Introduction of a Single Electron-Withdrawing Carbon-Substituent into π-Deficient N-Heteroarenes by Means of Free-Radical Reactions: Strategies and Synthetic Applications

Abstract

π-Deficient N-heteroarenes, having two or more ring carbonatoms free that can be attacked by a nucleophilic carbon-centered radical, in general yield polysubstituted products in homolytic alkoxycarbonylation and homolytic aroylation reactions. It is demonstrated that the problem of multiple substitution can be overcome by performing the alkoxycarbonylation reaction in the presence of an appropriate organic solvent. Moreover it was found in pyrazine and pyridazine series that also the homolytic aroylation can be restricted to monosubstitution by employing N-heteroarene carboxylic acids; decarboxylation of the keto acids obtained provides convenient access to diaza-benzophenones. Substitution reactions by carbon-centered radicals thus are shown to be of high preparative value also with regard to the introduction of a single electron-withdrawing substituent into N-heteroarenes.