Abstract
The mesomeric betaine imidazolium-1-ylphenolate forms a borane adduct with tris(pentafluorophenyl)borane by coordination with the phenolate oxygen, whereas its NHC tautomer 1-(2-phenol)imidazol-2-ylidene reacts with (triphenylphosphine)gold(I) chloride to give the cationic NHC complex [Au(NHC)2][Cl] by coordination with the carbene carbon atom. The anionic N-heterocyclic carbene 1-(2-phenolate)imidazol-2-ylidene gives the complexes [K][Au(NHC−)2], [Rh(NHC−)3] and [Ni(NHC−)2], respectively. Results of four single crystal analyses are presented.
Highlights
Since the first isolation of a stable N-heterocyclic carbene (NHC) [1] in 1991 this compound class has provided numerous highly efficient ligands of NHC-metal catalysts for cross-coupling reactions [2,3,4,5,6,7], versatile organocatalysts [8,9,10], and starting materials for heterocycle syntheses [11,12,13]
Deprotonation with potassium carbonate resulted in the formation of the tautomeric equilibrium of the mesomeric betaine 6A and its NHC 6B (Scheme 1)
The NMR spectra measured in a variety of solvents (DMSO-d6, MeCN-d3, MeOD-d4, D2O, THF-d8) show that the polar mesomeric betaine 6A is the only detectable tautomer under these conditions; the addition of less polar solvents induces a precipitation from solution so that detection of the NHC tautomer by NMR spectroscopy is not possible under these conditions
Summary
Since the first isolation of a stable N-heterocyclic carbene (NHC) [1] in 1991 this compound class has provided numerous highly efficient ligands of NHC-metal catalysts for cross-coupling reactions [2,3,4,5,6,7], versatile organocatalysts [8,9,10], and starting materials for heterocycle syntheses [11,12,13].
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