Journal of the American Chemical Society vol:126 issue:36 pages:11202-11212
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 S2N2(CH)(4) (8), all with 10 pi 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 pi-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)-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 S6N12(CH)(6), formally derived from benzene, to be aromatic on the basis of this criterion.