Preparation of Bicyclopyrazolium Scaffolds and Their Application toward 1,3-Difunctionalization

Abstract A photocatalyzed method for synthesizing 2,3-dihydro-1H-pyrazolo[1,2-a]pyrazol-4-ium compounds from pyrazole and 3-phenylpropyl halides is reported. The strategy employs a formal hydride abstraction process achieved through a stepwise hydrogen atom transfer and oxidative radical–polar crossover process to generate a benzylic carbocation. Capture of the carbocation with pyrazole generates an intermediate N-benzylpyrazole, which undergoes cyclization with a pendant halide to form a fused bicyclic pyrazolium scaffold. We demonstrate that the pyrazolium structure can serve as an intermediate toward accomplishing 1,3-difunctionalization through further reaction with various nucleophiles.