Divergent Electrochemical Synthesis of Indoles through pKa Regulation of Amides: Synthetic and Mechanistic Insights.

A divergent synthetic approach to access highly substituted indole scaffolds is illustrated. By virtue of a tunable electrochemical strategy, distinct control over the C-3 substitution pattern was achieved by employing two analogous 2-styrylaniline precursors. The chemoselectivity is governed by the fine-tuning of the acidity of the amide proton, relying on the appropriate selection of N-protecting groups, and assisted by the reactivity of the electrogenerated intermediates. Detailed mechanistic investigations based on cyclic voltametric experiments and computational studies revealed the crucial role of water additive, which assists the proton-coupled electron transfer event for highly acidic amide precursors, followed by an energetically favorable intramolecular C-N coupling, causing exclusive fabrication of the C-3 unsubstituted indoles. Alternatively, the implementation of an electrogenerated cationic olefin activator delivers the C-3 substituted indoles through the preferential nucleophilic nature of the N-acyl amides. This electrochemical approach of judicious selection of N-protecting groups to regulate pKa/E° provides an expansion in the domain of switchable generation of heterocyclic derivatives in a sustainable fashion, with high regio- and chemoselectivity.

Visible-Light-Mediated Synthesis of Phenanthrenes through Successive Photosensitization and Photoredox by a Single Organocatalyst.

An efficient approach for the synthesis of phenanthrene scaffolds by utilizing the dual catalytic activity of an organo-photocatalyst is documented. The controlled cascade transformation proceeds via in situ diazotization followed by olefin isomerization and subsequent arene radical generation through photoreduction. The overall process demonstrates both the photosensitization and photoredox properties of a single organo-photocatalyst and facilitates the desired intramolecular annulation with high precision and efficacy. In this context, the underexplored organocatalyst acridine orange base is employed and the photophysical interactions between the catalyst and the substrates along with the detailed reaction kinetics are documented.

Regioselective Synthesis of N2-Aryl 1,2,3-Triazoles via Electro-oxidative Coupling of Enamines and Aryldiazonium Salts.

An efficient synthetic route for the construction of N2-aryl 1,2,3-triazoles is reported via sequential C-N bond formation and electro-oxidative N-N coupling under metal-free conditions. Readily accessible 2-aminoacrylates and aryldiazonium salts were used as starting materials, and the developed protocol displays excellent functional group tolerance, allowing an extensive range of substrate scope up to 91% isolated yield. Various mechanistic studies, along with the isolation of an intermediate adduct, refer to successive ionic and radical reaction sequences.

Cobalt-Catalyzed Sustainable Synthesis of Benzimidazoles by Redox-Economical Coupling of o-Nitroanilines and Alcohols.

This study reveals cobalt-catalyzed sustainable synthesis of benzimidazoles by redox-economical coupling of o-nitroanilines and alcohols. The major advantage of this report is the use of a commercially available cheap cobalt catalyst to produce a wide variety of 2-substituted benzimidazoles by hydrogen autotransfer without using any additional external redox reagent and costly ligand system. A thorough mechanistic insight of the reaction is proposed by performing a series of control experiments.