Group 6 metal alkylidene and alkylidyne N-heterocyclic carbene complexes for olefin and alkyne metathesis

Abstract

The concept of metal imido and metal oxo alkylidene N-heterocyclic carbene (NHC) catalysts for olefin metathesis is outlined. This includes the synthetic concepts for their synthesis, their reactivity, the role of the metal, the origin of functional group tolerance as well as relevant aspects of regio- and stereoselectivity. Attention is also devoted to the role of the NHC in the generation and stabilization of cationic metal complexes. Indeed, the novel concept allows for the preparation of neutral, pentacoordinate pre-catalysts that serve as excellent progenitors to the corresponding tetracoordinate cationic systems. The activation process entails the release of an anionic ligand and can be regulated by the careful choice of ligands. This allows for the design of precursor catalysts that are fully thermally latent in the presence of monomer, which can be used in single-component, dicyclopentadiene (DCPD) based resin systems for reaction injection molding (RIM) or resin transfer molding (RTM). Mechanistic aspects, i.e. an associative vs. a dissociative mechanism, and their importance for this particular feature are outlined, too. Also, concepts for preparing all-cis or all-trans-tactic polymers and for accomplishing E- and Z-selective olefin metathesis reactions are summarized. In addition, the immobilization of this new type of olefin metathesis catalysts on silica, the reactivity of the resulting supported catalysts, both in olefin or alkyne metathesis, is described. In addition, aspects of biphasic catalysis, for which cationic catalysts can be used, are addressed. Finally, the extension of this concept to neutral metal alkylidyne NHC catalysts for alkyne metathesis, their state of the art and remaining challenges are outlined.