2
A. MUTHUVINOTHINI AND S. STELLA
[
9,10]
producing a number of resonance forms.
Hence, guanidines are used as base cata-
[
11,12]
[13,14]
lyst in organic transformation reactions
such as addition,
epoxidation and
[15,16]
condensation reactions,
etc. On account of the tremendous use of guanidines and
its derivatives, the synthesis of guanidines gets greater attention among researchers.
Guanylation, the addition of amines to carbodiimides, is one of the important methods
[
17,18]
in the synthesis of guanidines due to its simple reaction conditions.
Guanylation reac-
[
19]
tion is reportedly catalyzed by metal complexes (Zinc complexes,
transition metal com-
[
20]
[21]
[22]
plexes,
rare-earth metal complexes,
etc.), transition metals
and nanoparticles
[23–25]
(NPs).
Among these, NPs catalyzed synthesis has drawn much attention due to their
[26]
greater activity in both homogeneous and heterogeneous medium.
However, the usage
of toxic raw materials and elimination of waste during the preparation of NPs are major
concern. Hence, nanoparticles prepared from nontoxic chemicals are highly acknowledged
for the synthesis and development of valuable organic products.
Zinc NPs are widely used due to their chemical stability, catalytic activity, cost-effective-
[27,28]
ness and exhibit greater optical, antibacterial and antifungal properties.
The contribu-
tion of zinc as a catalyst for the guanylation reaction is enormous as it produces excellent
yield and very less waste. Zn catalysts like fN-(aryl)iminoacenapthenone) ZnCl com-
2
[29]
[30]
[31]
[32]
plexes,
Zn-Al hydrotalcite
commercially available ZnEt2
and nano ZnO g were
used for the guanylation reaction. Most of these catalysts require 8 ꢀ 12 h for the guanidine
synthesis and in some cases, the method of catalyst synthesis involved a lengthy process.
Hence, an efficient catalyst prepared by an economic method under simple laboratory con-
ditions is highly desirable. In this work, we have synthesized an eco-friendly catalyst L-cyst-
eine-capped Zn NPs and explored its catalytic efficiency in the formation of guanidines.
Results and discussion
[
33–35]
Since zinc NPs as Lewis acid are efficient catalyst,
we have synthesized Zinc NPs
in the presence of L-cysteine. Being water-soluble, less toxic and the presence of highly
active sulfur atom, L-cysteine is selected as the capping and reducing agent. The forma-
tion of the NPs mainly rely on the pH of the reaction mixture and the NPs were
obtained at pH ¼ 5. In our earlier work, the synthesized NPs were characterized by
using various spectral techniques, such as UV–Vis, FT-IR, XRD and SEM which proved
the formation of Zn NPs through the thiol-metal bond. The particle size of the synthe-
[
36]
sized NPs was 58 nm as calculated from the most intense peak of the XRD spectrum.
The structure of the synthesized L-cysteine-capped Zn NPs is further confirmed by
EDAX and HR-TEM analytical methods.
EDAX spectrum of the as-prepared NPs (Fig. 1) confirms the presence of Zn
(17.29%) and Cysteine molecule [sulfur (21.86%) and carbon (60.85%)] which proved
the formation of cysteine capped Zn NPs. HR-TEM analysis of the synthesized NPs is
shown in Figure 2. TEM images at 20 nm and 10 nm scale show that the metal atom
(Zn) is completely surrounded by the L-Cysteine molecule.
Optimization studies
The catalytic ability of the Cys-Zn NPs was explored in guanylation reaction. The reac-
0
tion was carried out using 4-Chloroaniline (1a) and N,N -dicyclohexylcarbodiimide