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

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

Asymmetric enamine catalysis.

Asymmetric intramolecular crossed-benzoin reactions by N-heterocyclic carbene catalysis.

The enantioselective synthesis of 3-hydroxy-4-chromanones bearing a quaternary stereocenter via an N-heterocyclic carbene catalyzed intramolecular crossed-benzoin reaction is described.

Asymmetric Synthesis of Chromanones via N-Heterocyclic Carbene Catalyzed Intramolecular Crossed-Benzoin Reactions

Intramolecular crossed aldehyde-ketone benzoin-type reactions catalyzed by nucleophilic carbenes, easily generated from commercially available thiazolium salts as precatalysts, are described. Five- and six-membered cyclicacyloins are obtained in moderate to good yields. Depending on the structure of the aldehyde-ketone substrate, an interchange of the alcohol and ketone function of the resulting acyloin is possible. Simple aldehyde-ketones are not good substrates due to the competing intermolecular reaction. Starting from biphenyl-2,2'-dicarbaldehyde, the intermediate acyloin is converted to 9,10-phenanthrenequinone by mild air oxidation.

Oliver Niemeier

Papers

Organocatalysis by N-Heterocyclic Carbenes

Asymmetric intramolecular crossed-benzoin reactions by N-heterocyclic carbene catalysis.

Asymmetric Synthesis of Chromanones via N-Heterocyclic Carbene Catalyzed Intramolecular Crossed-Benzoin Reactions

Thiazol-2-ylidene Catalysis in Intramolecular Crossed Aldehyde-Ketone Benzoin Reactions

Asymmetric Organocatalytic Synthesis of cis-Substituted Dihydrobenzofuranols via Intramolecular Aldol Reactions

Oliver Niemeier

Papers

Organocatalysis by N-Heterocyclic Carbenes

Asymmetric intramolecular crossed-benzoin reactions by N-heterocyclic carbene catalysis.

Asymmetric Synthesis of Chromanones via N-Heterocyclic Carbene Catalyzed Intramolecular Crossed-Benzoin Reactions

Thiazol-2-ylidene Catalysis in Intramolecular Crossed Aldehyde-Ketone Benzoin Reactions

Asymmetric Organocatalytic Synthesis of cis-Substituted ­Dihydrobenzofuranols via Intramolecular Aldol Reactions

Asymmetric Organocatalytic Synthesis of cis-Substituted Dihydrobenzofuranols via Intramolecular Aldol Reactions