Synthesis, X-ray crystal structure and optical properties of novel 2-aryl-3-ethoxycarbonyl-4-phenylpyrido[1,2-a]benzimidazoles

Article abstract of DOI:10.1002/bio.1362

A series of novel 2-aryl-3-ethoxycarbonyl-4-phenylpyrido[1,2-a] benzimidazole derivatives were synthesized by the tandem reaction of 2-benzoyl benzimidazole and (Z)-ethyl 4-bromo-3-arylbut-2-enoate in the presence of potassium carbonate. The compounds were characterized using IR, ¹H-NMR, ¹³C-NMR, HRMS and the structure of 6f was further determined by X-ray crystallography. Both absorption and fluorescence spectra characteristics of the compounds were investigated in acetonitrile and dichloromethane. The results showed that the absorption maxima of the compounds varied from 220 to 284 nm, depending on the structure of 2-aryl group. The fluorescence results revealed that these compounds exhibited blue-green fluorescence (463-475 nm) in dilute solutions and showed acceptable fluorescence quantum yields (_PL = 0.13-0.73) in dichloromethane. Copyright

Full text of DOI:10.1002/bio.1362

Research article
Received: 20 July 2011,
Revised: 29 August 2011,
Accepted: 31 August 2011,
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.1362
Synthesis, X-ray crystal structure and optical
properties of novel 2-aryl-3-ethoxycarbonyl-
4-phenylpyrido[1,2-a]benzimidazoles
Xia Chen,a He Yang,a Yanqing Ge,c Lei Feng,a Jiong Jiaa,b* and
Jianwu Wanga*
ABSTRACT: A series of novel 2-aryl-3-ethoxycarbonyl-4-phenylpyrido[1,2-a]benzimidazole derivatives were synthesized by the
tandem reaction of 2-benzoyl benzimidazole and (Z)-ethyl 4-bromo-3-arylbut-2-enoate in the presence of potassium carbonate.
The compounds were characterized using IR, 1H-NMR, 13C-NMR, HRMS and the structure of 6f was further determined by X-ray
crystallography. Both absorption and fluorescence spectra characteristics of the compounds were investigated in acetonitrile
and dichloromethane. The results showed that the absorption maxima of the compounds varied from 220 to 284 nm, depending
on the structure of 2-aryl group. The fluorescence results revealed that these compounds exhibited blue-green fluorescence
(463–475 nm) in dilute solutions and showed acceptable fluorescence quantum yields (ФPL = 0.13–0.73) in dichloromethane.
Copyright © 2011 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: pyrido[1,2-a]benzimidazole; synthesis; X-ray; absorption; fluorescence
to the C-2 position of the pyrido[1,2-a]benzimidazole ring.
Introduction
Herein, we report the synthesis, X-ray crystal structure and
Nitrogen bridge-head heterocycles are of special interest, due to
optical properties of novel 2-aryl-3-ethoxycarbonyl-4-
their wide variety biological activities and optical/electrical prop-
phenylpyrido[1,2-a]benzimidazoles.
erties, and many of attractive methods have been developed for
synthesizing this class of compounds. Of these heterocycles,
compounds containing imidazo[1,2-a]pyridine ring systems
Experimental
Thin-layer chromatography (TLC) was conducted on silica gel
(1–3) and related structures such as imidazo[1,2-a]pyrimidine,
imidazo[1,2-a]quinoxaline or imidazo[2,1-a]isoquinoline (4–6)
1
have been reported to possess a wide variety of biological and
pharmacological properties. In contrast, the related pyrido[1,2-
a]benzimidazole ring system has not received much attention
until the past two decades (7–11) when some of its derivatives
found pharmaceutical applications, based on their anti-anxiety
(12), anti-tumour (13–15) and antiviral activities (16). Further-
more, some of these compounds were also found to display
interesting photophysical and fluorescent properties (17–19).
However, as a class of compounds their spectroscopic properties
and biological activity have remained largely unexplored, which
in part can be attributed to the lengths of the preparative routes,
the formation of regioisomeric mixtures of products, limited sub-
strate scope and unsatisfactory yields (20).
Recently, there is a great interest to vary the structure of
conjugated molecules in order to tune and acquire more
favourable physical properties and functional properties (21).
In previous work, we have reported the synthesis of pyrido
[1,2-a]benzimidazole containing different substituents, which
emit blue-green fluorescence and have acceptable fluores-
cence quantum yields (22,23). In order to investigate in detail
the photophysical properties of compounds containing the
pyrido[1,2-a]benzimidazole moiety and extend the conjugated
p-electron system, a series of aromatic rings, such as a phenyl
ring, a naphthyl ring and a thiophene ring, were introduced
GF254 plates (Merck KGaA). H-NMR spectra were recorded on
a Bruker Avance 300 (300 MHz) spectrometer, using CDCl3 as
the solvent and tetramethylsilane (TMS) as the internal
standard. Melting points were determined on an XD-4 digital
micro melting point apparatus. IR spectra were recorded with
an IR spectrophotometer VERTEX 70 FT–IR (Bruker Optics).
HRMS spectra were recorded on a Q-TOF6510 spectrograph
(Agilent). UV-vis spectra were recorded on a U-4100 (Hitachi).
Fluorescent measurements were recorded on a Perkin-Elmer
LS-55 luminescence spectrophotometer.
* Correspondence to: J. Jia, School of Chemistry and Chemical Engineering,
Shandong University, Jinan 250100, Shandong, People’s Republic of China.
E-mail: jiongjia@sdu.edu.cn
* J. Wang, School of Chemistry and Chemical Engineering, Shandong University,
Jinan 250100, Shandong, People’s Republic of China. E-mail: jwwang@sdu.edu.cn
a
School of Chemistry and Chemical Engineering, Shandong University,
Jinan, 250100, Shandong, People’s Republic of China
b
State Key Laboratory of Crystal Materials, Shandong University, Jinan,
250100, People’s Republic of China
c
School of Chemistry and Chemical Engineering, Taishan Medical University,
Taian, Shandong, 271016, People’s Republic of China
Luminescence 2011
Copyright © 2011 John Wiley & Sons, Ltd.

X. Chen et al.
(75 MHz, CDCl3): d 167.1, 154.5, 141.1, 135.5, 129.2, 128.1, 118.0,
60.4, 18.3, 14.8; HRMS calcd for [M + H]+ C12H14ClO2, 225.0682;
found, 225.0679.
General experimental procedure for the synthesis of
a,b-unsaturated esters 3a–h
As shown in Fig. 1, compounds 3a–h were synthesized according
to the literature method (24,25). To a suspension of sodium
hydride (0.24 g, 10 mmol) in dry THF (100 mL) was added dropwise
triethyl phosphonoacetate (2.34 g, 10mmol) at 0 ꢀC under nitro-
gen atmosphere. The mixture was stirred for 1 h at 0 ꢀC and then
the appropriate ketone (6.76 mmol) in 15mL THF was added drop-
wise. After stirring at room temperature for 2 days and then cooled
with a water bath, a saturated aqueous ammonium chloride
solution (20 mL) was added dropwise to the cold mixture. The
aqueous phase was extracted with ethyl acetate (3Â 40 mL) and
the combined organic phase was washed with brine (3Â 30 mL),
dried over sodium sulphate and concentrated under reduced
pressure. The crude residue was purified by column chromatogra-
phy on silica gel to afford compounds 3a–h in 70–85% yield. Here
only the E-isomer of the products were isolated and characterized.
(E)-Ethyl 3-(4-bromophenyl)but-2-enoate (3d)
1
Colourless oil, yield 82%. H-NMR (300 MHz, CDCl3): d 7.47–7.50
(m, 2H), 7.32–7.34 (m, 2H), 6.11 (d, 1H, J= 1.2 Hz), 4.18–4.25 (q, 2H,
J= 7.2 Hz), 2.54 (d, 3H, J= 1.2 Hz ), 1.29–1.34 (t, 3H, J=7.2Hz); 13C-
NMR (75 MHz, CDCl3): d 167.1, 154.5, 141.5, 132.1, 128.4, 123.7,
118.1, 60.4, 18.2, 14.8; HRMS calcd for [M + H]+ C12H14BrO2,
269.0177; found, 269.0148.
(E)-Ethyl 3-(4-(trifluoromethyl)phenyl)but-2-enoate (3e)
Colourless oil, yield 80%. 1H-NMR (300 MHz, CDCl3): d 7.55–7.64 (m,
4H), 6.15 (d, 1H, J= 1.5 Hz), 4.20–4.27 (q, 2H, J= 7.2 Hz), 2.58 (d, 3H,
J= 1.2 Hz ), 1.30–1.35 (t, 3H, J= 7.2Hz); 13C-NMR (75 MHz, CDCl3): d
166.9, 154.2, 146.3, 127.1, 126.2, 125.9, 122.6, 119.5, 60.6, 18.4,
14.8; HRMS calcd for [M+ H]+ C13H14F3O2, 259.0946; found,
259.0935.
(E)-Ethyl 3-phenylbut-2-enoate (3a)
1
Colourless oil, yield 85%. H-NMR (300 MHz, CDCl3): d 7.33–7.45
(m, 5H), 6.13–6.14 (q, 1H, J = 1.5 Hz), 4.18–4.25 (q, 2H, J = 7.2 Hz),
2.58 (d, 3H, J = 1.5 Hz ), 1.29–1.34 (t, 3H, J = 7.2 Hz); 13C-NMR
(75 MHz, CDCl3): d 167.3, 156.0, 142.7, 129.5, 129.0, 126.8, 117.7,
78.0, 77.6, 77.2, 60.3, 18.4, 14.8; HRMS calcd for [M + H]+
C12H15O2, 191.1072; found, 191.1065.
(E)-Ethyl 3-(4-methoxyphenyl)but-2-enoate (3f)
1
Colourless oil, yield 75%. H-NMR (300MHz, CDCl3): d 7.43–7.46
(m, 2H), 6.87–6.90 (m, 2H), 6.10–6.11 (d, 1H, J = 1.2Hz), 4.17–4.24
(q, 2H, J = 7.2Hz), 3.81 (s, 3H), 2.56 (d, 3H, J = 1.2Hz ), 1.28–1.33 (t,
3H, J = 7.2 Hz); 13C-NMR (75 MHz, CDCl3): d 167.5, 160.9, 155.3,
134.8, 128.1, 115.8, 114.3, 60.2, 55.8, 18.1, 14.9; HRMS calcd for
[M+ H]+ C13H17O3, 221.1178; found, 221.1176.
(E)-Ethyl 3-(4-fluorophenyl)but-2-enoate (3b)
1
Colourless oil, yield 80%. H-NMR (300 MHz, CDCl3): d 7.44–7.48
(m, 2H), 7.02–7.08 (m, 2H), 6.09 (d, 1H, J = 1.2 Hz), 4.18–4.25 (q,
2H, J = 7.2 Hz), 2.55–2.56 (d, 3H, J = 1.2 Hz), 1.29–1.34 (t, 3H,
J = 7.2 Hz); 13C-NMR (75 MHz, CDCl3): d 167.2, 165.4, 162.1,
154.7, 138.7, 128.6, 117.6, 60.4, 18.4, 14.8; HRMS calcd for
[M + H]+ C12H14FO2 209.0978; found, 209.0968.
(E)-Ethyl 3-(naphthalen-2-yl)but-2-enoate (3g)
Colourless oil, yield 85%. 1H-NMR (300 MHz, CDCl3): d 7.93–7.79 (m,
4H), 7.60–7.56 (m, 1H), 7.50–7.45 (m, 2H), 6.28 (s, 1H), 4.27–4.20 (q,
2H, J= 7.2 Hz), 2.68 (s, 3H), 1.35–1.30 (t, 3H, J=7.2Hz); 13C-NMR
(75 MHz, CDCl3): d 155.7, 139.9, 134.0, 133.6, 129.0, 128.7, 128.1,
127.2, 127.0, 126.4, 124.5, 118.0, 60.4, 18.4, 14.9; HRMS calcd for
[M + H]+ C16H17O2, 241.1229; found, 241.1228.
(E)-Ethyl 3-(4-chlorophenyl)but-2-enoate (3c)
1
Colourless oil, yield 70%. H-NMR (300 MHz, CDCl3): d 7.32–7.42
(m, 4H), 6.10–6.12 (q, 1H, J = 1.2 Hz), 4.18–4.25 (q, 2H, J = 7.2 Hz),
(E)-Ethyl 3-(thiophen-2-yl)but-2-enoate (3h)
2.55 (d, 3H, J = 1.2 Hz ), 1.29–1.34 (t, 3H, J = 7.2 Hz); 13C-NMR
Colourless oil, yield 80%. 1H-NMR (300 MHz, CDCl3): d 7.31–7.26 (m,
2H), 7.05–7.02 (m, 1H), 6.25 (s, 1H), 4.24–4.16 (q, 2H, J = 7.2Hz), 2.61
(s, 3H), 1.33–1.29 (t, 3H, J = 7.2 Hz); 13C-NMR (75 MHz, CDCl3): d
167.2, 148.2, 146.1, 128.4, 127.5, 127.2, 114.8, 78.0, 77.6, 77.1, 60.3,
17.8, 14.8; HRMS calcd for [M + H]+ C10H13O2S, 197.0636; found,
197.0632.
General experimental procedure for the synthesis of
g-bromo-a,b-unsaturated esters 4a–h
A solution of a,b-unsaturated ester 3 (1 mmol), NBS (1.05 mmol)
and benzoyl peroxide (0.04 mmol) in dry CCl4 (35 mL) was refluxed
for 8–10h. The resulting reaction mixture was cooled to room
temperature and then filtered through a sintered funnel to
separate succinimide formed during the reaction. The filtrate was
concentrated under reduced pressure to obtain crude products,
which were then purified by column chromatography on silica
gel to afford compounds 4a–h in 70–80% yield.
(Z)-Ethyl 4-bromo-3-phenylbut-2-enoate (4a)
Yellow oil, yield 80%. 1H-NMR (300 MHz, CDCl3): d 7.52–7.57 (m, 2H),
7.39–7.44 (m, 3H), 6.21 (s, 1H), 4.98 (s, 2H), 4.23–4.30 (q, 2H, J=7.2Hz),
1.31–1.36 (t, 3H, J=7.2Hz); 13C-NMR (75 MHz, CDCl3): d 166.0, 153.7,
Figure 1. Synthesis of 2-aryl-3-ethoxycarbonyl-4-phenylpyrido[1,2-a]benzimidazoles.
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Luminescence 2011

Properties of pyrido[1,2-a]benzimidazoles
139.0, 130.2, 129.3, 127.1, 126.8, 120.3, 61.0, 27.1, 14.7; HRMS calcd for
[M + H]+ C12H14BrO2, 269.0099; found, 269.0174.
General procedure for the synthesis of compounds 6a–h
A mixture of compound 5 (1 mmol), compound 4 (1.2 mmol) and
potassium carbonate (0.28 g, 2.05 mmol) in DMF (10 mL) was
stirred at 75 ꢀC for 12–14 h and then filtered. The filtrate was
poured into water (40 mL) and extracted with ethyl acetate
(3 Â 30 mL). The combined extracts were washed with brine,
dried over sodium sulphate, and concentrated under reduced
pressure. The crude residue was purified by column chromatog-
raphy on silica gel to afford compounds 6a–h in 60–78% yield.
(Z)-Ethyl 4-bromo-3-(4-fluorophenyl)but-2-enoate (4b)
1
Yellow oil, yield 78%. H-NMR (300 MHz, CDCl3): d 7.39–7.54 (m,
2H), 7.01–7.13 (m, 2H), 6.16 (s, 1H), 4.95 (s, 2H), 4.18–4.30 m, 2H),
1.29–1.36 (m, 3H); 13C-NMR (75 MHz, CDCl3): d 138.7, 135.0, 129.1,
128.6, 120.2, 117.6, 116.5, 115.8, 61.1, 26.9, 18.4, 14.8; HRMS calcd for
[M + H]+ C12H13BrFO2, 287.0083; found, 287.0092.
2,4-Diphenyl-3-ethoxycarbonylpyrido[1,2-a]
benzimidazole (6a)
(Z)-Ethyl 4-bromo-3-(4-chlorophenyl)but-2-enoate (4c)
Yellow solid, yield 75%, mp 182–184 ꢀC. 1H-NMR (300 MHz, CDCl3): d
8.42 (s, 1H), 7.97–8.00 (d, 1H, J=8.4Hz), 7.88–7.90 (d, 1H, J=8.1Hz),
7.64–7.68 (m, 2H), 7.35–7.54 (m, 10H), 3.84–3.91 (q, 2H, J=7.2Hz),
0.79–0.84 (t, 3H, J=7.2Hz); 13C-NMR (75 MHz, CDCl3): d 136.9, 134.9,
134.8, 130.2, 129.6, 129.5, 129.4, 129.3, 129.1, 129.0, 128.7, 126.4,
124.1, 124.0, 122.4, 121.3, 111.1, 78.0, 77.6, 77.1, 61.9, 13.9; IR (KBr) n/
cmÀ1: 3053, 2985, 1727, 1481, 1463, 1321, 1253, 1121, 1018, 847,
736, 699; HRMS calcd for [M + H]+ C26H20N2O2, 393.1603; found,
393.1598.
Yellow oil, yield 75%. 1H-NMR (300MHz, CDCl3): d 7.46–7.51
(m, 2H), 7.32–7.42 (m, 2H), 6.18 (s, 1H), 4.94 (s, 2H), 4.18–4.30 (m,
2H), 1.29–1.36 (m, 3H); 13C-NMR (75 MHz, CDCl3): d 165.8, 152.4,
137.4, 136.3, 129.5, 129.2, 128.4, 128.1, 120.6, 61.2, 26.7, 14.8; HRMS
calcd for [M+ H]+ C12H13BrClO2, 302.9787; found, 302.9788.
(Z)-Ethyl 4-bromo-3-(4-bromophenyl)but-2-enoate (4d)
1
Yellow oil, yield 75%. H-NMR (300 MHz,CDCl3): d 7.48–7.57 (m,
2H), 7.32–7.44 (m, 2H), 6.18 (s, 1H), 4.93 (s, 2H), 4.18–4.30 (m,
2H), 1.29–1.36 (m, 3H); 13C-NMR (75 MHz, CDCl3): d 165.8, 152.5,
137.9, 132.5, 128.7, 124.6, 120.6, 118.1, 61.2, 26.7, 18.3, 14.8; HRMS
calcd for [M + H]+ C12H13Br2O2, 346.9282; found, 346.9268.
4-Phenyl-2-(4-fluorophenyl)-3-ethoxycarbonylpyrido[1,2-a]
benzimidazole (6b)
Yellow solid, yield 60%, mp 156–158 ꢀC. 1H-NMR (300 MHz,
CDCl3): d 8.40 (s, 1H), 7.98–8.01 (d, 1H, J = 8.4 Hz), 7.89–7.92 (d,
1H, J = 8.4 Hz), 7.63–7.67 (m, 2H), 7.37–7.55 (m, 7H), 7.10–7.18
(m, 2H), 3.85–3.92 (q, 2H, J = 7.2 Hz), 0.82–0.87 (t, 3H, J = 7.2 Hz);
13C-NMR (75 MHz, CDCl3): d 167.4, 165.0, 161.7, 147.1, 145.8,
134.8, 132.8, 131.3, 131.2. 130.1, 129.6, 129.4, 129.0, 126.5,
124.0, 123.0, 122.5, 121.3, 116.2, 111.1, 62.0, 14.0; IR (KBr) n/
cmÀ1: 3052, 2979, 1731, 1483, 1461, 1318, 1244, 1118, 1015,
841, 746, 700; HRMS calcd for [M + H]+ C26H19FN2O2, 411.1509;
found, 411.1513.
(Z)-Ethyl 4-bromo-3-(4-(trifluoromethyl)phenyl)
but-2-enoate (4e)
1
Yellow oil, yield 73%. H-NMR (300 MHz, CDCl3): d 7.55–7.70 (m,
4H), 6.21 (s, 1H), 4.20–4.32 (m, 2H), 2.58 (d, 2H, J = 1.2 Hz), 1.30–
1.37 (t, 3H, J = 7.2Hz); 13C-NMR (75 MHz, CDCl3): d 166.9, 165.6,
154.2, 131.1, 127.1, 126.2, 122.0, 119.5, 60.6, 26.6, 18.4; HRMS calcd
for [M + H]+ C12H13BrF3O2, 337.0051; found, 337.0042.
4-Phenyl-2-(4-chlorophenyl)-3-ethoxycarbonylpyrido[1,2-a]
benzimidazole (6c)
(Z)-Ethyl 4-bromo-3-(4-methoxyphenyl)but-2-enoate (4f)
Yellow solid, yield 78%, mp 84–85 ꢀC. 1H-NMR (300 MHz, CDCl3):
d 7.44–7.55 (m, 2H), 6.88–6.95 (m, 2H), 6.18 (s, 1H), 4.98 (s, 2H),
4.19–4.29 (m, 2H), 3.83–3.84 (d, 3H, J = 1.2 Hz), 1.31–1.35 (t, 3H,
J = 7.2 Hz); 13C-NMR (75 MHz, CDCl3): d 166.3, 161.5, 153.0, 130.9,
128.5, 118.2, 114.7, 60.9, 55.9, 26.9, 14.7; HRMS calcd for
[M + H]+ C13H16BrO3, 299.0283; found, 299.0271.
Yellow solid, yield 72%, mp 248–250 ꢀC. 1H-NMR (300 MHz, CDCl3):
d 8.41 (s, 1H), 7.99–8.02(d, 1H, J = 8.1 Hz ), 7.91–7.94 (d, 1H,
J = 8.1Hz ), 7.64–7.67 (m, 2H), 7.39–7.57 (m, 9H), 3.86–3.93 (q, 2H,
J = 7.2Hz ), 0.83–0.87 (t, 3H, J = 7.2 Hz); 13C-NMR (75 MHz, CDCl3):
d 167.3, 147.0, 145.7, 135.3, 135.0, 134.7, 134.5, 130.8, 130.2,
130.1, 129.8, 129.4, 129.0, 128.7, 126.6, 124.0, 123.0, 122.6, 121.4,
111.1, 62.1, 14.0; IR (KBr) n/cmÀ1: 3010, 2982, 1729, 1480, 1462,
1319, 1247, 1120, 1019, 832, 739, 697; HRMS calcd for [M + H]+
C26H19ClN2O2, 427.1213; found, 427.1202.
(Z)-Ethyl 4-bromo-3-(naphthalen-2-yl)but-2-enoate (4g)
Yellow oil, yield 70%. 1H-NMR (300 MHz, CDCl3): d 7.93–7.80 (m, 4H),
7.60–7.56 (m, 1H), 7.50–7.46 (m, 2H), 6.33 (s, 1H), 5.08 (s, 2H), 4.27–
4.20 (q, 2H, J= 7.2 Hz), 1.35–1.30 (t, 3H, J=7.2Hz); 13C-NMR
(75 MHz, CDCl3): d 167.4, 155.7, 139.9, 134.0, 129.2, 128.2, 127.6,
127.1, 126.4, 124.5, 120.5, 118.1, 61.1, 27.0, 18.4, 14.9; HRMS calcd
for [M + H]+ C16H16BrFO2, 319.0334; found, 319.0336.
4-Phenyl-2-(4-bromophenyl)-3-ethoxycarbonylpyrido[1,2-a]
benzimidazole (6d)
Yellow solid, yield 63%, mp 206–207 ꢀC. 1H-NMR (300 MHz,
CDCl3): d 8.42 (s, 1H), 7.99–8.02 (d, 1H, J = 8.4 Hz ), 7.91–7.94 (d,
1H, J = 8.4Hz ), 7.63–7.67 (m, 2H), 7.42–7.61 (m, 7H), 7.36–7.40
(m, 2H), 3.86–3.93 (q, 2H, J = 7.2Hz ), 0.83–0.88 (t, 3H, J = 7.2Hz);
13C-NMR (75 MHz, CDCl3): d 167.3, 151.0, 147.0, 145.7, 135.8,
134.6, 134.4, 132.3, 131.1, 130.5, 130.1, 129.8, 129.4, 129.0,
126.6, 123.9, 123.2, 122.6, 121.4, 111.1, 62.1, 14.0; IR (KBr)
n/cmÀ1: 3012, 2981, 1730, 1478, 1462, 1320, 1248, 1120,
1017, 829, 738, 696; HRMS calcd for [M + H]+ C26H19BrN2O2,
471.0708; found, 471.0696.
(Z)-Ethyl 4-bromo-3-(thiophen-2-yl)but-2-enoate (4h)
Yellow oil, yield 75%. 1H-NMR (300 MHz, CDCl3): d 7.45–7.44 (m, 1H),
7.37–7.35 (m, 1H), 7.10–7.07 (m, 1H), 6.29 (s, 1H), 4.96 (s, 2H), 4.29–
4.22 (q, 2H, J= 7.2 Hz), 1.35–1.31 (t, 3H, J=7.2Hz); 13C-NMR
(75 MHz, CDCl3): d 166.0, 146.9, 142.2, 128.4, 116.8, 61.0, 25.9, 14.7;
HRMS calcd for [M + H]+ C10H12BrO2S, 274.9741; found, 274.9734.
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X. Chen et al.
parent atoms, with C–H = 0.93 Å and Uiso(H) = 1.2 Ueq (C). A summary
of the crystallographic data and structure refinement details is given
in Table 1 (see also Supporting information).
4-Phenyl-2-(4-(trifluoromethyl)phenyl)-3-ethoxycarbonylpyrido
[1,2-a]benzimidazole (6e)
Yellow solid, yield 74%, mp 330–332 ꢀC. 1H-NMR (300 MHz, CDCl3): d
8.46 (s, 1H), 8.02–8.05 (d, 1H, J= 8.1 Hz), 7.92–7.95 (d, 1H, J= 8.1Hz),
7.72–7.75 (d, 2H, J= 8.1 Hz), 7.62–7.66 (m, 4H), 7.42–7.59 (m, 5H),
3.86–3.93 (q, 2H, J=7.2Hz), 0.80–0.85 (t, 3H, J=7.2Hz); 13C-NMR
(75 MHz, CDCl3): d 167.1, 140.5, 134.4, 131.2, 130.8, 130.0, 129.8,
129.6, 129.3, 129.1, 126.9, 126.3, 126.2, 126.1, 126.0, 124.3, 123.1,
122.9, 122.7, 121.3, 111.2, 62.2, 13.9; IR (KBr) n/cmÀ1: 3057, 3011,
1726, 1486, 1464, 1322, 1251, 1127, 1066, 845, 739, 697; HRMS calcd
for [M + H]+ C27H19F3N2O2, 461.1477; found, 461.1467.
Results and discussion
Synthesis
The synthetic route to the target heterocycles is shown in Fig. 1.
Firstly, the a,b-unsaturated esters 3a–h were obtained from the
reaction of aryl ketones 1a–h and triethyl phosphonoacetate 2,
which were readily available, in the presence of sodium hydride
from 0 ꢀC to room temperature (24,25). After that, allylic bromina-
tion of compounds 3a–h with NBS afforded the brominated
derivatives 4a–h in good yields after work-up, which was the key
step in the synthesis of these target products. Finally, compounds
4a–h reacted with 2-benzoyl benzimidazole 5 in the presence of
K2CO3 at 75 ꢀC for 12–14 h to afford pyrido[1,2-a]benzimidazole
derivatives in good yield via the tandem reaction we reported
before (26) It should be noted that the reaction between
compound 5 and (Z)-ethyl 4-bromo-3-arylbut-2-enoate requires a
higher temperature than that between compound 5 with (E)-ethyl
4-bromobut-2-enoate we previously reported. Within the (Z)-ethyl
4-bromo-3-arylbut-2-enoate derivatives, the bromomethyl and
4-Phenyl-2-(4-methoxyphenyl)-3-ethoxycarbonylpyrido
[1,2-a]benzimidazole (6f)
Yellow solid, yield 78%, mp 130–132 ꢀC. 1H-NMR (300 MHz, CDCl3):
d 8.39 (s, 1H), 7.97–8.00 (d, 1H, J=8.4Hz), 7.88–7.91 (d, 1H,
J= 8.4 Hz), 7.64–7.68 (m, 2H), 7.35–7.53 (m, 7H), 6.95–6.99 (m, 2H),
3.86–3.93 (q, 2H, J= 7.2 Hz), 3.85 (s, 3H), 0.83–0.87 (t, 3H, J=7.2Hz);
13C-NMR (75 MHz, CDCl3): d 167.6, 160.2, 147.1, 145.7, 135.1, 134.9,
130.6, 130.2, 129.5, 129.3, 129.1, 128.9, 126.4, 123.8, 123.7, 122.3,
121.2, 114.5, 111.1, 61.9, 55.9, 14.0; IR (KBr) n/cmÀ1: 3044, 2981,
2961, 1727, 1483, 1461, 1344, 1246, 1181, 1040, 836, 748, 701; HRMS
calcd for [M + H]+ C27H22N2O3, 423.1709; found, 423.1697.
4-Phenyl-2-(naphthalen-2-yl)-3-ethoxycarbonylpyrido[1,2-a]
benzimidazole (6g)
Table 1. Crystal data and structure refinement for 6f
Yellow solid, yield 67%, mp 198–200 ꢀC. 1H-NMR (300 MHz, CDCl3):
d 8.49 (s, 1H), 8.00–8.02 (d, 1H, J= 8.4Hz), 7.91–7.95 (m, 2H),
7.84–7.88 (m, 3H), 7.76–7.70 (m, 2H), 7.36–7.59(m, 7H), 7.24 (s,
6f
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
C27H22N2O3
422.48
295(2) K
0.71073 Å
Monoclinic
P21/c
13
1H), 3.81–3.88 (q, 2H, J = 7.2 Hz), 0.74–0.79 (t, 3H, J = 7.2Hz);
C-NMR (75 MHz, CDCl3): d 167.6, 147.2, 145.8, 134.9, 134.3, 133.7,
133.3, 130.2, 129.7, 129.6, 129.3, 129.0, 128.8, 128.5, 128.3, 127.2,
127.1, 127.0, 124.3, 124.1, 122.4, 121.3, 111.2, 62.0, 14.0; IR (KBr) n/
cmÀ1: 3052, 2975, 1733, 1490, 1460, 1314, 1247, 1111, 1014, 854,
744, 698; HRMS calcd for [M+ H]+ C30H22N2O2, 443.1760; found,
443.1754.
Unit cell dimensions
a = 13.0331(17) Å, a = 90.00ꢀ
b = 15.9620(2) Å, b = 104.169(2)ꢀ
c = 12.4647(17) Å, g = 90.00ꢀ
2514.2(6) Å3
Volume
Z
Calculated density
Absorption coefficient
F(000)
4-Phenyl-2-(thiophen-2-yl)-3-ethoxycarbonylpyrido[1,2-a]
benzimidazole (6h)
4
1.214 mg/m3
Yellow solid, yield 70%, mp: 157–159 ꢀC. 1H-NMR (300 MHz, CDCl3):
d 8.55 (s, 1H), 7.98–8.01 (d, 1H, J = 8.1 Hz), 7.92–7.94 (d, 1H,
J = 8.1 Hz), 7.65–7.68 (m, 2H), 7.39–7.56 (m, 6H), 7.19–7.20
(m, 1H), 7.10–7.13 (m, 1H), 3.94–4.01 (q, 2H, J = 7.2 H), 0.88–0.93
(t, 3H, J = 7.2 Hz); 13C-NMR (75 MHz, CDCl3): d 167.2, 147.1, 145.6,
137.2, 134.9, 134.5, 130.2, 129.4, 129.0, 128.3, 128.2, 127.0, 126.6,
124.5, 122.6, 121.3, 116.8, 114.1, 62.1, 14.0; IR (KBr) n/cmÀ1: 3051,
3003, 2922, 1731, 1484, 1461, 1313, 1247, 1110, 1019, 839, 740,
698; HRMS calcd for [M + H]+ C24H18N2O2S, 399.1167; found,
399.1167.
0.080 mm–1
972
Crystal size
0.28 Â 0.18 Â 0.14 mm
θ range for data collection 2.06 to 25.50ꢀ
Limiting indices
À15 ≤ h ≤ 15, –19 ≤ k ≤ 8, –15
≤ l ≤ 14
Reflections collected/
unique
12674/4646 [R (int) = 0.0266]
Completeness to
99.5%
θ = 25.50ꢀ
Absorption correction
Max. and min.
transmission
Refinement method
Data/restraints/parameters 4646/3/314
None
0.9889 and 0.9779
X-ray crystallography
Suitable single crystals of 6f for X-ray structure analysis were
obtained by slow evaporation of a solution of the solid in ethyl
acetate. The diffraction data were collected with a Bruker–Nonius
SMART APEX IICCD diffractometer using a graphite monochro-
mated Mo Ka radiation (l = 0.71073Å) at 295(2) K. The structures
were solved by direct methods using the SHELXS-97 program
and refinements on F2 were performed with the SHELXL-97
program by full-matrix least-aquares techniques, with anisotropic
thermal parameters for the non-hydrogen atoms. All H atoms were
initially located in a difference Fourier map and were placed in
geometrically idealized positions and constrained to ride on their
Full-matrix least-squares on F2
Goodness-of-fit on F2
Final R indices [I > 2s(I)]
R indices (all data)
Extinction coefficient
Largest diff. peak and
hole
1.065
R1 = 0.0641, wR2 = 0.1871
R1 = 0.0905, wR2 = 0.2090
0.006(2)
0.683 and À0.363 e. Å–3
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Luminescence 2011

Properties of pyrido[1,2-a]benzimidazoles
ester group adopt a syn configuration, resulting in certain degree
of steric effect, which may hinder the reaction between compound
5 and the brominated ester derivatives.
Absorption spectra
The absorption spectra of the compounds 6a-h measured in
acetonitrile solution with the concentration of 5 Â 10–6 mol/L
are shown in Fig. 4. And the optical characteristics are summa-
rized in Table 2. As shown in Fig. 4, compounds 6a–h display
similar absorption characteristics, with absorption peaks in the
range 220–284 nm, which are attributed to the p–p* transition
of conjugate backbone.
Structure characterization
The assumed structures of target products were proved by
IR, 1H-NMR and HRMS spectra. For example, compound 6f,
obtained as yellow solid, gave an [M + H]-ion peak at m/z
423.1697 in the HRMS, in accord with the molecular formula
C27H22N2O3. The IR spectra of compound 6f showed the charac-
teristic absorption bands at 3044 (ArH), 1721 (C = O), 1483
(C = C), 1246 (C–O–C). The 1H-NMR spectra of compound 6f
revealed two singlet peaks at d 3.93 (3H, CH3) and 8.39 (1H,
pyridine moiety) which were readily assigned to the hydrogen
of methoxy and pyridine moiety, respectively. Moreover,
compound 6f showed peaks at d 0.85 (t, 3H, J = 7.2 Hz) and
3.90 (q, 2H, J = 7.2 Hz), assigned to the protons of ethoxycarbonyl
group. Compound 6f also showed peaks between d 6.95–8.00
(13H), assigned to the protons on aromatic rings. All other
signals are consistent with the structure of 6f.
Crystal structure
The spatial structures of compound 6f were determined by
using X-ray diffraction analysis. A summary of crystallographic
data collection parameters and refinement parameters for 6f
are compiled in Table 1.
The single crystal structure and packing diagram of 6f are
shown in Figs 2 and 3. The structure of compound 6f is crystal-
lized in monoclinic space group P21/c. One benzene moiety
and one p-methoxy phenyl moiety are bonded to the pyrido
[1,2-a]benzimidazole ring at the atoms of C8 and C10, respec-
tively. Consistent with a pronounced electronic interaction, the
bond lengths of C8–C12 and C10–C21 are significantly shorter
as would be expected for a normal carbon-carbon single bond.
The dihedral angle between the N1/C7/N2/C6/C1 ring and N1/
C7/C8/C9/C10/C11 ring is 1.985ꢀ, and 0.693ꢀ for the C1/C2/C3/
C4/C5/C6 ring; thus, the three rings in 6f are nearly in the same
plane and this coplanar conformation provides a large conju-
gated system. In contrast, the pyridine moiety makes dihedral
angles with benzene ring and p-methoxyphenyl ring of 61.963ꢀ
and 53.111ꢀ, respectively. The torsion angles apparently result
from the steric effect between the aryl rings and ester group.
Figure 3. Crystal packing diagram of 6f along the a axis (A) and the c axis (B).
Figure 2. The molecular strcture of 6f with displacement ellipsoids drawn at the
30% probability level.
Figure 4. UV-vis absorbtion spectrum of compounds 6a–h in dilute (5 Â 10–6 mol/L)
acetonitrile solution.
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X. Chen et al.
of 2-thienyl moiety. In addition, compound 6g also displays the
absorption peak at 220 nm, which is attributed to p–p* transition
of the naphthyl ring.
Table 2. The absorption characteristics of compounds 6a-h
Compounds
lmax (nm)
Furthermore, it is noted that when an electron-donating
substituent, such as a bromo group or a methoxyl group is
located on the 2-phenyl ring, the absorption peaks of 6d and
6f are at longer wavelengths (281 and 284 nm) than that of
compound 6a. In contrast, the electron-withdrawing substituent
located on the 2-phenyl ring could not obviously affect the
absorption spectra of compounds 6.
Possible influence of the solvent on the absorption behaviour was
investigated. The absorption spectra of 6a, 6c, 6e and 6g, as exam-
ples, in three different solvents of cyclohexane, dichloromethane
and acetonitrile, at a concentration of 5 Â 10–6 mol/L are shown in
Fig. 5. It was observed that the absorption spectra changed very little
with the increase of solvent polarity although there was a tendency
of a shorter lmax in acetonitrile, indicating that there was no charge
transfer in the ground state.
Comparing with the 3-ethoxycarbonyl-4-phenylpyrido
[1,2-a]benzimidazole derivatives (22) the absorption maxima
of compounds 6, in which an aryl group is bonded to the
C-2 position of pyrido[1,2-a]benzimidazole ring, are red-
shifted, with the maximum shift of 53 nm. The difference in
absorption spectra is due to the fact that there is an aryl
group in the C-2 position of pyrido[1,2-a]benzimidazole ring.
The aryl group enhances the extent of conjugation in
compounds 6 and thus shifts the absorption spectra to
longer wavelengths.
6a–h
6a
6b
6c
6d
6e
6f
Cyclohexane Dichloromethane Acetonitrile
274
272
275
276
274
283
215
259
283
277
275
277
281
275
284
223
260
281
274
273
275
281
272
284
220
261
283
6g
6h
The data indicated that the structure of 2-aryl ring has definite
effects on the absorption bands. The absorption maximum of
compound 6a with the 2-phenyl ring is at 274 nm. As the steric
hindrance of the 2-aryl ring increased, the maximum absorption
of compound 6g with the 2-naphthyl ring is blue-shifted to
261 nm compared to compound 6a, which indicates that the
conjugated p-system between the 2-aryl ring and pyrido[1,2-a]
benzimidazole ring decreases due to steric hindrance effect.
Compound 6h with the 2-thienyl exhibits the maximum absorp-
tion wavelength at 283 nm, resulting in a 9 nm red shift com-
pared to compound 6a. This result may be attributed to the
enhancement of conjugation between the 2-aryl ring and pyrido
[1,2-a]benzimidazole ring and the strong electron-donating effect
Figure 5. UV-vis absorption spectrum of compounds 6a, 6c, 6e, 6g in different solvents.
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Copyright © 2011 John Wiley & Sons, Ltd.
Luminescence 2011

Properties of pyrido[1,2-a]benzimidazoles
attributed to the different conjugation degree and different elec-
tronic effects in these compounds. Moreover, the trifluoromethyl
on the 2-phenyl ring is an electron-attracting group, which causes
a shift of fluorescence band to longer wavelengths.
Fluorescence quantum yields (ФPL) of 6a–h in dichloromethane
were measured using quinine sulphate (Ф = 0.55) as standard (27)
The highest ФPL value of 0.73 is observed for 6a, which is higher
than that of other compounds. The fluorescence quantum yields
of the other compounds are in the range of 0.13–0.61. This dif-
ference of quantum yields might be due to the change of the
electronic push–pull substitution of the conjugated part in
the molecules. From Table 3, it can be also seen that the Stokes’
shifts of compounds 6a–h between absorption and emission
maximum are significant. The efficient p-conjugation in
molecule is known to be responsible for the charge-transfer
nature of the emissive excited state and causing the observed
relatively large Stokes’ shifts.
Moreover, the solvent effect on the fluorescence characteris-
tics was also observed (Fig. 7). The emission peaks of compound
6d were red-shifted from 467 nm in dichloromethane to 469 nm,
472 nm in acetonitrile and in DMF, respectively, which indicated
that the emission wavelength of the compound was red-shifted
with the increase of solvent polarity (Fig. 7).
Figure 6. Emission spectrum of compounds 6a–h in dilute (5 Â 10–6 mol/L)
dichloromethane solution.
Table 3. Fluorescence spectral data for 6a–h in dichloromethane
Compounds lex (nm) lem (nm) Stokes’ shift (nm) ФPL
6a
6b
6c
6d
6e
6f
317
315
317
321
315
324
300
321
466
475
463
467
471
475
469
470
149
160
146
146
156
151
169
149
0.73
0.33
0.61
0.30
0.50
0.54
0.27
0.13
Conclusion
A series of novel 2-aryl-3-ethoxycarbonyl-4-phenylpyrido[1,2-a]
benzimidazole derivatives have been synthesized by the tan-
dem reaction of 2-benzoyl benzimidazole and g-bromo-a,2b-
unsaturated esters in the presence of potassium carbonate at
75 ꢀC. The structures of the compounds obtained were deter-
mined by IR, 1H-NMR, 13C-NMR and HRMS spectra, and the spatial
structure of compound 6f was confirmed by X-ray crystallogra-
phy. It can be concluded that the absorption and fluorescence
6g
6h
characteristics of compounds 6a–h show
a significant
dependence on the structure of 2-aryl ring in the C-2 position
of pyrido[1,2-a] benzimidazole. These compounds exhibited
blue-green fluorescence (463–475 nm) in dilute solutions and
showed acceptable quantum yields of fluorescence in dichloro-
methane, with the highest ФPL value of 0.73 for compound 6a.
These observations indicate that the high conjugated molecule
structure enhances the light absorption and the fluorescence
emission ability of compounds 6a–h due to the contribution of
2-aryl ring.
Supplementary information
The following supplementary information (file CCDC 813941) may
be found in the online version of this article:
Figure 7. The fluoroscence spectra of compound 6d in different solvents.
Supplementary crystallographic data for this paper (these data can
be obtained free of charge via http://www.ccdc.cam.ac.uk/data_
request/cif, by emailing data_request@ccdc.cam.ac.uk, or by
contacting the Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033).
Fluorescence
Figure 6 displayed the emission spectra of compounds 6a–h in
dichloromethane solution (5 Â 10–6 mol/L). These compounds
showed a blue-green fluorescence with the maximum emission
peaks varying in the range 463–475 nm. Their maximum emission
peaks and the intensity of fluorescence are dependent on 2-aryl
ring bonded to pyrido[1,2-a]benzimidazole ring. The emission
maximum of 6f, in which a methoxyl group bonded to 2-phenyl
ring, is red-shifted by 9 nm compared to 6a, which might be
Acknowledgement
The authors thank the Shandong Natural Science Foundation
(Grant No. Y2008B40) and Shandong Excellent Young and Mid-
aged Scientist Promotive Foundation (Grant No. 2008BS04024)
for financial support of this work.
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Copyright © 2011 John Wiley & Sons, Ltd.
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X. Chen et al.
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14. Badawey E, Kappe T. Benzimidazole condensed ring system. IX.
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