Open-chain and cyclic nitrenium ions: generation, characterization, structure and (bio)chemical reactivity

Nitrenium ions (NIs) are divalent nitrogen species with an N^⊕ centre; many of them are reactive intermediates, and the number of identified stable, catalytic NIs has been surging. Open-chain nitrenium ions (ONIs) generally have very short half-lives and are non-isolable. Therefore, they can only be generated and characterized under suitable reaction conditions. Only a few ONIs have been isolated and structurally characterized. Many ONIs exhibit carcinogenic properties under biological conditions. A few ONIs are therapeutically important. On the other hand, cyclic nitrenium ions (CNIs) are bench-stable and are available in various ring sizes. Several CNIs have been characterized by single-crystal X-ray diffraction and electronic structure analyses. A few CNIs were reported to exhibit catalytic properties (Lewis acid catalysis, phase-transfer catalysis and photocatalysis), and they are also valuable cationic species in biomedical applications. A few biocatalytic CNIs are recognised, and their analogues are designed as useful organocatalysts. Some CNIs participate as reactive intermediates in various biochemical reactions. CNIs are isoelectronic to N-heterocyclic carbenes (NHCs), but their chemical characteristics are distinct. This review provides a comprehensive account of NIs, whose importance in chemical research is proliferating across all branches of chemistry.