Exploring Synthetic Strategies for 1H-Indazoles and Their N-Oxides: Electrochemical Synthesis of 1H-Indazole N-Oxides and Their Divergent C-H Functionalizations.

The selective electrochemical synthesis of 1H-indazoles and their N-oxides and the subsequent C-H functionalization of the 1H-indazole N-oxides are described. The electrochemical outcomes were determined by the nature of the cathode material. When a reticulated vitreous carbon cathode was used, a wide range of 1H-indazole N-oxides were selectively synthesized, and the electrosynthesis products were deoxygenated to N-heteroaromatics, owing to cathodic cleavage of the N-O bond via paired electrolysis, when a Zn cathode was used. The scope of this electrochemical protocol is broad, as both electron-rich and electron-poor substrates were tolerated. The potency of this electrochemical strategy was demonstrated through the late-stage functionalization of various bioactive molecules, making this reaction attractive for the synthesis of 1H-indazole derivatives for pharmaceutical research and development. Detailed mechanistic investigations involving electron paramagnetic resonance spectroscopy and cyclic voltammetry suggested a radical pathway featuring iminoxyl radicals. Owing to the rich reactivity of 1H-indazole N-oxides, diverse C-H functionalization reactions were performed. We demonstrated the synthetic utility of 1H-indazole N-oxides by synthesizing the pharmaceutical molecules lificiguat and YD (3); key intermediates for bendazac, benzydamine, norepinephrine/serotonin reuptake inhibitors, SAM-531, and gamendazole analogues; and a precursor for organic light-emitting diodes.

Visible light-mediated photocatalytic oxidative cleavage of activated alkynes via hydroamination: a direct approach to oxamates.

The direct oxidative cleavage of activated alkynes via hydroamination has been described using organic photocatalyst under visible-light irradiation at room temperature. In this reaction, the single electron oxidation of an in situ formed enamine followed by radical coupling with an oxidant finally delivers the oxamate. The key features of this photocatalytic reaction are the mild reaction conditions, metal-free organic dye as a photocatalyst, and TBHP playing a dual role as "O" source and for the regeneration of the photocatalyst.

A Direct Cycloaminative Approach to Imidazole Derivatives via Dual C-H Functionalization.

Organoiodine(III)-promoted C(sp3)-H azidation was a key step for the cycloaminative process. An unprecedented method for metal-free dehydrogenative N-incorporation into C(sp3)-H and C(sp2)-H bonds for the synthesis of diverse imidazoles has been disclosed. The overall transformation involves the construction of four C-N bonds through hydroamination-azidation-cyclization sequence. The reaction can be easily handled and proceeds under mild conditions. Further, the potential of the present strategy is revealed by the practical synthesis of N-heterocyclic carbene (NHC) precursors.

Nickel-Catalyzed Aerobic Oxidative Isocyanide Insertion: Access to Benzimidazoquinazoline Derivatives via a Sequential Double Annulation Cascade (SDAC) Strategy.

An efficient protocol for the synthesis of quinazoline derivatives through nickel-catalyzed ligand-/base-free oxidative isocyanide insertion under aerobic conditions with intramolecular bis-amine nucleophiles has been developed. A one-pot sequential double annulation cascade (SDAC) strategy involving an opening of isatoic anhydride and annulation to benzimidazole and further nickel-catalyzed intramolecular isocyanide insertion has also been demonstrated. The method is operationally simple to implement with a wide variety of substrates and represents a new approach for multiple C-N bond formations. The methodology has been successfully applied to the syntheses of hitherto unreported imidazo-fused benzimidazoquinazolines via a deprotection-GBB reaction sequence. Further, a florescence study reveals the potential of the present strategy for the discovery of highly fluorescent probes.