Metal-free construction of tricyclic or tetracyclic compounds—acid-promoted synthesis of benzo[4,5]imidazo[2,1-a]isoindole and 1,2-dialkyl-2,3-dihydrobenzimidazoles

A One-Pot Cascade Strategy toward Organocatalytic Enantioselective Construction of Fused Benzimidazoles

Herein, we describe an asymmetric assembly of ortho-aromatic diamines and formyl tethered Michael acceptors forming chiral fused benzimidazoles. A cinchona-alkaloid-derived bifunctional squaramide catalyst enables the methodology through on-site dihydrobenzimidazole formation followed by an aza-Michael addition/oxidation cascade. This protocol stands out for its excellent catalytic efficiency over the background reaction and its mild conditions, making it more practical. Various Michael acceptors, including enones, ester, and thioester, were successful substrates in this study. Additionally, this methodology has demonstrated scalability and successfully showcased postsynthetic transformations.

Iodine(III)-Catalyzed Oxidative Cyclization of Aryl Amines to Construct N-Alkylbenzimidazoles

An I(III)-catalyzed oxidative cyclization reaction using selectfluor as the oxidant was developed that converts ortho-substituted anilines to benzimidazoles. The mild reaction requires as little as 0.5 mol % of iodobenzene, and its scope is broad: electron-withdrawing or electron-releasing groups on the aniline portion are tolerated, and cyclic or acyclic N-alkylamines are permitted as ortho-substituents. Preliminary mechanistic investigations suggest that benzimidazole formation occurs via cationic reactive intermediates, and an intramolecular kinetic isotope effect of 1.98 ± 0.05 was measured.

Copper-Catalyzed Coupling between ortho-Haloanilines and Lactams/Amides: Synthesis of Benzimidazoles and Telmisartan

An efficient copper-catalyzed synthesis of (annelated) benzimidazoles is reported. This transformation is based on a simple and straightforward one-pot sequence involving a copper-catalyzed cross coupling between o-haloanilines and lactams/amides followed by a subsequent cyclization under acidic conditions. A variety of (annelated) benzimidazoles could be easily obtained in high yields from readily available starting materials, and this procedure could be further applied to the synthesis of the antihypertensive blockbuster drug telmisartan.

Aromatic heterobicycle-fused porphyrins: impact on aromaticity and excited state electron transfer leading to long-lived charge separation

A new synthetic method to fuse benzo[4,5]imidazo[2,1-a]isoindole to the porphyrin periphery at the β,β-positions has been developed, and its impact on the aromaticity and electronic structures is investigated. Reactivity investigation of the fused benzoimidazo-isoindole component reveals fluorescence quenching of a zinc porphyrin (AMIm-2) upon treatment with a Brønsted acid. The reaction of the zinc porphyrin (AMIm-2) with methyl iodide initiated a new organic transformation, resulting in the ring-opening of isoindole with the formation of an aldehyde and dimethylation of the benzoimidazo component. The fused benzoimidazo-isoindole component acted as a good ligand to bind platinum(ii), forming novel homobimetallic and heterobimetallic porphyrin complexes. The fusion of benzoimidazo-isoindole on the porphyrin ring resulted in bathochromically shifted absorptions and emissions, reflecting the extended conjugation of the porphyrin π-system. Time-resolved emission and transient absorption spectroscopy revealed stable excited state species of the benzoimidazo-isoindole fused porphyrins. Zinc porphyrin AMIm-2 promoted excited state electron transfer upon coordinating with an electron acceptor, C₆₀, generating a long-lived charge-separated state, in the order of 37.4 μs. The formation of the exceptionally long-lived charge-separated state is attributed to the involvement of both singlet and triplet excited states of AMIm-2, which is rarely reported in porphyrins.

π-Extended porphyrins fused with benzo[4,5]imidazo[2,1-a]isoindole, showing unique electronic and photophysical properties, were newly synthesized. A long-lived charge-separated state was revealed upon coordination of C₆₀ to zinc porphyrin AMIm-2.

Photocatalyst-, metal- and additive-free, regioselective radical cascade sulfonylation/cyclization of benzimidazoles derivatives with sulfonyl chlorides induced by visible light

Herein, an environmental and practical protocol for the visible-light-triggered regioselective radical cascade sulfonylation/cyclization of unactivated alkenes towards synthesis of polycyclic benzimidazoles containing sulfone group has been developed. Notably, the control...

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.

Ruthenium(ii)-catalyzed C–H alkenylation/annulation cascade for the rapid synthesis of benzoimidazoisoindoles

A one pot synthesis of benzimidazole-fused isoindoles from 2-arylbenzimidazole and conjugated alkenes has been explored. The strategy involved the ruthenium-catalyzed formation of C–C and C–N bonds and a new five-membered ring.

1,1-Hydroboration of Fused Azole-Isoindole Analogues as an Approach for Construction of B,N-Heterocycles and Azole-Fused B,N-Naphthalenes

Three isoelectronic analogues of pyrido[2,1-a]isoindole have been found to undergo a facile 1,1-hydroboration with HBMes2 borane, which provides a new and convenient method for the synthesis of B,N-heterocycles 1a-3a in high yields. Compounds 1a-3a can undergo photoelimination upon irradiation at 300 nm, generating heterocycle-fused B,N-naphthalene molecules 1b-3b, which display distinct yellow-green and blue fluorescent colors, respectively. Compound 1a undergoes thermal elimination, producing 1b at 280 °C, while compound 2a only undergoes partial elimination, forming 2b at 320 °C. Compound 3a is thermally stable up to 320 °C.

A new [4 + 1]/[4 + 2]bicycliaztion strategy for accessing functionalized indeno[1,2-b]pyran-2-ones

A new base-promoted bicyclization strategy for the synthesis of functionalized indeno[1,2-b]pyran-2-ones has been established from readily accessible o-phthalaldehydes (OPA) and the preformed 1-arylethylidenemalononitriles.

Larger π-extended anti-/syn-aroylenediimidazole polyaromatic compounds: synthesis, physical properties, self-assembly, and quasi-linear conjugation effect

Four polyaromatic compounds with 11- or 13-fused rings have been synthesized and their physical properties have been studied.