Rhodium-Catalyzed RegioselectiveOrthoC−H Olefination of 2-Arylindoles via NH-Indole-Directed C−H Bond Cleavage

Diversity-Oriented Synthesis of Indole-Fused Polycyclic Scaffolds via Rhodium-Catalyzed NH-Indole-Directed C-H Coupling of 2-Phenyl-1H-indoles with Propargylic Alcohol Derivatives

Diversity-oriented synthesis strategy for the efficient assembly of indole-fused polycyclic scaffolds via rhodium-catalyzed NH-indole-directed C-H coupling with propargylic alcohol derivatives in a regioselective manner was developed. Five 2-phenyl-1H-indole-embedded core skeletons were synthesized. In particular, three different indole-fused exo-olefin-containing polycycles were realized, which may be manipulated for further chemistry.

Cascade Electrochemical Aerobic Oxygenation of 2-Substituted Indoles and Electrochemical [5 + 3] Annulation with Amidines: Access to Eight-Membered Benzo[1,3,5]triazocin-6(5H)-ones

The cascade electrochemical C3-selective aerobic oxygenation of 2-substituted indoles and electrochemical [5 + 3] annulation with amidines through an undivided cell galvanostatic method employing molecular oxygen and "electricity" as green oxidants was developed. This protocol provides an efficient and direct approach to eight-membered benzo[1,3,5]triazocin-6(5H)-ones. Mechanistic studies suggested that two subsequent electrochemical processes both proceeded through radical pathways.

Substrate-controlled Rh(III)-catalyzed regiodivergent annulation towards fused and spiro benzimidazoles

A Rh(III)-catalyzed cascade C-H activation and cyclization of 2-aryl benzimidazoles with maleimides for the synthesis of benzimidazole-fused isoquinolines and benzimidazole-spiro isoindoles is reported. Switchable selectivity towards the formation of these two distinct products can be achieved using unsubstituted and substituted benzimidazoles at the ortho-position of the phenyl ring. Mechanistically, C-H activation followed by migratory insertion of maleimide forms a Heck-type intermediate. Unsubstituted benzimidazole undergoes aza-Michael addition to form a (4 + 2) fused product, whereas ortho-substituted phenyl benzimidazole causes steric clash to deliver a (4 + 1) spiro-adduct favorably via acid-catalyzed intramolecular annulation.

Recent advances in rhodium-catalysed cross-dehydrogenative-coupling between two C(sp2)-H bonds

Rhodium-catalysed cross-dehydrogenative coupling (CDC) has received considerable attention in recent years. This modern technology has been considered as an attractive synthetic tool for selective C−C bond formation due to (1)...

Synthesis of Indole-Fused Six-, Seven-, or Eight-Membered N,O-Heterocycles via Rhodium-Catalyzed NH-Indole-Directed C-H Acetoxylation/Hydrolysis/Annulation

We report herein the facile synthesis of indole-fused six-, seven-, or eight-membered N,O-heterocycles through rhodium-catalyzed C-H acetoxylation/hydrolysis/annulation. The notable features of this method include C-H acetoxylation using NH-indole as the intrinsic directing group, high functional group compatibility, and construction of indole-fused medium-sized rings.

Recent advances in the synthesis of pyrrolo[1,2-a]indoles and their derivatives

The pyrrolo[1,2-a]indole unit is a privileged heterocycle found in numerous natural products and has been shown to exhibit diverse pharmacological properties. Thus, recent years have witnessed immense interest from the synthesis community on the synthesis of this scaffold. In light of the ever-increasing demand for pyrrolo[1,2-a]indoles in drug discovery, this review provides an overview of recent synthesis methods for the preparation of pyrrolo[1,2-a]indoles and their derivatives. The mechanistic pathway and stereo-electronic factors affecting the yield and selectivity of the product are briefly explained. Furthermore, we have attempted to demonstrate the utility of the developed methods in the synthesis of bioactive molecules and natural products, wherever offered.

Rhodium-Catalyzed Oxidative Annulation of 2- or 7-Arylindoles with Alkenes/Alkynes Using Molecular Oxygen as the Sole Oxidant Enabled by Quaternary Ammonium Salt

Developing an efficient catalytic system using molecular oxygen as the oxidant for rhodium-catalyzed cross-dehydrogenative coupling remains highly desirable. Herein, rhodium-catalyzed oxidative annulation of 2- or 7-phenyl-1H-indoles with alkenes or alkynes to assemble valuable 6H-isoindolo[2,1-a]indoles, pyrrolo[3,2,1-de]phenanthridines, or indolo[2,1-a]isoquinolines using the atmospheric pressure of air as the sole oxidant enabled by quaternary ammonium salt has been accomplished. Mechanistic studies provided evidence for the fast intramolecular aza-Michael reaction and aerobic reoxidation of Rh(I)/Rh(III), facilitated by the addition of quaternary ammonium salt.

Synthesis of Indolo[2,1-a]benzazepinones through Rhodium-Catalyzed Cascade Reactions of 2-Arylindoles with Allyl Alcohols

An efficient synthesis of indolo[2,1-a]benzazepinones through rhodium-catalyzed cascade reactions of 2-arylindoles with allyl alcohols has been developed. This work expands the scope of products that are available through C-H activation/intramolecular annulation reactions of 2-arylindoles in organic synthesis.

Chemoselective Installation of Diverse Succinimides on Fused Benzimidazoles via Rhodium-Catalyzed C-H Activation/Annulation: Chemosensor for Heavy Metals

A novel Rh-catalyzed cascade C-H activation/annulation of 2-arylbenzimidazoles with maleimides is reported. Rapid chemoselective access to two structurally distinct succinimide-bearing benzoimidazoisoquinolinones is achieved, depending on the acidic and basic conditions. This atom- and step-economic strategy features a wide substrate scope, excellent functional group tolerance, and site-specific functionalization. Application of the methodology yields a novel benzimidazole-based probe as a fluorescent chemosensor for the nanomolar detection of Hg²⁺, Cu²⁺, and Fe³⁺ ions.

Rh(iii)-catalyzed regioselective C-H activation dialkenylation/annulation cascade for rapid access to 6H-isoindolo[2,1-a]indole

6H-isoindolo[2,1-a]indoles were accessed via a Rh(iii)-catalyzed N-H free indole directed C-H activation dialkenylation/annulation cascade in moderate to excellent yields. This protocol also features: reaction procedures that are insensitive to air and moisture, excellent regioselectivity and good functional group tolerance.

Recent development in transition metal-catalysed C-H olefination

Transition metal-catalysed functionalizations of inert C-H bonds to construct C-C bonds represent an ideal route in the synthesis of valuable organic molecules. Fine tuning of directing groups, catalysts and ligands has played a crucial role in selective C-H bond (sp2 or sp3) activation. Recent developments in these areas have assured a high level of regioselectivity in C-H olefination reactions. In this review, we have summarized the recent progress in the oxidative olefination of sp2 and sp3 C-H bonds with special emphasis on distal, atroposelective, non-directed sp2 and directed sp3 C-H olefination. The scope, limitation, and mechanism of various transition metal-catalysed olefination reactions have been described briefly.

Construction of Bridged Carbocycles and Heterocycles via Rh(III)-Catalyzed C-H Alkylation/Michael Addition of 2-Arylindoles with Quinone Monoacetals

A rhodium-catalyzed formal [4 + 5] annulation reaction of 2-arylindoles with quinone monoacetals for the selective preparation of bridged nine-membered carbocyclic and heterocyclic compounds is developed. When 2-aryl substituted indoles are employed, this annulation reaction affords bridged nine-membered carbocyclic compounds with excellent indolyl C3 selectivity. On the other hand, with 2-aryl-3-substituted indoles as the substrates, bridged nine-membered heterocyclic compounds are exclusively formed via N1 annulation. Further transformations of the obtained products into new bridged compounds showcase the synthetic potential of this protocol.

Access to [4,3,1]-Bridged Carbocycles via Rhodium(III)-Catalyzed C-H Activation of 2-Arylindoles and Annulation with Quinone Monoacetals

Reported herein is the Rh(III)-catalyzed annulation of N-unprotected 2-arylindoles with quinone monoacetals for the straightforward synthesis of [4,3,1]-bridged carbocycles with exclusive C(3) selectivity. Mechanistic studies, particularly deuterium-labeling experiments, suggest that the coupling likely proceeds via two-fold C-H activation with two-fold migratory insertion into the enone moieties.

Revisiting the mechanism of the Fujiwara–Moritani reaction

A new perspective of the Fujiwara–Moritani reaction is presented, including its kinetic profile and catalyst deactivation studies.

One pot synthesis of pyrrolo[3,2,1-de]phenanthridines from 7-phenylindoles via tandem C–H olefination/aza-Michael addition

An unprecedented one-pot C–H olefination/aza-Michael addition tandem process has been developed for the synthesis of pyrrolo[3,2,1-de]phenanthridines from 7-phenylindoles and alkenes using a [Cp*RhCl₂]₂/AgOAc/Me₄NOAc catalytic system.

Access to 5H-benzo[a]carbazol-6-ols and benzo[6,7]cyclohepta[1,2-b]indol-6-ols via rhodium-catalyzed C–H activation/carbenoid insertion/aldol-type cyclization

The rhodium-catalyzed mono-ortho C–H activation/carbenoid insertion/aldol-type cyclization of 3-aldehyde-2-phenyl-1H-indoles with diazo compounds has been developed.

Rhodium-Catalyzed Selective Oxidative (Spiro)annulation of 2-Arylindoles by Using Benzoquinone as a C2 or C1 Synthon

Rhodium-catalyzed substrate-tunable oxidative annulation and spiroannulation reactions of 2-arylindoles with benzoquinone leading to 9H-dibenzo[a,c]carbazol-3-ols and new spirocyclic products are reported. Intriguingly, with 2-aryl-substituted indoles, benzoquinone could act as a C2 synthon to afford dibenzo[a,c]carbazoles. On the contrary, when 2-aryl-3-substituted indoles were used, benzoquinone switched to act as a C1 synthon to furnish spirocyclic compounds. In addition, further transformations of the obtained products demonstrate the synthetic utility of the present protocol.