Keteniminium Ions: Unique and Versatile Reactive Intermediates for Chemical Synthesis

Abstract

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

Keteniminium Salts as Key Intermediates for the Efficient Synthesis of 3‐Amino‐Indoles and ‐Benzofurans

“…With longer reaction time, we only observed slow decomposition of the remaining starting materials. To overcome this issue, we replaced the triflating reagent by the well‐known amide activator POCl 3 which cannot formed efficiently a keteniminium due to the nucleophilic counter anion chloride . As expected, no aniline cyclization onto the iminium intermediate was observed with or without base, even with higher temperature and longer reaction time.…”

“…Based on our previous work on the direct formation of 3‐amino(benzo)thiophenes C from easily accessible thioacetamides A , through a 6 π ‐electrocyclization involving a keteniminium salt intermediate B , we are pleased to report herein the expansion of this method towards the facile synthesis of 3‐aminoindoles and 3‐aminobenzofurans E ( Scheme ) …”

Efficient and Divergent Synthesis of α‐Halogenated Amides and Esters by Double Electrophilic Activation of Ynamides

“…Indeed, the high nucleophilicity of ynamides enables their easy activation with an electrophile—most commonly a proton, a metal, or a halogenium salt—resulting in the generation of these unique highly reactive intermediates which have been used to trigger a number of remarkably efficient and innovative transformations. Most of these reactions rely on the trapping of these activated keteniminium ions 2 with a nucleophile yielding highly substituted enamides 3 that, in addition, can then participate in other processes . Based on the reactivity and electrophilicity of enamides and on our long‐standing interest in the chemistry of ynamides, we envisioned the possibility of merging the cationic chemistry of ynamides and enamides.…”

Diels–Alder and Formal Diels–Alder Cycloaddition Reactions of Ynamines and Ynamides

“…Nitrogen‐substituted alkynes such as ynamines and ynamides are versatile partners in pericyclic reactions and for the past 50 years numerous groups have capitalized on their unique π‐systems. Among these pericyclic transformations, Diels–Alder cycloadditions and their variants [Scheme , Equations (1)–(3)] are among the most popular, being powerful tools with which to increase molecular complexity from simple starting materials.…”