Sunlight‐promoted Direct Irradiation of N‐centred Anion: The Photocatalyst‐free Synthesis of Pyrazoles in Water

A practical method through sunlight mediated annulation of α,β‐unsaturated hydrazones has been developed for the synthesis of pyrazole. Based on the analysis of UV‐Vis absorption of the substrate, the reaction was designed to avoid the use of external photocatalysis and proceeds via direct irradiation of N‐centred anion by sunlight. The key features of this reaction include operational simplicity, readily available reagents, and amenability to gram‐scale synthesis.