The Development of Water-Soluble Olefin Metathesis Catalysts Containing an N-Heterocyclic Carbene Ligand

Citation

Jordan, Jason Paul (2008) The Development of Water-Soluble Olefin Metathesis Catalysts Containing an N-Heterocyclic Carbene Ligand. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/B57A-ZS49. https://resolver.caltech.edu/CaltechETD:etd-07182007-154911

Abstract

This research describes the development of new olefin metathesis catalysts that are stable and active in water. Earlier water-soluble metathesis catalysts rely on phosphine ligands functionalized with ionic groups. In contrast to these bis(phosphine) complexes, the catalysts reported in this research harness the increased stability and activity provided by N-heterocyclic carbene (NHC) ligands. As a result, these catalysts display an activity and stability that are unprecedented in aqueous olefin metathesis.

Initial efforts to produce the desired water-soluble metathesis catalysts employed poly(ethylene glycol) (PEG) to generate a complex that was soluble in water (Chapter 2). This catalyst was capable of the ring-opening metathesis polymerization (ROMP) of a challenging endo-norbornene monomer, which an earlier bis(phosphine) complex catalyzed poorly. While demonstrating the potential of NHC ligands to improve the activity of water-soluble metathesis catalysts, this catalyst was not sufficiently stable to mediate metathesis transformations involving acyclic olefins in water.

A careful examination of the described PEG catalyst inspired a few strategies to produce olefin metathesis catalysts with improved activities in water (Chapter 3). This strategic vision was honed by studies examining the effect of water on the decomposition of catalysts that contain an NHC ligand (Chapter 4). These studies indicated that phosphine ligands play an active role in the aqueous decomposition of ruthenium methylidene complexes, which are vital complexes for metathesis reactions involving terminal olefins. With these results in hand, incorporating NHC ligands into phosphine-free ruthenium complexes was pursued as a promising approach to producing active metathesis catalysts that are stable in water.

Catalysts supported by both isopropoxybenzylidene and NHC ligands were modified to include ammonium salts (Chapter 5). The water-soluble catalysts produced were stable in water and competently initiated aqueous ROMP. More importantly, these catalysts readily catalyzed the ring-closing metathesis of [alpha],[omega]-dienes in water. The described catalysts were also able to homodimerize allyl alcohol and homoallyl alcohol in water, which are among the few known examples of cross metathesis in neat water.

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