Abstract
Alkyl radicals and acyl radicals have a nucleophilic character; therefore, radical alkylation and acylation of aromatics show the opposite reactivity and selectivity to polar alkylation and acylation with the Friedel–Crafts reaction. Thus, alkyl radicals and acyl radicals do not react with anisole, but may react with pyridine. Generally, radical alkylation or radical acylation onto aromatics is not a radical chain reaction, because it is just a substitution reaction of a hydrogen atom of aromatics by an alkyl radical or an acyl radical through the addition–elimination reaction. Therefore, the intermediate adduct radical (σ complex) must be re-aromatized to form a product and a hydrogen atom (or H+ and e–). Thus, this type of reactions proceeds effectively under oxidative conditions. The Friedel–Crafts reaction is polar (ionic) alkylation or acylation of electron-rich aromatics by alkyl cation or acyl cation species, derived from the reactions of alkyl halides or acyl halides with AlCl3. Therefore, electron-rich aromatics such as anisole are very reactive, but electron-deficient aromatics such as pyridine are inert. From the practical point of view, alkylation of heteroaromatics with Barton decarboxylation of N-acyloxy-2-thiopyridones prepared from carboxylic acids and N-hydroxy-2-thiopyridone is very useful, because it can be used for various kinds of carboxylic acids such as sugars and nucleosides. This reaction comprises the initial homolytic cleavage of the N–O bond in N-acyloxy-2-thiopyridone to form an acyloxyl radical and PyS˙, β-cleavage of the acyloxyl radical to generate an alkyl radical and CO2, in addition to the electron-deficient position of heteroaromatics by the alkyl radical to form the adduct, and finally, abstraction of a hydrogen atom from the adduct by PyS˙.
Published Version
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