Metal-Free Double Csp2–H Bond Functionalization: Strategy for Synthesizing Benzo[a]carbazoles from 2-Arylindoles and Acetophenones/Alkynes

Synthesis of Benzo[a]carbazoles and Dibenzo[c,g]carbazoles via Sequential Gold Catalysis and Photomediated Cyclization

Herein, we report a reaction protocol for the construction of benzo[a]carbazole and dibenzo[c,g]carbazole frameworks. The detailed gold catalytic reaction conditions developed for the challenging intermolecular carbon nucleophilic addition to internal alkynes are realized, giving the desired alkyne hydroarylation products in good yields. The resulting trisubstituted alkenes are able to undergo photomediated cyclization, furnishing the desired carbazole molecules with excellent yields and high efficiency.

Aryldiazonium Salt-Triggered [2 + 2 + 1] Heteroannulation of Indoles by an Arylhydrazone Radical-Relayed 1,5-Hydrogen Atom Transfer

An unprecedented three-component [2 + 2 + 1] annulation cascade of indoles with aryldiazonium salts and polyhalomethanes or acetone is presented by dual hydrogen atom transfer (HAT) and C-H functionalization. By employing readily accessible aryldiazonium salts as the radical initiators and electrophiles and polyhalomethanes and acetone as the C1 units, this method unprecedentedly constructs a pyrazole ring on an indole ring skeleton through the formation of two C-N bonds and a C-C bond in a single reaction.

Palladium-Catalyzed Tandem Nucleophilic Addition/C-H Functionalization of Anilines and Bromoalkynes for the Synthesis of 2-Phenylindoles

A novel and efficient palladium-catalyzed annulation of anilines with bromoalkynes for the synthesis of 2-phenylindoles has been described. This approach features excellent regio- and stereoselectivities and good functional group tolerance. Preliminary mechanistic studies indicate that anilines undergo anti-nucleophilic addition to bromoalkynes to generate (Z)-N-(2-bromo-1-phenylvinyl) anilines, followed by sequential C-H functionalization to deliver different substituted 2-phenylindoles. This method provides potential applications for the construction of various biologically active compounds.

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.

Metal- and Solvent-Free Synthesis of Tetrahydrobenzo[c]carbazolones through NaI-Catalyzed Formal [4 + 2] Annulation

A novel strategy for the preparation of functional carbazoles through NaI-catalyzed formal [4 + 2] annulation of 2-(indol-3-yl)cyclohexanones and alkynes/alkenes has been developed. The present approach started from easily available raw materials and provided a variety of tetrahydrobenzo[c]carbazolones in satisfactory yields under metal- and solvent-free conditions. Furthermore, the products could be further transformed into structurally valuable carbazole-based conjugated derivatives.

Cu-Catalyzed synthesis of spiroimidazole derivatives via an indolyl mediated cyclization–rearrangement reaction

A novel Cu-catalyzed synthesis of structurally valuable spiroimidazole derivatives from 2-(indol-3-yl)cyclohexanones and amidines via an indolyl mediated cyclization–rearrangement reaction is described.

Rhodium(iii)-catalyzed successive C(sp2)–H and C(sp2)–C(sp2) bond activation of aryl oximes: synthetic and mechanistic studies

A rhodium(iii)-catalyzed redox-neutral reaction of aryl oximes and internal alkynes to generate novel N-(2-cyanoaryl) indanone imines.

Ten Years of Glory in the α-Functionalizations of Acetophenones: Progress Through Kornblum Oxidation and C-H Functionalization

This review article focuses on the α-functionalization of acetophenones involving Kornblum oxidation and C-H functionalizations. Although various other strategies, such as classical approaches, enamine approaches and umpolung strategy are also known for this functionalization, here we discuss mainly the Kornblum oxidation approach and C-H functionalization strategy as they have advantages over the others. In Kornblum oxidation, the reaction uses iodine and dimethylsulfoxide and proceeds through the formation of arylglyoxal as the key intermediate. In C-H functionalization, the reaction requires metal, or metal-free catalyst, and generates radical intermediate in most cases. α-Functionalization of acetophenones is very important because of their huge applications in the synthesis of various natural products and pharmaceuticals and, therefore, a number of research articles have been published in this area. However, no review articles are available so far. In this article, we present a succinct discussion of various important and novel reactions, along with their mechanisms, published since 2012 to date. We believe that this first review article in this field will give readers one-stop information on this topic and encourage further intriguing work in this area.

Efficient synthesis of polyfunctionalized carbazoles and pyrrolo[3,4-c]carbazoles via domino Diels-Alder reaction

The p-TsOH-catalyzed Diels–Alder reaction of 3-(indol-3-yl)maleimides with chalcone in toluene at 60 °C afforded two diastereoisomers of tetrahydropyrrolo[3,4-c]carbazoles, which can be dehydrogenated by DDQ oxidation in acetonitrile at room temperature to give the aromatized pyrrolo[3,4-c]carbazoles in high yields. On the other hand, the one-pot reaction of 3-(indol-3-yl)-1,3-diphenylpropan-1-ones with chalcones or benzylideneacetone in acetonitrile in the presence of p-TsOH and DDQ resulted in polyfunctionalized carbazoles in satisfactory yields. The reaction mechanism included the DDQ oxidative dehydrogenation of 3-(indol-3-yl)-1,3-diphenylpropan-1-ones to the corresponding 3-vinylindoles, their acid-catalyzed Diels–Alder reaction and sequential aromatization process.

Water Modulated Diastereoselective Synthesis of cis/trans-Spiro[indoline-3,6'-naphtho[2,3-c]carbazoles]

p-TsOH catalyzed Diels-Alder reaction of 2-(1-alkylindol-3-yl)naphthalene-1,4-diones and 3-phenacylideneoxindoles showed fascinating diastereoselectivity. The reaction with the hydrated p-TsOH afforded trans-isomers of dihydrospiro[indoline-3,6'-naphtho[2,3-c]carbazoles] as major products. Alternatively, the reaction with anhydrous p-TsOH under a Dean and Stark apparatus predominately gave cis-isomer of dihydrospiro[indoline-3,6'-naphtho[2,3-c]carbazoles]. On the other hand, the similar p-TsOH catalyzed reaction of 2-(indol-3-yl)naphthalene-1,4-diones with 3-arylideneindolin-2-ones afforded cis/trans-isomers of dihydrospiro[indoline-3,6'-naphtho[2,3-c]carbazoles]. Additionally, the p-TsOH catalyzed reaction of 2-(indol-3-yl)naphthalene-1,4-diones with 2-arylidene-1,3-indanediones gave the expected spiro[indene-2,6'-naphtho[2,3-c]carbazoles] in satisfactory yields.

Cascade annulative π-extension for the rapid construction of carbazole based polyaromatic hydrocarbons

A Brønsted acid catalyzed cascade benzannulation strategy for the one-pot synthesis of densely populated poly-aryl benzo[a]carbazole architectures is disclosed from easily affordable fundamental commodities. The efficacy of this technique was further validated via the concise synthesis of structurally unique carbazole based poly-aromatic hydrocarbons. Furthermore, the photo-physical properties of the synthesized compounds are thoroughly investigated.

DDQ dehydrogenative Diels–Alder reaction for the synthesis of functionalized spiro[carbazole-1,3′-indolines] and spiro[carbazole-1,5′-pyrimidines]

Reactive 3-vinylindoles were generated by in situ DDQ oxidative dehydrogenation of 3-(indol-3-yl)-1,3-diphenylpropan-1-ones, and underwent sequential Diels–Alder reaction to give novel heterocyclic spirocarbazoles.

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.

Cobalt-catalyzed intramolecular decarbonylative coupling of acylindoles and diarylketones through the cleavage of C-C bonds

We report here cobalt-N-heterocyclic carbene catalytic systems for the intramolecular decarbonylative coupling through the chelation-assisted C-C bond cleavage of acylindoles and diarylketones. The reaction tolerates a wide range of functional groups such as alkyl, aryl, and heteroaryl groups, giving the decarbonylative products in moderate to excellent yields. This transformation involves the cleavage of two C-C bonds and formation of a new C-C bond without the use of noble metals, thus reinforcing the potential application of decarbonylation as an effective tool for C-C bond formation.

One-pot access to tetrahydrobenzo[c]carbazoles from simple ketones by using O2 as an oxidant

An effective and operationally simple one-pot Brønsted acid catalyzed cascade method is demonstrated for the synthesis of diversely functionalized carbazole frameworks starting from protecting group free 2-alkenyl indoles.