Visible-Light-Promoted Intermolecular Annulation Toward Efficient Synthesis of Multisubstituted Pyrazoles

A visible-light-promoted intermolecular annulation between aryl hydrazines and α-bromoketones for synthesizing 1,3,5-trisubstituted pyrazoles is described from commercially available starting materials. This protocol features a broad substrate scope, transition metal catalyst-free conditions, and operational simplicity. The approach proceeds under mild reaction conditions using air as the oxidant. Gram-scale experiments and postsynthetic modifications demonstrate its practical utility. Mechanistic studies indicate that α-bromoketones undergo self-coupling, forming 1,2-diacylvinylene intermediates that subsequently cyclize with aryl hydrazines under visible-light irradiation to afford the target products.