C(sp)-H Carbonylative Cyclization of Hydrazones with CO: Synthesis of Pyrazolone Derivatives.

A novel method is reported to synthesize various pyrazolones through transition-metal-free and redox-neutral 1°, 2°, or 3° C(sp)-H carbonylative cyclization using 1 atm of CO as a green carbonyl source, featuring good functional group tolerance, a broad substrate scope, facile scalability, and easy product transformation. The utility of this method could be demonstrated by the applications in preparing useful synthetic intermediates and bioactive compounds.