Thiathiophthens and Oxygenated Derivatives

Thiathiophthens and Oxygenated Derivatives. Contradictory interpretations of experimental studies of the heterocyclic compound C5H4S3 have stimulated a number of theoretical studies. Following the X-ray study and radio-frequency spectroscopy, the bicyclic structure of the 6a-thiathiophthen (trithia[l,6,6a]pentalene) (111), with C2. symmetry, seemed to be assured. (For a recent electron-diffraction study see ref. 187). This conclusion was supported by X-ray p.e. spectroscopy. A reinvestigation of the X-ray and u.v.-p.e. spectra, however, favoured the C. symmetric structure (111a), and the same conclusion was drawn from a study of the electronic polarization spectrum of the 2,5-dimethyl derivative.  

Unfortunately, theoretical studies have not allowed a clear-cut decision to be made between (111) and (111a) as there is little difference in energy 

Gleiter, V. Honiung, B. Lindberg, S. Hdgberg, and N. Lozac h, Chem. Phys. Letters, 1971, 

Clark et aU have carried out a study on the chemical reactivities of (111) and its methyl-substituted derivatives (PPP approximation). Results for localization energies and charge densities led to certain predictions that the 3-position is preferred in electrophilic attack, but electrophilic attack at the sulphur may be a competitive reaction, that the 2-position is preferred in nucleophilic attack, and that the 2-methyl-substituted (111) has higher acidity than (111). All these predictions are in good agreement with experimental findings. 

CNDO/2-calculations of substituted 6a-thiathiophthens have also been performed to explain the effect of the methyl and phenyl group on the geometry. The calculations indicated that introduction of a 2-methyl or 2-phenyl group lengthened the S-1—S-6a bond of (111), but the introduction of 3-methyl and 6-phenyl groups shortened it. [For a discussion of steric effect on the electronic spectrum of substituted (111) see ref. 174.] 

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