Graduate Thesis Or Dissertation
 

Atom abstraction from excoyclic postitons of derivatives of methylated pyridines and quinolines : an evaluation of change separation in the transition states of these reactions

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  • The relative rates of hydrogen atom abstraction from a series of 13 homoaryl and heteroarylmethanes by the nucleophilic undecyl radical, thermally generated from lauroyl peroxide, were determined at 70 °C. A reactivity range of 15 was obtained. Substantial differences in reactivity among isomers were found. The relative reactivities of the compounds studied were correlated with SCF-PPP calculated energy differences. Modest correlation was obtained when a carbanion model was used for the transitions state (r = 0.88). Much poorer correlations were obtained when either a carbocation or radical was used to model the transition state. Best correlations were obtained when the logarithms of the relative rates of hydrogen atom abstraction are plotted against those for base catalyzed hydrogen-deuterium exchange (r = 0.91). These results are suggestive of substantial negative charge development in the transition state. The free-radical reduction of all isomers of chioromethylpyridine and chloromethylquinoline with triphenyltin hydride at 70 °C was also investigated. Small differences in reactivity among isomers were found. A reactivity range of ca. 4.5 was obtained. This is much smaller than the reactivity range found for hydrogen atom abstraction from methylpyridines and methylquinolines by undecyl radical. This is rather surprising since Hammett studies suggest that the selectivities in these two systems should be at least comparable. It is felt that the rate determining step might involve an electron transfer rather than direct atom abstraction. Good results (r = 0.94) are obtained for correlation involving plotting the logarithms of the experimental relative rates against the logarithms of the corresponding calculated relative rates using a four parameter equation based on SCF relative LUMO energy differences of the parent unsubstituted systems and SCF total energy differences for a carbanion model.
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