Organocatalysis by N-heterocyclic carbenes.

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 Carbenes belong to the most investigated reactive species in the field of organic chemistry. As typical structural features, all carbenes are neutral and possess a bivalent carbon atom with an electron sextet. First evidence for the existence of carbenes is found in the pioneering work of Buchner and Curtius and Staudinger and Kupfer in the late 19th and early 20th century.5 Because of their pronounced reactivity, carbenes could not be isolated until recently and were regarded as reactive intermediates.6,7 Since the first reports of stable nucleophilic carbenes by Bertrand and co-workers8 and Arduengo et al.9 in the late 1980s and early 1990s, the broad application of N-heterocyclic carbenes (NHCs) in organic synthesis has been impressively demonstrated. Beside their role as excellent ligands in metal-based catalytic reactions,10 organocatalytic carbene catalysis has emerged as an exceptionally fruitful research area in synthetic organic chemistry.11 This review highlights the extensive applications and reaction pathways of thiazol(A), triazol(B), imidazol(C), and imidazolin2-ylidenes (D) as versatile synthetic methods (Figure 1).