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

The 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, -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 (approximate to 37%) and Biginelli dihydropyrimidinone 5 (approximate to 49%), whereas the same reaction with 2 (catalyst loading of 1mol% as well) furnishes the corresponding aldolic product methyl-2-benzylidene-3-oxobutanoate 6 as the major product (approximate to 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.