀN), 157.3 (ArꢀC),
160.4 (C=N), 166.4 (C=O); MS (EI): m/z 296 (M+).
4-(2-Methylbenzylidene)-2-phenyloxazol-5-one (Table 3, product entry 5g)13a: MP 142 °C
(lit.,141 °C)13b; IR (KBr): υ = 2921, 1798, 1640, 1586, 1544, 1487, 1456, 1324, 1294, 1252,
1
1227, 1165, 987, 866 cmꢀ1; H NMR (300 MHz, CDCl3): δ 8.71ꢀ8.76 (m, 1H, ArꢀH), 8.14ꢀ8.19
(m, 2H, ArꢀH), 7.57ꢀ7.63 (m, 1H, ArꢀH), 7.54 (dd, 2H, J = 5.7, 4.4 Hz, ArꢀH), 7.47 (dd, 1H, J =
5.0, 3.5 Hz, ArꢀH), 7.29ꢀ7.33 (m, 2H, ArꢀH), 7.30 (s, 1H, CH), 2.56 (s, 3H, CH3); 13C NMR (75
MHz, CDCl3): 20.4 (CH3), 125.2 (CꢀH), 126.4 (ArꢀC), 128.2 (ArꢀC), 128.7 (ArꢀC), 129.0 (Arꢀ
C), 130.4 (ArꢀC), 130.9 (CꢀN), 132.1 (ArꢀC), 133.4 (ArꢀC), 133.4 (ArꢀC), 133.5 (ArꢀC), 139.7
(ArꢀC), 163.9 (C=N), 167.4 (C=O); MS (EI): m/z 263 (M+).
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4-(4-Methylbenzylidene)-2-phenyloxazol-5-one (Table 3, product entry 5h)21: MP 147 °C
(lit.,145 °C)20b;IR (KBr): υ = 2926, 1795, 1647, 1667, 1556, 1490, 1330, 1227, 1162, 985, 860
1
cmꢀ1; H NMR (300 MHz, CDCl3): δ 8.17ꢀ8.20 (m, 2H, ArꢀH), 8.08 (d, 2H, J = 8.0 Hz, ArꢀH),
7.57 (dd, 1H, J = 4.7, 10.4 Hz, ArꢀH), 7.50 (dd, 2H, J = 7.1, 8.0 Hz, ArꢀH),7.32 (d, 2H, J = 8.1
Hz, ArꢀH), 7.20 (s, 1H, CꢀH), 2.39 (s, 3H, CH3); 13C NMR (75 MHz, CDCl3): 22.1 (CH3), 125.9
(CꢀH), 128.1 (ArꢀC), 128.7 (ArꢀC), 128.9 (ArꢀC), 130.4 (ArꢀC), 132.2 (CꢀN), 133.5 (ArꢀC),
133.3 (ArꢀC), 133.7 (ArꢀC), 142.4 (ArꢀC), 163.2 (C=N), 167.9 (C=O); MS (EI): m/z 263 (M+).
4-(4-Methoxybenzylidene)-2-phenyloxazol-5-one (Table 3, product entry 5i)23a: MP 167°C
(lit.,165 °C)20b;IR (KBr): υ = 2907, 1740, 1670, 1594, 1555, 1480, 1111, 957, 845 cmꢀ1; 1H NMR
(300 MHz, CDCl3): δ 8.22 (d, 2H, J = 7.5 Hz, ArꢀH), 8.18 (d, 2H, J = 7.5 Hz, ArꢀH), 7.40ꢀ7.58
13
(m, 3H, ArꢀH), 7.36 (s, 1H, CꢀH), 6.96ꢀ7.00 (d, 2H, J = 8.9 Hz, ArꢀH), 3.84 (s, 3H, CH3); C
NMR (75 MHz, CDCl3): 56.1 (CH3), 115.0 (CꢀH), 126.0 (ArꢀC), 126.9 (ArꢀC), 128.4 (ArꢀC),
128.9 (ArꢀC), 131.3 (CꢀN), 131.7 (ArꢀC), 133.2 (ArꢀC), 134.4 (ArꢀC), 162.4 (ArꢀC), 162.7
(C=N), 167.8 (C=O); MS (EI): m/z 264 (M+).
4-[(4-Dimethylaminophenyl)methylene]-2-phenyloxazol-5-one (Table 3, product entry 5j)23c:
MP 212 °C (lit.,213 °C)20b; IR (KBr): υ = 3422, 2920, 1764, 1660, 1620, 1381, 1138, 923, 870
cmꢀ1; 1H NMR (300 MHz, CDCl3): δ 3.11 (s, 6H, (ꢀCH3)2), 5.9 (s, 1H, CꢀH), 6.2ꢀ6.7 (m, 9H, Arꢀ
13
H); C NMR (75 MHz, CDCl3): 40.3 (CH3), 112.2 (CꢀH), 121.5 (ArꢀC), 125.7 (ArꢀC), 127.3
(ArꢀC), 128.2 (ArꢀC), 129.1 (ArꢀC), 132.4 (CꢀN), 133.6 (ArꢀC), 134.5 (ArꢀC), 152.4 (ArꢀC),
160.7 (C=N), 168.7 (C=O); MS (EI): m/z 292 (M+).
2-Phenyl-4-((thiophen-2-yl)methylene)oxazol-5-one (Table 3, product entry 5k)13a: MP 178 °C
(lit.,175 °C)13c; IR (KBr): υ = 2927, 1789, 1639, 1553, 1490, 1327, 1235, 1151, 1051, 978, 854
cmꢀ1; 1H NMR (300 MHz, CDCl3): δ 7.17ꢀ7.20 (m, 1H, CꢀH), 7.51ꢀ7.66 (m, 5H, ArꢀH), 7.75 (d,
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1H, J = 5.1 Hz, ArꢀH), 8.18ꢀ8.22 (m, 2H, ArꢀH); 13C NMR (75 MHz, CDCl3): 124.8 (CꢀH),
125.6 (ArꢀC), 127.9 (ArꢀC), 128.3 (ArꢀC), 128.9 (ArꢀC), 130.9 (CꢀN), 133.1 (ArꢀC), 134.9 (Arꢀ
C), 135.3 (ArꢀC), 137.6 (ArꢀC), 162.5 (C=N), 166.9 (C=O); MS (EI): m/z 255 (M+).
2-Phenyl-4-(4-pyridinyl)-oxazol-5-one (Table 3, product entry 5l)13a: MP 170 °C (lit.,168ꢀ169
1
°C)13a; IR (KBr): υ = 3470, 2915, 1798, 1770, 1660, 1594, 1559, 869, 814 cmꢀ1; H NMR (300
MHz, CDCl3): δ 8.66 (d, 2H, J = 5.7 Hz, ArꢀH), 8.18 (d, 2H, J = 7.4 Hz, ArꢀH), 8.00 (d, 2H, J =
5.7 Hz, ArꢀH), 7.69 (t, 1H, J = 7.3 Hz, ArꢀH), 7.58 (t, 2H, J = 7.5 Hz, ArꢀH), 7.11 (s, 1H, CꢀH);
13C NMR (75 MHz, CDCl3): 125.4 (CꢀH), 125.9 (ArꢀC), 127.9 (ArꢀC), 128.9 (ArꢀC), 129.0 (Arꢀ
C), 134.5 (ArꢀC), 137.4 (CꢀN), 140.3 (ArꢀC), 150.2 (ArꢀC), 165.8 (C=N), 166.7 (C=O); MS
(EI): m/z 250 (M+).
1-(4-Nitrophenyl)-2-nitroethanol (Table 6, product entry 8a)24: MP 80 °C (lit.,82 °C)24; IR
(KBr): υ = 3410, 2962, 1712, 1602, 1555, 1522, 1384, 1347, 1261, 1098, 856, 801, 698 cmꢀ1; 1H
NMR (300 MHz, CDCl3): δ 8.23 (d, 2H, J = 8.7 Hz, ArꢀH), 7.63 (d, 2H, J = 8.7 Hz, ArꢀH), 5.59ꢀ
13
5.63 (m, 1H, CꢀH), 4.60 (t, 2H, J = 3.3, 4.5 Hz, CH2), 3.60 (s, 1H, OH); C NMR (75 MHz,
CDCl3): 69.9 (CH), 80.7 (CH2), 124.1 (ArꢀC), 127.0 (ArꢀC), 145.4 (ArꢀC), 147.9 (ArꢀC); GCꢀ
MS m/z : 194 (M+ꢀH2O).
1-(3-Nitrophenyl)-2-nitroethanol (Table 6, product entry 8b)24: MP 74 °C (lit.,70 °C)24; IR
(KBr): υ = 3410, 2928, 1712, 1616, 1531, 1351, 1261, 1098, 806, 735 cmꢀ1; 1H NMR (300 MHz,
CDCl3): δ 8.33 (s, 1H, ArꢀH), 8.23 (d, 1H, J = 8.1 Hz, ArꢀH), 7.79 (d, 1H, J = 7.8 Hz, ArꢀH),
7.62 (t, 1H, J = 7.8 Hz, ArꢀH), 5.62 (q, 1H, CꢀH), 4.63 (t, 2H, CH2), 3.45 (s, 1H, OH); 13C NMR
(75 MHz, CDCl3): 69.84 (CH), 80.72 (CH2), 121.15 (ArꢀC), 123.79 (ArꢀC), 130.10 (ArꢀC),
131.99 (ArꢀC), 140.32 (ArꢀC), 148.58 (ArꢀC); GCꢀMS m/z : 194 (M+ꢀH2O).
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1-(2-Nitrophenyl)-2-nitroethanol (Table 6, product entry 8c)25a: MP 68 °C (lit.,69 °C)25b; IR
1
(KBr): υ = 3517, 2923, 1553, 1525, 1430, 1371, 1255, 1084, 899 cmꢀ1; H NMR (300 MHz,
CDCl3): δ 7.51ꢀ8.12 (m, 4H, ArꢀH), 6.04 (dd, 1H, J = 9.2, 2.6 Hz, CH), 4.87 (dd, 1H, J = 13.2,
2.5 Hz, CH2), 4.52 (dd, 1H, J = 9.1, 13.2 Hz, CH2), 3.57 (s, 1H, OH); 13C NMR (75 MHz,
CDCl3): 69.98 (CꢀH), 80.74 (CH2), 121.11 (ArꢀC), 127.10 (ArꢀC), 128.2 (ArꢀC), 134.2 (ArꢀC),
145.43 (ArꢀC), 147.93 (ArꢀC); GCꢀMS m/z : 194 (M+ꢀH2O).
4-((1-Hydroxy)-2-nitroethyl)phenol (Table 6, product entry 8d)26: MP 74 °C; IR (KBr): υ =
3356, 2929, 2844, 1687, 1596, 1547, 1523, 1485, 1456, 1374, 1255, 1160, 1104, 954, 847 cmꢀ1;
1H NMR (300 MHz, CDCl3): δ 7.30 (d, 2H, J = 8.6 Hz, ArꢀH), 6.86 (d, 2H, J = 8.6 Hz, ArꢀH),
13
5.44ꢀ5.47 (m, 1H, OH), 4.96 (s, 1H, CH), 4.39ꢀ4.55 (m, 2H, CH2), 2.69 (s, 1H, OH); C NMR
(75 MHz, CDCl3): 155.9 (ArꢀC), 132.2 (ArꢀC), 129.9 (ArꢀC), 116.7 (ArꢀC), 81.5 (CH2), 70.4
(CH); GCꢀMS m/z : 165 (M+ꢀH2O).
2-((1-Hydroxy)-2-nitroethyl)phenol (Table 6, product entry 8e)26: MP 87 °C; IR (KBr): υ =
1
3416, 3036, 2931, 1546, 1517, 1444, 1426, 1377, 1265, 1232, 1115, 854, 647 cmꢀ1; H NMR
(300 MHz, CDCl3): δ 7.12ꢀ7.21 (m, 2H, ArꢀH), 6.87ꢀ6.92 (m, 1H, ArꢀH), 6.54 (d, 1H, J = 8.2
13
Hz, ArꢀH), 5.76 (s, 2H, CH2), 4.77ꢀ4.87 (m, 2H, CH, OH), 4.37 (t, 1H, J = 6.7 Hz, OH); C
NMR (75 MHz, CDCl3): 153.4 (ArꢀC), 131.2 (ArꢀC), 130.5 (ArꢀC), 121.6 (ArꢀC), 120.2 (ArꢀC),
116.7 (ArꢀC), 89.3 (CH2), 74.6 (CH); GCꢀMS m/z : 165 (M+ꢀH2O).
1-Phenyl-2-nitroethanol (Table 6, product entry 8f)24: MP 67 °C; IR (KBr): υ = 3452, 2924,
2853, 1600, 1561, 1454, 1384, 1271, 701, 526 cmꢀ1; 1H NMR (300 MHz, CDCl3): δ 7.39 (s, 5H,
ArꢀH), 5.41ꢀ5.45 (m, 1H, CH2), 4.59 (m, 1H, CH2), 4.49 (dd, 1H, J = 13.2, 9.9 Hz, CH), 3.15 (s,
1H, OH); 13C NMR (75 MHz, CDCl3): 71.0 (CH), 81.2 (CH2), 125.9 (ArꢀC), 128.9 (ArꢀC),
129.0 (ArꢀC), 138.2 (ArꢀC); GCꢀMS m/z : 149 (M+ꢀH2O).
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2-Nitro-1-p-tolylethanol (Table 6, product entry 8g)27a: MP 49 °C (lit.,47 °C)24; IR (KBr): υ =
1
3551, 3035, 2938, 1559, 1509, 1450, 1370, 1278, 1067, 920, 816 cmꢀ1; H NMR (300 MHz,
CDCl3): δ 7.26 (d, 2H, J = 9.0 Hz, ArꢀH), 7.20ꢀ7.23 (d, 2H, J = 9.0 Hz, ArꢀH), 5.44ꢀ5.56 (m, 1H,
CH), 4.40ꢀ4.63 (m, 2H, CH2), 2.87 (s, 1H, OH), 2.29 (s, 3H, CH3); 13C NMR (75 MHz, CDCl3):
138.7 (ArꢀC), 135.4 (ArꢀC), 129.6 (ArꢀC), 125.7 (ArꢀC), 81.6 (CH2), 70.6 (CH), 21.5 (CH3);
GCꢀMS m/z : 163 (M+ꢀH2O).
1-(4-Methoxyphenyl)-2-nitroethanol (Table 6, product entry 8h)27a: MP 41 °C (lit.,43 °C)27b; IR
1
(KBr): υ = 3436, 2925, 1549, 1510, 1456, 1377 , 1307, 1255, 1167, 1034, 867, 734 cmꢀ1; H
NMR (300 MHz, CDCl3): δ 2.67 (s, 1H, OH), 3.87 (s, 3H, ꢀOꢀCH3), 4.40 (dd, 1H, J = 3.7, 13.3
Hz, CH), 4.55 (dd, 1H, J = 9.1, 13.3 Hz, CH2), 5.38ꢀ5.41 (m, 1H, CH2), 6.88 (d, 2H, J = 8.7 Hz,
ArꢀH), 7.36 (d, 2H, J = 8.7 Hz, ArꢀH); 13C NMR (75 MHz, CDCl3): 160.0 (ArꢀC), 130.1 (ArꢀC),
127.2 (ArꢀC), 114.1 (ArꢀC), 81.7 (CH2), 70.2 (CH), 55.9 (OCH3); GCꢀMS m/z : 179 (M+ꢀH2O).
1-(3,4,5-Trimethoxyphenyl)-2-nitroethanol (Table 6, product entry 8i)27c: MP 106 °C (lit.,108ꢀ
1
109°C)27d; IR (KBr): υ = 3466, 2917, 1506, 1447, 1403, 1379, 1260, 1216, 1056, 857 cmꢀ1; H
NMR (300 MHz, CDCl3): δ 6.58 (s, 2H, ArꢀH), 5.36 (dd, 1H, J = 9.2, 3.2 Hz, CH2), 4.54 (dd,
1H, J = 13.1, 9.2 Hz, CH2), 4.44 (dd, 1H, J = 13.1, 3.2 Hz, CH), 3.81 (s, 6H, (OCH3)2), 3.74 (s,
13
3H, OCH3), 3.34 (s, 1H, OH); C NMR (75 MHz, CDCl3): 153.0 (ArꢀC), 137.4 (ArꢀC), 133.9
(ArꢀC), 102.4 (ArꢀC), 81.5 (CH2), 71.6 (CH), 55.1 (OCH3), 55.7 (OCH3); GCꢀMS m/z : 239
(M+ꢀH2O).
1-(2-Pyridyl)-2-nitroethanol (Table 6, product entry 8j)17a: MP 68 °C (lit.,69 °C)27e; IR (KBr): υ
1
= 3435, 2955, 1529, 1409, 1366, 1105, 1024, 866 cmꢀ1; H NMR (300 MHz, CDCl3): δ 4.12 (s,
1H, OH), 4.67ꢀ4.88 (m, 2H, CH2), 5.43ꢀ5.57 (m, 1H, CH), 7.15ꢀ7.44 (m, 2H, ArꢀH), 7.68ꢀ7.80
(m, 1H, ArꢀH), 8.47ꢀ8.51 (m, 1H, ArꢀH); 13C NMR (75 MHz, CDCl3): 157.4 (ArꢀC), 149.1 (Arꢀ
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C), 137.7 (ArꢀC), 123.7 (ArꢀC), 121.1 (ArꢀC), 81.6 (CH2), 70.6 (CH); GCꢀMS m/z: 150 (M+ꢀ
H2O).
1-(4-Pyridyl)-2-nitroethanol (Table 6, product entry 8k)28a: MP 93 °C (lit.,95 °C)28b; IR (KBr): υ
= 3427, 2945, 1529, 1460, 1357, 1109, 1010, 856 cmꢀ1; 1H NMR (300 MHz, CDCl3): δ 8.64 (dd,
2H, J = 5.0, 1.6 Hz, ArꢀH), 7.35 (m, 2H, ArꢀH), 5.54 (dd, 1H, J = 7.5, 4.6 Hz, CH), 4.55 (m, 2H,
CH2), 3.64 (s, 1H, OH); 13C NMR (75 MHz, CDCl3): 149.6 (ArꢀC), 147.0 (ArꢀC), 120.0 (ArꢀC),
79.0 (CH2), 68.9 (CH); GCꢀMS m/z : 150 (M+ꢀH2O).
Result and Discussion
Our initial task was to accomplish a reliable preparation of ionic liquid gel. [Bmim]OH
was chosen as a prototype IL for synthesis of ionic liquid gel. Our interest in employing
[Bmim]OH stems from its basic nature and ease of preparation. Owing to these properties,
[Bmim]OH has been one of the most commonly employed IL in organic synthesis.29aꢀc Initially,
a series of experiments were undertaken, in which different concentrations of [Bmim]OH (1ꢀ25
% w/v) were dissolved in a varying amount of agar in water. After a substantial experimentation,
we found that 10 g of agar containing 2.8 % w/v [Bmim]OH resulted in the formation of soft
gel, that served as a ionic liquid gel (acronymed as [Bmim]OH@agar (1)). [Bmim]OH@agar has
a transparent gel like appearance and could be easily cut into pieces (Fig. 1).
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Fig. 1 Photograph of [Bmim]OH@agar (1)
Based on literature reports,29dꢀe the plausible structure of [Bmim]OH@agar (
established in Fig. 2 and is supported by interaction between IL and agar as evidenced by FTꢀIR
analysis. The [Bmim]OH@agar ( ) exhibits ion dipole interaction between oxygens of agarose
1) is
1
with imidazolium cations and hydrogen bonding between agarose oxygen with C2ꢀH of
imidazolium ring as revealed from FTꢀIR spectroscopic analysis. FTꢀIR spectra of [Bmim]OH,
agar and
1 were recorded to study the interaction between IL and agar. The FTꢀIR spectrum of
[Bmim]OH displayed characteristic peaks at 3406 cmꢀ1 (OꢀH stretching vibrations), 1644 cmꢀ1
(inꢀplane CꢀC and CꢀN stretching vibrations of imidazolium), 1403 cmꢀ1 (CꢀH bending vibrations
of ꢀCH3) and 831 cmꢀ1 (CꢀH in plane vibrations of imidazolium) whereas that of pure agar gel
showed characteristic peaks at 3434 cmꢀ1 (OꢀH stretching vibrations of agarose and water), 1636
cmꢀ1 (bending vibrations of water), 1252 and 1074 cmꢀ1 (CꢀOꢀC and the glycosidic linkage of
agarose). On contrary, FTꢀIR spectrum of [Bmim]OH@agar (
1
) displayed peaks at 3401 cmꢀ1
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(OꢀH stretching vibrations of agarose, IL and water), 1634 cmꢀ1 (inꢀplane CꢀC and CꢀN
stretching vibrations of imidazolium), 1373 cmꢀ1 (CꢀH bending vibrations of ꢀCH3), 1260 cmꢀ1
(symmetrical CꢀH vibrations of imidazolium), 1175 cmꢀ1 (in plane CꢀH deformation vibrations of
imidazolium), 1070 cmꢀ1 (CꢀOꢀC and the glycosidic linkage of agarose) and 890 cmꢀ1 (CꢀH in
plane vibrations of imidazolium). A red shift in the position of OꢀH of pure agar gel from 3434
cmꢀ1 to 3401 cmꢀ1 in
1074 cmꢀ1 to 1070 cmꢀ1 in
1
and CꢀOꢀC stretching vibrations of glycosidic linkage of agarose from
1
are indicative of hydrogen bonding interactions.
Fig. 2 Structure of [Bmim]OH@agar (
1)
Next, we studied the effect of solvents on the physical nature of [Bmim]OH@agar (
1
). A
gel was cut into 2.5 x 2.5 x 2.0 mm3 pieces and stirred in different solvents at room temperature
for 24 h. Subsequently, the gel was filtered and its physical dimensions were measured. The
nature of gel remained intact in ethanol, methanol and acetone. The noticeable shrinkage in
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dimensions was found in dichloromethane and water. On the contrary, gel network collapsed in
toluene.
A thermal profile of [Bmim]OH@agar (1) was studied by thermogravimetric analysis
(TGA) and differential thermal analysis (DTA) (Fig. 3). An initial weight loss of 95% up to 162
°C in TGA curve is ascribed to desorption of water as evident from a strong endothermic peak in
o
o
o
DTA at 90 C. The second weight loss from 162 C to 308 C is attributed to the thermal
decomposition of [Bmim]OH along with the agar into their corresponding carbonaceous
material. The TGA analysis is consistent with the TGA profile of [Bmim]OH reported in the
literature30a revealing no obvious changes in the thermal stability of [Bmim]OH even after
entrapment.
Fig. 3 TGA and DTA curve of [Bmim]OH@agar (
Surface morphology of [Bmim]OH@agar ( ) was studied by Scanning Electron Microscopy
(SEM) analysis (Fig. 4). The SEM micrograph of
1)
1
1
revealed uniform gel type morphology with
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smooth surface which mimicked the morphology of gel obtained from only agar. This clearly
indicates that morphology of agar gel is preserved even after entrapment of [Bmim]OH into agar
gel.
Fig. 4 SEM micrographs of (
a
) agar; (
b
) [Bmim]OH@agar (
1
)
Our next task was to assess the catalytic efficiency of [Bmim]OH@agar (
1
) in the synthesis
of azlactones (Scheme 1). Initial studies to examine the effect of catalyst loading were carried
out using a model reaction between hippuric acid (1 mmol), ꢀnitrobenzaldehyde 3a (1 mmol),
acetic anhydride (1.2 mmol) in ethanol. The reaction proceeded smoothly affording the
expected azlactone viz. 4ꢀ[(4ꢀnitrophenyl)methylene]ꢀ2ꢀphenylꢀ5(4H)ꢀoxazolone (5a) in 75%
yield. When the quantity of ( ) was increased from 50 mg (0.025 mmol) to 100 mg (0.051
2
p
4
1
mmol), yield of the product (5a) was elevated significantly from 75% to 90% (Table 1, entries
1−2). However, further increase in catalyst quantity did not markedly influence yield of the
product (Table 1, entries 3ꢀ4).
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O
CHO
O
O
N
[Bmim]OH@agar (1)
EtOH, RT
N
COOH
+
+
Ac2O
H
R
R
5a-l
3a-l
2
4
Scheme 1 [Bmim]OH@agar (1) catalyzed azlactones synthesis
Table 1. Optimization of catalyst loading in synthesis of azlactonesa
O
CHO
O
O
[Bmim]OH@agar (1)
N
N
COOH
+
Ac2O
+
H
NO2
Ethanol, RT
NO2
5a
3a
2
4
Entry
Catalyst
(mg)
Time
Yieldb
(%)
75
(min.)
35
1
2
3
4
50 (0.025 mmol)
100 (0.051 mmol)
150 (0.077 mmol)
200 (0.103 mmol)
20
90
25
91
24
92
aReaction conditions: 2 (1 mmol), 3a (1 mmol), 4 (1.2 mmol) and EtOH (5 mL).
bIsolated yields after purification by column chromatography.
Next, we probed the effect of solvents on a model reaction by employing an array of
solvents. The model reaction afforded good yields in polar protic solvents such as methanol and
ethanol (Table 2, entries 1ꢀ2) while moderate yields were acquired in nonꢀpolar solvents such as
CH3CN, CH2Cl2, 1,4ꢀdioxane, CHCl3 and toluene (Table 2, entries 3ꢀ7). The reaction did not
proceed to the synthetically useful degree in solvent like DMF (Table 2, entry 8). Among all the
screened solvents, ethanol was found to furnish excellent yield of the product in shorter reaction
time (Table 2, entry 2).
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Table 2. Solvent optimization in the synthesis of azlactonesa
O
CHO
O
[Bmim]OH@agar (1)
O
N
(100 mg)
N
COOH
+
+
Ac2O
H
NO2
Solvent, RT
NO2
5a
2
3a
4
Entry
Solvent
Time
(Min.)
25
Yieldb
(%)
78
90
68
55
40
45
60
35
1
2
3
4
5
6
7
8
Methanol
Ethanol
CH3CN
CH2Cl2
20
40
45
1, 4ꢀdioxane
CHCl3
55
65
Toluene
DMF
80
70
aReaction conditions: 2 (1 mmol), 3a (1 mmol), 4 (1.2 mmol), [Bmim]OH@agar (1) (100 mg) and solvent (5 mL).
bIsolated yields after purification by column chromatography.
With an optimal set of parameters in hand, we turned our focus on surveying scope of
reaction by reacting structurally diverse aldehydes (3a–l) with hippuric acid (
2) and acetic
anhydride ( ) under the optimized reaction conditions. The results are illustrated in Table 3. The
4
present methodology is amenable to the presence of a variety of functional groups on aldehydes
and provided the anticipated products in moderate to good yields (Table 3, entries 5a–l).
Notably, aldehydes with electronꢀwithdrawing substituents (Table 3, entries 3a–c) were found to
be suitable for this transformation as corresponding products were obtained in high yields than
aldehydes with electronꢀdonating substituents (Table 3, entries 3f, 3hꢀ3j). More significantly,
sterically hindered aldehydes such as salicylaldehyde and
o
ꢀtolualdehyde also performed well
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providing the desired azlactones in relatively good yields (Table 3, entries 3e, 3g). It is
noteworthy to mention that heteroaromatic aldehydes such as 2ꢀthiophenecarboxaldehyde and 4ꢀ
pyridinecarboxaldehyde also reacted efficiently furnishing the corresponding azlactones in good
yields (Table 3, entries 3k–l).
Table 3. [Bmim]OH@agar (
1
) mediated synthesis of azlactonesa
O
CHO
O
[Bmim]OH@agar (1)
O
(100 mg)
N
N
COOH
+
+
Ac2O
H
EtOH, RT
R
R
5a-l
3a-l
2
4
Entry
Aldehyde
(3)
Product
(5)
Time
(min.)
Yieldb
(%)
O
CHO
O
N
20
90
O2N
O2N
a
CHO
O
O
N
25
35
82
87
NO2
b
NO2
CHO
O
O
F
N
c
F
CHO
O
O
N
d
40
82
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CHO
OH
O
90
76
e
N
O
O
OH
CHO
O
HO
N
f
105
115
80
77
HO
CHO
O
g
O
N
CHO
O
h
90
95
86
84
O
N
CHO
O
i
O
O
N
O
CHO
O
60
80
O
N
N
j
k
l
N
O
S
S
CHO
50
65
78
73
O
N
O
CHO
O
N
N
N
a Reaction conditions: 2 (1 mmol), 3 (1 mmol), 4 (1.2 mmol), [Bmim]OH@agar (1) (100 mg) and EtOH (5 mL).
b Isolated yields after purification by column chromatography.
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The successful application of [Bmim]OH@agar (
1
) in the synthesis of azlactones
encouraged us to explore its compatibility in Henry reaction for synthesis of
Scheme 2). To evaluate optimal reaction parameters, nitromethane ( ) (1 mmol) and
benzaldehyde (7a) (1 mmol) were chosen as model substrates in presence of [Bmim]OH@agar
) in ethanol. The impact of catalyst loading and solvents was investigated by employing
different amounts of [Bmim]OH@agar ( ) and variety of solvents (Table 4 and 5). The best
results were obtained by performing a model reaction in the presence of 150 mg of
β
ꢀnitro alcohols
(
6
p
ꢀnitro
(1
1
[Bmim]OH@agar (
1
) (0.077 mmol) in ethanol as a solvent.
OH
[Bmim]OH@agar (1)
CHO
NO2
(150mg)
CH3
NO2
+
EtOH, RT
R
R
6
7a-7k
8a-k
Scheme 2 [Bmim]OH@agar (1) catalyzed βꢀnitro alcohols synthesis
Table 4. Optimization of catalyst loading in synthesis of
β
ꢀnitro alcoholsa
OH
CHO
NO2
[Bmim]OH@agar (1)
CH3
NO2
+
NO2
EtOH, RT
NO2
6
7a
8a
Entry
Catalyst
(mg)
Time
(Min.)
160
55
Yieldb
(%)
50
1
2
3
4
50 (0.025 mmol)
100 (0.051 mmol)
150 (0.077 mmol)
200 (0.103 mmol)
70
25
94
25
95
aReaction conditions: 6 (1 mmol), 7a (1 mmol) and EtOH (5 mL).
bIsolated yields after purification by column chromatography.
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Table 5. Solvent optimization in the synthesis of β
ꢀnitro alcoholsa
OH
[Bmim]OH@agar (1)
CHO
NO2
(150mg)
CH3
NO2
+
NO2
Solvent, RT
NO2
6
7a
8a
Entry
Solvent
Time
Yieldb
(min.)
60
(%)
60
57
88
94
30
47
50
35
1
2
3
4
5
6
7
8
DMF
THF
45
Methanol
Ethanol
CHCl3
35
25
55
CH3CN
CH2Cl2
Toluene
48
45
65
aReaction conditions: 6 (1 mmol), 7a (1 mmol), [Bmim]OH@agar (1) (150 mg) and solvent (5 mL).
bIsolated yields after purification by column chromatography.
To establish generality of the method, substrate scope for the reaction was investigated
under optimized reaction conditions by reacting structurally and electronically diverse aldehydes
(
7aꢀk) with nitromethane (
6
), the results of which are depicted in Table 6. In all the cases,
ꢀnitro alcohols in good to excellent
reactions proceeded smoothly affording the corresponding
β
yields. In general, an electronic nature of the substituents attached to the aromatic ring had an
impact on the yields of the products as aryl aldehydes with electron poor substituents (Table 6,
entries 7a–b) required shorter reaction time and afforded the anticipated products in high yield
than those with electron rich substituents (Table 6, entries 7d, 7gꢀ7i). In addition, ortho
substituted benzaldehydes such as ꢀnitrobenzaldehyde, salicylaldehyde (Table 6, entries 7c, 7e
ꢀ
)
o
underwent smooth reaction affording corresponding products in good yields. Moreover,
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heterocyclic aldehydes such as 2ꢀpyridinecarboxaldeyde, 4ꢀpyridinecarboxaldehyde (Table 6,
entries 7jꢀ7k) were found to react well furnishing related products in better yields.
Table 6. [Bmim]OH@agar (1) mediated synthesis of β
ꢀnitro alcoholsa
OH
[Bmim]OH@agar (1)
CHO
NO2
(150mg)
CH3
NO2
+
EtOH, RT
R
R
6
7a-7k
8a-k
Entry
Aldehyde
(7)
Product
(8)
Time
(min.)
Yieldb
(%)
OH
CHO
NO2
a
25
35
94
90
O2N
O2N
CHO
OH
NO2
b
c
d
NO2
NO2
CHO
NO2
OH
NO2
40
40
85
75
NO2
OH
NO2
CHO
HO
HO
CHO
OH
OH
e
f
NO2
55
72
79
OH
OH
CHO
NO2
120
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CHO
OH
NO2
125
130
76
77
g
h
OH
NO2
CHO
O
O
OH
O
O
NO2
O
O
CHO
i
185
75
O
N
O
OH
NO2
N
CHO
j
20
15
75
78
OH
NO2
CHO
N
N
K
aReaction conditions: 6 (1mmol), 7 (1 mmol), [Bmim]OH@agar (1) (150 mg) and EtOH (5 mL).
bIsolated yields after purification by column chromatography.
1
Identity of all the compounds was ascertained on the basis of IR, H NMR, 13C NMR
spectroscopy and mass spectrometry. Spectroscopic data are consistent with the proposed
structures and in harmony with the literature.
In order to gain insight into the main catalytic component of [Bmim]OH@agar (1), both the
model reactions were carried out in presence of exclusively agar gel and [Bmim]OH. The agar
gel failed to initiate both the reactions. On the contrary, [Bmim]OH could catalyze both
ErlenmeyerꢀPlochl reaction (87% yield with 249 mol% catalyst loading) as well as Henry
reaction (91% yield with 10 mol% catalyst loading). This is conclusive evidence for suggesting
the prime role of [Bmim]OH in catalysis indicating that agar is unreactive immobilizing
medium.
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[Bmim]OH@agar (1)
O
H3C
NO2
..
N
H
OH
O
(III)
O
O
O
O
Henry
O
..
N
(Nitroaldol reaction)
H
O
O
Erlenmeyer Plochl
azalactone synthesis
O
O
H2C
NO2
N
O
(I)
H
R
O
O
N
O
O
H
NO2
R
O
O
N
R
(IV)
OH
H
(II)
R
OH
O
NO2
O
N
R
R
[Bmim]OH@agar (1)
Scheme 3 A plausible mechanism for the synthesis of azlactones and
β
ꢀnitro alcohols
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On the basis of literature reports,30bꢀd tentative mechanistic rational for the
[Bmim]OH@agar ( ) promoted ErlenmeyerꢀPlochl and Henry reaction is postulated in Scheme
. In an initial step in the ErlenmeyerꢀPlochl reaction, cyclocondensation of hippuric acid in the
presence of acetic anhydride generates intermediate 2ꢀphenyl oxazolꢀ5ꢀone ( ). The formation of
1
3
I
intermediate
I was confirmed by real time HRMS analysis of reaction mixture. The HRMS of
reaction mixture displayed a peak at m/z = 160.98, which represents molecular ion (M+) of
intermediate (I). In the next step, [Bmim]OH from 1 abstracts proton of C2 in I leading to the
formation of carbanion which subsequently shows the nucleophilic attack on C=O group of
aldehyde leading to the formation of II. In the last step, II eliminates water molecule furnishing
the corresponding azlactone (5). In case of Henry reaction, initially, [Bmim]OH from 1 abstracts
proton from nitromethane (III) leading to the formation of carbanion which subsequently shows
the nucleophilic attack on C=O group of aldehyde generating intermediate IV. Finally, IV
undergoes protonation to afford corresponding
βꢀnitro alcohol (8).
In order to confirm the heterogeneous nature of [Bmim]OH@agar (
1
), a split test was
performed with model ErlenmeyerꢀPlochl and Henry reaction. A catalyst was filtered off when
50% conversion was achieved (GC). The filtrate of the reaction mixture was stirred for
additional 1 h. There was no increase in the amount of the corresponding product beyond 50%.
These results revealed that entrapped IL is not being leached out from the agar matrix during the
course of reaction. This greatly simplified the purification step and resulted in quantitative yields
of product, without IL impurities.
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Fig. 5 Reusability of [Bmim]OH@agar (1) in the synthesis ofꢀ(1) βꢀnitro alcohols;
(2) Azlactones
A recyclability of the catalyst is a striking feature of heterogeneous catalysis and is a crucial
parameter which designates the efficiency of the catalyst. Reusability of [Bmim]OH@agar (1)
was tested for model ErlenmeyerꢀPlochl and Henry reactions. At the end of the reaction, the
catalyst was isolated with the aid of simple filtration, washed several times with ethanol and
subsequently reused in another catalytic cycle with identical substrates. The catalyst could be
reused for five catalytic cycles, without a substantial decrease in catalytic activity and yield of
product (Fig. 5).
In order to show merits of [Bmim]OH@agar (1) in comparison with other reported catalysts,
we have summarized some of the previous reports for the preparation of 4ꢀ[(4ꢀ
nitrophenyl)methylene]ꢀ2ꢀphenyloxazolꢀ5ꢀone and 1ꢀ(4ꢀnitrophenyl)ꢀ2ꢀnitroethanol in Table 7
and 8 respectively. It is evident from these result that [Bmim]OH@agar (
1) is more effective as
compared to many of the reported catalysts in terms of yields as well as reaction time.
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Table 7.
Comparison of various catalysts for synthesis of 4ꢀ[(4ꢀnitrophenyl)methylene]ꢀ2ꢀ
phenyloxazolꢀ5ꢀone (5a
)
Sr.
Catalyst
Quantity
Temp (oC)
Time
Yield
(%)
Ref.
No.
1
2
3
[Bnmim]H2PO4
[Et3NH][HSO4]
Mg/Al2O3
2 mol%
10 mol%
10 mol%
US, RT
100 oC
40 min.
20 min.
5 min.
90
96
90
12a
12b
31
MW
(300 W)
80 oC
4
5
K3PO4
Iodine
5 mol%
5 mol%
30 min.
65 sec.
93
94
11f
MW
11g
(320 W)
6
7
[Bmim]3PW12O40
[Bmim]4W10O32
5 mol%
5 mol%
5 mol%
80 oC
80 oC
75 min.
65 min.
24 min.
89
90
85
12c
12e
[C6CmIm)2]2W10O32.
2H2O
US
(RT)
RT
8
[Bmim]OH
249 mol%
90 min.
87
11e
Present
work
9
[Bmim]OH@agar (1)
5.37 mol%
RT
20 min.
90
29