Stereoelectronic effects in α-carbanions of conformationally constrained sulfides, sulfoxides, and sulfones

摘要

Stabilizing effects in oxygenated thiane- and 1,3-dithiane-derived α-carbanions were investigated computationally. All isomers bearing sulfide, sulfoxide, and sulfone functional groups and combinations thereof were investigated by DFT calculations and NBO analyses. Their stabilities and the stereoelectronic effects present in these compounds were compared. Stabilizing effects of the respective functional groups were further estimated by calculation of isodesmic reactions. It turned out that n_C → σ*_(S-O) interactions, where both the n_C and the S - O bond are in an antiperiplanar conformation, have the highest stabilizing effects. Similar stabilizing interactions are effective in carbanions with an equatorial lone pair at the carbon; here a productive n_C → σ*_(S-C) interaction is possible. n_C → σ*_(S-O) interactions, where the S - O bond is part of a sulfoxide are significantly more effective than in sulfones. Calculations of isodesmic reactions show similar trends but suggest the presence of additional electrostatic effects and possibly, to some extent, steric effects.