The metal-mediated (catalytic) activation of strong and typically unreactive bonds under mild conditions requires the development of powerful ligand sets. Two particularly useful strategies have been developed during the last few years. [1] The first relies on electron-deficient systems with high-valent early transition metals, often in a d 0 configuration. [2] Such metal centers have been shown to activate unreactive bonds through agostic interactions and subsequent s-bond metathesis. In a second strategy, electronrich metal centers are used to promote bond activation through oxidative addition, and late transition metals such as the platinum group metals typically accommodate the required electron density for these reactions. [3] The relative basicity can be further increased by using acidic media [4] and by installing strongly donating nontransferable ligands in the metal coordination sphere. [5]

a Palladation of C2-protected diimidazolium salts with Pd(OAc) 2 afforded complexes comprising C4-bound N-heterocyclic dicarbene ligands. The reactivity of these complexes towards Lewis acids (AgBF 4 , AgOAc) and Brønsted acids (H 2 SO 4 , H 3 PO 4 , HOAc) revealed that abnormal C4 bonding of the carbenes markedly increases the nucleophilicity of the coordinated palladium center as compared to C2 bonding. Despite its formal +2 charge, the palladium center in these complexes is best described as a Lewis base. The abnormal carbene bonding mode induces new reaction patterns such as the formation of a Pd-Ag adduct. Based on metallation studies including the palladation of a dissymmetric diimidazolium salt, a rationale for the selective activation of the C4-H bond in the diimidazolium precursor salts is proposed.

No abstract

The metal-mediated (catalytic) activation of strong and typically unreactive bonds under mild conditions requires the development of powerful ligand sets. Two particularly useful strategies have been developed during the last few years.[1] The first relies on electron-deficient systems with high-valent early transition metals, often in a d 0 configuration.[2] Such metal centers have been shown to activate unreactive bonds through agostic interactions and subsequent s-bond metathesis. In a second strategy, electronrich metal centers are used to promote bond activation through oxidative addition, and late transition metals such as the platinum group metals typically accommodate the required electron density for these reactions.[3] The relative basicity can be further increased by using acidic media [4] and by installing strongly donating nontransferable ligands in the metal coordination sphere.[5]

Oxidative addition of donor‐functionalised 4‐iodoimidazolium salts to palladium(0) provides a selective route for the preparation of abnormal chelating N‐heterocyclic carbene complexes and enables the introduction of a variety of donor groups. The activation of the C4 position does not necessitate protection of the imidazolium C2 position, thereby leaving this site available for further modification. While metallation of the unsubstituted C2 position of the N‐heterocyclic carbene ligand was unsuccessful when palladium was bound to the C4 carbon atom, sequential metallation of first the C2 position, by means of transmetallation, followed by C4–I oxidative addition, afforded a dimetallic complex comprised of two palladium centres bridged by a single NHC ligand.