Ionic Liquid Effect over the Biginelli Reaction under Homogeneous and Heterogeneous Catalysis

Bronsted and Lewis acid catalysts with ionic tags under homogeneous and heterogeneous conditions have been tested to perform the Biginelli synthesis of 3,4-dihydropyrimidin-2(1H)-one (DHPMs). Metal-containing ionic liquids were evaluated as the catalysts with reasonable results (homogeneous systems). Heterogeneous catalysts (zeolite β and H3PW12O40 supported on zeolite β) have been tested, as well, with good to excellent results. The use of a functionalized acid ionic liquid with a heteropolyacid in the anion moiety (homogeneous system) gave the best results using no excess of the three reagents. The preferred mechanistic pathway was investigated by electrospray ionization (tandem) mass spectrometry in both positive and negative ion modes with very elucidative results. The ionic liquid effect and its origin were also investigated using mass spectrometry and DFT calculations.