Ru(II)-Catalyzed [4 + 2]-Annulation of 2-Alkenyl/Arylimidazoles with N-Substituted Maleimides and 1,4-Naphthoquinones: Access to Imidazo-Fused Polyheterocycles

Synthesis of imidazo-fused polyheterocyclic molecular frameworks, viz. imidazo[1,2-a]pyrrolo[3,4-e]pyridines, imidazo[2,1-a]pyrrolo[3,4-c]isoquinolines, and benzo[g]imidazo[1,2-a]quinoline-6,11-diones, has been achieved by the ruthenium(II)-catalyzed [4 + 2] C-H/N-H annulation of 2-alkenyl/2-arylimidazoles with N-substituted maleimides and 1,4-naphthoquinones. The developed protocol is operationally simple, exhibits broad substrate scope with excellent functional group tolerance, and provides the desired products in moderate to good yields. The mechanistic studies suggest that the reaction involves the formation of a C-C bond through Ru-catalyzed C(sp2)-H bond activation followed by intramolecular cyclization.

Visible-light-promoted radical cascade alkylation/cyclization: access to alkylated indolo/benzoimidazo[2,1-a]isoquinolin-6(5H)-ones

A new protocol is herein described for the direct generation of alkylated indolo/benzoimidazo[2,1-a]isoquinolin-6(5H)-one derivatives by using Hantzsch esters as alkylation radical precursors using a photoredox/K₂S₂O₈ system. This oxidative alkylation of active alkenes involves a radical cascade cyclization process and a sequence of Hantzsch ester single electron oxidation, C-C bond cleavage, alkylation, arylation and oxidative deprotonation.

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.

Ni-catalyzed hydroarylation of alkynes with unactivated β-C(sp2)-H bonds

Hydroarylation of alkynes with unactivated C(sp²)-H bonds via chelated C-H metalation mainly occurs at γ-position to the coordinating atom of directing groups via stable 5-membered metallacycles, while β-C(sp²)-H bond-involved hydroarylation has been a formidable challenge. Herein, we used a phosphine oxide-ligated Ni-Al bimetallic catalyst to enable β-C-H bond-involved hydroarylations of alkynes via a rare 7-membered nickelacycle.

Visible-light-induced acylation/cyclization of active alkenes: facile access to acylated isoquinolinones

Nitrogen heterocycles, especially polycyclic compounds, are significant skeletons in valuable molecules. Herein, we developed an efficient and practical visible-light-induced acylation/cyclization of active alkenes with acyl oxime derivatives for constructing acylated indolo/benzimidazo-[2,1,a]isoquinolin-6(5H) ones. This reaction was compatible with various functional groups and a series of fused indole/imidazole skeletons were prepared in up to 95% yield at room temperature.

Palladium(II) catalyzed site-selective C-H olefination of imidazo[1,2-a]pyridines

Herein, we disclose an efficient Pd(II)-catalyzed site selective C8 alkenylation of imidazo[1,2-a]pyridines with electronically biased olefinic substrates. Notably, besides the presence of four C-H sites available, selective mono-alkenylation was achieved by N-chelation overriding O-chelation. The versatility and scalability of the catalysis enabled the selective late-stage functionalization of a marketed drug, zolimidine. Various substituted heteroaryl alkenes can be afforded with moderate to good yields with high C8 regioselectivity.

Advances in the Synthesis of Ring-Fused Benzimidazoles and Imidazobenzimidazoles

This review article provides a perspective on the synthesis of alicyclic and heterocyclic ring-fused benzimidazoles, imidazo[4,5-f]benzimidazoles, and imidazo[5,4-f]benzimidazoles. These heterocycles have a plethora of biological activities with the iminoquinone and quinone derivatives displaying potent bioreductive antitumor activity. Synthesis is categorized according to the cyclization reaction and mechanisms are detailed. Nitrobenzene reduction, cyclization of aryl amidines, lactams and isothiocyanates are described. Protocols include condensation, cross-dehydrogenative coupling with transition metal catalysis, annulation onto benzimidazole, often using CuI-catalysis, and radical cyclization with homolytic aromatic substitution. Many oxidative transformations are under metal-free conditions, including using thermal, photochemical, and electrochemical methods. Syntheses of diazole analogues of mitomycin C derivatives are described. Traditional oxidations of o-(cycloamino)anilines using peroxides in acid via the t-amino effect remain popular.

Electricity-induced micro-flow C–H/N–H alkyne annulation: a greener approach to access heteroaromatic compounds

Fast access to structurally diverse heteroaromatic compounds is demonstrated in an integrated continuous-flow manner without employing any expensive oxidant.

Practical Synthesis of Benzimidazo[1,2-a]quinolines via Rh(III)-Catalyzed C-H Activation Cascade Reaction from Imidamides and Anthranils

We report a novel and practical one-pot Rh(III)-catalyzed strategy to construct benzimidazo[1,2-a]quinolines from readily available imidamides and anthranils. The cascade reaction proceeds via a C-H amination-cyclization-cyclization process in ionic liquid without any additives and possesses simple operation, moderate-to-high yield, and broad substrate scope features, which will provide the reference for the construction of biologically active fused benzimidazoles.