Photochemical Conversion of Indazoles into Benzimidazoles.

Fragment-based drug discovery relies on preparing diverse libraries of advanced building blocks, often incorporating heteroaromatic motifs. Altering the core of heteroaromatics to maximise library diversity typically requires de novo synthesis of each system. This can be often challenging when specific substitution patterns are needed. Here, we introduce a photochemical strategy for the direct permutation of 1H- and 2H-indazoles into benzimidazoles. This transformation exploits the distinct photochemical properties of these heteroaromatics and proceeds under mild conditions. Through systematic experimental and computational studies, we have elucidated a two-step mechanism involving excited-state tautomerization of 1H-indazoles, followed by photochemical rearrangement of the resulting 2H-isomers. This approach demonstrates broad substrate scope, high yields, and compatibility with a variety of functional groups. This method can expand the structural diversity of heterocycle-based libraries through the concept of chemical permutation for heteroaromatic interconversion.