Download or read book Novel Olefin Metathesis Catalysts Bearing a Hemi-labile NHC Ligand written by Nicolas Cena and published by . This book was released on 2019 with total page 79 pages. Available in PDF, EPUB and Kindle. Book excerpt: The field of olefin metathesis has grown appreciatively in recent decades. Elucidation of the mechanism and a deeper understanding of the key intermediates have enabled chemists to design catalysts, which exhibit greater activity, stability, and selectivity towards a variety of substrates. However, the economic impact of performing this reaction on the industrial scale is often governed by the high cost of the catalysts in comparison to modest turnover numbers (TON). The lifetime of an active catalyst species depends on the stability of intermediates in the catalytic cycle, and this ultimately determines the TON. One of the remaining goals in the field is to further stabilize the intermediates of the catalytic cycle in order to prolong the catalyst lifetimes and increase turnover numbers (TON). The ruthenium catalytic species cycle through several electron deficient intermediates, which lead towards decomposition pathways. These decomposition pathways are directed towards reactions that provide the metal with additional electron density. The primary focus of this thesis project was to develop a new generation of olefin metathesis catalysts employing a tridentate N-heterocyclic carbene (NHC) ligand bearing a hemi labile pyridine arm in the ortho position of the aromatic ring. The rationale behind incorporating these functional groups was to stabilize the reactive intermediates of the catalytic cycle via electron donation from the ether O → Ru and the pyridine N → Ru. These ligands increase the likelihood of stabilization of the metal center by chelation of electron-donating substituents from the O and N, thus adding electron density back into the electron-deficient metal center. The ligand not only donates electron density back to the metal but also shields the sterically open position trans to the NHC. The hypothesis was that stabilizing these reactive intermediates would prolong the lifetime of the active catalyst and thus increase TON and allow for a much lower catalyst loading for industrial applications, thus vastly impacting the economical aspect of olefin metathesis processes. A novel set of two catalysts bearing tridentate NHC ligands with hemi labile pyridine arms were synthesized. The ligands differed in one aromatic ring containing either a 2,6 diisopropyl phenyl (DIPP) or mesityl (Mes) moiety. The result of the x-ray crystallographic analysis revealed the NHC ligand coordinated in the proposed tridentate meridional fashion around the central Ru atom. This coordination was proposed in order to affect a hinge-like mechanism in which the pyridine arm's hemi-labile nature would be in close proximity to the electron deficient metal center, so that it could bind reversibly in order to satiate the metal's desire for electron density while still allowing reactivity upon dissociation. NMR spectroscopy revealed information about the proposed structure in solution and revealed that the ligand was bound in solution to the metal center in one orientation, owing to the coordination of the O and N to the metal center. Catalyst decomposition studies were performed using the methylidene variant of the catalysts at elevated temperatures under inert conditions as well as under an atmosphere of ethylene gas. The purpose of intentionally decomposing the catalyst was to generate the electron deficient Ru center and probe the stabilizing effects of the pyridine arm coordination. These reactive intermediates are electronically and sterically more similar to the 14-electron active catalyst than the 16-electron pre-catalyst, giving insight into how the catalysts behave in solution upon metathesis active conditions. Decomposition products of the DIPP variant were analyzed by NMR and x-ray crystallography, giving insight into possible decomposition pathways for these novel catalysts. The catalysts were screened for metathesis activity in ring-closing metathesis (RCM) and ring opening metathesis polymerization (ROMP). Both showed noticeable differences from previous generations of Grubbs and Hoveyda-Grubbs catalysts in overall efficacy. The prolonged lifetimes of these new catalysts were competitive with commercially available catalysts in terms of lifetime and TON. Though slightly lower in TON, these catalysts lasted much longer in solution at elevated temperatures than their predecessors, thus indicating much more stabilized reactive intermediates. The data gathered from decomposition studies and metathesis activity along with NMR and x-ray crystallography allowed for potential structure-activity relationships and mechanistic pathways to be proposed. The insight into olefin metathesis catalyst structure, performance, and design provided by these studies may assist in future endeavors in the field of olefin metathesis catalyst design and employment.