A. Pramanik et al. / Tetrahedron Letters 55 (2014) 1771–1777
1773
Figure 2. SEM images of (a) neutral alumina, (b) freshly prepared alumina-sulfuric acid, (c) recycled alumina-sulfuric acid.
Figure 3. AFM images of (a) neutral alumina, (b) freshly prepared alumina-sulfuric acid (reproduced with permission of Elsevier from Ref. 9b (Catal. Commum., 2012, 20, 17–
24), (c) recycled alumina-sulfuric acid.
Different aryl aldehydes 2 bearing both electron-donating (en-
tries 1, 3–6, 9, 14–16 and 18) and electron-withdrawing (entries
7, 8, 12 and 17) substituents underwent facile condensation with
1 and produced 2-aryl-1-arylmethyl-1H-benzimidazoles 3 in good
yield having a wide range of substitution patterns. The structures
corresponding to the compound numbers can be found in Figure 6.
Unsubstituted benzaldehyde 2b, highly susceptible to aerial oxida-
tion, also reacted very efficiently with 85% yield without any ove-
roxidation to benzoic acid (entry 2). The reactions with acid-labile
heteroaryl aldehydes 2j and 2k (entries 10 and 11) went cleanly
with satisfactory yields without much side-reactions. Acid sensi-
tive methylenedioxy moiety (entries 9 and 18) also survived dur-
ing this transformation. Interestingly, 2-hydroxybenzaldehyde
2m yielded the bis-imine 4m at room temperature (entry 13) but
produced the 1,2-disubstituted benzimidazole 3m under reflux
(entry 14). So the present alumina-sulfuric acid catalyzed method
provided a facile access to 2-aryl-1-arylmethylbenzimidazoles
with a wide range of structural diversity. As an exception, 2-nitro-
benzaldehyde 2n yielded monosubstituted benzimidazole 5n as
the exclusive product (entry 19). Only the bis-imine 40 was ob-
tained from the aldehyde 2c and acyclic ethylene diamine 1c with-
out any cyclization. After completion of the reaction, the reaction
mixture was diluted with ethanol to dissolve the precipitated
product and alumina-sulfuric acid was separated simply by filtra-
tion, washed with ethanol and dried at 130 °C. The catalyst was
recovered more easily than PEG-OSO3H7 and recycled very effi-
ciently using 1a (1 mmol), 2a (2 mmol) and catalyst (0.1 g) at room
temperature for 3 h in dry ethanol with little variation of yield
(Fig. 1).
surface, which provided greater surface area with proper chemical
polarization towards the substrates to accomplish facile bimolecu-
lar condensation. The AFM images of three aforesaid solids (Fig. 3)
also provided supportive evidence.
From the EDAX spectra of the above-mentioned three solids
(Fig. 4) and comparative profile of the atom composition (Table 4)
it was evident that sulfur was tightly linked with the alumina sup-
port of both freshly prepared and recycled alumina-sulfuric acid.
There was extremely marginal loss of sulfur atom during the
reaction and recycling process. So it can be concluded that in alu-
mina-sulfuric acid, SO3H moiety remains immobilized7 on alumina
support probably through the oxygen atoms (acting as linkers) of
alumina oxide.
Finally, line scan on the image of alumina-sulfuric acid (Fig. 5)
confirmed the immobilization of a sulfur-bearing moiety on the so-
lid support.
The aforesaid reaction has been studied in the presence of var-
ious catalysts and the results are presented in Table 5. When 1a
and 2a reacted in the absence of any catalyst, only the bis-imine
product 4a was obtained. The aldehyde 2a was partially converted
to 3a with alumina-sulfuric acid within a limited period of time
without any formation of monosubstituted imidazole 5a. When
the reaction was run for longer time, 2a was completely consumed
and 3a was formed exclusively. With SiO2, supported with or with-
out any other acid catalyst, the reaction remained incomplete and
the product 3a was obtained with lower yield. Aldehyde 2a was
converted to a mixture of 3a and bis-imine product 4a using zeolite
as catalyst. It is important to note that the 2a was totally consumed
in the presence of neutral alumina and a mixture of 3a and 4a was
produced where 4a was preponderant. In contrast, when concd
H2SO4 was used as a homogeneous catalyst, 2a reacted completely
but produced a mixture 3a and 4a, with 3a in lesser amount. Alu-
mina-sulfuric acid was found totally different from concd H2SO4 on
alumina in terms of product spread and isolated yield of the de-
sired product 3a. With concd H2SO4 adsorbed on neutral alumina,
3a was obtained as the minor product with predominant formation
of the by-product 4a. A similar trend was also observed with
From the SEM images (Fig. 2), it was evident that when neutral
alumina (Fig. 2a) was converted to alumina-sulfuric acid (Fig. 2b), a
clear change in the morphology of the surface was observed; the
roughness of the surface was increased and thoroughly distributed
over the entire area. This was also maintained in the recycled alu-
mina-sulfuric acid (Fig. 2c).
The remarkable catalytic activity of alumina-sulfuric acid might
be ascribed to this widely distributed roughness of the catalyst