Rice Husk SiO 2 (NPs) Supported-BO 3 H 3 :A Highly Active, Solvent-Free and Recyclable Catalyst to Dihydropyrimidin-2(1 H )ones- (Thiones) and Coumarin-3-Carboxylic Acid Synthesis

3,4-Dihydropyrimidin-2(1H)ones(thiones) (DHPM) and coumarin-3-carboxylic acid are obtained in excellent to good yield by employing green catalyst under sol-vent-free condition. The condensation of substituted arylaldehyde, 1,3-diketoester and urea/thiourea in the presence of green catalyst after 1 h of stirring at 50°C resulted in DHPM. The reaction of substituted o-hydroxybenzaldehyde with Meldrum’s acid in the presence of catalyst under sonication for a few minutes gave coumarin-3-carboxylic acid. Here, we have used Lewis acid catalyst RHA–SiO₂(NPs)–BO₃H₃ derived from the agro-waste of rice husk, a heteroge-neous catalyst for important organic scaffold synthe-sis. The reaction required low catalyst loading (1.2 mg) to achieve a target product under solvent-free condition. A series of other derivatives of heterogene-ous catalysts synthesized are RHA–SiO₂, RHA–SiO₂(NPs), RHA–SiO₂–BO₃H₃. We examined their catalytic activity in the synthesis of DHPM and cou-marin-3-carboxylic acid. Only the reaction catalysed by RHA–SiO₂(NPs)–BO₃H₃ gave excellent yield of the product. The final isolated pure product has been fully characterized by various spectroscopic methods and confirmed.