Bronsted Acidic Ionic Liquid-Catalyzed Regioselective Synthesis of Pyrazolopyrimidines and Their Photophysical Properties

A convenient and straightforward regioselective synthesis of pi-conjugated pyrazolo[1,5-a]pyrimidines has been achieved via Bronsted acidic ionic liquid catalyzed tandem cyclization between 3-aminopyrazole and chalcone derivatives in high yields. Task specific ionic liquid [1-methyl-3-(4-sulfobutyl)imidazolium-4-methylbenzenesulfonate] is found to be an effective catalyst for the cyclization, and the presence of both C2-H of imidazolium moiety and acidic proton drastically enhance the catalytic activity. Substituted aminopyrazoles reacted with a wide range of functionalized alpha,beta-unsaturated ketones to variety of pyrazolo[1,5-a]pyrimidine derivatives. The present protocol offers a broad substrates scope, use of non-hazardous reagent, metal-free environmentally benign conditions and catalyst recyclability. Comparative photophysical studies have shown high quantum yields for certain derivatives.