Copper-decorated chitosan organocatalyst for sonophotocatalytic CH arylation of pyrazoles: Mechanistic insights and eco-friendly perspective

Chitosan-modified catalysts offer a significant opportunity for efficient organic synthesis. Catalysis is crucial in synthesizing pyrazole derivatives, which are integral components of many biologically active molecules. Recent advancements have shown the high potential of sonophotocatalysis to efficiently and eco-friendly intensify catalytic reactions. However, the specific reaction mechanisms and the role of external energy sources remain partially understood. In our previous study, we first validated a sonophotoreactor for Cu-catalyzed CH arylation of pyrazoles, demonstrating the superior performance of the sonophotochemical approach. In this study, we analyzed the morphology, structure, and chemical composition of the Cu-Chitosan catalyst before and after different catalysis experiments utilizing both micro-scale and nano-scale characterization to verify the reaction mechanism we proposed previously. Conventional analysis techniques confirmed the presence of KHCO₃, deprotonation side product, as well as higher Cu(I)/Cu(II) ratio only in the catalysts after ultrasound-assisted reactions, proving that ultrasound-enhanced deprotonation reaction as well as Cu(II) conversion into catalytically more active Cu(I). Infrared nanospectroscopy (nano-FTIR) supports our finding for the catalyst after sonophotoreaction. Thus, our results indicate that ultrasound-enhanced transmetalation and blue LED helped to complete the catalytic reaction and provide regenerated catalysts, leading to a synergistic effect. The impressive Turnover Number (TON) and Turnover Frequency (TOF) of the catalyst underscore its remarkable efficiency in driving the transformation of reactants into products.