Ruthenium(II) complexes bearing chelating Carboxylate-anchored normal and abnormal Carbenes: Synthesis, characterizations and catalytic applications

Abstract

Functionalized abnormal/mesoionic NHC (NHC = N-heterocyclic carbene) complexes have been much less investigated in contrast to their normal NHC counterparts. In this work, a series of [Ccarbene^Ocarboxylate]- and [Ccarbene^C’phenyl]-type Ru(II) complexes bearing normal and abnormal NHCs have been synthesized. Their spectroscopic features, solid-state structure and aquation reactivity have also been reported. All the as-synthesized complexes have served as precatalysts to catalyze intramolecular or intermolecular carboxylic acid-to-alkyne addition reactions. It was found that the catalytic behavior of abnormal NHC complexes is superior to that of the normal NHC analogs. An anti-Markovnikov addition reactivity, leading to the formation of an E-type enol ester as the major product, was observed. A few stoichiometric experiments have also been carried out to provide preliminary insight into the catalytic mechanism.