Keteniminium ions have been demonstrated to be remarkably useful and versatile reactive intermediates in chemical synthesis. These unique heterocumulenes are pivotal electrophilic species involved in a number of efficient and selective transformations. More recently, even more reactive ‘activated’ keteniminium ions bearing an additional electron-withdrawing group on the nitrogen atom have been extensively investigated. The chemistry of these unique reactive intermediates, including representative methods for their in situ generation, will be overviewed in this review article.1 Introduction2 The Chemistry of Keteniminium Ions3 The Chemistry of Activated Keteniminium Ions4 Keteniminium Ions: Pivotal Intermediates for the Synthesis of Natural and/or Biologically Relevant Molecules5 Conclusions and Perspectives

A method for electrophilic sulfenylation by organophosphorus‐catalyzed deoxygenative O‐atom transfer from sulfonyl chlorides is reported. This C−S bond‐forming reaction is catalyzed by a readily available small‐ring phosphine (phosphetane) in conjunction with a hydrosilane terminal reductant to afford a general entry to sulfenyl electrophiles, including valuable trifluoromethyl, perfluoroalkyl, and heteroaryl derivatives that are otherwise difficult to access. Mechanistic investigations indicate that the twofold deoxygenation of the sulfonyl substrate proceeds by the intervention of an off‐cycle resting state thiophosphonium ion. The catalytic method represents an operationally simple protocol using a stable phosphine oxide as a precatalyst and exhibits broad functional‐group tolerance.

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A method for the annulation of amines and carboxylic acids to form pharmaceutically relevant azaheterocycles via organophosphorus P III /P V redox catalysis is reported. The method employs a phosphetane catalyst together with a mild bromenium oxidant and terminal hydrosilane reductant to drive successive C–N and C–C bond-forming dehydration events via the serial action of a catalytic bromophosphonium intermediate. These results demonstrate the capacity of P III /P V redox catalysis to enable iterative redox-neutral transformations in complement to the common reductive driving force of the P III /P V couple.

An efficient, modular and straightforward entry to tetrahydropyridines and piperidines is reported. This reaction is based on a formal intramolecular hydroalkylation of readily available, properly substituted ynamides which, upon simple activation under acidic conditions, generate highly reactive activated keteniminium ions whose reactivity can be finely controlled to induce a remarkably efficient [1,5]‐hydride shift from unactivated C−H bonds and trigger a cationic cyclization which is complete within minutes.