Radical umpolung reactivity

In Chapter 30, you came across the idea of umpolung, the inversion of the usual reactivity pattern of a molecule. You may have already noticed that radicals often have an umpolung reactivity pattern. Alkyl halides are electrophiles in polar reactions yet they generate nucleophilic radicals that react with electrophilic alkenes. 

A synthetically very potent and unique feature of organic electrosynthesis is the oxidative or reductive Umpolung of reactivity. Reactive acceptors are anodically available as radical cations in a wide variety by the oxidative Umpolung of donors. This way two donors can be coupled in one step if one of them is converted to an acceptor at the electrode. Chemically, at least two additional... 

Reactive donors can be generated ca-thodically by reductive cleavage of hahdes to carbanions or by reduction of double bonds to radical anions. Using these methods, two acceptors can be dimerized in one step by reductive Umpolung , for example, two molecules of acrylonitrile to adipodini-trile - a reaction, which normally needs two or more steps. 

The lack of reactivity of 3-halo substituents under non-radical nucleophilic substitution conditions allows differential functionalization of pyri-dines by 3-umpolung and 2-nucIeophilic substitution processes. Thus, treatment of 2-fluoro-3-iodopyridine (189) with oxygen or amine nucleophiles affords products 191 which, upon subjection of SRN1 reactions with carbon, phosphorus, and sulfur systems, give 2,3-difunctionalized pyri-dines 192 (Scheme 56) (88JOC2740). 

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