Improving the Selectivity toward Three‐Component Biginelli versus Hantzsch Reactions by Controlling the Catalyst Hydrophobic/Hydrophilic Surface Balance

Abstract

AbstractThe catalytic activities and selectivities of two kinds of mesoporous solid acids SBA‐15‐PrSO3H 1, SBA‐15‐Ph‐PrSO3H 2, and a periodic mesoporous organosilica (PMO) based solid acid Et‐PMO‐Me‐PrSO3H 3 that comprise different physicochemical surface properties were compared in an environmentally benign one‐pot, three‐component Biginelli reaction of aldehydes, β‐ketoesters and urea or thiourea under solvent‐free conditions. Among these mesoporous solid acid catalysts, 3, which has a hydrophobic/hydrophobic balance in the nanospaces (mesochannels) in which the active sites are located, is found to be a significantly more selective catalytic system in the Biginelli reaction; it produces the corresponding 3,4‐dihydropyrimidin‐2‐one\thione (DHPM) 5 derivatives in good to excellent yields and excellent selectivities. Notably, in the case of conducting the three‐component coupling reaction of benzaldehyde, metylacetoacetate and urea in the presence of 1 result in the generation of a mixture of Hantzsch dihydropyridine 4 (≈37 %) and Biginelli dihydropyrimidinone 5 (≈49 %), whereas the same reaction with 2 (catalyst loading of 1 mol % as well) furnishes the corresponding aldolic product methyl‐2‐benzylidene‐3‐oxobutanoate 6 as the major product (≈80 %) with concomitant formation of small amounts of 5 (<10 %) under essentially the same reaction conditions that are employed with catalyst 3. Water adsorption–desorption analysis of the catalysts is employed to possibly relate the observed selectivity to the difference in physicochemical properties of the materials.