New Vanadium Keggin Heteropolyacids Encapsulated in a Silica Framework: Recyclable Catalysts for the Synthesis of Highly Substituted Hexahydropyrimidines Under Suitable Conditions

Vanadium-doped phosphomolybdic acids as catalysts for geraniol oxidation with hydrogen peroxide

In this work, vanadium-doped phosphomolybdic acids were evaluated as catalysts in green oxidation routes of terpene alcohols with hydrogen peroxide. A series of phosphomolybdic acids containing a variable load of vanadium cations (i.e., V5+ ions) were synthesized, and tested as catalysts in geraniol oxidation, the model molecule selected. All the catalysts were characterized by powder X-ray diffraction, attenuated diffuse reflectance infrared spectroscopy, UV-Vis spectroscopy, thermogravimetric analysis, N2 adsorption-desorption isotherms, scanning electronic microscopy, X-ray dispersive spectroscopy, and n-butylamine potentiometric titration. Various catalysts were evaluated; phosphomolybdic acids with general formulae H3+n PMo12-n V n O40 (n = 0, 1, 2 and 3), and common Brønsted acids (i.e., H2SO4, H3PO4, and p-toluene sulfonic acid). Among them, vanadium monosubstituted phosphomolybdic acid was the most active catalyst and selective toward epoxide. The effect of main reaction variables, such as temperature, load catalyst, and reactant stoichiometry was assessed. Evaluating the effect of substrate, it was verified that only allylic alcohols such as geraniol and nerol were successfully epoxidized, demonstrating that this is a hydroxy group-assisted reaction. The effect of vanadium doping on the physicochemical properties of the phosphomolybdic acid catalysts was evaluated and used to explain their catalytic performance.

Vanadium-doped sodium phosphomolybdate salts as catalysts in the terpene alcohols oxidation with hydrogen peroxide

In this work, we have explored the catalytic activity of Keggin-type heteropolyanions PMo_12−nV_nO₄₀⁽³⁺ⁿ⁾⁻ (n = 0, 1, 2, or 3) in the form of sodium salts in green oxidation routes of terpene alcohols with hydrogen peroxide. Nerol was the model molecule selected to assess the impacts of the main reaction parameters, such as temperature, catalyst load, and stoichiometry of reactants. The impacts of the presence of vanadium at different proportions (i.e., V₁, V₂, and V₃ loads/per anion) in the structure of phosphomolybdate catalysts were assessed. All the catalysts were characterized by various techniques such as powder X-ray diffraction, attenuated diffuse reflectance infrared spectroscopy, ultraviolet-visible spectroscopy, thermogravimetric analysis, isotherms of adsorption–desorption of N₂ measurements of surface area, scanning electronic microscopy, energy-dispersive X-ray spectroscopy, and n-butylamine potentiometric titration. Among the catalysts assessed, Na₄PMo₁₁VO₄₀ was the most active and selective toward epoxides. The efficiency of this catalyst in the epoxidation of different terpene alcohols was investigated. Special attention was dedicated to correlating the composition and properties of the vanadium-doped phosphomolybdic catalysts with their catalytic activity.

In this work, we have explored the catalytic activity of Keggin-type heteropolyanions PMo_12−nV_nO₄₀⁽³⁺ⁿ⁾⁻ (n = 0, 1, 2, or 3) in the form of sodium salts in green oxidation routes of terpene alcohols with hydrogen peroxide.

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

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.

Tuning the Biginelli reaction mechanism by the ionic liquid effect: the combined role of supported heteropolyacid derivatives and acidic strength

Herein, a combination of heteropolyacids and ionic liquids as a catalytic system was studied for the Biginelli multicomponent reaction; the positive ionic liquid effect associated with the acidic strength of zeolite-supported heteropolyacids made this combination an efficient catalytic system for the multicomponent synthesis of 3,4-dihydropyrimidin-2(1H)-one/thione derivatives. The acidic strength effect was evaluated, and a range was determined in which the reaction provided better results. The mechanism of the reaction was also investigated in the presence and absence of ionic liquids, and two features of paramount importance were revealed: the mechanism could be tuned to proceed through only one reaction path among three possibilities and the kinetics of the reaction was significantly faster in the presence of an ionic liquid.

Heteropolyacids and ionic liquid effect allowed tuning of the Biginelli reaction mechanism and synthesis of 3,4-dihydropyrimidin-2(1H)-one/thione derivatives in an efficient, recyclable fashion. The role of acidic strength and supported heteropolyacid is disclosed.