Improving Selectivity in Olefin Metathesis for Small Molecule Synthesis and Materials Applications

Citation

Li, Jean (2012) Improving Selectivity in Olefin Metathesis for Small Molecule Synthesis and Materials Applications. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/JVC6-7A52. https://resolver.caltech.edu/CaltechTHESIS:10162011-170949582

Abstract

The olefin metathesis reaction has been studied extensively from the perspective of catalyst design and synthesis, as well as from that of reaction control and application in a variety of fields. Beginning with the design of enantioselective catalysts based on the “geared” C2-symmetric N-heterocyclic carbene (NHC) containing ruthenium catalysts, architectural modifications were envisioned and implemented in order to control the N-bound arene tilt angle (Chapter 2). From there, the asymmetric class of olefin metathesis reactions were explored and trends in enantioselectivities were obtained and reapplied in the further design of novel, chiral catalysts. These asymmetric catalysts were developed not only for their useful application in a range of asymmetric metathesis reactions, but also to provide insight into the spatial arrangement of the NHC ligand during the catalytic cycle.

Amidst this overall cyclical process, mechanistic understandings of the ruthenium-based olefin metathesis catalysts were garnered and integrated; and the concept of a covalentlylinked NHC ligand was born. In Chapter 3, both the cis-fused and trans-fused versions of this linked NHC were constructed, with each independent synthesis hinging on a key ringclosing metathesis reaction mediated by ruthenium catalysts. These novel NHCs were then translated into rhodium-bound complexes and their unique structural conformations were studied with X-Ray crystallography.

Chapter 4 explores the forefront of control and selectivity in olefin metathesis, specifically, in the selective reactivity of dienes in cross-metathesis reactions. The desire to synthesize conjugated dienes, thus limiting reactivity to one of two potentially reactive olefins, is both mechanistically intriguing and contains practical applications for the synthesis of linear pheromone natural products. These pheromones show great utility as a green, biorational pesticide with few ecological and biological side-effects. Thus, exploration of the general diene cross-metathesis reaction was focused on the actual synthesis of codlemone, one of the world’s most sought-after insecticides.

The potential of the olefin metathesis reaction for biomedical applications was further explored in the application of peptide-containing polynorbornenes formed from ringopening metathesis polymerization (ROMP). In order to apply synthetic materials made from ROMP in biological applications, a route towards ruthenium removal to the FDAapproved levels of 10 parts per million (ppm) was developed. Once low ppm remnant ruthenium content was obtained, the synthesis of varying monomers for crosslinking to bulk materials was explored.

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