Recent Applications of Heteropolyacids and Related Compounds in Heterocycle Synthesis. Contributions between 2010 and 2020

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.

Recent advances in the synthesis of 4H-chromen-4-ones (2012 − 2021)

“…The importance of specific reagents, namely, molecular iodine in the synthesis of 2-aryl-, 2-styryl-and 2-aryl-3-iodo-4H-chromen-4-ones 31 ; quinacetophenone as an easily accessible building block for the synthesis of various 2-and 3-substituted and 2,3-disubstituted 4H-chromen-4-ones; 32 the use of heteropolyacids as catalysts for the synthesis of 2-aryl-4H-chromen-4-ones; 33 2-hydroxyaryl tertiary enaminones for the synthesis of C-3-functionalized chromones via tandem vinyl CdH bond elaboration and chromone annulation reactions; 34 acyclic 2-hydroxyaryl enaminones for the synthesis of 3-acyl-4H-chromen-4-ones; 35 2-hydroxyaryl N,N-dimethylenaminones for the synthesis of 2,3-unsubstituted, 2-and 3-substituted 4H-chromen-4-ones; 36 and 2,3-unsubstituted 4H-chromen-4-ones for diversification of their chromone unit by direct CdH bond activation/functionalization at C-2 and C-3 sites 37 have been disclosed.…”

Response Surface Methodology Optimization of Friedel–Crafts Acylation Using Phosphotungstic Acid Encapsulation in a Flexible Nanoporous Material

“…Heteropoly acids are complex Bronsted acids consisting of heteropoly anions, which possess metal–oxygen octahedra as the basic structural units. − Heteropoly acid catalysts such as the Silica-coated magnetic NiFe 2 O 4 nanoparticle-supported phosphomolybdic acid are reported as non-toxic solid acids with high acidity and, therefore, significant catalytic activity, are a very good alternative to liquid mineral acids in industry. − Due to the very high solubility in polar solvents, these materials are mainly used heterogeneously (deposited on the porous solid matrix). , Phosphotungstic acid (PTA), as a heteropoly acid (HPA), has been regarded as a novel efficient reusable catalyst for Fries rearrangement of phenyl acetate in homogeneous or heterogeneous liquid-phase systems, the synthesis of amidoalkyl naphthols, which are generally synthesized via the three-component reaction of β-naphthol with an aldehyde and an amide in the presence of various promoting agents, and the synthesis of N , N ′-alkylidene bisamidesa using M-Fe 3 O 4 @HAL-SO 3 H. , Moreover, metal organic frameworks (MOFs) are a class of organic–inorganic structural porous nanomaterials whose crystal structure consists of metal groups or metal oxides coordinated into organic ligands to form regular lattice frameworks. , They have tunable open-metal centers and are able to hold active guest species in their pores. , MIL-53 is a typical class of MOFs, which is built up from Fe­(III) cations with 1,4-dicarboxylate as the linker molecule . In particular, the presence of iron has many advantages over other metals due to its low cost and non-toxicity, making it interesting as a catalyst.…”

“… 21 − 24 Due to the very high solubility in polar solvents, these materials are mainly used heterogeneously (deposited on the porous solid matrix). 25 , 26 Phosphotungstic acid (PTA), as a heteropoly acid (HPA), has been regarded as a novel efficient reusable catalyst for Fries rearrangement of phenyl acetate in homogeneous or heterogeneous liquid-phase systems, the synthesis of amidoalkyl naphthols, which are generally synthesized via the three-component reaction of β-naphthol with an aldehyde and an amide in the presence of various promoting agents, and the synthesis of N , N ′-alkylidene bisamidesa using M-Fe 3 O 4 @HAL-SO 3 H. 27 , 28 Moreover, metal organic frameworks (MOFs) are a class of organic–inorganic structural porous nanomaterials whose crystal structure consists of metal groups or metal oxides coordinated into organic ligands to form regular lattice frameworks. 29 , 30 They have tunable open-metal centers and are able to hold active guest species in their pores.…”

Cesium salt of 2-molybdo-10-tungstophosphoric acid as an efficient and reusable catalyst for the synthesis of uracil derivatives via a green route

“…The development in heterogeneous processes was more conscious, with a suspicious image of the future, and it was supported since the establishment of green chemistry principles. 1 Considering the past two decades, heteropoly acids have established a substantial dedication in heterogeneous catalysis due to their unique physicochemical attributes. [1][2][3][4][5] They have very strong Brønsted acidity in the region of a superacid, and also, they are efficient oxidizers under mild conditions.…”

“…1 Considering the past two decades, heteropoly acids have established a substantial dedication in heterogeneous catalysis due to their unique physicochemical attributes. [1][2][3][4][5] They have very strong Brønsted acidity in the region of a superacid, and also, they are efficient oxidizers under mild conditions. Their structural adaptability is also emphasized since they are complex molecules that can be modied by changing the constituting elements to model their size, charge density, redox potentials, acidity, and solubility.…”