Abstract
Over the past two decades, polyoxometalates (POM) have received considerable attention as solid catalysts, due to their unique physicochemical characteristics, since, first, they have very strong Bronsted acidity, approaching the region of a superacid, and second, they are efficient oxidizers that exhibit rapid redox transformations under fairly mild conditions. Their structural mobility is also highlighted, since they are complex molecules that can be modified by changing their structure or the elements that compose them to model their size, charge density, redox potentials, acidity, and solubility. Finally, they can be used in substoichiometric amounts and reused without an appreciable loss of catalytic activity, all of which postulate them as versatile, economic and ecological catalysts. Therefore, in 2009, we wrote a review article highlighting the great variety of organic reactions, mainly in the area of the synthesis of bioactive heterocycles in which they can be used, and this new review completes that article with the contributions made in the same area for the period 2010 to 2020. The synthesized heterocycles to be covered include pyrimidines, pyridines, pyrroles, indoles, chromenes, xanthenes, pyrans, azlactones, azoles, diazines, azepines, flavones, and formylchromones, among others.
Highlights
Environmental problems and social pressure currently poses the chemical industry with one of the greatest challenges throughout its history, which is to redesign existing industrial processes and design new ones in compliance with the basic rules of sustainability [1]
Blanco et al [111] prepared phosphomolybdic and tungstophosphoric acid catalysts supported on a silica–alumina material, using the sol-gel method, and their catalytic activity was evaluated in the synthesis of azlactones (4-benzylidene-2-phenyloxazolin-5ones and 4-alkylidene-2-phenyloxazolin-5-ones) by the Erlenmeyer method
It is well known that heterocyclic wide range of pharmaceutical and biological properties, new structures are expected to be developed in the following years
Summary
Environmental problems and social pressure currently poses the chemical industry with one of the greatest challenges throughout its history, which is to redesign existing industrial processes and design new ones in compliance with the basic rules of sustainability [1]. Organic transformations catalyzed by inorganic solids have gained importance throughout the world, and within these solids, heteropolyacids (HPA) and related compounds stand out [8] This is demonstrated in the large number of publications on basic and applied research in fine chemical processes, since, thanks to their ionic structure, they have several available protons, which makes them extremely promising in catalysis because it gives them a very strong Brønsted acidity and suitable redox properties, which are modifiable, changing the chemical composition of the polyoxometalate. HPAs have been used in a large number of highly selective organic reactions including esterification, dehydration, cyclization, oxidation of amines, epoxidation of olefins, and other processes such as photocatalysis, water decontamination, biomass conversion, and oxidative desulfurization of fuels, among others They have been used in both heterogeneous and homogeneous systems [9,10,11,12,13,14,15,16,17]. This review is the continuation of a previous review published in 2009 and brings together the reports on the advances and new designs of catalysts based on heteropolyacids and their application in the catalytic synthesis of het-erocycles [26], updated with the contributions made in the same area for the for the period 2010 to 2020
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