The structure of sulfonium ylides stabilized by the malonylide moiety

The crystal structures of four thioxanthenium bis(carbomethoxy)methylides and methylides of di-(p-nitro-phenyl)sulfonium and dimethylsulfonium have been determined. The carbanion moiety in all cases is planar and the pi-system is rotated 90° with respect to the sulfur lone pair. All compounds exhibit a pseudo-equatorial orientation of the carbanion system except for the 2,4-dimethylthioxanthenium ylide. The compounds are described in terms of the geometries around the sulfur atom, the interplanar angles and sulfur-oxygen and oxygen-oxygen interactions. Surprisingly, the sulfur atom is not symmetrically placed with respect to the two phenyl rings in the thioxanthenium compounds. The sulfur is coplanar with one phenyl ring but is significantly out of the plane of the other. The ¹³C and ¹H spectra of the ylides are discussed along with the temperature dependence which indicates restricted rotation about the ⁺S? C^? bond in several cases. The factors contributing to the stability of the ylides are analyzed.