Reactions of tert-butoxyl radicals with acyclic ethers studied by the radical trapping technique

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Abstract
1,1,3,3-Tetramethyl-2,3-dihydro-1H-isoindol-2-yloxyl has been used as a radical trap to investigate the pattern of hydrogen abstraction reactions occurring in a range of acyclic ethers by tert-butoxyl radicals. The results confirm the high degree of selectivity for abstraction at C atoms adjacent to ethereal O. The presence of two α ethereal O atoms is less effective in enhancing abstraction than one. This is possibly due to a stereoelectronic effect whereby the 1,3-oxygen orbital interactions reduce the ability of either oxygen to interact effectively with the developing radical centre. Whereas the presence of an α-oxygen enhances the rate of hydrogen abstraction, the presence of a β-oxygen retards the rate of hydrogen abstraction. Abstraction at a methine C atom adjacent to ethereal O occurs at about the same rate or a little faster than at a methylene C atom, whereas abstraction at methyl groups is much slower. A temperature study of the abstraction from methyl and methylene sites in dimethoxymethane shows that the difference in abstraction rates at these sites is predominantly an entropy effect. A lower entropy of activation due to the loss of the internal rotational mode of the methyl group in the formation of the transition state is the probable reason. Ethers appear to be less reactive than alcohols in hydrogen abstraction reactions.

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