A. Shaabani et al.
was chemically bonded or physically adsorbed to the por-
ous organic or inorganic solid surfaces [3–7]. In the case of
sulfonic acids (R–SO3H), two procedures for the produc-
tion of solid acids are reported: (1) the sulfonation of
support in a multi-step strategy in which sulfur trioxide
(SO3) reacts with functional groups of organic compounds
such as phenyl ring in the support to form a sulfur–carbon
bond [8–10]; (2) a multi-step strategy that usually involves
the loading of the –SH groups on the support and then
oxidizing to –SO3H groups [11, 12].
frequently unavoidable, the aggregation and agglomeration
of Fe3O4 nanoparticles into less active large particles and
bulk Fe3O4 during the reaction decrease its catalytic
activities [23, 24]. As mentioned above, wool contains
numerous amino acids units with –NH–CO–, –NH2, and –
S–S– functional groups, and the metal oxides nanoparticles
uptake by wool fibers can be done by these functional
groups; due to the structurally ordered amino acid chains,
the aggregation of Fe3O4 could be prevented.
Polyfunctionalized heterocycles play important roles in
the drug discovery process [25, 26]. For example, various
types of fused nitrogen-containing heterocycles such as
quinoxalinones [27, 28] and benzodiazepines [29, 30]
exhibit a wide variety of biological activity. Therefore, it is
not surprising that research in the field of synthesis of
heterocyclic compounds has received special attention.
As part of our ongoing program related to developing new
methods using solid acids and heterogeneous catalysts in
organic compound transformations [3, 4, 31], and based on our
previous investigations on multicomponent reaction [32–37],
wool-SO3H and Fe3O4@wool were synthesized and investi-
gated as catalysts for the synthesis of 3,4-dihydroquinoxalin-2-
amine, 1,6-dihydropyrazine-2,3-dicarbonitrile, (cyanophenyl-
amino)acetamide, tetrahydro-1H-1,5-benzodiazepine-2-car-
Wool is a chiral natural biopolymer composed of repet-
itive units of amino acids cross-linked by S–S bonds [13–
15]. Keratin as the main component of wool is insoluble in
any solvent, so this biopolymer could be used as natural
solid support. The suitable oxidizing agents can attack the
disulfide linkages, –S–S–, and they can easily oxidize them
to –SO3H groups in one step [16–18]. When the S–S bonds
in keratin are oxidized to the –SO3H groups, the acidic
active sites which were placed on this support may play the
role of a solid acid catalyst (Scheme 1). It is important to
note, in the case of wool, relatively high sulfonic acid pro-
vided on the catalyst (with respect to chemical
functionalization of inorganic and organic supports with
sulfonic acid groups) combined with its availability,
biodegradability, renewability, and being green makes this
material attractive as solid acid catalyst [19–22].
boxamide,
and
4,5,6,7-tetrahydro-1H-1,4-diazepine-5-
carboxamide derivatives (Scheme 2).
In recent decades, composite nanocatalysts, especially
supported magnetic metal oxides, have attracted consider-
able interest of the researchers, because of their potential
applications in chemical processes [23, 24]. Simple sepa-
ration (using an external magnet), availability, high
catalytic activity, high chemical stability, reusability, and
environmental friendliness are several important advan-
tages of these heterogeneous nanocatalysts. Since
unsupported nanoparticles are usually unstable and the
coagulation of the nanoparticles during the reaction is
Results and discussion
The synthetic route to produce the catalyst was based on
the modification of natural wool by oxidation of the –S–S–
to –SO3H groups, using KMnO4 as oxidant and then
loading MnO2 on the surface of the modified wool, fol-
lowed by the removal of MnO2 from the support. The
MnO2 was removed from the catalyst by washing with an
aqueous solution of Na2SO3 and acetic acid (Scheme 3).
Wool-supported Fe3O4 nanoparticles were prepared from
FeCl3 and FeCl2 solution at pH 11–12 as shown in
Scheme 4.
Scheme 1
The Fe content of the Fe3O4@wool catalyst was deter-
mined using the FAAS method and the amount of Fe3O4
was determined to be 15.1 % in this catalyst. The thermal
stability of the catalysts was verified by thermogravimetric
analysis. Thermogravimetric curves for wool, wool-SO3H,
and Fe3O4@wool are shown in Fig. 1 which revealed that
after an initial loss of adsorbed water at 90–236, 90–222,
and 90–245 °C, respectively, the main loss of weight
occurred. The decomposition step seems to include two
stages up to 800 °C. This loss was attributed to the first step
of the decomposition reaction at about 236, 222, and
245 °C and the second one at 372, 364, and 383 °C,
respectively.
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