Kinetics and mechanism of intermolecular enyne metathesis
Galan, Brandon Ryan
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In chapter one of this dissertation the kinetics and mechanism of intermolecular enyne metathesis were investigated. Kinetic data were obtained using FT-IR spectroscopy by monitoring the terminal C-H stretching frequency of the alkyne partner. The results suggest that the substitution pattern at the propargylic position of the alkyne influences the rate of metathesis. Tri-substituted alkynes underwent enyne cross metathesis approximately 20 times faster when compared to the di-substituted alkynes. Different rate laws were also observed when the alkene partner was changed from 1-hexene to ethylene. A mechanism for intermolecular enyne metathesis had been proposed based on our observations. In chapter two of this dissertation the decomposition of Grubbs' second generation carbenes was explored. Small ligands that contain acceptor properties such as carbon monoxide, arylisocyanides, and alkylphosphites promote decomposition of the ruthenium carbenes by a novel Buchner insertion reaction. The ruthenium carbene inserts into the mesityl group of the N -heterocyclic carbene ligand to yield a 1,3,5-cycloheptatriene moiety. Our results suggest that steric and electronic properties of both the starting material and incoming ligand play a large factor in whether Buchner insertion will occur. The insertion products were isolated and characterized using NMR, IR, and x-ray crystallography. A mechanism for the ligand promoted Buchner insertion has been proposed based on our observations. In chapter three of this dissertation a cleanup procedure for the removal of unreacted catalyst and ruthenium byproducts from metathesis reactions was developed using the knowledge of carbene reactivity obtained from Chapter two. A polar isocyanide derived from glycine was synthesized and used to quench metathesis reactions. The isocyanide rapidly quenched the active catalyst and the resulting polar ruthenium complex was easily removed by column chromatography. Ruthenium levels in the isolated products were comparable or better than the values reported in the literature using other clean-up procedures. A variety of commonly used metathesis catalysts were evaluated and found to work exceptionally well with our clean-up protocol.