Substitution, radical leaving group effects

Eq. 5.17 shows ipso substitution of imidazoles (30) at the 2-position by an sp3 carbon-centered radical. Here, the tosyl group plays an important role in both activation of the ipso 2-position and as an effective leaving group, Ts [40-46]. The phosphoryl group also shows the same leaving ability as the tosyl group. Preparation of [1.2-b]-fused bicyclic pyrazoles (33) from l-(phenylselenoalkyl)pyrazoles (32) also works well, as shown in eq. 5.18. 

Traylor et al. have described studies designed to trap the active species in the catalytic reaction of several iron porphyrins with peracid at room temperature using 2,4,6-tri-tert-butyl phenol (15) and observed an important role of acid on the rate of the oxidation of 15 (72). Lee and Bruice have used a similar method to examine the effect of peroxides on the rate of formation of the corresponding phenoxy radical catalyzed by metalloporphyrins (73). A good correlation between the pKa of the leaving group, a substituted benzoic acid, and the oxidation... 

The Skv I radical chain mechanism lor nucleophilic substitution [I70. as illustrated generally in eqs (2.64a-c), has been absent from our discussions. This mechanism has been shown to occur itt malty displacement reactions of leaving groups front both aromatic and aliphatic substrates. I low ever, in most of the aliphatic eases the substrates have a nitro or nilrophcnyl group (or other effective electron acceptor) in the a-position to the leaving group. The combination of I he nucleophile with the radical from the substrate must also lot m a relatively stable radical-anion in step 2.6lb. which is capable of propagating the chain... 

The synthesis of the asymmetrically substituted monomer, which used to be a major challenge, is found to be possible with an unexpected high selectivity [1001] (Fig. 84). The effect of the polarizer and leaving group on the polymerization reaction has been studied by Issaris et al. [1002]. Although at first indications were found for a free-radical mechanism in the polymerization process... 

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