4-Acyl-1,2-dithiole-3-thiones and their S–oxides. Synthesis, crystal structures, and UV and circular dichroism spectra

Abstract

When 6,6-dimethyl- and 6-phenyl-3-thioxo-6,7-dihydro-3H-1,2-benzodithiol-4(5H)-one was allowed to react with monomeric boron trisulphide under argon, the only products isolated after chromato-graphy on silica were the corresponding S-oxides in up to 37% yield based on reacted starting material. The same S-oxides were obtained in good yields on oxidation of the thiones with m-chloroperbenzoic acid (MCPBA). The structures of the 6,6-dimethyl derivatives, thione and S-oxide, were established by X-ray crystallography. The S-oxide displays a very short intramolecular distance, 2.81 Å, between the SO oxygen atom and the S-2 atom of the dithiole ring, which indicated an attractive interaction. The C₆ ring has a sofa conformation with one axial and one equatorial methyl group. Analysis of the ¹H NMR spectra of the 6-phenyl compounds leads to the same conformations with equatorial phenyl groups.

The 6-phenyl compounds are chiral and were able to be resolved into pure enantiomers by chromatography on triacetylcellulose. The UV spectra of all compounds and the CD spectra of the 6-phenyl derivatives are discussed in relation to the results of CNDO/S calculations. Bands in the CD spectrum of the thione S-oxide, which lack counterparts in the UV spectrum, are assigned to the SO and CO n—→π* transitions.