The synthesis, characterization and optical properties of novel pyrido[1,2-a]benzimidazole derivatives

Article abstract of DOI:10.1016/j.dyepig.2010.08.005

A series of novel, substituted, pyrido[1,2-a]benzimidazole derivatives were synthesized using a novel tandem annulation reaction between 2-acylbenzimidazole derivatives and 4-bromobut-2-enoic esters under mild conditions. The compounds were characterized using IR, ¹H NMR, ¹³C NMR and HRMS; the crystal structure of 2,7,8-trimethyl-3- ethoxycarbonyl-4-phenylpyrido[1,2-a]benzimidazole was determined as orthorhombic. The absorbance and fluorescence spectra of the pyrido[1,2-a]benzimidazoles were measured in dichloromethane; an intense absorption maxima was observed at 250 nm and emission maxima were noted at 460 nm. The absorption spectra and fluorescence characteristics of the pyrido[1,2-a]benzimidazole derivatives revealed that a phenyl and a methyl group attached to the pyrido[1,2-a]benzimidazole ring markedly influenced maximum emission.

Full text of DOI:10.1016/j.dyepig.2010.08.005

Dyes and Pigments 88 (2011) 344e349
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The synthesis, characterization and optical properties of novel
pyrido[1,2-a]benzimidazole derivatives
,
c
,
,
b
,
,
a,
*
Yan Qing Ge a 1, Jiong Jia a 1, He Yang a, Xu Tang Tao b , Jian Wu Wang
*
a School of Chemistry and Chemical Engineering, Shandong University, Jinan, Shandong, 250100, PR China
b State Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong, 250100, PR China
c School of Chemistry and Chemical Engineering, Taishan Medical University, Taian, Shandong, 271016, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel, substituted, pyrido[1,2-a]benzimidazole derivatives were synthesized using a novel
tandem annulation reaction between 2-acylbenzimidazole derivatives and 4-bromobut-2-enoic esters
under mild conditions. The compounds were characterized using IR, 1H NMR, 13C NMR and HRMS; the
crystal structure of 2,7,8-trimethyl-3-ethoxycarbonyl-4-phenylpyrido[1,2-a]benzimidazole was deter-
mined as orthorhombic. The absorbance and fluorescence spectra of the pyrido[1,2-a]benzimidazoles
were measured in dichloromethane; an intense absorption maxima was observed at 250 nm and
emission maxima were noted at 460 nm. The absorption spectra and fluorescence characteristics of the
pyrido[1,2-a]benzimidazole derivatives revealed that a phenyl and a methyl group attached to the pyrido
[1,2-a]benzimidazole ring markedly influenced maximum emission.
Received 5 May 2010
Received in revised form
2 August 2010
Accepted 8 August 2010
Available online 17 August 2010
Keywords:
Synthesis
UV absorption
Fluorescence
Ó 2010 Elsevier Ltd. All rights reserved.
Pyrido[1,2-a]benzimidazole
Imidazo[1,2-a]pyridine
Nitrogen-bridgehead
1. Introduction
properties. Continuing our efforts on extending the scope and
applications of the tandem reaction for the preparation of nitrogen-
Imidazo[1,2-a]pyridine derivatives (Ips) are common to several
diverse classes of compounds with medicinal value [1]. They are
also used as fluorescent ligands [2] and as components in organic
light-emitting diodes (OLED) [3] and in organic field effect tran-
sistors (FET) [4]. In contrast, the related pyrido[1,2-a]benzimidazole
ring system has not received much attention until the past two
decades when some of its derivatives were found to have phar-
maceutical applications [5e10]. Some of these compounds also
display interesting photophysical and fluorescent properties
[11e15]. However, as a class of compounds their spectroscopic
properties have remained largely unexplored perhaps as a conse-
quence of the lack of general synthetic methods to this class of
heterocycle from easily accessible precursors.
bridgehead heteroaromatics [16e18] and in order to search for
novel fluorescent organic compounds, herein, pyrido[1,2-a]benz-
imidazole derivatives were conveniently synthesized by a novel
tandem annulation reaction under very mild conditions and their
structure and optical properties were described.
2. Experimental
2.1. General
Thin-layer chromatography (TLC) was conducted on silica gel 60
F254 plates (Merck KGaA). 1H NMR spectra were recorded on
a Bruker Avance 300 (300 MHz) spectrometer, using CDCl3 or
DMSO-d6 as solvent and tetramethylsilane (TMS) as internal stan-
dard. 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). UVevis
It is promising that the introduction of additional
p-systems
into imidazo[1,2-a]pyridines influences their photophysical
* Corresponding authors. Tel.: þ86 531 88362708; fax: þ86 531 88364464.
E-mail addresses: txt@icm.sdu.edu.cn (X.T. Tao), jwwang@sdu.edu.cn (J.W.
Wang).
spectra were recorded on
a U-4100 (Hitachi). Fluorescent
measurements were recorded on a PerkineElmer LS-55 lumines-
cence spectrophotometer.
1
Equal contributors.
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.08.005

Y.Q. Ge et al. / Dyes and Pigments 88 (2011) 344e349
345
2.2. Synthesis
(s, 6H); 13C NMR (75 MHz, DMSO-d6):
d
191.9, 147.2, 142.3, 136.6,
133.3, 132.4, 121.5, 111.9, 107.5, 25.8, 20.8, 20.4; IR (KBr)
n
¼ 3307,
The synthetic approach for the preparation of substituted pyrido
[1,2-a]benzimidazole derivatives is shown in Fig. 1.
The hydroxybenzylimidazoles 5aed were synthesized by
condensing diamines 4aeb with an appropriate hydroxy acid via
Phillips condensation [20].
2981, 2919,1670,1572,1509,1441,1414,1352, 1236, 1001, 667 cmꢂ1
;
HRMS: m/z calcd for C11H13N2O [M þ H]þ 189.1028, found 189.1034.
2.2.2.4. (5,6-Dimethyl-1H-benzo[d]imidazol-2-yl)(phenyl)
methanone (1e). Yellow solid (78% yield): mp 190 ꢀC; 1H NMR
(300 MHz, DMSO-d6):
1H), 7.52e7.66 (m, 3H), 7.33 (s, 1H), 2.40 (s, 6H); 13C NMR (75 MHz,
DMSO-d6): 163.9, 147.2, 142.9, 136.6, 135.6, 133.7, 133.6, 133.2,
131.9, 131.3, 128.4, 121.8, 111.7, 20.8, 20.4; IR (KBr)
¼ 3305, 2968,
d
10.50 (s, 1H), 8.68 (d, J ¼ 8.1 Hz, 2H), 7.71 (s,
2.2.1. Preparation of 1a
The benzoimidazole-2-carbaldehyde 1a was synthesized
according to the literature [19], and was subjected to the following
steps without purification.
d
n
2902, 1629, 1596, 1570, 1505, 1433, 1407, 1328, 1264, 993, 734 cmꢂ1
;
HRMS: m/z calcd for C16H15N2O [M þ H]þ 251.1184, found 251.1178.
2.2.2. General procedure for the preparation of 1bee
A solution of chromium trioxide (caution: Stable. Very strong
oxidant; reacts with most organic material in a violent and often
explosive fashion; moisture sensitive: 0.60 g, 6.00 mmol) in water
2.2.3. General procedure for the preparation of 3aej
To a 50-mL round-bottomed flask were added 1 (1.00 mmol),
a,
b-unsaturated ester 2 (2.00 mmol), potassium carbonate (0.28 g,
(2 mL) was added dropwise to
a
solution of hydrox-
2.05 mmol) and dry DMF (10 mL). The mixture was stirred at r.t for
3 h and then filtered. The filtrate was poured into water (100 mL)
and extracted with CH2Cl2 (3 ꢁ 30 mL). The combined extracts were
washed with water, dried over anhydrous MgSO4 and filtered, and
the solvent was removed by rotary evaporation. The crude products
were purified by column chromatography to afford compound 3 in
42e88%.
ybenzylimidazoles 5 (8.00 mmol) in glacial acid (20 mL) at 90 ꢀC. The
reaction mixture was heated at 100 ꢀC for a further 5 min and then
poured into water (100 mL) and filtered. The filtrate was extracted
with dichloromethane (3 ꢁ 30 mL). The combined extracts were
washed with brine (2 ꢁ 20 mL), dried over MgSO4 and filtered, and
the solvent was removed by rotary evaporation. The crude products
1bee were purified by recrystallization from ethyl acetate.
2.2.3.1. 3-Ethoxycarbonylpyrido[1,2-a]benzimidazole (3a). Yellow
solid (55% yield): mp 203e204 ꢀC; 1H NMR (300 MHz, CDCl3):
2.2.2.1. 1-(1H-Benzo[d]imidazol-2-yl)ethanone (1b). White solid
(80% yield): mp 187e188 ꢀC (lit. 186e188 ꢀC); 1H NMR (300 MHz,
d
8.50 (d, J ¼ 7.2 Hz, 1H), 8.45 (s, 1H), 8.01(d, J ¼ 8.1 Hz, 1H), 7.95
DMSO-d6):
d
10.85 (s,1H), 7.91 (d, J ¼ 7.2 Hz, 2H), 7.36e7.55 (m, 3H),
(d, J ¼ 8.1 Hz, 1H), 7.59 (t, J ¼ 4.8 Hz, 1H), 7.45 (t, J ¼ 7.5 Hz, 2H),
2.85 (s, 3H); 13C NMR (75 MHz, DMSO-d6):
d
192.2, 147.8, 143.5,
4.46 (q, J ¼ 7.2 Hz, 2H), 1.46 (t, J ¼ 7.2 Hz, 3H); 13C NMR (75 MHz,
133.8, 126.6, 123.9, 121.9, 112.2, 26.0; IR (KBr)
n
¼ 3290, 3060, 2924,
CDCl3): d 164.9, 147.3, 145.5, 130.8, 128.7, 126.3, 124.9, 122.4, 120.9,
1675, 1578, 1507, 1485, 1442, 1420, 1313, 1236, 950, 733 cmꢂ1
;
120.7, 110.8, 109.3, 61.9, 14.3; IR (KBr)
n
¼ 3415, 3031, 2981, 1720,
HRMS: m/z calcd for C9H9N2O [M þ H]þ 161.1715, found 161.1704.
1519, 1474, 1330, 1230, 1082, 743 cmꢂ1; HRMS: m/z calcd for
C14H13N2O2 [M þ H]þ 241.0977, found 241.0976.
2.2.2.2. (1H-Benzo[d]imidazol-2-yl)(phenyl)methanone (1c). White
solid (85% yield): mp 213e215 ꢀC; 1H NMR (300 MHz, DMSO-d6):
2.2.3.2. 2-Methyl-3-ethoxycarbonylpyrido[1,2-a]benzimidazole
d
13.53 (s, 1H), 8.59 (d, J ¼ 7.5 Hz, 2H), 7.90 (d, J ¼ 8.1 Hz, 2H), 7.75
(3b). Yellow solid (42% yield): mp 134e136 ꢀC; 1H NMR (300 MHz,
(t, J ¼ 7.2 Hz, 1H), 7.61e7.66 (m, 3H), 7.44 (t, J ¼ 7.5 Hz, 1H), 7.35
CDCl3):
d
8.35 (s, 1H), 8.28 (s, 1H), 7.98 (d, J ¼ 8.1 Hz, 1H), 7.91 (d,
(t, J ¼ 7.5 Hz, 1H); 13C NMR (75 MHz, DMSO-d6):
d
183.5, 147.9,
J ¼ 8.1 Hz, 1H), 7.55 (t, J ¼ 3.0 Hz, 1H), 7.41 (t, J ¼ 3.0 Hz, 1H), 4.43
143.2, 135.6, 134.1, 133.6, 130.8, 128.4, 125.7, 123.1, 121.3, 112.8; IR
(q, J ¼ 7.2 Hz, 2H), 2.61 (s, 3H), 1.45 (t, J ¼ 7.2 Hz, 3H); 13C NMR
(KBr)
n
¼ 3302, 3051, 2854, 1657, 1593, 1514, 1486, 1431, 1318,
(75 MHz, CDCl3): d 165.7, 146.7, 145.4, 132.2, 128.4, 126.0, 124.1,
1262, 919, 739 cmꢂ1; HRMS: m/z calcd for C14H11N2O [M þ H]þ
122.0, 121.2, 120.5, 119.4, 110.8, 61.6, 18.5, 14.3; IR (KBr)
n
¼ 3451,
233.0871, found 233.0869.
3050, 2980, 1690, 1638, 1506, 1454, 1430, 1333, 1277, 1258, 1013,
748 cmꢂ1; HRMS: m/z calcd for C15H15N2O2 [M þ H]þ 255.1134,
found 255.1134.
2.2.2.3. 1-(5,6-Dimethyl-1H-benzo[d]imidazol-2-yl)ethanone
(1d). Yellow solid (75% yield): mp 179e180 ꢀC; 1H NMR (300 MHz,
DMSO-d6):
d
10.39 (s, 1H), 7.64 (s, 1H), 7.29 (s, 1H), 2.80 (s, 3H), 2.39
2.2.3.3. 3-Ethoxycarbonyl-4-methylpyrido[1,2-a]benzimidazole
(3c). Yellow solid (85% yield): mp 129e130 ꢀC; 1H NMR (300 MHz,
CDCl3):
d
8.33 (d, J ¼ 7.2 Hz, 1H), 8.03 (d, J ¼ 8.4 Hz, 1H), 7.91 (d,
J ¼ 8.1 Hz,1H), 7.57 (m,1H), 7.43 (m,1H), 7.32 (d, J ¼ 7.2 Hz,1H), 4.44
(q, J ¼ 7.2 Hz, 2H), 3.04 (s, 3H), 1.45 (t, J ¼ 7.2 Hz, 3H); 13C NMR
(75 MHz, CDCl3):
d 166.3, 148.8, 144.7, 132.2, 129.3, 128.0, 126.0,
122.2, 121.9, 120.5, 110.8, 110.6, 61.6, 15.3, 14.3; IR (KBr)
n
¼ 3391,
3126, 2991,1709,1478,1328,1244,1170,1058, 746 cmꢂ1; HRMS: m/z
calcd for C15H15N2O2 [M þ H]þ 255.1134, found 255.1134.
2.2.3.4. 2,4-Dimethyl-3-ethoxycarbonylpyrido[1,2-a]benzimidazole
(3d). Yellow solid (79% yield): mp 140e141 ꢀC; 1H NMR (300 MHz,
CDCl3):
d
8.12 (s, 1H), 7.98 (d, J ¼ 8.4 Hz, 1H), 7.83 (d, J ¼ 8.1 Hz, 1H),
7.50 (m, 1H), 7.36 (m, 1H), 4.47 (q, J ¼ 7.2 Hz, 2H), 2.70 (s, 3H), 2.36
(s, 3H), 1.45 (t, J ¼ 7.2 Hz, 3H); 13C NMR (75 MHz, CDCl3):
d 167.7,
147.6, 144.6, 134.3, 129.0, 125.6, 125.2, 121.5, 121.1, 120.2, 110.6, 61.7,
16.9,14.8,14.3; IR (KBr)
n
¼ 3425, 2975,1724,1490,1468,1450,1338,
1255, 1153, 1038, 756 cmꢂ1; HRMS: m/z calcd for C16H17N2O2
Fig. 1. Synthesis of substituted pyrido[1,2-a]benzimidazole derivatives.
[M þ H]þ 269.1290, found 269.1293.

346
Y.Q. Ge et al. / Dyes and Pigments 88 (2011) 344e349
Table 1
128.8, 128.7, 128.5, 125.6, 122.5, 121.6, 120.7, 117.1, 110.5, 61.6, 16.8,
Crystal data and structure refinement for 3j.
13.7; IR (KBr)
n
¼ 3438, 3051, 3010, 2927, 1730, 1641, 1640, 1490,
1340, 1244, 1182, 1087, 1016, 741, 697 cmꢂ1; HRMS: m/z calcd for
Empirical formula
Formula weight
Temperature
C23H22N2O2
358.43
293(2)K
C21H19N2O2 [M þ H]þ 331.1447, found 331.1445.
Wavelength
Crystal system, space group
Unit cell dimensions
0.71973 Å
Orthorhombic, Pbca
2.2.3.7. 3-Ethoxycarbonyl
azole (3g). Yellow solid (82% yield): mp 159e160 ꢀC; 1H NMR
(300 MHz, CDCl3):
-4,7,8-trimethylpyrido[1,2-a]benzimid-
a ¼ 16.4809(12) Å
a
¼ 99 deg.
b ¼ 12.4963(9) Å
c ¼ 19.1780(14) Å
3949.7(5) A3
8
b
¼ 99 deg.
d
8.20 (d, J ¼ 7.2 Hz, 1H), 7.76 (s, 1H), 7.61 (s, 1H),
g
¼ 90deg.
7.24 (d, J ¼ 7.2 Hz, 1H), 4.43 (q, J ¼ 7.2 Hz, 2H), 3.01 (s, 3H), 2.47 (s,
Volume
Z
Calculated density
Absorption coefficient
F(000)
3H), 2.46 (s, 3H), 1.44 (t, J ¼ 7.2 Hz, 3H); 13C NMR (75 MHz, CDCl3):
1.206 Mg/m3
0.077 mmꢂ1
1520
d
166.4, 148.3, 143.6, 135.5, 132.1, 131.9, 127.8, 127.0, 121.5, 120.2,
110.5,110.2, 61.4, 20.8, 20.7, 15.3,14.3; IR (KBr)
n
¼ 3407, 2979, 2920,
2853,1719, 1490, 1470,1309, 1235, 1147, 1055, 737 cmꢂ1; HRMS: m/z
calcd for C17H19N2O2 [M þ H]þ 283.1447, found 283.1448.
Crystal size
0.29 ꢁ 0.15 ꢁ 0.19 mm
q
range for data collection
2.12e27.54ꢀ
Limiting indices
Reflections collected/unique
Completeness to
Absorption correction
Max. and min. transmission
Refinement method
ꢂ29 ꢃ h ꢃ 21, ꢂ16 ꢃ k ꢃ 14, ꢂ12 ꢃ 1 ꢃ 24
21955/4518 [R(int) ¼ 0.0263]
99.2%
2.2.3.8. 2,4,7,8-Tetramethyl-3-ethoxycarbonylpyrido[1,2-a]benzimid-
q
¼ 27.54ꢀ
azole (3h). Yellow solid (70% yield): mp 84.5 ꢀC; 1H NMR (300 MHz,
Semi-empirical from equivalents
0.9923 and 0.9847
CDCl3):
2.68 (s, 3H), 2.46 (s, 3H), 2.44 (s, 3H), 2.34 (s, 3H), 1.44 (t, J ¼ 7.2 Hz,
3H); 13C NMR (75 MHz, CDCl3):
167.9, 147.0, 143.3, 135.0, 133.4,
131.1, 127.5, 125.1, 120.9, 119.9, 116.3,110.4, 61.6, 20.7, 20.6, 16.8,14.8,
d
8.05 (s,1H), 7.71 (s,1H), 7.58 (s,1H), 4.47 (q, J ¼ 7.2 Hz, 2H),
Full-matrix least-squares on F2
4518/0/245
d
Data/restraints/parameters
Goodness-of-fit on F2
1.048
Final R indices [I > 2
R indices (all data)
Largest diff. peak and hole
s
(I)]
R1 ¼ 0.0658, wR2 ¼ 0.2054
R1 ¼ 0.0884, wR2 ¼ 0.2307
0.536 and ꢂ0.352 e. Åꢂ3
14.3; IR (KBr)
n
¼ 3444, 3038, 2983, 2926, 1729, 1629, 1499, 1473,
1384,1266,1245,1180,1145,1064,1007, 852 cmꢂ1; HRMS: m/z calcd
for C18H21N2O2 [M þ H]þ 297.1603, found 297.1606.
2.2.3.9. 3-Ethoxycarbonyl-4-phenyl-7,8-dimethylpyrido[1,2-a]benz-
imidazole (3i). Yellow solid (83% yield): mp 179e180 ꢀC; 1H NMR
2.2.3.5. 3-Ethoxycarbonyl-4-phenylpyrido[1,2-a]benzimidazole
(3e). Yellow solid (88% yield): mp 165e166 ꢀC; 1H NMR (300 MHz,
(300 MHz, CDCl3):
d
8.40 (d, J ¼ 7.2 Hz, 1H), 7.74 (s, 1H), 7.68 (s, 1H),
CDCl3):
d
8.50 (d, J ¼ 7.2 Hz, 1H), 8.00 (d, J ¼ 8.4 Hz, 1H), 7.95 (d,
7.44e7.56 (m, 5H), 7.23 (d, J ¼ 7.2 Hz, 1H), 4.10 (q, J ¼ 7.2 Hz, 2H),
2.48 (s, 3H), 2.43 (s, 3H), 0.96 (t, J ¼ 7.2 Hz, 3H); 13C NMR (75 MHz,
J ¼ 8.1 Hz, 1H), 7.41e7.54 (m, 7H), 7.28 (d, J ¼ 7.2 Hz, 1H), 4.11 (q,
J ¼ 7.2 Hz, 2H), 0.97 (t, J ¼ 7.2 Hz, 3H); 13C NMR (75 MHz, CDCl3):
CDCl3):
d 167.3, 147.2, 144.2, 135.6, 135.5, 132.6, 131.9, 129.4, 129.2,
d
167.2, 147.8, 145.4, 135.4, 132.7, 130.1, 129.4, 129.0, 128.5, 128.3,
128.3, 128.2, 127.5, 123.6, 120.7, 110.4, 110.0, 61.4, 20.8, 20.7, 13.5; IR
126.0, 123.9, 122.2, 121.0, 110.7, 110.3, 61.6, 13.6; IR (KBr)
n
¼ 3410,
(KBr)
n
¼ 3452, 2974, 1706, 1644, 1618, 1564, 1462, 1326, 1212, 1157,
3031, 2984, 1717, 1468, 1369, 1327, 1257, 1135, 1011, 768, 697 cmꢂ1
;
1026, 847, 687 cmꢂ1; HRMS: m/z calcd for C22H21N2O2 [M þ H]þ
HRMS: m/z calcd for C20H17N2O2 [M þ H]þ 317.1290, found
345.1603, found 345.1607.
269.1294.
2.2.3.10. 2,7,8-Trimethyl-3-ethoxycarbonyl-4-phenylpyrido[1,2-a]
benzimidazole (3j). Yellow solid (74% yield): mp 163e164 ꢀC; 1H
2.2.3.6. 2-Methyl-3-ethoxycarbonyl-4-phenylpyrido[1,2-a]benzimid-
azole (3f). Yellow solid (77% yield): mp 136e138 ꢀC; 1H NMR
NMR (300 MHz, CDCl3):
d 8.20 (s, 1H), 7.71 (s, 1H), 7.63e7.60 (m,
(300 MHz, CDCl3):
d
8.29 (s, 1H), 7.96 (d, J ¼ 8.4 Hz, 1H), 7.88 (d,
3H), 7.39e7.50 (m, 3H), 4.08 (q, J ¼ 7.2 Hz, 2H), 2.47 (s, 3H), 2.42 (s,
3H), 2.40 (s, 3H), 0.97 (t, J ¼ 7.2 Hz, 3H); 13C NMR (75 MHz, CDCl3):
J ¼ 8.1 Hz, 1H), 7.61e7.64 (m, 2H), 7.35e7.52 (m, 5H), 4.10 (q,
J ¼ 7.2 Hz, 2H), 2.43 (s, 3H), 0.98 (t, J ¼ 7.2 Hz, 3H); 13C NMR
d
167.7, 146.2, 144.0, 135.0, 134.9, 133.9, 131.2, 129.6, 128.9, 128.5,
(75 MHz, CDCl3):
d 167.5, 146.7, 145.1, 134.9, 134.7, 129.6, 129.1,
128.4, 127.3, 122.4, 120.4, 116.6, 110.3, 61.4, 20.7, 20.6, 16.8, 13.7; IR
Table 2
Selected bond lengths and angles for 3j.
Bond lengths
N(l)-C(9)
C(10)-C(12)
C(7)-C(l)
C(13)-O(2)
1.372(2)
1.438(3)
1.491(3)
1.334(3)
N(l)-C(8)
1.389(2)
1.330(3)
1.387(3)
1.466(4)
N(l)-C(17)
C(8)-C(7)
C(16)-C(23)
C(21)-C(19)
1.391(3)
1.422(3)
1.399(3)
1.424(4)
C(9)-C(10)
C(7)-C(12)
C(13)-O(l)
C(14)-C(15)
1.349(3)
1.367(3)
1.188(3)
1.431(6)
C(8)-N(2)
N(2)-C(16)
O(2)-C(14)
Bond angles
C(9)-N(l)-C(8)
C(18)-C(17)-C(16)
N(2)-C(16)-C(17)
C(9)-N(l)-C(17)
N(2)-C(16)-C(23)
C(17)-C(16)-C(23)
C(7)-C(12)-C(10)
C(7)-C(12)-C(13)
C(10)-C(12)-C(13)
C(6)-C(l)-C(2)
C(6)-C(l)-C(7)
C(8)-N(2)-C(16)
O(l)-C(13)-O(2)
O(l)-C(13)-C(12)
123.58(17)
123.0(2)
111.54(19)
129.91(16)
129.9(2)
C(19)-C(18)-C(17)
C(8)-N(l)-C(17)
C(10)-C(9)-N(l)
C(9)-C(10)-C(12)
N(2)-C(8)-N(l)
N(2)-C(8)-C(7)
N(l)-C(8)-C(7)
C(12)-C(7)-C(8)
C(12)-C(7)-C(l)
C(8)-C(7)-C(l)
117.5(2)
106.34(16)
119.84(18)
118.24(18)
112.84(18)
129.46(18)
117.64(17)
118.21(18)
122.59(19)
119.17(18)
131.82(19)
105.06(17)
110.9(2)
118.6(2)
122.49(19)
118.59(18)
118.89(18)
118.7(2)
120.5(2)
104.18(17)
124.9(2)
N(l)-C(17)-C(18)
N(l)-C(17)-C(16)
O(2)-C(13)-C(12)
C(21)-C(23)-C(16)
124.2(2)
119.9(2)

Y.Q. Ge et al. / Dyes and Pigments 88 (2011) 344e349
347
Fig. 4. The dimeric structure of 1a.
3. Results and discussion
3.1. Synthesis
The desired pyrido[1,2-a]benzimidazole 3 was obtained by the
reactions of compounds 1 and the bromomethyl substituted esters
2 in the presence of K2CO3 at room temperature. Because of the low
solubility of the benzoimidazole-2-carbaldehyde 1a in DMF and the
formation of the dimeric structure (Fig. 4) [21], the yields of 3aeb
are low compared to those of other products. It is interesting to note
that the presence of a benzoyl or acetyl substituent of benzimid-
azole did not show any significant influence on the outcome of the
Fig. 2. The molecular structures of 3j, H atoms and the molecular structures of
solvents are omitted for clarity.
reaction but the presence of a methyl substituent on the
a, b-
(KBr)
n
¼ 3448, 3017, 2974, 1735, 1621, 1492, 1468, 1424, 1313, 1240,
unsaturated ester interfered with the reaction because of the
inductive effect. Additionally, increased temperature (50 ꢀC) and
extended reaction time (up to 9 h) were required to obtain the
desired product when 2-benzoylbenzoimidazoles 1bee were
reacted with ethyl 4-bromo-3-methylbut-2-enoate 2b. On the basis
of the above results and our previous work [16,17,18], we believe
that this reaction proceeds by an SN2 displacement of bromide ion,
deprotonation resulting in an ester stabilised anion which facili-
tates cyclisation and a subsequent dehydration as shown in Fig. 5.
1172, 1116, 1019, 857, 699 cmꢂ1; HRMS: m/z calcd for C23H23N2O2
[M þ H]þ 359.1760, found 359.1766.
2.3. X-ray crystallography
Single crystals of compound 3j suitable for X-ray diffraction
were obtained by slow evaporation of a solution of the solid in ethyl
acetate at room temperature for 5 days. A crystal with approximate
dimension of 0.20 mm ꢁ 0.15 mm ꢁ 0.10 mm was mounted on
a Bruker Smart Apex II CCD equipped with a graphite mono-
3.2. Structure characterization
The structures of products 3a-j were characterized by spectro-
scopic methods (1H and 13C NMR, IR, and HRMS). For example,
compound 3j, obtained as yellow crystals, gave an [M þ H]-ion peak
at m/z 359.1766 in the HRMS, in accord with the molecular formula
C23H23N2O2. The IR spectra of compound 3j showed the charac-
teristic absorption bands at 1715 (C]O), 1621 (C]C), 1240
(CeOeC) and the 1H NMR spectra (CDCl3) revealed five distinct
chromated MoK
a
radiation (
l
¼ 0.71073Å) by using
F and u scan
modes and the data were collected at 293(2) K. The structure of the
crystal was solved by direct methods and refined by full-matrix
least-squares techniques implemented in the SHELXTL-97 crystal-
lographic software. The non-hydrogen atoms were refined aniso-
tropically. The hydrogen atoms bound to carbon were located by
geometrical calculations, with their position and thermal parame-
ters being fixed during the structure refinement. A summary of the
crystallographic data and structure refinement details is given in
Table 1, and the selected bond lengths and angles are presented in
Table 2. The crystal structure and cell structure of 3j are shown in
Figs. 2 and 3, respectively.
singlets at
benzimidazole moiety) and 8.20 (1H, pyridine moiety). Moreover,
compound 3j showed peaks at
d 2.40 (3H, CH3), 2.42 (3H, CH3), 2.47 (3H, CH3), 7.71 (1H,
d
0.97 (t, 3H, J ¼ 7.2 Hz), 4.08 (q, 2H,
J ¼ 7.2 Hz), assigned to the protons of ethoxycarbonyl group. All
other signals are consistent with the structure of 3j.
3.3. Crystal structure
From Fig. 2 and from Table 1, the bond length of C1eC7 is, as
expected, shorter than a normal carbonecarbon single bond due to
Fig. 3. A packing diagram for 3j, H atoms and the molecular structures of solvents are
omitted for clarity.
Fig. 5. The mechanism of the tandem reaction.

348
Y.Q. Ge et al. / Dyes and Pigments 88 (2011) 344e349
Table 3
The optical characteristics of the compounds 3aej in dichloromethane.
Compounds lmax (nm) lex (nm) emax Fmax (nm) Stokes shift
(L molꢂ1 cmꢂ1
)
(nm)
3a
3b
3c
3d
3e
3f
3g
3h
3i
248
253
250
250
256
253
252
251
228
258
335
335
335
335
335
335
335
335
340
335
42213
49202
40822
55090
45612
48410
38855
47854
33071
46168
457
465
447
451
472
473
457
461
443
494
122
130
112
116
137
138
122
126
103
149
3j
the conjugation effect. The dihedral angle between the N1/C8/N2/
C16/C17 ring and N1/C9/C10/C12/C7/C8 ring is 3.38ꢀ, and 3.44ꢀ for
the C16/C174/C18/C19/C21/C23 ring, thus the three rings in 3j are
nearly in the same plane and this coplanar conformation provides
a large conjugated system. However, the dihedral angle between
the N1/C9/C10/C12/C7/C8 ring and C16/C17/C18/C19/C21/C23 ring
is 6.62ꢀ, and 71.32ꢀ for the C1/C2/C3/C4/C5/C6 ring thus overall
from the dihedral angle data compound 3j is not a planar molecule.
Fig. 7. The Fluorescence spectra of the compounds 3aej in dichloromethane.
to the pyrido[1,2-a]benzimidazole ring, are red-shifted by 15 and
8 nm, respectively, compared to 3a and 3b. The position of the
methyl groups has a different effect on the emission maxima: 3b is
red-shifted 8 nm while 3c is blue-shifted 10 nm compared to 3a.
Both 3g and 3h are red-shifted by 10 nm compared to 3c and 3d,
respectively. Stokes shifts of compounds 3aej (Table 3) are also
dependent on the groups bonded to pyrido[1,2-a]benzimidazole
ring. Fluorescence quantum yields (FF) in CH2Cl2 were determined
by a comparative method, using quinine sulfate (purchased from
Sigma-Aldrich) as a standard sample with FF ¼ 0.577 in 0.1 mol Lꢂ1
H2SO4 as the reference. The FF values for 3aej were listed in Table 3.
3.4. Absorption spectral characteristics of the compounds 3aej
For UVevisible absorption measurements, the dye concentra-
tion was 1 ꢁ10ꢂ5 mol Lꢂ1, and the absorption data are summarized
in Table 3. The UVevisible absorption spectra of compounds 3aej
are given in Fig. 6. Several absorption peaks could be observed in
the wavelength range from 225 to 425 nm, while almost no
absorption was observed beyond 425 nm. It is noted that 3aej
display very similar absorptions with a strong absorption band at
ca. 250 nm, which should originate from the benzene ring and
pyrido[1,2-a]benzimidazole ring and should be assigned to the
4. Conclusion
This paper describes the use of a novel tandem annulation
reaction to prepare pyrazolo[1,5-a]pyridine derivatives under mild
conditions in good yields. The structures of compounds obtained
were determined by IR, 1H NMR, 13C NMR and HRMS spectra, and
the spatial structure of compound 3j was conformed by X-ray
crystallography. Absorption and fluorescence spectral characteris-
tics of the compounds were investigated in dichloromethane
solution by UVevis absorption and emission spectra. The absorp-
tion spectra and fluorescence characteristics were correlated with
substituents on pyrido[1,2-a]benzimidazole rings.
pep* electronic transition. The relatively weak absorption bands
between 300 and 425 nm are assigned to the nep* electronic
transition (Fig. 7).
3.5. Fluorescence spectral characteristics
The emission spectra of compounds 3aej in dichloromethane
solution (1 ꢁ 10ꢂ5 mol Lꢂ1) are presented in Fig. 7 and their exci-
tation wavelengths are shown in Table 3. The fluorescence intensity
and maximum emission bands of 3aej are dependent on the
groups bonded to pyrido[1,2-a]benzimidazole rings. Emission
maxima of compounds 3aej range from 443 to 494 nm. The
emission maxima of 3e and 3f, in which a phenyl group is bonded
5. Supplementary material
CCDC 747673 contains the supplementary crystallographic data
forthispaper. Thesedatacanbeobtainedfreeofchargeviawww.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 CB21EZ, UK; fax: þ44 1223 336033.
Acknowledgement
The authors thank the Shandong Natural Science Foundation
(No. Y2008B40) and Shandong Excellent Young and Mid-aged
Scientist Promotive Foundation (No. 2008BS04024) for financial
support of this work.
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