Environmentally benign synthesis of heterocyclic compounds by combined microwave-assisted heterogeneous catalytic approaches^†

Abstract

Recent advances in the application of heterogeneous catalysis combined with microwave irradiation in the synthesis of heterocyclic compounds are reviewed. While a detailed summary of the different catalysts applied in the synthesis of heterocycles is provided, the work mainly focuses on the heterocyclic compounds and their synthesis and not the preparation or characterization of the catalyst. Due to the large number of N-containing heterocycles, the synthesis of these compounds dominates this account, however, the preparation of other heteroatom containing compounds is also covered in detail. The literature data are summarized based on the size of cycles and the number of heteroatoms in the compounds. Since the major goal of the work is to highlight the environmentally benign and sustainable nature of the combined microwave-assisted heterogeneous catalytic methods, the green aspects of the individual synthetic approaches will be emphasized.

Heteropoly acids-catalyzed organic reactions in water: doubly green reactions

“…The mechanism is started with protonation of the substrates and followed by conversion of the ionic intermediates to the related products. Misono and co-workers introduced two types of surface and bulk mechanism for heterogeneous catalysis (38). In surface type catalysis, the reactions occurred on the surface of bulk or supported heteropoly compounds and the catalytic activity frequently depended on the surface acidity of HPA.…”

Synthesis and Microbicidal Activity of Substituted 1,2,3-Triazoles having Amide and Hydroxy Functionality

“…But, lowering the catalyst concentration to < 1 % has lowered the catalytic efficiency and Table-1 shows the yields. It was observed and reported in literature that heterogeneous catalysis often requires certain % of catalyst by (w/v) and of theoretical catalytic surface area can be only be indicative [20][21][22][23]. Synthesis of pyrimidine by Knoevenagel condensation: Dihydropyrimidines and pyrimidines are known for then Ttype calcium channel blocking activity [1,2].…”

“…Though it is believed that pyrimidines are less utilized as pharmacological synthons compared to other rings like dihydropyrimidine [11,12], pyridines [13,14], furons [15,16], thiophenes [17], thiazides [18], etc., the evolution of newer medicinal chemistry leads are limited [19]. This is because the pyrimidine synthesis reactions usually result in lower reactions yields compared to other heterocyclics [20,21]. The recent spurt in interest of pyrimidine compounds necessitates development of viable synthetic methods.…”

Reduction of phenylacetylenes using Raney Ni–Al alloy, Al powder in the presence of noble metal catalysts in water

“…25 More recently, the application of a heterogeneous catalyst in combination with microwave irradiation has been employed as an environmentally benign tool for some contemporary organic syntheses. 26,27 Based on the application of Raney-type Ni-Al alloy in aqueous medium as the hydrogen source as well as solvent, the selective reduction of ketones and reductive amination of carbonyl compounds has been reported. 28,29 Tashiro et al have pioneered the use of various metal-Al alloys (Ni-Al, Co-Al, Cu-Al, Fe-Al) in water for the reduction of aryl ketones to the corresponding alcohols or alkane derivatives.…”