An evaluation of charge generation in hydrogen atom abstraction processes: the reaction of homoaryl and heteroarylmethanes with t-butoxy radical Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/cc08hh95g

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  • The relative rates of hydrogen atom abstraction from a series of 13 homoaryl and heteroarylmethanes by t-butoxy radical were determined at 70°C. The attacking radical was photochemically generated from di-t-butyl peroxide. A relatively small range of relative rates (factor of seven) has been found among the compounds studied. The relative reactivities of the homoaryl and heteroarylmethanes can be correlated with various calculated energy differences obtained from both HMO and standard semi-empirical SCF-MO methods. Fairly good correlations were obtained using a carbocation model for the transition state (correlation coefficient of 0.92). Much poorer correlations were obtained when either a radical model was used for the transition state (correlation coefficient of 0.46) or when ground state properties, such as charge density, were used (correlation coefficient of 0.53). This suggests, at best, only partial ground state control for this reaction. The relative reactivities of all compounds were also studied using empirically derived four parameter equations. A very poor correlation was obtained for that approach which completely neglected charge separation in the transition state. However, good correlations were obtained with those four parameter equations which included cationic character in the transition state. Present use of nonalternant heteroarylmethyl systems strongly suggests the importance of appreciable positive charge development in the transition state of hydrogen atom abstraction processes. Optimum correlation of the present data was with the relative reactivities for pyrolytic decomposition of corresponding 1-aryl and 1-heteroarylethyl acetates. The coefficient for this correlation was 0.97.
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