Basc-catalyzed hydrogen-deuterium exchange in benzo derivatives of five-membered aromatic heterocyeles. Part III. A comparative and systematic study of the i-beuzothiopbene,benzofuran, henzotbiazole and benzoxazole systems

The rates of base-catalyzed hydrogen-deuterium exchange at C-2 in 1-benzothiophene, benzofuran, benzothiazole and benzoxazole were studied. For benzothiophene benzofuran the relative second-order rale constants of hydrogen-deuterium exchange were 4:1, while for benzothiazole and benzoxazole they were 1:20. These data can be explained by assuming that the sulphur heteroatom interacts electronically with the atom in position 3 of the five-membered ring. While, very probably, the d-orbital conjugation cannot depend much on the: atom in position 3, the polarization phenomena can vary more with the atom in position 3. For this reason it was thought that the electronic activity ol the sulphur heleroatom is due to polarization phenomena rather that d-orbilal interaction.