Synthesis of 7-substituted 4-methyl coumarins by Pechmann reaction using nano-crystalline sulfated-zirconia

The nano-crystalline sulfated-zirconia catalysts, prepared by one-step as well as two-step sol–gel technique, showed excellent catalytic activity with a high substrate to catalyst weight ratio for the synthesis of 7-substituted 4-methyl coumarins via solvent free Pechmann reaction. The m-amino phenol was found to be more reactive than m-hydroxy phenol. The catalyst results 100% conversion of m-amino phenol with ∼100% selectivity of 7-amino 4-methyl coumarin at 110 °C within 2 min. For 7-hydroxy 4-methyl coumarin, 94% yield was obtained after 3 h at 170 °C with phenol to catalyst weight ratio of 80. Slow kinetics was observed in polar nitrobenzene as well as in non-polar toluene for both coumarin derivatives, due to the slow bulk diffusion of reactant molecules in presence of solvent. The solvent free microwave assisted synthesis seems advantageous way to synthesize the hydroxy derivative resulting excellent yield (99%) in much lesser time (15 min) at lower temperature (150 °C) as compared to thermal heating. The use of very small catalytic amount of sulfated-zirconia catalyst for the synthesis of coumarins and the reusability of the catalyst after simple activation for several times with similar catalytic activity are novel properties of the catalyst.

The nano-crystalline sulfated-zirconia catalysts showed excellent catalytic activity for the synthesis of 7-substituted 4-methyl coumarins via solvent free Pechmann reaction. Slow kinetics was observed in the presence of solvents. High substrate to catalyst ratio and the reusability of the catalyst after simple activation for several times with similar catalytic activity are the novel properties of the catalyst.Figure optionsDownload as PowerPoint slide