KF/Al2O3: an efficient solid heterogeneous base catalyst
845
Table 3 Comparison of KF/Al2O3 efficiency with other recently reported catalysts for the synthesis of 2-phenylbenzimidazole by the reaction o-
PDA with benzoyl chloride
Entry
Catalyst
Solvent
Temperature
Time
Yield/%
References
1
2
3
4
5
6
BF3ꢀEt2O
Dioxane (dry)
130 °C
RT
1–2.5 h
40 min
10 min
4 h
97
[14]
[bbim]BF4 (1 eq)
[Hbim]BF4 (1 eq)
Heteropolyacids (8.3 mol%)
HZSM (0.25 g)
–
92
[13]
–
RT
95
[13]
AcOH
Reflux
100 °C
RT
92.8–98.2
[11]
Dioxane (dry)
Acetonitrile (dry)
3 h
81
93
[12]
KF/Al2O3 (25 mol%)
1 h
Present work
2,20-Bis(4-chlorophenyl)-5,50-bis(1H-benzimidazole)
(3b, C26H16ClN4)
magnesium sulfate, while the reaction mixture was stirred
well at RT. Then, a solution of acid chloride, acid anhy-
dride, or ester (1 mmol) in 5 cm3 dry acetonitrile was
added dropwise to the reaction mixture. TLC monitoring
showed the completion of the reaction of acid chlorides and
acid anhydrides after 45–90 min and the completion of the
reaction of esters after 90 min. After filtration and washing
the solid phase with acetonitrile (2 9 10 cm3), the solvent
was evaporated and pure 2-substituted benzimidazole was
obtained with no need for any further purification. The
known benzimidazoles were characterized by comparing
their m.p., IR, 1H NMR, 13C NMR data with those reported
for authentic samples.
Colorless solid; m.p.: 249–251 °C; IR (KBr film):
m = 1,624 (C=N), 3,448 (NH) cm-1
;
1H NMR
ꢀ
(250 MHz, DMSO-d6): d = 7.47–7.53 (m, 4H), 7.71–
7.74 (m, 2H), 7.76–7.85 (m, 4H), 7.92–7.96 (m, 4H),
12.31 (br, NH) ppm; 13C NMR (62.9 MHz, DMSO-d6):
d = 113.4, 122.1, 123.7, 125.8, 130.4, 130.8, 134.9, 135.5,
138.2, 141.6, 152.7 ppm; MS (70 eV): m/z = 455 (M?).
2,20-Bis(4-methylphenyl)-5,50-bis(1H-benzimidazole)
(3d, C28H22N4)
Colorless solid; m.p.: 260–264 °C; IR (KBr film):
m = 1,630 (C=N), 3,559 (NH) cm-1
;
1H NMR
ꢀ
General procedure for the synthesis of 2,20-
disubstituted 5,50-bis-benzimidazoles
(250 MHz, DMSO-d6): d = 2.38 (s, 6H), 7.23–7.29 (m,
4H), 7.71–7.82 (m, 6H), 7.85–7.91 (m, 4H), 12.80 (br, NH)
ppm; 13C NMR (62.9 MHz, DMSO-d6): d = 21.6, 113.5,
121.8, 123.9, 125.2, 128.6, 133.3, 134.6, 138.0, 139.7,
142.2, 161.4 ppm; MS (70 eV): m/z = 414 (M?).
To a solution of 214 mg 3,30-diaminobenzidine (1 mmol)
in 8 cm3 dry acetonitrile in a 25-cm3 round-bottom flask
was added 80 mg KF/Al2O3 (0.5 mmol) and 200 mg
anhydrous magnesium sulfate, while the reaction mixture
was stirred well at RT. Then a solution of acid chloride or
acid anhydride (2 mmol) in 7 cm3 dry acetonitrile was
added dropwise to the reaction mixture. TLC monitoring
showed the completion of the reaction after 90 min. After
filtration and washing the solid phase with acetonitrile
(2 9 10 cm3), the solvent was evaporated and pure 2,20-
disubstituted 5,50-bis-benzimidazole was obtained with no
need for any further purification. The known bis-benzimi-
dazoles were characterized by comparing their m.p., IR, 1H
NMR, and 13C NMR data with those reported for authentic
samples.
2,20-Dibenzyl-5,50-bis(1H-benzimidazole) (3f, C28H22N4)
Colorless solid; m.p.: 260–264 °C; IR (KBr film):
m = 1,630 (C=N), 3,512 (NH) cm-1
;
1H NMR
ꢀ
(250 MHz, DMSO-d6): d = 4.37 (s, 4H), 7.12–7.30 (m,
10H), 7.65–7.71 (m, 2H), 8.05–8.11 (m, 4H), 13.36 (br,
NH) ppm; 13C NMR (62.9 MHz, DMSO-d6): d = 43.1,
113.4, 121.5, 123.0, 125.9, 129.5, 130.4, 134.2, 135.6,
139.5, 140.3, 161.8 ppm; MS (70 eV): m/z = 414 (M?).
2,20-Di(2-naphthyl)-5,50-bis(1H-benzimidazole)
(3g, C34H22N4)
ꢀ
White solid; m.p.: 285–289 °C; IR (KBr film): m = 1,634
(C=N), 3,461 (NH) cm-1; 1H NMR (250 MHz, DMSO-d6):
d = 7.44–7.59 (m, 4H), 7.71–7.89 (m, 8H), 7.88–7.92 (m,
2H), 8.09–8.26 (m, 4H), 8.71–8.86 (m, 2H), 13.72 (br, NH)
ppm; 13C NMR (62.9 MHz, DMSO-d6): d = 124.9, 125.1,
126.7, 127.2, 127.8, 128.0, 128.2, 128.9, 129.2, 129.5,
129.7, 130.5, 132.1, 132.7, 134.9, 162.7, 167.4 ppm; MS
(70 eV): m/z = 488 (M?).
2,20-Bis(4-nitrophenyl)-5,50-bis(1H-benzimidazole)
(3a, C26H16N6O4)
ꢀ
White solid; m.p.: 235–237 °C; IR (KBr film): m = 1,634
(C=N), 3,424 (NH) cm-1; 1H NMR (250 MHz, DMSO-d6):
d = 7.47–7.57 (m, 8H), 7.67–7.70 (d, 2H), 7.85 (s, 2H),
8.18–8.21 (d, 4H), 12.48 (br, NH) ppm; 13C NMR
(62.9 MHz, DMSO-d6): d = 114.2, 123.7, 125.5, 126.1,
129.7, 131.0, 135.9, 140.3, 141.2, 151.3, 155.4 ppm; MS
(70 eV): m/z = 476 (M?).
Acknowledgments We gratefully acknowledge the support of this
work by Shiraz University Research Council.
123