The Design, Synthesis, and Application of Ruthenium Metathesis Catalysts for the Preparation of Small Molecules and Polymers

Abstract

Olefin metathesis is a widely used method for constructing carbon–carbon double bonds. This methodology has broad applications in organic and polymer chemistry, and the continued design of highly efficient catalysts has been critical to the success of this reaction. The main goal of this thesis was to design and synthesize new catalysts for better selectivity and for improved properties for targeted applications, as well as to explore different ligand structures for optimal catalyst performance in olefin metathesis. The application of ruthenium catalysts for the ring-opening metathesis polymerization of challenging monomer 1,5-dimethyl-1,5-cyclooctadiene in the presence of a chain transfer agent is discussed in chapter 2. A variety of complexes were explored to find the ideal catalyst for this transformation, enabling the synthesis of telechelic polyisoprene, which has extensive applications in block copolymerization. Chiral N-alkyl, N-aryl NHC ruthenium catalysts were designed and synthesized to improve the enantioselectivity during asymmetric ring-opening cross-metathesis. Mechanistic studies of these catalysts revealed a preference for methylidene propagation compared to previous NHC catalysts. Chapter 3 describes these studies, in addition to the screening of a variety of chiral ligands for optimal enantioselectivity. Some of these catalysts gave very high enantioselectivity, comparable to the best reported ruthenium catalysts. Insights into the stability of these complexes as a propagating methylidene led to investigating them in applications where propagation as a methylidene is desirable. N-aryl, N-alkyl NHC ruthenium catalysts were designed and synthesized for improved selectivity during ethenolysis reactions, which require a ruthenium methylidene species to react with an internal olefin to yield a terminal olefin and a ruthenium alkylidene species. Subsequent reaction of this ruthenium alkylidene species with ethylene gives the other terminal olefin. This reaction can be applied to the internal olefin of seed oils to generate valuable products that are typically derived from petroleum sources, thus providing an environmentally friendly route to the same products. An important component of ethenolysis catalysts is stability to existing as a methylidene, a property of the N-aryl, N-alkyl NHC ruthenium catalysts described in chapter 4. Chapter 5 describes the design and synthesis of sterically hindered N-aryl, N-alkyl NHC ruthenium catalysts for application in latent metathesis. These complexes also show excellent stability at elevated temperatures for extended periods of time. Appendix A contains NMR spectra for catalysts described in chapter 4, as well as X-ray crystal structures for two of the catalysts. Appendix B contains NMR spectra for catalysts described in chapter 5, as well as X-ray crystal structures for two of those catalysts.