Clean and efficient synthesis of coumarins over modified metal oxides via Pechmann reaction

Abstract

The catalytic activity of solid acids such as zirconia, alumina and silica and their SO₄²⁻,PO₄³⁻ and BO₃³⁻ modified forms has been studied in the Pechmann reaction between resorcinol and ethylacetoacetate under solvent-free liquid phase reflux conditions and autogenous pressure.A systematic study of this reaction has been made by varying the following parameters: catalyst, amount of catalyst, duration and temperature of reaction and molar ratio of reactants. Under refluxing conditions, in the presence of sulphated zirconia, the percentage yield of the product, 7-hydroxy 4-methyl coumarin was in the range of 30-75% which, however, contained many impurities. When the same reaction was conducted in an autoclave at 150ºC, the clean product of yields upto 99% was obtained. Other phenols used in this work include catechol, hydroquinone, m-cresol, p-cresol, o-cresol, pyrogallol, phloroglucinol, orcinol, m-aminophenol and phenol.The sites of higher acid strength and perhaps super-acidic sites on the catalyst appear to be catalyzing the formation of coumarins via Pechmann reaction. Sulphated zirconia is found to be a promising catalyst for this reaction under autogenous conditions.