Reactions with Radical Reagents

Couplings requiring pyridyl organometallic species are best achieved with boron, zinc, or tin compounds the last are available either by reaction of pyridyl halides with sodium trialkylstannate or, in the opposite sense, by the reaction of a pyridyllithium with chlorotrimethylstannane.  

At temperatures where bromine (500°C) and chlorine (270°C) are appreciably dissociated into atoms, 2- and 2,6-dihalopyridines are obtained via radical substitution.  

This same preference for a-attack is demonstrated by phenyl radical attack, but the exact proportions of products depend on the method of generation of the radicals. Greater selectivity for phenylation at the 2- and 4-positions is found in pyridinium salts.  

Of more preparative value are the reactions of nucleophilic radicals, such as HOCH2 and R2NCO which can be easily generated under mild conditions. These substitutions are carried out on the pyridine protonic salt, which provides both increased reactivity and selectivity for an a-position the process is known as the Minisci reaction (cf. section 2.4.1). It is accelerated by electron-withdrawing substituents. 

Intermediate, reduced dimers can be trapped under milder conditions, and reduced monomers when the pyridine carries a 4-substituent.  

Phenyl radicals generated by the decomposition of dibenzoyl peroxide attack quinoline and isoquinoline with formation of mixtures of all the isomeric phenyl-substituted products. Much more discriminating substitutions can be achieved with more nucleophilic radicals in acid solution (cf. section 2.4.1).  

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Fig. (15). Photoreaction of artonin E (7) and the reaction with radical reagent.

Pyridines undergo a variety of reactions with radical reagents, and at surfaces many of these parallel the corresponding reactions of benzenes however, the reaction of nucleophilic radicals with protonated pyridines is important (the Minisci reaction). Electron uptake from a metal to form a radical anion occurs readily. 

Reactions with radical reagents reactions of pyridyl radicals... 

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