Alternative mechanisms of electrophilic substitution in azole series

Data on three mechanisms of aromatic electrophilic substitution in azole series have been presented. First of them is similar to an ordinary addition-elimination mechanism including the formation of a cationic sigma-complex (Wheland intermediate). The second mechanism is realized according to the elimination-addition scheme and includes a protonation or a formation of a complex on the "pyridine" N atom, proton abstraction from the C atom adjacent to the above N atom, and addition of a cationoid reagent to the ylide (carbene) formed. The third mechanism proposed by the present authors can be realized for azoles having three and two N atoms of the pyridine type. It does not require the preliminary N-protonation or complex formation and occurs due to a strong electron withdrawing effect of several "pyridine" atoms, resulting in the CH-deprotonation followed by the interaction of the carbanion formed with a cation under mild conditions.