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The successful isolation and characterization of an N-heterocyclic carbene in 1991 opened up a new class of organic compounds for investigation. From these beginnings as academic curiosities, N-heterocyclic carbenes today rank among the most powerful tools in organic chemistry, with numerous applications in commercially important processes. Here we provide a concise overview of N-heterocyclic carbenes in modern chemistry, summarizing their general properties and uses and highlighting how these features are being exploited in a selection of pioneering recent studies.
Defined as neutral compounds containing a divalent carbon atom with a six-electron valence shell, carbenes are an intriguing class of carbon-containing compounds. Their incomplete electron octet and coordinative unsaturation, however, render free carbenes inherently unstable and they have been traditionallyconsideredonlyashighlyreactive transient intermediates in organic transformations such as cyclopropana- tion. Despite attempted syntheses from as early as 1835 (ref. 1), the isola- tion and unambiguous characterization of a free, uncoordinated carbene remained elusive until pioneering studies in the late 1980s and early 1990s (ref.2).In aseminalpublicationin1988, Bertrand andco-workersreported the preparation of the first isolable carbene stabilized by favourable inter- actions with adjacent phosphorus and silicon substituents3.Threeyears later Arduengo et al. reported an isolable and 'bottleable' carbene incorpo- ratedintoanitrogenheterocycle4.Withstructuralfeaturesinspiredbyearlier insightfulstudiesbyWanzlick5 andÖfele6onmetal-carbenecomplexes,the remarkablestabilityandrelativelysimplesynthesisofthefirstN-heterocyclic carbene (NHC) 1,3-di(adamantyl)imidazol-2-ylidene (IAd, compound labelled1a)led toanexplosion ofexperimentalandtheoreticalstudies with libraries of novel NHCs being synthesized and analysed (see below). As a result of these investigations, NHCs have been elevated from mere labor- atory curiosities to compounds of enormous practical significance as more and more of the rich chemistry of these compounds has been revealed and exploited. As excellent ligands for transition metals, NHCs have found mul- tiple applications in some of the most important catalytic transformations in the chemical industry, while their reactivity upon coordination to main group elements and as organocatalysts has opened up new areas of research.
In this review, we aim to provide a concise overview of the properties and broad range of applications of NHCs, which we hope will serve as a useful introduction and reference guide for scientists interested in study- ing and applying these important compounds. After an initial summary of the general structure and properties of NHCs, the reactivity and appli- cations in modern chemistry are loosely categorized into three sections with a discussion of NHCs as ligands for...