Factors controlling the selectivity of ipso-attack in homolytic aromatic substitutions. Reactions of alkyl radicals with nitrothiophen derivatives

Abstract

The reactions of the nearly electroneutral methyl and of the nucleophilic 1-adamantyl radical with some selected nitrothiophen derivatives have been investigated in order to elucidate the factors which control the positional selectivity of radical addition to an aromatic substrate. With 5-nitro-2-X-thiophens (II), (V), and (VI) and 3,5-dinitro-2-methoxycarbonylthiophen (III) the adamantyl radical gave exclusively the products of ipso-attack, whereas the methyl radical selectively added at the unsubstituted 4-position. On the other hand, with 4-nitro-2-methoxy-carbonylthiophen (I) both radicals added at the 5-position and with 4,5-dinitro-2-methoxycarbonylthiophen (IV) both radicals gave the products of ipso-substitution by displacing the nitro-group from the 5-position. These changes in positional selectivity are explained by assuming that the nature of the transition state of the addition step changes as a function of the polar character of the radical and of the electron deficiency of the aromatic substrates.