Mechanistic Studies, Catalyst Development, and Reaction Design in Olefin Metathesis

Abstract

Chapter 1. Exploring Ligand Effects in Ruthenium Dithiolate Carbene Complexes. Ruthenium dithiolate metathesis catalysts discovered in the Hoveyda group have been a valuable addition to the field of olefin metathesis. While the catalyst shows unique selectivity and reactivity, quantifying and mapping key interactions in the catalyst framework to elucidate and explain causes is difficult. We, therefore, decided to use the neutral chelating or monodentate ligand, controlling initiation, as a structural probe. By altering its properties and observing changes in the catalyst, we sought to deepen our understanding of these complexes. We established a trans influence series with over 20 catalysts and correlated the impact on catalyst initiation. Further, we show that in the case of strongly σ-donating and π-accepting ligands such as phosphites and isonitriles, the complex exhibits fluxional behavior. The catalysts ground state is elevated to such a degree that thiolate Ruthenium bonds become labile and rapidly exchange. While Ruthenium dithiolate catalysts were readily applied to metathesis polymerization, their use in the synthesis of small molecules was initially less forthcoming. Specifically, reactions involving terminal olefins lead to rapid catalyst deactivation and only low conversion. We were able to determine that the potential energy stored in the trans-influence between the thiolate ligand and the NHC can be released in a sulfur shift to reactive Ruthenium methylidene species. Since methylidenes are formed by reaction with terminal olefins, use of an excess of internal olefins can prevent their formation. Chapter 2. Harnessing Catalyst Fluxionality in Olefin Metathesis. Depending on its use, material requirements can vary significantly. Materials that can easily be adapted to a given application, for example by varying tensile strength, melting point or solubility, are desirable. Controlling the polymers tacticity (the adjacent stereocenters in a polymer chain) is a straight forward way to achieve just that. Ru dithiolate catalysts should give highly syndiotactic polymers due to their single stereocenter undergoing inversion during every metathesis step. The fluxional nature of the catalyst allows for control of polymer tacticity from 50% (atactic) to ≥95% syndiotacticity by changing monomer concentration. We determined the factors which are responsible for fluxionality and synthesized complexes that give either high or low levels of tacticity over a broader range of monomer concentration. Chapter 3. Harnessing Catalyst Fluxionality in Olefin Metathesis. The importance of fluorine-containing molecules is hard to understate, keeping in mind the surge of new methodologies for their synthesis and the medical breakthroughs they enable. However, efficient and practical syntheses of stereodefined alkenyl fluorides are rare. In this context, we have developed enantioselective boryl allylic substitution of allylic fluorides, which yield enantioenriched γ-alkenyl fluoride substituted allyl boronate esters. The reaction is catalyzed by Cu-based catalysts that are prepared in-situ and delivered as products with high yield and enantioselectivity. Mechanistic inquiry shows the reaction is not a concerted allylic substitution. An intermediate Cu alkyl complex is formed after the Cu boron addition is made to the double bond, which only slowly undergoes β-fluorine elimination in the presence of a Lewis acid.