Keggin-type polyoxometalates as efficient catalysts for the synthesis of 4-methylcoumarins in solvent-free conditions, under conventional heating and microwave irradiations: Theoretical calculations and mechanism studies

An efficient solvent-free methodology, presented here, allowed the synthesis of 4-methylcoumarins substituted by hydroxyalkylation of phenols with ethylacetoacetate (via the Pechmann reaction) using a series of Keggin-type heteropolyacids (H4SiMo12O40, H4SiW12O40, H3PW12O40, H3PMo12O40 and H4PMo11VO40) and a series of Keggin-type heteropolysalts ((NH4)3PMo12O40, Ag3PMo12O40 and Cs3PW12O40) as green catalysts. Among them, H4SiMo12O40 was found to be the most effective with a 97% yield of 7-hydroxy-4-methylcoumarin. The purity of the reaction products was determined both by measuring their melting point and by spectral techniques (FT-IR, 1H-NMR, 13C-NMR). Theoretical calculations were optimized by the B3LYP method, using a 6-31G* set of bases, to allow confirmation of the proposed reaction mechanism and to predict their biological applications.© 2020 Elsevier B.V. All rights reserved.