TV-heterocyclic carbenes

A homoleptic methylene-bridged bis(carbene) dinuclear silver(i) complex, 55, was isolated in 81% yield from a two-step reaction, as shown in Scheme 13.100 The structure of 55 reveals a boat conformation with the two methylene groups protruding away from the Ag2 core in one direction. The Ag-C distances of 2.081(2)-2.095(2) A were in the normal range for silver(i) TV-heterocyclic carbene complexes, while an uncommonly short intramolecular Ag-Ag contact of 3.2039(3) A was observed in the homoleptic silver(i) carbene complexes. 

Coulembier O, Lohmeijer BG, Dove AP, Pratt RC, Mespouille L, Culkin DA, Benight SJ, Dubois P, Waymouth RM, Hedrick JL (2006) Alcohol adducts of TV-heterocyclic carbenes latent catalysts for the thermally-controlled living polymerization of cyclic esters. Macromolecules 39 5671-5628... 

Dove AP, Pratt RC, Lohmeijer BGG, Li H, Hagberg EC, Waymouth RM, Hedrick JL (2006) TV-Heterocyclic carbenes as organic catalysts. In Nolan SP (ed) TV-Heterocyclic carbenes in synthesis. Wiley-VCH, Weinheim, pp 275-296... 

Duong HA, Cross MJ, Louie J (2004) TV-Heterocyclic carbenes as highly efficient catalysts for the cyclotrimerization of isocyanates. Org Lett 6 4679-4681... 

Enders D, Balensiefer T, Niemeier O, Christmann M (2007a) Nucleophile TV-Heterocyclic carbenes in asymmetric organocatalysis. In Dalko PI (ed) Enantioselective Organocatalysis reactions and experimental procedures. Wiley-VCH, Weinheim, pp 331-355... 

Stability and reactivity of TV-heterocyclic carbene complexes 04CCR(248)2247. 

A -Alkylated thiazolium and benzothiazolium salts also experience base-promoted deprotonation at the 2-position to form ylides. Such compounds, often referred to as TV-heterocyclic carbene (NHC), are nucleophilic catalysts in benzoin condensation. In 1943, Ugai and co-workers reported that thiazolium salts catalyze self-condensation of benzaldehyde to generate benzoin via an umpoulong process. Breslow at Columbia University in 1958 proposed thiazolium ylide as the actual catalyst for this transformation. In this mechanism, the catalytically active species was represented as a thiazolium zwitterion, the resonance structure of an NHC, and the reaction was postulated to ensue via the enaminol or the Breslow intermediate. ... 

Structurally related species that exhibit C-H activation include the bis(pyrazolyl) borate complex in Scheme 21, for which (as in the above dmdpb system) protonation (or methide abstraction) generates an intermediate that reacts readily with benzene [74] the bis(azaindolyl)methane complex in Scheme 22, which activates both aromatic and benzylic C-H bonds [75,76] (some stable Pt(TV) complexes based on the same architectiRe have also been isolated and shown to undergo reductive elimination of MeX [77]) and complexes based on anionic bidentate ligands such as 2-(2 -pyridyl)indolide [78]. Intramolecular C-H activation was observed for one example of a series of A -heterocyclic carbene complexes of Pt(II) distortions induced by steric crowding appear to influence reactivity strongly [79]. 

Carbenoid addition of diazocarbonyl compounds to pyrrole, TV-alkylpyrroles, indole, N-alkylindoles, imidazole, and benzimidazole does not result in cyclopropanation, but leads to the formal products of carbene insertion into a heterocyclic C-H bond (see Houben-Weyl, Vol. E 19b, ppll58 and 1334). However, iV-acylpyrroles,315 316 and JV-acylindoles303,317 have successfully been converted into 2-azabicyclo[3.1.0]hex-3-ene-exn-6-carboxylates and alkyl-1,la,2,6b-tetrahydrocyclopropa[ >]indole-ex0-l-carboxylate, respectively (for an experimental procedure, see Houben-Weyl, Vol.E19b, p 1160). 

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