Microwave-assisted solvent-free synthesis and luminescence properties of 2-substituted-4,5-di(2-furyl)-1H-imidazoles

Article abstract of DOI:10.1515/chempap-2015-0014

A solvent-free microwave-assisted method for the synthesis of 2-substituted-4,5-di(2-furyl)-1H-imidazoles was developed. Imidazoles with moderate to good yields were produced by condensation of furil with aldehydes over acidic alumina impregnated with amm

Full text of DOI:10.1515/chempap-2015-0014

Chemical Papers 69 (2) 325–338 (2015)
DOI: 10.1515/chempap-2015-0014
ORIGINAL PAPER
Microwave-assisted solvent-free synthesis and luminescence
properties of 2-substituted-4,5-di(2-furyl)-1 -imidazoles
Jian Zhang, Tian-Qi Zhao, Yao Chen, Xiao-Dong Chen, Hai-Kun Chang,
Yu-Min Zhang*, Shu-Cheng Hua*
College of Chemistry, First Hospital, Jilin University, Qianjin Street 2699, Changchun 130012, China
Received 4 May 2014; Revised 19 June 2014; Accepted 21 June 2014
A solvent-free microwave-assisted method for the synthesis of 2-substituted-4,5-di(2-furyl)-1H-
imidazoles was developed. Imidazoles with moderate to good yields were produced by condensation
of furil with aldehydes over acidic alumina impregnated with ammonium acetate, and they were
characterized by FT-IR, HRMS, 1H NMR and 13C NMR spectroscopy. Crystal structure of 2,4,5-
tri-2-furyl-1H-imidazole (I ) in the orthorhombic space group Pbca was reported, which showed more
coplanarity than the reported crystal structure of I in the monoclinic space group Cc. Moreover, their
luminescent properties were investigated. It was found that the organic small molecule compounds
synthesized possess higher fluorescence quantum efficiency (up to 0.508) in a 0.1 M H2SO4 aqueous
solution dissolved in 0.5 mL of CH3OH; along with higher stability; also the emission of some
compounds synthesized in the solution was sensitive to the polarity of the solvents.
c
ꢀ 2014 Institute of Chemistry, Slovak Academy of Sciences
Keywords: crystal structure, furil, luminescence property, microwave-assisted, 2-substituted-4,5-
di(2-furyl)-1H-imidazole
Introduction
stituted imidazoles have been reported so far (Schu-
bert & Stodolka, 1963; Stoeck & Schunack, 1974; Lip-
shutz, & Morey, 1983; Tsuji et al., 1983; Consonni
et al., 1991; Evans & Lundy, 1992; Claiborne et al.,
1998). Some of them involve fussy treatment, long re-
action times and relatively low yields. Nevertheless,
two research groups have recently reported one-pot
condensation of benzil, aldehyde, amine and ammo-
nium acetate on alumina, or silica solid support un-
der microwave irradiation (Usyatinsky & Khmelnit-
sky, 2000; Balalaie & Arabanian, 2000). The method
provides many profound advantages over the conven-
tional methods in chemical transformations such as
enhanced reaction rates and yields, significant energy
and organic solvents savings.
Imidazole derivatives play an important role in bio-
chemistry (Bräse et al., 2002; Nicolaou et al., 2002;
Chen et al., 2005; Costantino & Barlocco, 2006; Joshi
& Viswanathan, 2006) as materials including lumines-
cent (Zhao et al., 2006; Fang et al., 2007) and proton
conductive materials (Schuster et al., 2004; Benhab-
bour et al., 2005), medicinal chemicals (Nefzi et al.,
1997; Frazén, 2000; Horton et al., 2003; Fantini et al.,
2010; Soh & Lam, 2010) and in the determination of
heavy metal ions (Han et al., 2011; Kang et al., 2011;
Li et al., 2012) as they possess better pharmacologi-
cal and luminescent properties. Recently, compounds
with furan-rings have been reported for the determina-
tion of trace amounts of Cu2+ with a limit of detection
(3σ) of 6.5 × 10−11 mol L−1 (Kang et al., 2011), which
has prompted us to explore the synthesis and lumines-
cence property of imidazoles bearing furan rings.
Many methods for the synthesis of highly sub-
In the present work, a synthetic strategy provid-
ing not only advantaged preparation of 2-substituted-
4,5-di(2-furyl)-1H-imidazoles from furil and aldehy-
des according to the reported procedure (Usyatinsky
& Khmelnitsky, 2000) (Table 1) but also reducing
*Corresponding author, e-mail: zhang ym@jlu.edu.cn, shuchenghua@eyou.com
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326
J. Zhang et al./Chemical Papers 69 (2) 325–338 (2015)
Table 1. Effects of different solid supports on the yield of 4-(4,5-di(furan-2-yl)-1H-imidazol-2-yl)phenol
Solid support
Yield/%
Acidic alumina (Al2O3) (pH 4.3
Neutral alumina Al2O3
Acidic SiO2
0.5)
68
31
24
41
Strong acidic resin (activated by HCl)
environmental pollution was designed. Although the
microwave-assisted method has been successfully ap-
plied for the synthesis of 2-substituted-4,5-diphenyl-
1H-imidazoles, it has seldom been used for the synthe-
sis of 2-substituted-4,5-di(2-furyl)-1H-imidazoles. Be-
cause compounds containing furan rings have inherent
disadvantages, such as easy ring-opening in some re-
action media, etc. Thus, it is necessary to explore the
stability of furan rings under microwave irradiation.
Furthermore, luminescent materials of imidazole
derivatives have emerged as attractive blue-emitting
materials (Fang et al., 2007). Our reported crystal
structure of 2,4,5-tri(furan-2-yl)-1H-imidazole showed
that two furan rings (4- and 5-substituted) of the or-
ganic fluorescent molecule are not coplanar with the
imidazole ring (Wang et al., 2009). It is reported
in literature (Gao et al., 1999; Chen et al., 2003;
Zhao et al., 2006; Fang et al., 2007) that the non-
coplanarity could prevent intermolecular electrostatic
interactions between fluorescent molecules. Addition-
ally, imidazole moieties are electron rich and electron
donating due to the lone-pair electrons in the sp3 ni-
trogen atoms. Thus, an electron donating group con-
tributes to the charge transfer of a molecule. As a
result, imidazole derivatives possess better fluorescent
and thermal stability properties. Therefore, lumines-
cent and thermal stability properties of the synthe-
sized imidazole derivatives bearing furan rings have
been investigated here. Also, the fluorescence quantum
yields in a 0.1 M H2SO4 aqueous solution dissolved
in 0.5 mL of CH3OH were determined. The effect of
the different solutions on the fluorescence spectra of
compounds 2,4,5-tri(furan-2-yl)-1H-imidazole (I ), 4,5-
di(furan-2-yl)-2-phenyl-1H-imidazolen (II ) and 4,5-
di(furan-2-yl)-2-styryl-1H-imidazole (XIII ) were stud-
ied. Besides, the fluorescence emission of compounds
I and II was compared with that of 4,5-di(furan-2-yl)-
1-(4-methylbenzyl)-2-phenyl-1H-imidazole (Chen et
al., 2013), suggesting that the 2-position substituted
group of imidazole plays a key role in the fluorescence
emission shift in the same solution.
Experimental
A MCL-3-type microwave reactor (Sichuan Univer-
sity, China) with a thermometer for microwave ap-
plication was used in all experiments. All reagents
obtained from commercial companies (Tianjin Fuyu
Reagent, China; Aladdin-reagent, China) were of an-
alytical reagent (AR) quality and were used with-
out further purification. Furoin and furil were syn-
thesized according to the Li method (Li et al., 2006)
and the Gao method (Gao et al., 1998), respectively.
Furfural was distilled under reduced pressure before
its use. Thin layer chromatography was carried out
on silica gel (GF-254) TLC plates. All melting points
were determined on a XT-4 melting point apparatus
(China) without correction. IR spectra were collected
with an Nicolet 5PC FT–IR spectrometer (USA).
1H and 13C NMR spectra were measured using an
Varian Mercury-300 NMR spectrometer (USA) or a
Bruker AVANCE-500 NMR spectrometer (Germany)
with TMS as an internal standard. MS was collected
on an Agilent HP1100/6890 LC/MS spectrometer. El-
emental analyses were done using a FlashEA1112 el-
emental analyzer (Italy). TGA was measured on a
Shimadzu DTG-60H simultaneous DTA-TG appara-
tus (Japan) at the heating rate of 5◦C min−1 un-
der nitrogen atmosphere. The absorption spectra were
recorded on an Australian GBC Cintra 10e UV-VIS
Spectrophotometer within the wavelength range from
200 nm to 800 nm. Fluorescence spectra were recorded
on a Shimadzu RF-5301 spectrofluorimeter (Japan).
Fluorescence quantum yields (ΦF) were determined
against quinine sulfate in 0.1 M H2SO4 (ΦF = 0.55)
as the standard. Characterization data and spectral
data of prepared compounds are in Table 2 and Ta-
ble 3, respectively.
General procedure for the synthesis
of 2-substituted-4,5-di(2-furyl)-1H-imidazoles
A mixture of acidic alumina (1.02 g, 10 mmol),
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J. Zhang et al./Chemical Papers 69 (2) 325–338 (2015)
Table 2. Structures and yields of the synthesized compounds
327
Entry
1
R
Compound
Yielda/%
M.p./◦C
206–209b
ΦF
63
0.250
2
3
65
62
207–209b
0.376
0.366
217–219b
4
5
68
65
254–256
168–170
0.311
0.350
6
71
216–218b
0.310
7
8
79
86
224–226
186–187
0.220
0.213
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328
J. Zhang et al./Chemical Papers 69 (2) 325–338 (2015)
Table 2. (continued)
Entry
R
Compound
Yielda/%
M.p./◦C
197–198
ΦF
9
87
0.003c
10
78
189–191
0.325
11
12
13
82
47
49
132–134
244–246b
209–211b
0.357
0.008c
0.508
a) Yield refers to recrystallized product after column chromatography; b) according to Li et al. (2012); c) fluorescence quenching.
furil (0.19 g, 1 mmol), ammonium acetate (0.38 g,
5mmol) and the appropriate aldehydes (1.2 mmol) was
fully ground in a mortar. The reactive mixture was
transferred to a 50 mL dry two-neck round bottomed
flask containing a thermometer and a condenser, and
heated with microwave irradiation for 10 min (the re-
action temperature was 130◦C when the reaction was
over.). The procedure was repeated twice. The reac-
tion was monitored by TLC. The residue was directly
purified by column chromatography on Chemapol
(200–300 mesh) silica gel (eluent, petroleum ether–
EtOAc, 3 : 1 vol.) to provide 2-substituted-4,5-di(2-
furyl)-1H-imidazoles.
noted, hydrogen atoms were in idealized positions and
were allowed to ride. The details on crystallographic
collection and refinement data are given in Table 4.
CCDC–990908 (I ), contains the supplementary crys-
tallographic data for this paper. These data can be ob-
tained free of charge at www.ccdc.cam.ac.uk/ deposit
[or from the Cambridge Crystallographic Data Cen-
tre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax:
+ 44-1223-336-033; E-mail: deposit@ccdc.cam.ac.uk.
Results and discussion
Synthesis of 2-substituted-4,5-di(2-furyl)-1H-
imidazoles
Crystal structure determination
for compound I
It is reported that silica-gel and alumina play an
outstanding role in microwave assisted organic chem-
ical synthesis (Loupy et al., 1998; Varma, 1999). In
our previous work, the application of silica-gel and
alumina in microwave synthesis was extended. Some
2-substituted-4,5-di(2-furyl)-1H-imidazoles were syn-
thesized starting from 1,2-di(furan-2-yl)-2-oxoethyl
The structures were solved by a direct method
(Sheldrick, 1997a) and refined by full-matrix least
squares based on F2 using the SHELXTL 5.1 soft-
ware package (Sheldrick, 1997b). All non-hydrogen
atoms were refined anisotropically. Unless otherwise
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Table 3. Characterization data of synthesized compounds
329
Compound
Characterizing data
I
Yield: 63 % (0.14 g)
M.p.: 206–209◦C
1H NMR (500 MHz, CDCl3), δ: 10.48 (s, 1H, imidazole NH), 7.43 (s, 2H, furyl H5), 7.37 (s, 1H, furyl H5), 6.93 (d,
J = 3.3 Hz, 3H, furyl H3), 6.46 (dd, J = 3.0 Hz, J = 1.7 Hz, 2H, furyl H4), 6.41 (dd, J = 3.1 Hz, J = 1.6 Hz, 1H,
furyl H3
13C NMR (125 MHz, CDCl3), δ: 146.9, 145.0, 143.0 (furyl C2), 141.6 (imidazole C2), 139.2 (imidazole C4, C5),
112.2 (furyl C5), 111.8 (furyl C5), 108.6 (furyl C4), 107.8 (furyl C3)
MS, m/z: 267.9 (Calc. 267.26) [M + H]+
wi(found)/%: C, 67.53; H, 3.71; N, 10.45
wi(calc.)/% for C15H10N2O3: C, 67.67; H, 3.79; N, 10.52
II
Yield: 65 % (0.18 g)
M.p.: 207–209◦C
1H NMR (300 MHz, CDCl3), δ: 7.92 (t, J = 1.9 Hz, 1H, phenyl H2), 7.89 (t, J = 1.9 Hz, 1H, phenyl H6), 7.51–7.37
(m, 5H, furyl H4, H5, phenyl H3), 6.98 (d, J = 3.3 Hz, 2H, phenyl H4, H5), 6.52 (dd, J = 3.4 Hz, 1.8 Hz, 2H, furyl
H3). NH proton signal of imidazole was not observed
13C NMR (75 MHz, CDCl3), δ: 146.4 (imidazole C2), 129.4(furyl C2, phenyl C1, imidazole C4, C5), 129.2 (furyl
C5), 129.0 (phenyl C4), 125.7 (phenyl C2, C3, C5, C6), 111.8 (furyl C4), 107.0 (furyl C3)
MS, m/z: 277.9 (Calc. 277.30) [M + H]+
wi(found)/%: C, 73.78; H, 4.29; N, 9.97
wi (calc.)/% for C17H12N2O2: C, 73.90; H, 4.38; N, 10.14
III
Yield: 62 % (0.18 g)
M.p.: 217–219◦C
1H NMR (300 MHz, CDCl3), δ: 7.83 (d, J = 8.1 Hz, 2H, phenyl H2, H6), 7.50 (d, J = 1.2 Hz, 2H, furyl H5), 7.23
(s, 2H, furyl H3), 7.01 (d, J = 3.2 Hz, 2H, phenyl H3, H5), 6.53 (dd, J = 3.3 Hz, 1.8 Hz, 2H, furyl H4), 2.38 (s, 3H,
CH3). NH proton signal of imidazole was not observed
13C NMR (75 MHz, CDCl3), δ: 146.5 (imidazole C2), 139.4 (imidazole C4, C5, furyl C2), 129.7 (furyl C5), 126.6
(phenyl C1, C4), 125.5 (phenyl C2, C3, C5, C6), 112.0 (furyl C4), 107.6 (furyl C3), 21.5 (CH3)
MS, m/z: 291.2 (Calc. 291.32) [M + H]+
wi (found)/%: C, 74.52; H, 4.76; N, 9.85
wi (calc.)/% for C18H14N2O2: C, 74.47; H, 4.86; N, 9.65
IV
Yield: 68 % (0.20 g)
M.p.: 254–256◦C
IR (KBr), ν˜ /cm−1: 3444 (phenyl—OH), 3409, 3123 (imidazole—NH), 1612, 1532, 1470, 1452, 1386 (C C, C N),
—
—
—
—
1284 (C—N), 1203, 1026 (C—O—C), 993, 887, 841, 729, 592
1H NMR (300 MHz, DMSO), δ: 12.57 (s, 1H, imidazole—NH), 9.75 (s, 1H, phenyl—OH), 7.94–7.91 (m, 1H, phenyl
H6), 7.90–7.88 (m, 1H, phenyl H2), 7.81 (dd, J = 1.8 Hz, 0.7 Hz, 1H, furyl H5), 7.70 (dd, J = 1.8 Hz, 0.8 Hz, 1H,
furyl H5), 6.92 (dd, J = 3.4 HZ, 0.7 Hz, 1H, furyl H3), 6.88–6.85 (m, 1H, furyl H3), 6.85–6.82 (m, 1H, furyl H4),
6.71 (dd, J = 3.3 Hz, 0.8 Hz, 1H, furyl H4), 6.65 (dd, J = 3.4 Hz, 1.8 Hz, 1H, phenyl H5), 6.57 (dd, J = 3.3 Hz,
1.8 Hz, 1H, phenyl H3)
13C NMR (75 MHz, DMSO), δ: 158.1 (imidazole C2), 149.5 (phenyl C4), 146.8 (furyl C2), 127.2 (furyl C5), 120.9
(imidazole C4, C5), 118.6 (phenyl C1), 115.4 (phenyl C2, C6), 111.8 (phenyl C5), 111.3 (phenyl C3), 108.0 (furyl
C4), 106.6 (furyl C3)
MS, m/z: 293.9 (Calc. 293.30) [M + H]+
wi (found)/%: C, 70.13; H, 4.32; N, 9.62
wi (calc.)/% for C17H12N2O3: C, 69.86; H, 4.14; N, 9.58
V
Yield: 65 % (0.19 g)
M.p.: 168–170◦C
IR (KBr), ν˜ /cm−1: 3125 (imidazole—NH), 3068, 3026 ( C—H), 1611, 1492, 1442, 1419, 1365(C C, C N), 1232
—
—
—
—
—
—
(C—N), 1201, 1014(C—O—C), 976, 885, 841, 736, 592
1H NMR (300 MHz, CDCl3), δ: 7.90–7.84 (m, 2H, furyl H5), 7.48 (dd, J = 1.8 Hz, 0.7 Hz, 2H, phenyl H2, H6), 7.09
(t, J = 8.7 Hz, 2H, furyl H3), 6.96 (dd, J = 3.4 Hz, 0.6 Hz, 2H, phenyl H3, H5), 6.51 (dd, J = 3.4 Hz, 1.8 Hz, 2H,
furyl H4). NH proton signal of imidazole was not observed
13C NMR (75MHz, CDCl3), δ: 164.7 (imidazole C2), 162.7 (phenyl C4), 147.3 (furyl C2), 145.9 (imidazole C4, C5),
141.9 (furyl C5), 127.9 (phenyl C1), 125.9 (phenyl C2, C6), 116.4 (phenyl C5), 116.2 (phenyl C3), 112.1 (furyl C4),
108.1 (furyl C3)
MS, m/z: 295.8 (Calc. 295.29) [M + H]+
wi (found)/%: C, 69.21; H, 4.02; N, 9.46
wi (calc.)/% for C17H11FN2O2: C, 69.38; H, 3.77; N, 9.52
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J. Zhang et al./Chemical Papers 69 (2) 325–338 (2015)
Table 3. (continued)
Compound
Characterizing data
VI
Yield: 71 % (0.22 g)
M.p.: 216–218◦C
1H NMR (300 MHz, CDCl3), δ: 7.90–7.87 (m, 1H, phenyl H6), 7.87–7.84 (m, 1H, phenyl H2), 7.51 (d, J = 1.7 Hz,
2H, furyl H5), 7.46–7.39 (m, 2H, furyl H3), 7.01 (d, J = 3.2 Hz, 2H, phenyl H3, H5), 6.54 (dd, J = 3.4, 1.8 Hz, 2H,
furyl H4). NH proton signal of imidazole was not observed
13C NMR (75 MHz, DMSO), δ: 145.2(imidazole C2), 141.7 (furyl C2), 135.3 (imidazole C4, C5), 129.5 (furyl C5),
127.3 (phenyl C1, C4), 127.7 (phenyl C3, C5), 126.9 (phenyl C2, C6), 112.0 (furyl C4), 108.0 (furyl C3)
MS, m/z: 312.8 (Calc. 311.74) [M + 2]+ , 311.8 (Calc. 311.74) [M + H]+
wi (found)/%: C, 65.46; H, 3.65; N, 8.73
wi (calc.)/% for C17H11ClN2O2: C, 65.71; H, 3.57; N, 9.02
VII
Yield: 79 % (0.28 g)
M.p.: 224–226◦C
IR (KBr), ν˜ /cm−1: 3114(imidazole—NH), 3052, 3013 ( C—H), 1602, 1550, 1481, 1430, 1373 (C C, C N), 1260
—
—
—
—
—
—
(C—N), 1201, 1071 (C—O—C), 830, 732, 590
1H NMR (300 MHz, CDCl3), δ: 7.82–7.77 (m, 2H, furyl H5), 7.61–7.56 (m, 2H, phenyl H2, H6), 7.51 (dd, J =
1.8 Hz, 0.7 Hz, 2H, phenyl H3, H5), 7.00 (d, J = 3.3 Hz, 2H, furyl H3), 6.54 (dd, J = 3.4 Hz, 1.8 Hz, 2H, furyl H4).
NH proton signal of imidazole was not observed
13C NMR (75 MHz, DMSO), δ: 145.4 (imidazole C2), 142.0 (furyl C2), 132.5 (furyl C5), 128.5(imidazole C4, C5,
phenyl C1), 127.3 (phenyl C2, C3, C5, C6), 123.8 (phenyl C4), 112.2 (furyl C4), 108.2 (furyl C3)
MS, m/z: 357.3(Calc.345.00) [M + 2 + H]+ , 355.1 (Calc. 354.00) [M + H]+
wi (found)/%: C, 57.24; H, 2.98; N, 7.49
wi (calc.)/% for C17H11BrN2O2: C, 57.49; H, 3.12; N, 7.89
VIII
Yield: 86 % (0.23 g)
M.p.: 186–187◦C
IR (KBr), ν˜ /cm−1: 3404, 3116 (imidazole—NH), 3063, 3003 ( C—H), 2227 (C—N), 1689, 1609, 1559, 1525, 1489,
—
—
—
—
—
—
—
—
1437, 1372 (C C, C N), 1283 (C—N), 1203, 1012(C—O—C), 885, 846, 732, 595
1H NMR (300 MHz, CDCl3), δ: 8.02 (t, J = 1.7 Hz, 1H, furyl H5), 7.99 (d, J = 1.5 Hz, 1H, furyl H5), 7.73–7.70
(m, 1H, phenyl H6), 7.70–7.67 (m, 1H, phenyl H2), 7.55–7.49 (m, 2H, phenyl H3, H5), 7.04–7.00 (m, 2H, furyl H3),
6.55 (dd, J = 3.4 Hz, 1.8 Hz, 2H, furyl H4). NH proton signal of imidazole was not observed
13C NMR (75 MHz, DMSO), δ: 146.5 (imidazole C2), 144.0 (furyl C2), 142.0 (phenyl C1), 133.1 (imidazole C4, C5,
—
—C—N), 132.7 (furyl C2), 125.6 (phenyl C3, C5), 118.7 (phenyl C4), 112.4 (phenyl C2, C6), 112.3 (furyl C4), 108.5
—
(furyl C3)
MS, m/z: 302.9 (Calc. 302.31) [M + H]+
wi (found)/%: C, 71.53; H, 3.94; N, 13.67
wi (calc.)/%for C18H11N3O2: C, 71.75; H, 3.68; N, 13.95
IX
Yield: 87 % (0.28 g)
M.p.: 197–198◦C
IR (KBr), ν˜ /cm−1: 3152 (imidazole—NH), 3095, 3032 ( C—H), 1600, 1520, 1469, 1345 (C C, C N), 1232
—
—
—
—
—
—
(C—N), 1202, 1069 (C—O—C), 915, 871, 812, 739, 715, 591
1H NMR (300 MHz, CDCl3), δ: 8.72 (t, J = 1.9 Hz, 1H, phenyl H5), 8.36 (ddd, J = 7.8 Hz, 1.6 Hz, 1.1 Hz, 1H,
phenyl H3), 8.23 (ddd, J = 8.2 Hz, 2.2 Hz, 1.0 Hz, 1H, phenyl H6), 7.64 (t, J = 8.0 Hz, 1H, phenyl H2), 7.53 (dd,
J = 1.8 Hz, 0.7 Hz, 2H, furyl H5), 7.05 (d, J = 3.0 Hz, 2H, furyl H3), 6.56 (dd, J = 3.4, 1.8 Hz, 2H, furyl H4). NH
proton signal of imidazole was not observed
13C NMR (75 MHz, DMSO), δ: 148.8 (imidazole C2), 143.6 (furyl C2), 142.0 (phenyl C4), 131.6 (furyl C5), 130.9
(imidazole C4, C5, phenyl C1), 130.2 (phenyl C2, C6), 123.7 (phenyl C5), 120.1 (phenyl C3), 112.08 (furyl C4),
108.5 (furyl C3)
MS, m/z: 322.9(Calc. 321.29) [M + H]+
wi (found)/%: C, 63.25; H, 3.52; N, 12.98
wi (calc.)/% for C17H11N3O4: C, 63.55; H, 3.45; N, 13.08
carboxylates (Li et al, 2012). Herein, chemical re-
actions were accelerated which might be caused by
microwaves decreasing the activation energy of the
reactions (Langa et al., 1997). Furthermore, some
corresponding tetra-imidazoles containing furan rings
were synthesized from 2-substituted-4,5-di(2-furyl)-
1H-imidazoles. Luminescence properties of the syn-
thesized tetra-imidazoles were studied (Chen et al.,
2013). As a continuation of our work, in order to ex-
plore the possibility of 2-substituted-4,5-di(2-furyl)-
1H-imidazoles synthesis from furil and aldehydes un-
der microwave irradiation, optimization of solid sup-
ports, inorganic oxide and acid resins, was attempted.
Effects of different solid supports on the yield of
4-(4,5-di(furan-2-yl)-1H-imidazol-2-yl)phenol were in-
vestigated by employing 4-hydroxybenzaldehyde and
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J. Zhang et al./Chemical Papers 69 (2) 325–338 (2015)
331
Table 3. (continued)
Compound
Characterizing data
X
Yield: 78 % (0.26 g)
M.p.: 189–191◦C
IR (KBr), ν˜ /cm−1: 3122 (imidazole—NH), 3068, 3020 ( C—H), 2962, 2860 (—CH3), 1602, 1528, 1491, 1426, 1364
—
—
—
—
(C C, C N), 1268 (C—N), 1201, 1080 (C—O—C), 971, 886, 839, 732, 593
—
—
1H NMR (300 MHz,CDCl3), δ: 7.89–7.85 (m, 1H, phenyl H6), 7.85–7.83 (m, 1H, phenyl H2), 7.49 (dd, J = 1.8 Hz,
0.7 Hz, 2H, furyl H5), 7.47 (d, J = 2.0 Hz, 1H, furyl H3), 7.45–7.43 (m, 1H, furyl H2), 7.00 (d, J = 3.3 Hz, 2H,
phenyl H3, H5), 6.52 (dd, J = 3.4 Hz, 1.8 Hz, 2H, furyl H4), 1.34 (s, 9H, 3 × CH3). NH proton signal of imidazole
was not observed
13C NMR (75MHz, CDCl3), δ: 152.6 (imidazole C2), 146.4 (furyl C2, phenyl C4), 126.6 (imidazole C4, C5, phenyl
C1), 126.0 (furyl C5), 125.4 (phenyl C2, C3, C5, C6), 112.0 (furyl C4), 107.7 (furyl C3), 34.9 (—C(CH3)3), 31.4
(3 × CH3)
MS, m/z: 333.2 (Calc. 333.40) [M + H]+
wi (found)/%: C, 75.81; H, 6.20; N, 8.59
wi (calc.)/% for C21H20N2O2: C, 75.88; H, 6.06; N, 8.43
XI
Yield: 82 % (0.25 g)
M.p.: 132–134◦C
IR (KBr), ν˜ /cm−1: 3431, 3116 (imidazole—NH), 3073, 3042 ( C—–H), 2973, 2842 (CH3), 1586, 1542, 1483, 1438,
—
—
—
—
1394 (C C, C N), 1262(C—N), 1203, 1020(C—O—C), 886, 736, 594
—
—
1H NMR (300 MHz, CDCl3), δ: 8.58 (d, J = 1.6 Hz, 1H, phenyl H5), 7.52 (d, J = 1.1 Hz, 2H, furyl H5), 7.43–7.32
(m, 1H, phenyl H4), 7.18–6.98 (m, 4H, furyl H3, phenyl H3,H5), 6.54 (dd, J = 3.3 Hz, 1.8 Hz, 2H, furyl H4), 4.09
(s, 3H, —O—CH3). NH proton signal of imidazole was not observed.
13C NMR (75 MHz, DMSO), δ: 156.3 (furyl C2, phenyl C6), 144.6 (imidazole C2), 141.7 (imidazole C4, C5), 130.4
(furyl C5), 129.3 (phenyl C2, C4), 122.0 (phenyl C3), 117.9 (phenyl C1), 112.1 (phenyl C5), 111.6 (furyl C4), 107.7
(furyl C3), 56.3 (—O—CH3)
MS, m/z: 307.9 (Calc. 307.32) [M + H]+
wi (found)/%: C, 70.42; H, 4.85; N, 8.89
wi (calc.)/% for C18H14N2O3: C, 70.58; H, 4.61; N, 9.15
XII
Yield: 47 % (0.13 g)
M.p.: 244–246◦C
1H NMR (300 MHz, DMSO), δ: 9.27 (s, 1H, pyridine H2), 8.61 (dd, J = 5.0 Hz, 1.5 Hz, 1H, pyridine H6), 8.49–8.40
(m, 1H, pyridine H4), 7.80 (d, J = 1.2 Hz, 2H, furyl H5), 7.55 (dd, J = 8.0 Hz, 4.8 Hz, 1H, pyridine H5), 6.89 (d, J
= 2.9 Hz, 2H, furyl H3), 6.64 (dd, J = 3.4 Hz, 1.8 Hz, 2H, furyl H4). NH proton signal of imidazole was not observed
13C NMR (75MHz, DMSO), δ: 148.9 (pyridine C2), 146.3 (pyridine C6), 143.5 (furyl C2, imidazole C2, 142.4 (furyl
C5), 133.3 (pyridine C4), 125.8 (imidazole C4, C5, pyridine C3), 123.9 (pyridine C5), 111.7 (furyl C4), 108.0 (furyl
C3)
MS, m/z: 278.8 (Calc. 278.29) [M + H]+
wi (found)/%: C, 69.19; H, 4.05; N, 14.93
wi (calc.)/% for C16H11N3O2: C,69.31; H, 4.00; N, 15.15
XIII
Yied: 49 % (0.15 g)
M.p.: 209–211◦C
1H NMR (500 MHz, DMSO ), δ: 12.75 (s, 1H , imidazole—NH), 7.78 (d, J = 48.9 Hz, 2H, furyl H5), 7.59 (d, J =
7.5 Hz, 2H, phenyl H2, H6), 7.52 (d, J = 16.5 Hz, 1H, phenyl H4), 7.41 (t, J = 7.6 Hz, 2H, furyl H3), 7.32 (t, J =
7.3 Hz, 1H, furyl H4), 7.06 (d, J = 16.5 Hz, 1H, furyl H4), 6.97 (s, 1H, phenyl H5), 6.66 (dd, J = 44.5 Hz, 31.3 Hz,
—
3H, phenyl H3, —CH CH—)
—
13C NMR (125 MHz, DMSO), δ: 146.7 (furyl C2), 137.1 (imidazole C4, C5, imidazole C2, phenyl C6), 129.8 (furyl
—
—
C5), 127.5 (phenyl—CH ), 117.4 (phenyl C3, C5), 113.4 (phenyl C2, C6), 112.9 (phenyl C4), 112.3 ( CH—
—
—
imidazole), 108.9 (furyl C4), 107.6 (furyl C3)
MS, m/z: 303.0 (Calc. 303.34) [M + H]+
wi (found)/%: C, 75.82; H, 4.81; N, 9.45
wi (calc.)/% for C19H14N2O2: C, 75.58; H, 4.67; N, 9.27
furil under uniform mixing of solid supports under mi-
crowave irradiation conditions for 10 min and in the
absence of any organic solvent. The experiment results
are summarized in Table 1.
The results indicate that the yield of 4-(4,5-
di(furan-2-yl)-1H-imidazol-2-yl)phenol was higher
when an identical loading of acidic alumina was used
as the supported medium. The reason might be that
acidic alumina provides more appropriate acidity to
catalyze the reaction than acidic SiO2. Nonetheless,
acidity of a strong acidic resin is too strong. There-
fore, acidic alumina was applied as the solid support
to synthesize imidazole derivatives with furan rings.
The conventional method for synthesizing imida-
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Table 4. Crystal data and structure refinement for compound I
Data
I
Formula
C15H10N2O3
Molecular mass
Temperature/K
Crystal system
Space group
266.25
293(2)
Orthorhombic
Fdd2
˚
a/A
14.854(3)
˚
b/A
32.981(7)
˚
c/A
10.523(2)
α/◦
β/◦
γ/◦
90
90
90
3
˚
Volume/A
Z
Dcalcd/(mg m−3
F(000)
θ range for data collection/◦
Limiting indices
Data/restraints/parameters
Goodness-of-fit on F2
5154.8(18)
16
1.372
2208
)
3.01–27.47
–19 ≤ h ≤ 18 –42 ≤ k ≤ 42 –13 ≤ l ≤ 13
2933/99/223
1.101
Final R indices [I ¿ 2σ(I)]
R indices (all data)
Largest diff. peak and hole/(e A−3
R1a = 0.0406 wRb = 0.0960
R1a = 0.0566 wRb = 0.1024
0.191 and –0.133
)
ꢁ
ꢀ
ꢁ
ꢂ
ꢁ
ꢂꢂ
ꢀ
ꢀ
ꢀ
1/2
a) R1 =
||Fo|–|Fc||/
|Fo|; b) wR =
w(Fo2 − Fc2)2
/
w(Fo2)2
.
zole derivatives includes 1,2-diketone and appropriate
aldehydes reflux in the presence of ammonium acetate
and glacial acetic acid (Wang et al., 2003; Parveen et
al., 2007; Wang et al., 2011). In fact, these reactions
commonly require higher temperature (over 118◦C)
and the reaction time of 5–6 hours, whereas similar
or higher yields and cleaner reaction profiles can be
obtained using microwave heating for 10 minutes on
solid support acidic alumina under solvent-free condi-
tions (Table 2).
Fig. 1. Resonance structures of product XI: XIa (left) and XIb
Comparing the new approach to 2-substituted-
4,5-di(2-furyl)-1H-imidazoles synthesis by the conven-
tional method and by our new reported method (Li
et al., 2012), microwave-assisted solvent-free assem-
bled process represents a much more convergent route
by utilizing a common intermediate and introduc-
ing solid support acidic alumina. Herein, the nucle-
ophilic addition of aldehydes and furil and the self-
cycloaddition reaction with ammonium acetate was
readily carried out and 2-substituted-4,5-di(2-furyl)-
1H-imidazoles were obtained in moderate and prefer-
able yields. However, the compounds synthesized us-
ing our reported method (Li et al., 2012) were also
prepared by this method. A possible reason is that
solid support acidic alumina prevents ammonia from
forming the ammonium ion without nucleophilic re-
activity and acts as the catalyst and dispersant in
the reaction. As shown in Table 2, the yields of prod-
ucts with stronger electron-withdrawing substituted
groups (—CN, —NO2) (entries 8 and 9) on the aro-
matic ring are higher than those of the products with
electron-donating substituted groups (—CH3, —OH,
t-butyl and styryl) (entries 3, 4, 10 and 13) and with
(right).
weak electron-withdrawing substituted groups (—F,
—Cl and —Br) (entries 5, 6 and 7) on the aro-
matic ring, probably due to the electron-withdrawing
substituted groups enhancing the electrophilicity of
the aldehyde carbonyl group. However, the yield was
also higher when the reaction was preformed with
2-methoxyl benzaldehyde with an electron-donating
substituted group and furil as the starting materi-
als. A possible reason is that the target product, XI
(entry 11), showed better stabilization of the stable
resonance structure XIb formed in an acidic medium
(Jayabharathi et al., 2012a) (Fig. 1). Besides, when
solid support acidic alumina was five times recycled to
synthesize XIII, the yields of the obtained XIII were
45.5 %, 47.6 %, 49.3 %, 48.9 % and 47.4 %, respec-
tively, suggesting that acidic alumina can be recycled.
Single crystal determination of product I
Recently, another synthesis method for trisubsti-
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333
Fig. 2. Molecular structure of compound I in the monoclinic space system (Wang et al., 2009) (a); molecular structure of compound
I in the orthorhombic space system (b); thermal ellipsoids are drawn at the 30 % probability levels.
Table 5. Hydrogen bond geometries and C—H· · · · π interaction parameters for I
α(DHA)/◦
˚
˚
˚
Structure
D—H· · · · A
d(D—H)/A
d(H· · · · A)/A
d(D· · · · A)/A
N(1)—H(100)· · · · N(2)a
C(3)—H(3A)· · · · O(2)
C(8)—H(8)· · · · Cg(1)b
0.88(2)
0.93
0.93
2.08(2)
2.54
2.7369(4)
2.946(2)
3.093(9)
3.5834(5)
168.1(18)
118.8
151.780(12)
I
a) x + 1/4, –y + 1/4, z + 1/4; b) –x, –y + 1/2, z – 1/2. Cg(1) represents the centroid of the imidazole ring (N1/C5/C6/N2/C11).
tuted imidazoles containing furan rings has been re-
ported (Li et al., 2012; Wang et al., 2009). In this
study, product I was recrystallized after the column
chromatography to obtain two kinds of single crystals
with different polymorphs, one of which, in monoclinic
system, was reported in our previous work (Wang
et al., 2009). Here, the crystal structure of product
I in the orthorhombic system is reported. Three fu-
ran rings and an imidazole ring in the orthorhom-
bic system are nearly coplanar. The dihedral angles
between the three furan rings, C1/C2/C3/C4/O1,
C7/C8/C9/C10/O2, C12/C13/C14/C15/O3 and the
imidazole ring, N1/C5/C6/N2/C11, are 1.5◦, 7.7◦ and
11.7◦, respectively, suggesting that the three furan
rings and the imidazole ring of compound I are more
coplanar than those in the reported single crystal
structure (Wang et al., 2009), which is probably the
reason for the higher melting point of compound I.
An S(7) ring motif (Bernstein et al., 1995) is formed
due to the intramolecular C3—H3· · · · O2 interaction
(Fig. 2 and Table 5). In the monoclinic system, the
ring atoms in all three furan rings are all oriented
clockwise while in the orthorhombic system, the ring
atoms in two furan rings are oriented clockwise and
those in the third furan ring are oriented anticlock-
wise (Fig. 2). Similarly to the packing of the molecule
in the monoclinic polymorph, neighboring molecules
in the orthorhombic system are also nearly vertical
to each other with the dihedral angle of 84.0◦ (98.0◦
in the monoclinic polymorph) and linked together by
intermolecular N—H· · · · N hydrogen bonds into 1-
D infinite chains (Table 5, Fig. 2a). The 1-D infinite
chains are further linked by weak intermolecular C—
H· · · · π interactions (Table 5, Fig. 2b) to form a 3-D
network. The furan ring in the 5-position of the imi-
dazole ring is disordered over two positions with the
site occupation factors of 0.448 and 0.552.
Studies of thermal stability and photolumines-
cence properties
In general, thermal stability of organic materials
is an important factor influencing their application.
Herein, thermal properties of imidazoles (entries 1,
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334
J. Zhang et al./Chemical Papers 69 (2) 325–338 (2015)
Fig. 3. Packing of molecule I: packing of the molecules showing intermolecular N—H· · · · N interactions forming a 1-D chain (a);
packing of molecules showing a 3-D network formed by intermolecular N—H· · · · N interactions and C—H· · · · π interactions
(b). Color scheme: blue – nitrogen; red – oxygen; light grey – carbon; grey – hydrogen. Green points represent the centroids
˚
of the imidazole rings (N1/C5/C6/N2/C11): d(C10· · · · Cg) = 3.5834(5) A. Hydrogen bonds are indicated by dashed lines.
Yellow dashed lines represent N—H····N and C—H····O interactions. Purple dashed lines represent C—H····π interactions.
imidazoles (1.6 × 10−6 mol L−1) in a 0.1 M H2SO4
aqueous solution dissolved in 0.5 mL of CH3OH were
measured against quinine sulfate in 0.1 M H2SO4 (ΦF
= 0.55) (Table 2). ΦF were measured against quinine
sulfate in 0.1 M H2SO4 (ΦF = 0.55) referring to the
reported literature (Fletcher, 1969; Gill, 1969; Molard
et al., 2006; Chen et al., 2013).
As shown in Table 2, ΦF of the synthesized com-
pounds (I–XIII ) were 0.213–0.508 except for those
for compounds IX (0.003) and XII (0.008) fluores-
cence quenching in a 0.1 M H2SO4 aqueous solution
dissolved in 0.5 mL of CH3OH. In fact, ΦF deter-
mined was the ΦF of imidazoles (I–XIII ) salifying with
sulfuric acid. Compared with non-substituted ben-
Fig. 4. TGA curves of I, II and XII.
zene, when benzene was replaced with other electron-
donating substituted groups (entries 2, 3, 4, 10 and
11) and electron-withdrawing substituents (entries 5,
2 and 12) were investigated by the thermogravimet-
ric (TGA) analyses (Fig. 4). The temperatures found
are 227◦C (I ), 243◦C (II ) and 245◦C (XII ) when the
weight of the compounds decomposed by 5 %, which
shows that thermal stability of the synthesized 2-
pyridyl and 2-phenyl substituted imidazoles is higher.
In our previous work (Chen et al., 2013), pho-
toluminescence properties of 1,2,4,5-tetrasubstituted
imidazoles containing a furan ring were studied.
However, the photoluminescence properties of 2,4,5-
trisubstituted imidazoles containing a furan ring
have only rarely been studied. The value of ΦF
of the synthesized 2-substituted-4,5-di(2-furyl)-1H-
6, 7, 8 and 9), the influence on their ΦF is low except
for compound XIII (0.508). This can be attributed
to styryl with a long conjugated chain which was in-
troduced into the molecule, suggesting that the 2-
position substituent groups had a significant influence
on the luminescence properties of the synthesized com-
pounds. Moreover, ΦF of benzene with the hydroxyl
(entry 4) and t-butyl (entry 10) group slightly de-
creased probably due to the hydroxyl group being pro-
tonized in the acidic solvent and t-butyl excessively in-
creasing the vibrational and rotational degrees of the
imidazole derivatives. However, when benzene was re-
placed with low electron-withdrawing halogen (entries
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Table 6. Emission wavelengths of I, II and XIII in solvents with different polarities (λex = 320 nm)
λ/nm
Solvent
I
II
XIII
XIV
Cyclohexane
Toluene
THF
Methanol
Ethanol/water
379
394
388
399
405
396
401
406
410
418
441
452
459
464
472
394
402
409
416
423
XIV = 4,5-di(furan-2-yl)-1-(4-methylbenzyl)-2-phenyl-1H-imidazole from literature (Chen et al., 2013).
Fig. 6. Normalized emission spectra of II in solvents with dif-
ferent polarities (λex = 320 nm).
Fig. 5. Normalized emission spectra of I in solvents with dif-
ferent polarities (λex = 320 nm).
5, 6 and 7), ΦF of the synthesized compounds grad-
ually decreased with the 2-position of trisubstituted
imidazoles bearing 4-fluorophenyl, 4-chlorophenyl and
4-bromophenyl, which can be ascribed to a heavy
atom introduced into the molecule and resulting in
a great enhancement of the rate of the S1–T1 spin-
forbidden process with the decrease of ΦF (Chandra
et al., 1978; Chen et al., 1990; Chen & Tong, 2014).
Besides, comparing the effect of 2-substituted phenyl,
pyridyl and furyl groups on ΦF, ΦF value of the ben-
zene ring as a substituent group was the highest (entry
2), followed by that of the furan ring (entry 1), while
that of the pyridine ring as a substituent group (en-
try 12) possesses the lowest ΦF. A possible reason is
the π–π* transition energy of the benzene ring being
much higher than the n–π* and π–π* energy of the
furan ring. Moreover, the pyridine ring possesses an
n-electron of the electronic spin-forbidden transition.
Finally, ΦF values of the synthesized trisubstituded
imidazoles were higher than those of our reported cor-
responding tetrasubstituted imidazoles (Chen et al.,
2013) as the 1-position substituent groups made the
plane of the trisubstituted imidazole (Zhao et al.,
2006; Wang et al., 2009) twist by a smaller angle
(Jayabharathi et al., 2012b), which resulted in bad
coplanarity of the tetrasubstituted imidazoles.
Fig. 7. Normalized emission spectra of XIII in solvents with
different polarities (λex = 320 nm).
spectively. The corresponding data are summarized in
Table 6.
Emission bands of compounds II and XIII were
significantly red-shifted with an increase of the sol-
vent polarity. In cyclohexane, the emissions of II and
XIII were at 396 nm (Fig. 6, Table 6) and 441 nm
(Fig. 7, Table 6), respectively. However, in ethanol–
water (1 : 1, vol.), they were red-shifted to 418 nm
(II ) and 472 nm (XIII ) (Table 6), respectively. The
red-shift of an emission band might be caused by the
stronger interaction between the solvent and the ex-
cited state molecules (Jayabharathi et al., 2012a). Po-
lar solvents stabilized the excited state of II and XIII
(Mi et al., 2003; Jayabharathi et al., 2012a). There-
Fluorescence emission spectra of I, II and XIII (1.6
× 10−6 mol L−1) in solvents with different polarities
(cyclohexane, toluene, THF, methanol and ethanol–
water 1 : 1, vol.) are shown in Figs. 5, 6 and 7, re-
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J. Zhang et al./Chemical Papers 69 (2) 325–338 (2015)
fore, the emission spectra of both compounds, II and
XIII, are broad and solvent dependent (Loiseau et al.,
2005; Yang et al., 2009). With regard to compound
a, its emission was basically red-shifted with an in-
crease of the solvent polarity. However, in a low po-
lar solvent, like toluene and THF, its emission spectra
broke the red-shifted rule with the solvent polarity and
were observed at 394 nm in toluene and at 388 nm in
THF, respectively (Fig. 5, Table 6). This might re-
sult from compound a being solvated by the THF sol-
vent according to the principle of similar miscibility
and/or an H-bond being formed between compound
I and THF leading to the decreased stability of the
excited state. It is well known that excited state hy-
drogen bonding of N-heterocyclic compounds lumines-
cence depends on the excited singlet state I (n, π*) or
(π, π*). The ability to form hydrogen bonds between
compounds in the (π, π*) excited singlet state and
THF is weaker than that of compounds in the (n, π*)
ground state and THF.
ganic small molecule optoelectronic materials. Also,
emissions of imidazole compounds in solutions can
be controlled by varying the polarity of the solvents.
A 1-substituent group attached to the nitrogen atom
had only a small influence on the emission spectra.
Therefore, the synthesized solvent dependent fluores-
cence compounds might have latent applications in
the detection of environments, microenvironments and
material science fields, especially in biological pro-
cesses. Besides, a study on the applications of 2,4,5-
trisubstituted imidazoles in the metal ion detection is
in progress.
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