Radicals 2-hydroxyphenoxyl

Radical reduction is followed by a rapid reaction of the 2-hydroxyphenoxyl radical with the boronate 46. In this manner, chain propagation is ensured by the regeneration of the initial alkyl radical and the formation of Meulenhoff s free acid 47 (Scheme 40). 

Quantitatively, however, the meta isomer is different in that the spin density on its oxygen atoms is at least 3 times smaller that the 60-65% spin density found on the oxygens of o- and p-semiquinones. Protonation of the destroys this equivalence so that the hydroxyphenoxyl radicals shown below ... 

Intermolecular proton exchange between 3,6-di-f-butyl-2-hydroxyphenoxyl radical and a variety of organic acids and bases was studied under various conditions . Line broadening was analyzed in terms of a mechanism shown in equation 30 for... 

Substituents exert pronounced effects on the absorption spectra. In general, meta- and /tara-substituted phenoxyl radicals exhibit absorption maxima at higher wavelengths than the ortho analogues. On the other hand, the /tara-substituted phenoxyl radicals have extinction coefficients considerably higher than those of either the ortho or the meta analogues. These general trends hold for the methyl-, bromo-, methoxy- and hydroxyphenoxyl radicals, and for the semiquinones. 

The m-benzosemiquinone radical anion exhibits a Raman spectrum that has a CO stretching frequency similar to that of phenoxyl radical and another band at a much lower frequency that is ascribed to a second CO stretching. This suggested that the two CO groups are not equivalent and that the m-benzosemiquinone anion is more similar to a 3-hydroxyphenoxyl. ESR spectra of the m-semiquinone anion, however, indicate complete symmetry, probably due to rapid spin interchange. 

The first ttvo examples are the H-transfers in 2-hydroxyphenoxyl radicals which have been studied using dynamic EPR spectroscopy. When 3,6-di-tert-butyl-2-hydroxyphenoxyl and its deuterated analog are dissolved in heptane the Arrhenius diagram of Fig. 6.50 was obtained by Bubnov et al. [85]. The kinetic isotope effect is about 10 at room temperature. Setting the pre-exponential factor to lO 6 (Table... 

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