Experimental and Theoretical Investigation of Ligand Effects in Ruthenium-Catalyzed Olefin Metathesis

Abstract

Olefin metathesis, a powerful means of assembling carbon frameworks, was recognized with the 2005 Nobel Prize in Chemistry. Metathesis catalyst RuCl(OC6Br5)(IMes)(py)(=CHPh) (C) exhibits outstanding performance in macrocyclization and enyne metathesis. This thesis established the trans-anionic geometry of C, and undertook a computational study of its cycloaddition behaviour relative to models RuCl 2L(PMe3)(=CH2) (A: L = PMe3 , B: IMe). Both A and B undergo loss of a stabilizing Cl-Ru pi-donation during cycloaddition, but this is almost completely compensated for in B by enhanced Cl-Ru electrostatic attraction (a consequence of the poor charge-donor capacity of the NHC ligand, which results in a more electropositive Ru center). This poor donor capacity is also important in C, as the NHC ligand is unable to offset the electron-withdrawing effect of the aryloxide. Instead, stronger ethylene binding results. This flattens the energy profile for cycloaddition, accounting for the very high reactivity of C once initiation has commenced.