Benzimidazoline and benzimidazolium redox couples: fundamental properties and diverse applications

Abstract

Electron transfer is a fundamental chemical process and frequently involved in various chemical and biological reductions and oxidations (redox). Benzimidazolines (BIHs) are benzo-fused five-membered N-heterocycles and aromatic amines. Their oxidised form (i. e. benzimidazolium (BI⁺)) is obtained via the loss of an electron and/or H as hydride, H atom, or proton from BIHs. As these processes are similar to the conversion of the reduced form of nicotinamide adenine dinucleotide (NADH) to corresponding oxidised form (NAD⁺) in biological redox reactions, a pair of BIH and BI⁺ is often called an artificial analogue of NADH and NAD⁺ redox couple. In this review, fundamental properties of BIH and BI⁺ couples and applications of these couples in diverse fields of chemistry are described. Applications of these couples include comprehensive thermodynamic and kinetic investigations, hydride reductions of organic substrates, and reductive transformations of small inorganic molecules such as O₂ and CO. Moreover, doping effects of BIH and BI⁺ on the conductivities of organic semiconductors are discussed. Participations of these substances in photochemical processes to achieve several organic conversions are highlighted. Photocatalytic systems utilising transition metal photocatalysts (PCs) and BIH as a sacrificial electron donor are favourably employed for CO₂ and H₂O (H⁺) reductions. Unprecedented protocols using photoactivated BIH as a photoreductant and corresponding photocatalytic protocols in which BIH is regenerated in situ from BI⁺ are explained. Potential ability of BI⁺ as PC is explored by examining the tolerance of BI⁺ to various cooperating substances. Radical cation (BIH^•+) and radical (BI^•) generated during interconversion between BIH and BI⁺ are key intermediates, and thus, appropriate manipulation of these species is crucial to realise desired redox reactions. Finally, miscellaneous examples of the applications of BIH, BI⁺, and other related substances in molecular transformations and materials chemistry are briefly introduced.