Stereoselective Ruthenium-Catalyzed Olefin Metathesis

Citation

Ward, Donald William (2004) Stereoselective Ruthenium-Catalyzed Olefin Metathesis. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/zxw4-av70. https://resolver.caltech.edu/CaltechETD:etd-07152003-183214

Abstract

Ruthenium complexes ligated with N-heterocyclic carbenes, such as (IMesH2)(PCy3)(Cl)2Ru=CHPh [IMesH2 = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene], constitute the latest class of olefin metathesis catalysts and are particularly desirable for their high activity and tolerance of most common functional groups. This thesis primarily describes studies aimed at understanding and controlling the stereoselectivity of this type of complex, including enantioselectivity and cis/trans selectivity.

Chapter 2 describes the basic design of novel chiral N-heterocyclic carbene ruthenium complexes active for olefin metathesis. A series of catalysts based on this design is prepared and studied by variable-temperature NMR spectroscopy and X-ray crystallography.

Chapter 3 relates the utilization of these chiral ruthenium-based olefin metathesis catalysts in enantioselective desymmetrization reactions of achiral trienes to produce cyclic ethers. Selectivity trends are identified and catalysts are optimized with the best result showing a 90% enantiomeric excess of product. A stereochemical model is proposed based on the outcome of these reactions.

Chapter 4 relates efforts to develop a useful test for measuring the inherent stereoselectivity of a wide array of ruthenium-based olefin metathesis catalysts by converting them to relatively inactive Fischer-alkylidenes in a single-turnover reaction with dihydrofuran. A group of approximately twenty olefin metathesis catalysts is tested with this technique, and the resulting data are found to correlate well with the results of the ring-closing metathesis of macrocycles. Several trends are discussed, and a stereochemical model consistent with the results of these reactions is described.

Chapter 5 details a novel route for the synthesis of telechelic polymers through the ring-opening metathesis polymerization (ROMP) of highly strained bicyclic monomers.

Thesis Files