Platinum-Mediated Oxidative Addition and Reductive Elimination of Imidazolium Salts at C4 and C5

摘要

Nucleophilic N-heterocyclic carbenes (NHCs) have attracted considerable interest as ligands in organometallic chemistry and catalysis.This attention is largely because of the unique combination of strong sigma-donor,poor pi-acceptor,and steric properties.Typical transition-metal NHC complexes show "normal" binding at C2 because of the acidity of the C2 proton in the precursor azolium salt."Abnormal" metalation at C5 of the NHC ring was first reported in elegant studies by Crabtree and co-workers for pyridyl- and picolyl-NHC dihydride complexes of Ir~(III),blocking C2 with a substituent,such as Me or Ph groups,will also encourage binding at C5.Nolan and co-workers have recently reported the structure and interesting catalytic reactivity of [PdCl_2-(NHC)_2] complexes containing NHC ligands bound the "wrong way" that are derived from imidazolium salts.In both these studies,the carbene bound at C4/5 was generated through deprotonation by the addition of a base or use of basic ligands on the metal.