Effects of conjugation and aromaticity on the sulfoxide bond

An ab initio computational study on a series of sulfoxides is reported. The SO bond dissociation energy (BDE) of sulfoxides in which the sulfur atom is included in a formally aromatic ring (e.g., thiophene sulfoxide) is found to be decreased by as much as 25 kcal/mol, compared to DMSO. A complementary effect is observed for sulfoxides in which the sulfur is included in a formally antiaromatic ring (e.g., thiirene sulfoxide), in which SO BDEs are increased by as much as 15 kcal/mol. Both effects are attenuated by benzannulation. Examination of calculated geometries and isodesmic reactions with pure hydrocarbons leads to the conclusion that the observed effects are due to a severe disruption of the (anti) aromaticity of the sulfur-containing ring on oxidation. The cyclic sulfoxides appear to be neither significantly aromatic nor antiaromatic by energetic considerations. No significant SO bond strength effect is observed for simple conjugation. Inversion barriers for several sulfoxides are also calculated.