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 sul fur atom is included in a formally aromatic ring (e.g., thiophene sulf oxide) 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 sul foxide), in which SO BDEs are increased by as much as 15 kcal/mol. Bot h 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 o f the (anti) aromaticity of the sulfur-containing ring on oxidation. T he cyclic sulfoxides appear to be neither significantly aromatic nor a ntiaromatic by energetic considerations. No significant SO bond streng th effect is observed for simple conjugation. Inversion barriers for s everal sulfoxides are also calculated.