Five-membered heterocycles. Part V. Impact of heteroatoms on aromaticity of five-membered heterocycles

The aromaticity(in the form of HOMA index) of various five-membered heterorings systems and their bicyclic derivatives containing N, S, 0, and P were studied on the basis of statistical data from Cambridge Structural Database (CSD). The calculations of HOMA were done for pyrroles, furans, thiophenes, phospholes, imidazoles, oxazoles, thiazoles, phosphazoles as well as for 6 + 5 fused systems: benzimidazoles, benzoxazoles, benzothiazoles, and benzophosphazoles. Detailed analysis showed that relationship between classical aromaticity and electronegativity of heteroatom is described by squared equation. An optimal value of electronegativity is about 2.9 (Pauling's scale) for which aromaticity of heterorings reaches the highest level. For fused systems, the presence of benzene ring causes the: evident increasing of average value of HOMA indices.