Olefin Metathesis Reactions Promoted by Ru-Catalysts with a Syn Substituted N-heterocyclic Carbene Backbone

Abstract

The development of efficient catalytic systems dedicated to the formation of C−C double bonds from simple to highly functionalized alkenes represents a great challenge in modern organic synthesis. In addressing this challenge, olefin metathesis has become an extremely versatile tool simplifying synthetic routes to numerous and valuable natural and unnatural products dramatically. Breakthroughs made in the last two decades have applied mechanistic understanding in the design of innovative and well-defined homogeneous pre-catalysts, increasing the appeal of this highly atom efficient reaction methodology. Ruthenium based metathesis catalysts have received considerable attention because of their tolerance to oxygen, moisture and many functional group, but even more because it is possible to control catalyst activity by fine tuning of the steric and electronic properties of the ligands. Major improvements in Ru-based pre-catalysts were achieved through the incorporation of N-heterocyclic carbene (NHC) as ancillary ligand. In general, N-aryl bulk was found to increase activity, whereas increased backbone substitution decreased activity but increased catalyst lifetime. The development of this class of complexes has broadened the scope and the utility of the olefin metathesis reaction in both organic synthesis and polymer science. The research group in which I developed my PhD thesis, recently focused on the investigation of NHC ligand symmetry in ruthenium based complexes underlining the pivotal role played by NHC backbone substitution in the design of highly active pre-catalyst for the RCM of hindered di-olefins. This thesis was aimed to furnish a deeper understanding of the effect of syn backbone substitution on catalyst activity by creating a properly designed catalyst library. In particular, besides testing their activity toward many different metathesis reactions, we performed conformational studies of these newly synthesized complexes (via NMR and DFT calculations). Voltammetric studies were mpreover conducted to better elucidate on electrochemical complex properties... [edited by author]