M. Bhardwaj et al.
three important aspects in current synthetic chemistry,
(a) one-pot procedures: the in situ generation of organic
azides from anilines by diazotization followed by the
addition of sodium azide to acetylenes, which minimizes
hazards derived from the isolation and handling of azides
because of their toxic nature [28]; (b) magnetic heteroge-
neous catalysis, especially involving copper nanoparticles
supported on magnetic supports, which offers several
advantages compared to their solution counterparts, such as
reusability, efficiency, enhanced stability and magnetic
retrieval of the catalyst [29]; and (c) reactions in water,
which offers key advantages, such as rate enhancement and
insolubility of the final products, thus making synthetic
processes cheaper, safer, and greener [30, 31]. Recently,
copper nanoparticles (Cu NPs) due to their small sizes and
large surface areas are being explored as click catalysts in
the hope of improving reusability and efficiency for the
synthesis of triazoles [32–34].
1 Introduction
Nanotechnology in the recent years has enabled revolu-
tionary developments in various fields including environ-
mental science, medicine and importantly, catalysis.
Nanoparticles, being small, present close to a homoge-
neous experience, thus bridging the gap between homo-
geneous and heterogeneous catalysis [1, 2]. Hence,
synthetic protocols can be made economic, greener and
more sustainable by designing nanoparticle based hetero-
geneous catalytic systems. Recently, Cu(0) nanoparticles
have drawn attention of chemists owing to their inexpen-
sive nature, easy preparation and the ability to replace other
noble metal nanoparticles, such as Ag, Au and Pt
nanoparticles [3–5]. However, the use of Cu(0) nanopar-
ticles is often complicated by issues associated with their
oxidation and agglomeration in the absence of stabilizers,
thereby diminishing their activity [6, 7]. Therefore, efforts
have been made to stabilize nanoparticles by immobiliza-
tion on different solid supports such as polymers, silica,
and zeolites [8–10]. One of the most attractive alternatives
to stabilize nanoparticles is the magnetic support, and their
paramagnetic and insoluble properties enable the catalyst
to be easily separated with an external magnetic field [11–
13]. Further, the coating of magnetic nanoparticles or in
general inorganic cores with functional outer shells of sil-
ica, zeolites and biocompatible polymeric materials like
cellulose is a facile way to improve the solution stability
for such hybrid core shell structures and prevents
agglomeration, leading to stable and finely dispersed active
species [14, 15]. Recently, much attention has been focused
on amine functionalization of the core shell based magnetic
substrates, since amines are well known to stabilise the
nanoparticles against aggregation without disturbing the
desired properties, and also recognised to increase their
catalytic activity [16, 17].
Further, benzothiazoles and their derivatives are present
in many pharmaceuticals and exhibit remarkable biological
and therapeutic activities [35–38]. Thus, the attractive
biological profiles of this group of compounds stimulate
chemists to explore efficient methods for the synthesis of
benzothiazoles and their structural analogues. Synthesis of
derivatives with a benzothiazole framework, involves
commonly used approaches comprises of the condensations
of 2-aminothiophenol with carboxylic acids, acid chlorides,
nitriles and aldehydes [39–42]. But, these conventional
reactions often suffers from drawbacks such as the use of
strongly oxidizing or toxic reagents, multistep synthetic
methodology, tedious workup procedures, and the insta-
bility of starting materials including 2-aminothiophenol
[43–45]. Subsequently, the development of a simple, con-
venient and an eco-friendly method for the synthesis of
benzothiazoles would be highly enviable. Despite the
spectacular success in the field of triazole and benzothia-
zole synthesis, there is always much scope available for the
improvement at several levels.
1,2,3-Triazoles are an important class of organic com-
pounds which have risen to prominence in recent years as
superbly versatile five membered nitrogen heterocycles.
The Cu(I)-catalysed (CuAAC) synthesis of 1,2,3-triazoles
via 1,3-dipolar Azide–Alkyne cycloaddition click reaction
introduced independently by Sharpless [18] and Meldal
[19] in 2002, finds utility in many disciplines beyond
synthetic organic chemistry [20–22]. Furthermore, 1,2,3-
triazole moieties are emerging as powerful pharma-
cophores in their own right. As a result, a plethora of
methods have been developed for this powerful conjuga-
tion tool with the aim to increase efficiency. Both
homogenous and heterogeneous copper mediated methods
have been developed for the synthesis of 1,4-disubstituted-
1,2,3-triazoles [23–27]. Among them, special attention is
needed to be given to those which combine the following
In this paper, we developed a facile and novel
methodology for the synthesis of copper nanoparticles onto
ethylene diamine functionalized inorganic/organic mag-
netic composite [Cu(0)–Fe3O4@SiO2/NH2cel] which rep-
resent an excellent example of synergism of the properties
of both inorganic and organic components (Scheme 1).
Further, Cu(0)–Fe3O4@SiO2/NH2cel was efficiently
employed for the one-pot synthesis of 1,4-disubstituted-
1,2,3-triazoles and 2-substituted benzothiazoles. Because
of the magnetic nature of the catalyst, it can be retrieved
using an external magnet, which eliminates the obligation
of catalyst filtration after completion of the reaction and
could be reused up to six times with high efficiency. To the
best of our knowledge, the use of copper nanoparticles onto
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Bhardwaj, Madhvi
