Five-membered heterocycles. Part IV: Impact of heteroatom on benzazole aromaticity

To complete a set of 3-D data for phenoxymethyl–benzazoles, molecular structures of respective benzothiazole and benzimidazole were determined using X-ray crystallography. Based on that, the structure and conformation of the benzazole bicycle (6 + 5) with various heteroatoms in position 3 – nitrogen, sulphur or oxygen – are discussed. Subsequently, aromaticity of (6 + 5) bicycles was analyzed on the basis of crystallographic data (our and from CSD) and HOMA index, quantitative estimation of heteroring aromaticity, was calculated. The rings aromaticity is amplified from oxygen to sulphur and nitrogen as the second heteroatom. The dependences between HOMAav indices and respective heteroatom electronegativity were roughly established as second-degree polynomial with maximum in the vicinity of nitrogen. Similar relationship was found between HOMAav and lipophilicity descriptor log D (at pH 7.4) for phenoxymethyl–benzoxazole, –benzothiazole, –benzimidazole. Both relationships explain the categorization of the species with various Y according to the location in periodic table of the elements.