Synthetic and Mechanistic Studies in Ruthenium-catalyzed Olefin Metathesis

Abstract

Ruthenium - catalyzed olefin metathesis is now an invaluable tool in organic synthesis. However, routes to the dominant metathesis catalysts, the second - generation Grubbs and Hoveyda catalysts (RuCl 2 (PCy 3 )(H 2 IMes)(=CHPh) and RuCl 2 (H 2 IMes)[= CH( o - O i Pr)C 6 H 4 ], respectively) are plagued with problems. The common reliance on in situ methods to generate the N - heterocyclic carbene H 2 IMes severely limits stoichiometric control, and results in contamination by byproducts, some of which are readily overlooked, and some of which are difficult to remove. Both can affect batch - to - batch reproducibility in catalysis. This thesis work demonstrated that widespread perceptions of the instability of free H 2 IMes are erroneous, and that the free carbene is readily handled under water - free conditions. Clean, convenient, near - quantitative routes were developed to these second - generation catalysts by ligand exchange of their first - gen eration counterparts RuCl 2 (PCy 3 ) 2 (=CHPh), RuCl 2 (PCy 3 )[= CH( o - O i Pr)C 6 H 4 ] with free H 2 IMes, with sequestration of the liberated phosphine by an ion - exchange resin. A second focus was examination of a much - debated hypothesis in olefin metathesis: that is, the extent to which the high productivity of the Hoveyda catalysts reflects re - uptake of the styrenyl ether functionality released in the initial cycle of metathesis. Current evidence for and against this "boomerang" hypothesis is critically examined, and new approaches to examining its operation are described. Specifically, the rate of decomposition, vs. re - uptake, is examined for the active species RuCl 2 (PCy 3 )(=CH 2 ), and background exchange of the parent catalyst with free styrenyl ether is measured by use of a 13 C - labelled styrenyl ether. These studies confirm the relevance of the boomerang mechanism for first - generation Hoveyda catalysts.