Rice Husk SiO<sub>2</sub> (NPs) Supported-BO<sub>3</sub>H<sub>3</sub>:A Highly Active, Solvent-Free and Recyclable Catalyst to Dihydropyrimidin-2(1<i>H</i>)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 solvent-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 heterogeneous catalyst for important organic scaffold synthesis.The reaction required low catalyst loading (1.2 mg) to achieve a target product under solvent-free condition.A series of other derivatives of heterogeneous catalysts synthesized are RHA-SiO2, RHA-SiO2(NPs), RHA-SiO2-BO3H3.We examined their catalytic activity in the synthesis of DHPM and coumarin-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.