An eco-friendly, simple, and effective protocol is developed
for
the synthesis of various multisubstituted 2-aminothiophenes. In the
presence of a catalytic amount of ZnO (5 mol %), ketones or
aldehydes, malononitrile and elemental sulfur were converted to the
corresponding 2-aminothiophene derivatives in moderate to high yields
(27%–70%) under solvent-free conditions at 100 °C. Zinc
oxide as an efficient, readily available, and reusable catalyst, showed
very good catalytic activity for the synthesis of 2-aminothiophene
derivatives. Thus far, little research has been reported on the Gewald
reaction under solvent-free conditions; to the best of our knowledge,
this is the first time that it has been proposed as an inexpensive
and readily available catalyst for use in the target synthetic route.
In the present study, synthesis and decomposition of
zinc hydroxynitrates,
i.e., Zn(NO3)(OH)·H2O, Zn5(NO3)2(OH)8·2H2O, and Zn3(OH)4(NO3)2 into ZnO nanoparticles
were investigated at supercritical conditions by combination of Taguchi
experiment design method, analysis of variance (ANOVA), and complementary
verifying experiments. First, the effect of several important parameters
such as temperature, residence time, and initial nitrate salt concentration
and its pH on “purity” of synthesis particles (e.g.,
ZnO) was studied using an orthogonal experiment design methodology
coupled with instrumental analysis such as powder X-ray diffraction,
Fourier-transform infrared spectroscopy, transmission electron microscopy,
and thermo-gravimetric analysis. Then ANOVA was performed on the data
and the best conditions for the synthesis of pure ZnO nanoparticles
were attained. The second aim was the elucidation of formation and
decomposition mechanism of hydroxinitrates by combining the statistical
results and additional experiments which influential parameters were
investigated independently. It was found that the initial pH of zinc
nitrate alongside temperature had the most effects on formation of
Zn5(NO3)2(OH)8·2H2O as the first product of the formation reaction, while at
higher temperatures it was converted to Zn3(OH)4(NO3)2 which in turn decomposed to ZnO nanoparticles
as an ultimate product.
TiO2/nanoclinoptilolite (TiO2/NCP) was prepared by the mediation of hexadecyltrimethylammonium (HDTMA) as surfactant and used as an effective heterogeneous nanocatalyst for the preparation of substituted 2‐aminotiophenes. The modified HDTMA/NCP was impregnated with titanium(IV) chloride solution followed by calcination at 500 °C for 20 h. The obtained nanocomposite was characterized using Fourier transform infrared spectroscopy, field emission scanning electron microscopy, inductively coupled plasma optical emission spectroscopy and X‐ray diffraction. Moreover, the prepared nanocomposite had high stability and recoverability under mild and solvent‐free conditions.
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