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.

An overview of N-heterocyclic carbenes

N-Heterocyclic Carbenes in Late Transition Metal Catalysis

The chemistry of heterocyclic carbenes has experienced a rapid development over the last years. In addition to the imidazolin-2-ylidenes, a large number of cyclic diaminocarbenes with different ring sizes have been described. Aside from diaminocarbenes, P-heterocyclic carbenes, and derivatives with only one, or even no heteroatom within the carbene ring are known. New methods for the synthesis of complexes with N-heterocyclic carbene ligands such as the oxidative addition or the metal atom template controlled cyclization of β-functionalized isocyanides have been developed recently. This review summarizes the new developments regarding the synthesis of N-heterocyclic carbenes and their metal complexes.

Heterocyclic Carbenes: Synthesis and Coordination Chemistry

The area of transition-metal-catalyzed direct arylation through cleavage of CH bonds has undergone rapid development in recent years, and is becoming an increasingly viable alternative to traditional cross-coupling reactions with organometallic reagents In particular, palladium and ruthenium catalysts have been described that enable the direct arylation of (hetero)arenes with challenging coupling partners—including electrophilic aryl chlorides and tosylates as well as simple arenes in cross-dehydrogenative arylations Furthermore, less expensive copper, iron, and nickel complexes were recently shown to be effective for economically attractive direct arylations

Transition-metal-catalyzed direct arylation of (hetero)arenes by C-H bond cleavage.

In the investigation of efficient chemical transformations, the carbon-carbon bond-forming reactions play an outstanding role. In this context, organocatalytic processes have achieved considerable attention. 1 Beside their facile reaction course, selectivity, and environmental friendliness, new synthetic strategies are made possible. Particularly, the inversion of the classical reactivity (Umpolung) opens up new synthetic pathways. 2 In nature, the coenzyme thiamine (vitamin B1), a natural thiazolium salt, utilizes a catalytic variant of this concept in biochemical processes as nucleophilic acylations. 3 The catalytically active species is a nucleophilic carbene. 4

Organocatalysis by N-Heterocyclic Carbenes

Organocatalyzed reactions represent an attractive alternative to metal-catalyzed processes notably because of their lower cost and benign environmental impact in comparison to organometallic catalysis. In this context, N-heterocyclic carbenes (NHCs) have been studied for their ability to promote primarily the benzoin condensation. Lately, dramatic progress in understanding their intrinsic properties and in their synthesis have made them available to organic chemists. This has resulted in a tremendous increase of their scope and in a true explosion of the number of papers reporting NHC-catalyzed reactions. Here, we highlight the ever-increasing number of reactions that can be promoted by N-heterocyclic carbenes.

N‐Heterocyclic Carbenes as Organocatalysts

Stereoelectronic parameters associated with N-heterocyclic carbene (NHC) ligands: A quest for understanding

Vor allem wegen der geringeren Kosten und der besseren Umweltvertraglichkeit stellen organokatalytische Reaktionen eine vielversprechende Alternative zu metallkatalysierten Prozessen dar. N-heterocyclische Carbene (NHCs) wurden dahingehend untersucht, inwieweit sie (vor allem) die Benzoinkondensation katalysieren konnen. Die grosen Fortschritte beim Verstandnis der Eigenschaften von NHCs und bei deren Synthese haben die Aufmerksamkeit der Organiker nun auf diese Stoffklasse gelenkt. Folgerichtig nahmen auch die Moglichkeiten und Einsatzgebiete der NHCs enorm zu, und unzahlige Beitrage haben sich mit NHC-katalysierten Reaktionen befasst. Hier werden wir unser Augenmerk auf die wachsende Zahl an Umsetzungen richten, die von NHCs katalysiert werden konnen.

N‐heterocyclische Carbene als Organokatalysatoren

A versatile and highly efficient catalyst for the Huisgen cycloaddition reaction has been developed. Previously isolated or in situ generated azides yielded 1,2,3-triazoles with differently substituted alkynes in the presence of a [(NHC)CuBr] complex (NHC = N-heterocyclic carbene). Extremely high reaction rates and excellent yields were obtained in all cases. This catalytic system fulfils the requirements of "click chemistry" with its mild and convenient conditions, notably in water or solvent free reactions and simple isolation with no purification step. Furthermore, for the first time, an internal alkyne was successfully used in this copper-catalyzed cycloaddition reaction. DFT calculations on this particular system allowed for the proposition of a new mechanistic pathway for disubstituted alkynes.

Silvia Díez-González

Papers

N-Heterocyclic Carbenes in Late Transition Metal Catalysis

N‐Heterocyclic Carbenes as Organocatalysts

Stereoelectronic parameters associated with N-heterocyclic carbene (NHC) ligands: A quest for understanding

N‐heterocyclische Carbene als Organokatalysatoren

(NHC)Copper(I)‐Catalyzed [3+2] Cycloaddition of Azides and Mono‐ or Disubstituted Alkynes