Aromaticity of organic heterocyclothiazenes and analogues.

Members of a series of carbon-poor sulfur-nitrogen heterocycles and polycycles are shown by direct ab initio ipsocentric calculation to support diatropic ring currents and hence to be aromatic on the basis of magnetic criteria. They include 7-cycles S(3)N(2)(CH)(2), S(3)N(3)(CH), and S(3)N(4) and 8-cycles S(2)N(4)(CH)(2) and S(2)N(2)(CH)(4), all with 10 pi electrons. The unknown trithiatetrazepine S(3)N(4) is predicted to be at least as aromatic as its known diaza and triaza homologues. Angular-momentum arguments show that the pi-electron-rich nature of (4n + 2) SN heterocycles is the key to their diatropic current. The Woodward dithiatetrazocine parent framework S(2)N(4)(CH)(2) supports a diatropic ring current, as does its analogue in which N and CH groups are formally exchanged. Formal expansion of (4n + 2)-pi carbocyclic systems by insertion of NSN motifs in every CC bond is predicted to lead to structures that support diatropic ring currents: explicit ab initio calculation of magnetic response predicts the 24-center, 30-pi-electron heterocycle S(6)N(12)(CH)(6), formally derived from benzene, to be aromatic on the basis of this criterion.