Crystal structure and nuclear magnetic resonance and infrared spectra of 9,9-dideuteriothioxanthenium bis(carboethoxy)methylide, dimethyl 9-(1,4-dimethylthioxanthenyl)malonate, thioxanthonium bis(carbomethoxy)methylide, and related compounds

Article abstract of DOI:10.1021/jo00322a008

The single-crystal X-ray structures of the sulfonium ylides 9,9-dideuteriothioxanthenium bis(carboethoxy)-methylide and thioxanthonium bis(carbomethoxy)methylide reveal that both molecules are folded about an imaginary line connecting C(9) and S (138° and 169°, respectively). Like the corresponding sulfoxides, the methylide carbon is pseudoequatorial (e′) in both ylides. The malonylide fragment resides in the plane which bisects the "angle of the fold" of the heterocycle. The oxygen of one carbonyl group ("endo") is arrayed essentially trans to the nonbonding electron pair on sulfur. Dimethyl 9-(1,4-dimethylthioxanthenyl)malonate exists with the malonyl fragment in the pseudoaxial (a′) position. ¹³C resonances of a′ methylide carbons in these thioxanthenium ylides occur downfield of corresponding e′ methylide carbons (67 vs. 54 ppm). The methylide carbons of related thioxanthonium ylides resonate near 60 ppm, reflecting their intermediate geometry. Both ¹H and ¹³C NMR spectra indicate a relatively high rotational barrier about the S⁺-C^- bond in bis(carbomethoxy)methylides derived from thioxanthone, 2-chlorothioxanthone, and 2,4-dimethylthioxanthone. Infrared spectra can be used to distinguish isomeric ylides and thioxanthenes.