The Role of NHC Ligands in Oxidation Catalysis

Abstract

AbstractDuring the last decade N-heterocyclic carbenes (NHC) have become a very important class of ligands for catalytic applications. They have been employed in reactions like C-C/C-N coupling, olefin metathesis, hydrosilylation and hydrogenation. Recently, also in the field of oxidation catalysis significant progress has been achieved. NHC ligands are known to efficiently activate metal centers and they are surprisingly resistant, even in acidic media. In contrast to phosphine ligands, the metal-NHC bond is also stable under oxidizing conditions. This review covers the oxidation of organic substrates by NHC catalysts as well as the fixation and activation of O2 and CO2 by NHC complexes. A variety of oxidation reactions have been studied using NHC catalysts, but the oxidation of alcohols and olefins was of particular interest. Most of the work in this area was conducted with palladium catalysts, which even allow the efficient oxidation of alcohols at ambient temperature using molecular oxygen. Bi- and tridentate NHC ligands are described as well as a variety of complexes with different metals other than palladium. Reactive intermediates like peroxo complexes have been characterized experimentally, while DFT calculations have been used to explain the mechanisms of the oxidation chemistry of transition metal NHC complexes. CatalysisDFT calculations N-heterocyclic carbenesOxidationPalladium