Intramolecular transfer of complexed radicals as a possible explanation for long-range substituent effects observed in hydrogen abstraction reactions Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/tm70mz469

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  • The relative rates of hydrogen abstraction from 1-phenylalkanes and a,w-diphenylalkanes using N-bromosuccinimide and bromotrichloromethane were determined at 70°C. In both cases the reactions were photoinitiated. Only benzylic bromination was observed. The 1-phenylalkanes reacted with both reagents to give rates largely independent of substrate. Similarly, the rate of the reaction between the a, w-diphenylalkanes and bromine atom showed little dependence on substrate structure. However, for the reaction of the a, w-diphenylalkanes with trichloromethyl radical, the rates were strongly dependent upon the structure of the substrate. A maximum was found for 1, 6-diphenylhexane. These last results suggest the possibility of hydrogen abstraction by a route where the radical is first complexed with the aromatic ring at one end of the substrate and then intramolecularly transferred to the site of hydrogen abstraction at the opposite end of the substrate. The relative rates of hydrogen abstraction from 4-methyl-1-phenyl-4-(substitutedphenyl)pentanes by trichloromethyl radical were also determined under the same conditions. Application of the Hammett equation yielded a rho value of -0.40 for this reaction. Application of the Taft equation yielded rho values of -1.34 or -1.54 depending upon the Taft substituent constants used. The results for both the Hammett and Taft equations suggest a significant long range substituent effect. To explore the source of this effect, the relative rates of hydrogen abstraction from 4-phenyl-1-substitutedbutanes by trichloromethyl radical were also determined. Application of the Taft equation yielded a rho value of only -0.32. This substituent effect probably arises from inductive interactions between the substituent and the transition state of hydrogen abstraction. The large difference between the Taft rho values obtained for the reaction of the 4-methyl-1-phenyl-4-(substitutedphenyl)pentanes and the 4-phenyl-1-substitutedbutanes with bromotrichloromethane suggest an additional mechanism operative in the former system. A significant amount of that reaction may follow a path where the abstracting radical first forms a pi complex with the substituted phenyl ring of the substrate and then is intramolecularly transferred to the site of hydrogen abstraction.
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