Copper-Catalyzed Synthesis of Indazole N-Oxides and Catalyst-Free C3-Alkylation via [3+2] Cycloaddition

Indazole derivatives are privileged scaffolds in drug discovery, yet selective C3-functionalization remains a formidable challenge due to poor regioselectivity and intrinsic low reactivity. Herein, we report a practical and scalable copper-catalyzed aerobic oxidative protocol for synthesizing indazole N-oxides via intramolecular N─N bond formation and catalyst-free C3-alkylation of indazoles through a unique [3 + 2] cycloaddition enabled by the N-oxide functionality. This method employs inexpensive CuBr and molecular oxygen under mild conditions, delivering products in good yield and demonstrating excellent functional group tolerance. This complementary strategy uses conventional tools and operates efficiently on a gram scale through a sequential one-pot process, making it broadly accessible. The [3 + 2] cycloaddition transformation proceeds under thermal conditions without additives, exploiting spontaneous N─O bond cleavage to furnish highly valuable C3-alkylated indazoles in good to excellent yields. The dual approach combining sustainable copper catalysis with a novel catalyst-free alkylation, offers a unified platform for late-stage diversification of indazole frameworks. These findings open new opportunities for medicinal chemistry and materials science by providing cost-effective, scalable, and environmentally benign routes to complex indazole architectures.