Neutral Ambident Radicals

The best-known neutral ambident radicals are phenoxy radicals. Many substituted phenoxy radicals couple to give the polymers in wood, and dimerise or couple intramolecularly in the biosynthesis of a number of alkaloids, It is believed that many phenoxy radical couplings are actually the coupling of radical with radical and not of radical with neutral molecule, although the attack of a radical on a phenate ion may occasionally be an important pathway. A radical coupling with a radical is inherently a very fast process, since it is so exothermic, but only 

One simple example of much-preferred para attack is in the Elbs persulfate oxidation of phenol. The reaction is usually written as the combination of a phenoxy radical 7.123 with the persulfate radical anion 7.126, but it could be phenoxide ion undergoing electrophilic substitution in any event, para attack is substantially favoured over ortho attack.1 

A phenoxy radical conjugated into a side chain 7.127 is derived from coniferyl alcohol. Here we would expect the /3-carbon of the styrene unit to be the site of highest odd-electron population. The three major products, 7.128, 7.129 and 7.130, shown with the critical bonds in bold, are formed by reaction at this site in steps that are important because they model the reactions involved in lignification.1054 

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Some neutral radicals are ambident, but not much is known about this subject. The argument is a simple one—the site of reactivity should be largely determined by the coefficients of the SOMO. 

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