ABSTRACT
The 1,3- and 1,2-azoles each contain one hetero atom in an environment analogous to that of the nitrogen in pyridine - an azomethine nitrogen - and one hetero atom in the environment of the nitrogen in pyrrole, the sulfur in thiophene, or the oxygen in furan, respectively. The 1,2-azoles tend to be less nucleophilic and less basic at the azomethine nitrogen than their 1,3-isomers. Resonance contributors to the intermediates for such substitutions make the reason for this plain: the azomethine nitrogen can act as an electron sink for attack only at these positions. In further analogy with pyridines, the quaternisation of the azomethine nitrogen makes such deprotona-tions even easier; the resulting enamines react with electrophiles at the side-chain carbon. Continuing the analogy with pyridine reactivity, methyl groups at the 2-positions of 1,3-azoles and the 3-positions of 1,2-azoles carry acidified hydrogen atoms and can be deprotonated with strong bases.
