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Selective Reduction of Azines to Benzyl Hydrazones with Sodium Borohydride Catalyzed by Mesoporous Silica‐Supported Silver Nanoparticles: A Catalytic Route towards Pyrazole Synthesis
Author(s) -
Charistoudi Evangelia,
Kallitsakis Michael G.,
Charisteidis Ioannis,
Triantafyllidis Kostas S.,
Lykakis Ioannis N.
Publication year - 2017
Publication title -
advanced synthesis and catalysis
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.541
H-Index - 155
eISSN - 1615-4169
pISSN - 1615-4150
DOI - 10.1002/adsc.201700442
Subject(s) - chemistry , sodium borohydride , catalysis , pyrazole , mesoporous silica , reducing agent , selective reduction , transfer hydrogenation , selective catalytic reduction , mesoporous material , combinatorial chemistry , organic chemistry , ruthenium
The catalytic activity of supported silver nanoparticles on mesoporous silica was studied, for the selective reduction of azines into benzyl hydrazones using sodium borohydride as mild reducing agent. Different sizes of silver nanoparticles supported on mesoporous silica (Ag/HMS) were successfully prepared by two methods, i.e., wet impregnation followed by reduction with hydrogen at 350 °C and in situ deposition/reduction with a mixture of amines (ethanolamine and ethylenediamine). The Ag/HMS (amines) catalyst was found to promote the selective 1,2‐reduction of aryl‐substituted azines, compared to the corresponding 1,4‐reduction that occurs in general reduction processes. This catalytic transfer hydrogenation process found to be clean, fast and quantitative (>99% yields and selectivity) towards benzyl hydrazone synthesis under mild conditions. Of great importance is that under the present catalytic conditions reducible functional groups remain intact. Formal kinetics, support the in situ formation of silver hydride species being responsible for the reduction process. The presence of protic polar methanol enhanced the catalytic activity of Ag/HMS. Based on the recycling studies the catalytic system Ag/HMS‐NaBH 4 was found to catalyze the selective reduction of azines nine times without significant loss of its activity. Finally, a one‐pot reaction between the in situ produced benzyl hydrazones and a series of nitrostyrenes readily provided the regioselective synthesis of 1,3,5‐subtituted pyrazoles, highlighting a useful synthetic application of the catalytic protocol.