A Photochemical Route to Benzo[a]carbazolesviaDomino Elimination/Electrocyclization of 2-Aryl-3-(1-tosylalkyl)indoles

Preparation of Carbazoles Involving 6π-Electrocyclization, Photoredox-, Electrochemical-, and Thermal Cyclization Reactions: Mechanistic Insights

Carbazole is a heterocyclic motif that can be found in a diverse array of natural and unnatural products displaying a wide range of biological and physiological properties. Furthermore, this heterocycle is part of electronic materials like photoconducting polymers and organic optoelectronic materials owing to its excellent photophysical characteristics. Consequently, the development of synthetic strategies for carbazole scaffolds holds potential significance in biological and material fields. In this regard, a variety of preparation methods has been developed to exploit their efficient and distinct formation of new C-C and C-heteroatom bonds under mild conditions and enabling broad substrate diversity and functional group tolerance. Therefore, this review focuses on the synthesis of a set of carbazole derivatives describing a variety of methodologies that involve direct irradiation, photosensitization, photoredox, electrochemical and thermal cyclization reactions.

Cascade of C(sp2)-H Addition to Carbonyl and C(sp2)-CN/C(sp2)-H Coupling Enabled by Brønsted Acid: Construction of Benzo[a]carbazole Frameworks

Herein, we report an unprecedented cascade reaction of C(sp²)-H addition to carbonyl and the C(sp²)-CN/C(sp²)-H coupling of 2-(2-oxo-2-arylethyl)benzonitriles with indoles enabled by commercially available TsOH·H₂O. The protocol represents the first metal-free C(sp²)-CN/C(sp²)-H coupling, affording a new route for the synthesis of various benzo[a]carbazole derivatives with a broad substrate scope, high yields, and simple conditions.

Benzannulation strategies for the synthesis of carbazoles, indolocarbazoles, benzocarbazoles, and carbolines

This review presents a critical and authoritative analysis of several exciting benzannulation approaches developed in the past decade for the construction of carbazoles, indolocarbazoles, benzocarbazoles, and carbolines.

Palladium-Catalyzed Regioselective Coupling Cyclohexenone into Indoles: Atom-Economic Synthesis of β-Indolyl Cyclohexenones and Derivatization Applications

Herein, we report a palladium-catalyzed dehydrogenative cross-coupling of indoles with cyclic enones to give β-indolyl cyclic enones under mild and neutral reaction conditions. The key to the success is to explore a mild condition, which ensures the indole C-H activation and subsequent syn β-hydride elimination through rapid enolization isomerization of Pd(II)-enolate while suppressing other undesired side reactions. Synthetic utility has also been demonstrated in the flexible transformation of the coupling products to meta-phenols and benzo[a]carbazoles.

Synthesis of benzo[a]carbazoles via cyanide-catalyzed imino-Stetter reaction/Friedel–Crafts reaction sequence

A new sequential protocol for the synthesis of benzo[a]carbazoles was developed via the cyanide-catalyzed imino-Stetter reaction followed by Friedel–Crafts reaction.

Rhodium(iii)-catalyzed indole-directed carbenoid aryl C–H insertion/cyclization: access to 1,2-benzocarbazoles

A rhodium(iii)-catalyzed indole-directed aryl C–H bond carbenoid insertion cascade of 2-arylindoles with diazo compounds has been developed.

Regio-selective synthesis of diversely substituted benzo[a]carbazoles through Rh(iii)-catalyzed annulation of 2-arylindoles with α-diazo carbonyl compounds

A novel synthetic approach toward benzo[a]carbazoles and 6-aminobenzo[a]carbazoles containing an unprotected NH unit through Rh(iii)-catalyzed cascade reactions of 2-arylindoles or 2-arylindole-3-carbonitriles with α-diazo carbonyl compounds is presented.

Synthesis of dibenzo[a,c]carbazoles from 2-(2-halophenyl)-indoles and iodobenzenes via palladium-catalyzed dual C–H functionalization

We report an efficient approach to synthesize dibenzo[a,c]carbazoles via a palladium-catalyzed cross-coupling tandem reaction.

Sulfonyl Azoles in the Synthesis of 3-Functionalized Azole Derivatives

Sulfonyl indoles, as well as related azolyl derivatives, have been recently introduced in synthesis as stable precursors of reactive indolenine intermediates. This personal account reports on the discovery of sulfonyl azoles and their practical utilization in many synthetic processes for the preparation of functionalized 3-substituted indoles, indazoles, and pyrroles. The indolenine intermediates obtained by treatment of sulfonyl azoles with Brønsted bases or Lewis acids can be considered as vinylogous imino derivatives that can be made to react with different nucleophilic reagents. These include organometallic reagents, reducing agents, stabilized carbanions, and heteronucleophiles. The controlled and mild conditions for the generation of indolenines from sulfonyl azoles make these substrates particularly useful in asymmetric synthesis, exploiting organo- or metal-catalyzed processes. Although less exploited, sulfonyl indoles can also be involved in photochemical processes for the preparation of polycyclic derivatives.

A highly enantioselective thiolation of sulfonyl indoles to access 3-sec-sulfur-substituted indoles in water

Highly enantioselective organocatalytic Michael addition of thiol to vinylogous imine intermediates generated in situ from sulfonylindoles in aqueous medium is presented.

Synthesis of benzo[a]carbazole derivatives from 3-ethylindoles by exploiting the dual character of benzoquinone as an oxidizing agent and dienophile

The dual character of benzoquinone is exploited for the synthesis of benzo[a]carbazole derivatives starting from 3-ethylindole derivatives.