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 S3N2(CH)2 (1), S3N3(CH) (2), and S3N4 (3) and 8-cycles S2N4(CH)2 (7) and S2N 2(CH)4 (8), all with 10 π electrons. The unknown trithiatetrazepine S3N4 is predicted to be at least as aromatic as its known diaza and triaza homologues. Angular-momentum arguments show that the π-electron-rich nature of (4n + 2) SN heterocycles is the key to their diatropic current. The Woodward dithiatetrazocine parent framework S2N4(CH)2 (7) supports a diatropic ring current, as does its analogue in which N and CH groups are formally exchanged. Formal expansion of (4n + 2)-π 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-π-electron heterocycle S6N12(CH) 6, formally derived from benzene, to be aromatic on the basis of this criterion.