N. Anbu, et al.
MolecularCatalysisxxx(xxxx)xxxx
chitosan. In addition, C–H bending is also observed at around
1450 cm−1. These observed data are in agreement with earlier reports
and further matches with the structural components of chitosan. Fur-
ther, the elemental analysis of chitosan was measured and the atomic
weight percentages of C, N, O are almost identical to the reported value
[17]. All these results suggest that there is a free amino group in chit-
osan which can be readily employed as a solid base catalyst in pro-
moting base catalyzed reactions.
After ascertaining the structural features and the availability of
basic sites in chitosan, the prime objective of this work is to utilize these
basic sites to promote Knoevenagel-Doebner condensation reaction
under mild reaction conditions. As commented earlier, although this
reaction has been reported with many homogeneous base catalysts with
serious limitations, no attempts were made to develop heterogeneous
catalysts for this reaction. Hence, the present work aims to report en-
vironmentally benign and biodegradable heterogeneous basic catalyst
for the synthesis of α,β-unsaturated carboxylic acids from 1 and 2 as
model substrates under different experimental conditions. The attained
results are presented in Table 1. The catalyst loading was fixed at 25 mg
to optimize a suitable solvent to achieve higher selectivity and yield.
The Knoevenagel-Doebner reaction between 1 and 2 gave 38 and 47%
yields of 3 in toluene and benzene respectively, at 70 °C after 18 h
(entries 1-2, Table 1). Further, the polar protic solvents like methanol,
ethanol and water afforded 14–23 % yields of 3 under identical con-
ditions (entries 3-5, Table 1). The use of polar aprotic solvents such as
1,2-dichloroethane (DCE), tetrahydrofuran (THF), 1,4-dioxane, acet-
onitrile and dimethylsulfoxide resulted in 14–70 % yields (entries 6-10,
Table 1). Interestingly, the desired product 3 was obtained in 94 %
On the contrary, a blank control experiment in the absence of chitosan
solid furnished 28 % yield under similar reaction conditions (entry 12,
Table 1). Therefore, the optimum conditions to accomplish the max-
imum yield of 3 were the use of DMF as a convenient solvent at 70 °C
after 18 h. Corma and coworkers have reported that the reaction rate of
the Knoevenagel condensation is significantly reduced in non-polar
solvents than in polar solvents in the presence of solid catalysts [51]. A
similar effect was also observed by Gascon and coworkers exhibiting
higher reaction rate with polar solvents [52] and in another report the
Knoevenagel condensation involving a polymer-based catalyst showed
higher rate in ethanol [53]. Although these precedents have clearly
shown remarkable enhancement in the reaction rate in polar solvents
than non-polar solvents, the higher yield of Knoevenagel-Doebner
product (3) using chitosan in DMF is due to the higher solubility of 2
compared to other tested solvents. These results are in agreement with
an earlier report for an identical reaction [41].
Fig. 4. Time yield plots for the Knoevenagel-Doebner condensation between 1
and 2 (a) in the presence of chitosan as catalyst (b) hot filtration experiment
and (c) control experiment in the absence of catalyst.
Fig. 5. Effect of catalyst loading for the Knoevenagel-Doebner condensation
between 1 and 2 with (a) 5 mg (b) 10 mg (c) 15 mg, (d) 20 mg and (e) 25 mg
solid catalyst.
condensation between 1 and 2 in DMF which indicated that the reac-
tion is efficiently accelerated in the presence of chitosan and affording
94% yield of 3 after 18 h. Further, another experiment was conducted
under similar conditions to confirm heterogeneity of the reaction and to
make sure the absence of homogeneous species in the reaction mixture
under identical conditions. Hence, the reaction was initiated under the
optimized reaction conditions and the solid catalyst was filtered off
while in hot after 3 h from the reaction mixture. Later, the filtrate in the
absence of solid was continued for the remaining time under optimum
conditions. The analysis of the reaction mixture indicated that the yield
of 3 did not improve further after removal of the solid catalyst which
clearly proved that the solid catalyst is truly behaving as heterogeneous
and the contribution of dissolved homogeneous species is ruled out
under the optimized conditions. Furthermore, the control experiment in
the absence of chitosan provided a lower yield of 3 under similar ex-
perimental conditions, which suggests that catalyst is essential to
achieve high yield.
catalyst loading was reduced in the range of 25 to 5 mg and the ob-
served results are presented in Fig. 5. As it can be seen in Fig. 5, the
from 25 to 5 mg which clearly explains the observed catalytic activity.
This decrease of activity upon reducing the catalyst loading indicates
that the density of active sites are significantly decreased, thus lowering
the yield of the product. On other hand, the reaction temperature was
also crucial in determining the final yield of 3. As shown in Fig. 6, the
initial reaction rate as well as the final product yield is decreased as a
function of reducing the reaction temperature from 70 to 50 °C and
further to room temperature.
One of the main goals of employing heterogeneous catalysts in or-
ganic transformations is their easy recovery from the reaction and their
recyclability in the successive cycles which also provides valuable in-
sights into the stability of catalyst under the performed experimental
conditions. Therefore, the recyclability of chitosan was investigated by
separating it from the reaction medium through centrifugation and
washed with acetonitrile then evaluated the catalytic activity using
fresh substrates 1 and 2 under the optimized reaction conditions. Fig. 7
After optimizing the solvent to reach the maximum yield of the
expected product, the next parameters were the effect of catalyst
loading and temperature. In order to optimize catalyst dosage, the
4