Selective Reduction of Azines to Benzyl Hydrazones with Sodium Borohydride Catalyzed by Mesoporous Silica‐Supported Silver Nanoparticles: A Catalytic Route towards Pyrazole Synthesis

Abstract

Thec atalytic activity of supported silver nanoparticles on mesoporous silica was studied, for the selectiver eduction 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 impregnationf ollowed by reductionw ith hydrogena t3 50 8 8Ca nd in situ deposition/reduction with am ixture of amines (ethanolamine and ethylenediamine).T he Ag/HMS (amines) catalyst was found to promote the selective 1,2-reduction of aryl-substituted azines, comparedt o the corresponding 1,4-reductiont hat occurs in general reduction processes. This catalytic transfer hydrogenation process found to be clean, fast andq uantitative (> 99% yields and selectivity) towards benzyl hydrazone synthesis under mild conditions.O fg reat importance is that under the presentc atalytic condi-tions reducible functional groups remain intact. Formal kinetics, support the in situ formation of silver hydride species being responsible for the reductionp rocess.T he presence of protic polarm ethanol enhanced the catalytic activity of Ag/HMS. Basedo nt he recycling studies the catalytic system Ag/HMS-NaBH 4 wasf ound to catalyze the selective reductiono fa zines nine times without significant loss of its activity.F inally,ao ne-pot reactionb etween the in situ produced benzylh ydrazones and a series of nitrostyrenes readilyp rovided the regioselective synthesiso f1 ,3,5-subtituted pyrazoles,h ighlightingau seful synthetic application of the catalytic protocol.

Sustainable access to renewable N-containing chemicals from reductive amination of biomass-derived platform compounds

“…Sodium borohydride is a common low-cost and environmental friendly reductant in organic synthesis [77,78]. The employment of NaBH 4 as reductant for the reductive amination has been verified to be feasible [79,80].…”

Selective Synthesis of Benzimidazoles from o-Phenylenediamine and Aldehydes Promoted by Supported Gold Nanoparticles

“…Given the importance of this type of transformation and in terms of sustainability, the use of ambient and more eco-friendly heterogeneous conditions for the synthesis of 2-aryl and 2-alkyl benzimidazole derivatives continues to be a long-standing goal of chemical research. In light of our ongoing research directions on developing sustainable catalytic processes to construct N-heterocyclic organic molecules of high biological interest [72][73][74], and metal nanoparticle-catalyzed transfer hydrogenation processes for conversion of nitroarenes into amines [75][76][77][78][79][80], herein we report the synthesis of a library of 2-aryl and 2-alkyl benzimidazoles using Au/TiO2 as a catalyst at ambient conditions (Scheme 2). Scheme 2.…”

Application of Silver Nanoparticles in the Multicomponent Reaction Domain: A Combined Catalytic Reduction Methodology to Efficiently Access Potential Hypertension or Inflammation Inhibitors

“…For the catalytic reductions, we employed commercially available supported AuNPs Au/TiO 2 , as well as the salts AgNO 3 and AgOTf and the synthesized mesoporous catalysts Ag/TiO 2 and Ag/HMS. 21 , 22 The commercial catalyst Au/TiO 2 features a ca. 1 wt % Au loading and exhibits an average AuNP size of about 2–3 nm.…”

“… 21 Also, Ag/HMS with AgNP loading amounts of 10, 30, and 50 wt % and the size of particles ranged between 15 and 30 nm ( Table S1 ) were synthesized by the in situ deposition/reduction with a mixture of ethanolamine and ethylenediamine, described in details in our previous work on the selective reduction of azines to benzyl hydrazones. 22 For selected transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images, see also Supporting Information (Figure S1).…”