Second Generation Hoveyda-Grubbs Olefin Metathesis Catalysts Bearing Hemilabile Polyether Arms PDF Download

Author: Jordan David Carter
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Languages : en
Pages : 147

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Book Description
The field of olefin metathesis has grown tremendously in recent decades. Elucidation of the mechanism and an improved understanding of the structure-activity relationship has enabled chemists to design catalysts which exhibit greater activity, stability, and selectivity. However, the economic feasibility of performing this reaction on the industrial scale is often restricted by the high cost of the catalysts in combination with 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. Therefore, the various decomposition pathways leading to catalyst inactivation represents one of the greatest challenges still remaining in the field. The electron deficient Ru intermediates of the catalytic cycle are susceptible to decomposition through any reaction which provides the metal with additional electron density. The primary objective of this thesis project was to develop 2nd generation Hoveyda-Grubbs catalysts which contain an N-heterocyclic carbene (NHC) bearing hemilabile polyether arms in the ortho position of the aromatic rings. The purpose of incorporating these functional groups was to stabilize the fragile intermediates of the catalytic cycle via O → Ru coordination. The polyether arms have the potential to wrap around Ru via chelation and therefore provide electron density while simultaneously shielding the sterically wide open position trans to the NHC. The hypothesis was that stabilizing these intermediates would prolong the lifetime of the active catalyst, thus leading to increased TON and allowing for lowering loadings. A novel series of 5 catalysts bearing NHC ligands with hemilabile polyether arms were synthesized. The NHC ligands differed by the number of arms (1,2, or 4) and the length of each arm (2 or 3 carbons between the ether oxygens). The structures of the catalysts and NHC ligand precursors were investigated by x-ray crystallography and NMR spectroscopy. The results of x-ray diffraction revealed that the 4-armed ligands adopt a propeller conformation, and their corresponding tetra-substituted catalysts displayed a single ortho O → Ru interaction in the solid state. This coordination forces the NHC to adopt a highly contorted conformation which is twisted far away from the default position seen for the unsubstituted commercial catalyst bearing an NHC with 2 mesitylene rings but no electron donor groups. NMR spectroscopy provided information about the conformation adopted in solution, and revealed that the NHC has restricted rotation once bound to Ru but does not display broken molecular symmetry. The combination of O → Ru coordination in the solid state but retention of symmetry in solution indicates that all of the available polyether arms take turns interacting with the metal faster than the NMR timescale. Catalyst decomposition studies were performed under normal metathesis conditions with a standard alkene substrate or by forming the inactive Fischer carbene by treating the catalyst with a vinyl ether. The purpose of intentional catalyst destruction was to generate an electron deficient Ru center which is stabilized by coordination with the polyether arms. These species are electronically and sterically more similar to the 14 electron active catalyst than the 16 electron pre-catalyst, and therefore provide greater insight into how the catalyst actually behaves in solution upon initiation. Only the decomposition products obtained from the 2C-2arm catalyst 15 displayed chelation in the solid state, while every other catalyst formed a non-chelated dimer upon decomposing. A variety of unexpected degradation products were observed by x-ray crystallography, and a potential mechanism was proposed for each decomposition pathway. The catalysts were screened for ring-closing metathesis (RCM) and ring-opening metathesis polymerization (ROMP) performance. Only the tetra-substituted catalysts exhibited a rate enhancement for RCM when compared with the commercial catalyst bearing 2 mesitylene rings. However, the TON did not differ significantly from the commercial catalyst, thus indicating that the electron deficient intermediates of the catalytic cycle are not stabilized by coordination with the polyether arms. All of the polyether-substituted catalysts were significantly inferior than the commercial catalyst at performing ROMP, exhibiting both decreased TON and slower kinetics. The information gathered from testing catalyst performance in combination with the structural data obtained by analyzing the catalysts and decomposition products via x-ray diffraction and NMR spectroscopy allowed for potential structure-activity relationships to be proposed. The insight into olefin metathesis catalyst structure and performance provided by the studies of this work may assist in future catalyst design.