Free radical, arylation methylation

In general, if condensation polymers are prepared with methylated aryl repeat units, free radical halogenatlon can be used to introduce halomethyl active sites and the limitations of electrophilic aromatic substitution can be avoided. The halogenatlon technique recently described by Ford11, involving the use of a mixture of hypohalite and phase transfer catalyst to chlorinate poly(vinyl toluene) can be applied to suitably substituted condensation polymers. 

Free radical substitution of pyridines usually occurs principally at position 2 (Table 25), which is in agreement with theoretical calculations (69CCC1110). 2-Substitution is more favored in methylation than in phenylation of pyridine. This suggests that the methyl has more nucleophilic character than the phenyl radical. Furthermore, methylation of pyridine in acidic solution gives 13-fold excess of 2- over 4-substitution, although the overall yield is low. Alkyl and aryl radicals have been generated from diverse sources (Table 25). 

In the presence of iron(II) sulfate, /cH-butyl hydroperoxide decomposes to yield acetone and methyl radicals, For a discussion of the kinetics and mechanism of purine C8 methylation by methyl radicals see ref 50. Reaction of adenine with benzenediazonium ions yields ( )-6-(3-phenyl-2-triazen-l-yl)purinc (7, Ar = Ph), which undergoes a C-arylation reaction, probably involving an intermolecular free-radical substitution at C8, to give 8. " ... 

An example of conjugate free-radical addition to methyl acrylate mediated by a copper Lewis acid has been reported (Sch. 30) [65]. In this example the Lewis acid 127 activates the substrate for conjugate addition by the aryl radical which is followed by an enantioselective chlorine atom-transfer step. Chemical and optical yield for the transformation are both low. 

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