Molecular Recognition Utilizing Complexes with NH,NR‐Stabilized Carbene Ligands

Abstract

Tungsten complex [W(NHC)(CO)5] (1) (NHC=NH,N‐ethyl‐substituted benzimidazolin‐2‐ylidene ligand) was prepared by transmetallation of lithiated 1‐ethylbenzimidazole with the pentacarbonyltungsten complex fragment. The hydrogen‐bonding ability of the carbene‐NH group to substrates with carbonyl groups was investigated by NMR titration. Structural analogues to 1 with a catalytically active rhodium center show a distinct substrate‐selective hydrogenation behaviour.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Heterocyclic Carbenes: Synthesis and Coordination Chemistry

“…The subsequent reaction with HCl yields complex 186 with an NH,NRstabilized benzimidazolin-2-ylidene ligand (Scheme 97). [307] Rhodium complexes with similar benzimidazolin-2-ylidene ÀC coupling reaction of azoles with alkenes or aromatic compounds. [282] The formation of a hydrogen bond between the NH group of an NH,O-stabilized benzoxazolin-2-ylidene ligand and triphenylphosphine oxide has been described.…”

Ligand Exchange at a Gold(I) Carbene Complex

“…Less convenient, but highly useful, is the template-controlled intramolecular cyclization of metal-coordinated β -functionalized phenyl isocyanides [24 -28] as well as the oxidative addition of a C2-X bond (X = H, Cl, I) of benzimidazole derivatives to low-valent metal precursors [29 -34]. The latter two methods lead to complexes bearing NH,NH-or NH,NR-substituted benzimidazolin-2-ylidene ligands, which can either be further functionalized, or in selected cases, can function as molecular recognition units [35].…”

β‐Protic Pyrazole and N‐Heterocyclic Carbene Complexes: Synthesis, Properties, and Metal–Ligand Cooperative Bifunctional Catalysis

“…[69] Ellman, Bergman, and co-workers developed catalytic C À H bond functionalization of azoles, in which tautomerization of the substrate azoles to protic NHCs seems involved as a key step (vide infra). [67,70] As a variant of the tautomerization, transmetalation of C-metalated imidazole and subsequent N protonation also give the protic NHC complexes, [51,60,71,72] typified by Scheme 23.…”

“…[48, [59][60][61][62][63] The Staudinger reaction in the coordination sphere of the isocyanide complexes 49, for example, results in the formation of the protic NHC complexes 50 via an aminophenyl isocyanide complex (Scheme 20). [59] Deoxygenative condensation of carbonyl complexes with amino-functionalized phosphine imines followed by intramolecular cyclization also affords protic NHC complexes, [64,65] as exemplified by Scheme 21.…”