Substitutions Involving Aryl Free Radicals

SECTION 7.3 SUBSTITUTIONS INVOLVING ARYL FREE RADICALS... 

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. 

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. " ... 

The diazonium group can be replaced by a number of groups. " Some of these are nucleophilic substitutions, with S l mechanisms (p. 432), but others are free-radical reactions and are treated in Chapter 14. The solvent in all these reactions is usually water. With other solvents it has been shown that the Sj-jl mechanism is favored by solvents of low nucleophilicity, while those of high nucleophilicity favor free-radical mechanisms. The N2 group can be replaced by CP, Br, and CN, by a nucleophilic mechanism (see OS IV, 182), but the Sandmeyer reaction is much more useful (14-20). Transition metal catalyzed reactions are known involving aryl-diazonium salts, and diazonium variants of the Heck reaction (13-10) and Suzuki coupling (13-12) were discussed previously. As mentioned on p. 866 it must be... 

Vinyl and aryl halides also add to the metals such as Ni(0), Pd(0) and Pt(0) regiose-lectively. In oxidative addition of aryl halide, a nucleophilic substitution mechanism is also proposed. Sometimes electron transfer processes are also considered to be involved in oxidative addition, which gives anion radicals or free radicals. However, these processes sometimes lose stereoselectivity. In general, oxidative additions take place by a variety of mechanisms. 

An early report of the beneficial influence of 1,1-diphenylethylene (DPE) on the yields of alkylaryl ether obtained in the reaction of diaryliodonium salts with sodium alkoxides showed that radical chain reactions compete efficiently with the 0-arylation reaction. By contrast, addition of diphenylpicrylhydrazyl, a stable free radical species, had no significant influence on the yields of products obtained in e absence of additives. In this case, the 0-arylation reaction was considered to be a direct nucleophilic aromatic substitution reaction, without the involvement of any transient covalent intermediate. (Table 2.11)... 

Such a mechanism was unlikely as addition of an external trap, 1,1-diphenylethylene, had no effect on the course of the arylation of p-ketoesters.i l A second approach involved the use of an internal trapping system which had been successfully used in the study of the radical reactions of arenediazonium salts.The internal trap containing reagent, ( rf/io-allyloxyphenyl)lead triacetate (94), can be easily prepared from the corresponding boronic acid. 2 Reaction with various types of nucleophiles, such as ethyl 2-oxocyclopentanecarboxylate (86), mesitol (36), the sodium salt of nitropropane, iodide and azide always afforded the C-arylation products in high yield. No trace of the 3-substituted dihydrobenzofurans, expected in a mechanism involving the intermediacy of free radicals, could be detected. 

Even though the Jourdan-Ullmann condensation has been known for more than a century, the mechanism is still not quite clear. On the one hand, the reaction may proceed via a free-radical mechanism, pertaining to the reductive dehalogenation of aryl halides and the acceleration of the reaction rate by ultraviolet irradiation, as outlined in Scheme 1. On the other hand, the reaction may involve halonium and proceed as a simple aromatic nucleophilic substitution, as displayed in Scheme 2. However, for the reaction of 6>-halobenzoic acid, it is believed that the copper ion coordinates with both carboxyl and... 

---