Thiathiophthens and Related Compounds

Less work has been reported in this area during the past two years. The majority of papers have been concerned with thiabenzenes and nitrogen-containing derivatives. 6a-Thiathiophthens and related compounds are accorded separate treatment (Chapter 8) in view of the volume of published work on this subject. 

The popularity of 6a-thiathiophthens and related structures as targets for X-ray crystallography continues. Among the new studies is one on the parent compound of the series, which is shown to be symmetrical with S—S distances of 2.351 A. The molecular dimensions are generally similar to those of 2,5-dimethyl-6a-thiathiophthen, except that the C—S distances are appreciably shorter in the unsubstituted compound. 

Non-empirical calculations of the electronic structure of 6a-thiathiophthen and related l-oxa-6,6a-dithia-, l,6-dioxa-6a-thia-, and 6,6a-dithia-l-azapentalenes (see Sections 3 and S) appear to indicate that these molecules have little resonance energy, and are unlikely to be aromatic in the ordinary sense. Calculations of dipole moment for the same series of compounds give reasonable values when compared with the limited experimental data available. ... 

The Vilsmeier salt synthesis has been extended to the preparation of 2-methylthio-6a-thiathiophthens (15 R = H or Me). Full details have been published of the route to dithiolylidene ketones and thiathiophthens by way of 3-phenacylthio-l,2-dithiolium salts and related compounds. The 2-dimethylamino-6a-thiathiophthen (16), prepared by this method, is also obtained when the dithiolium salt (17) is treated with phosphorus pentasulphide, and the resulting mixture of isothiathiophthens (18 R = H, R = Ph) and (18 R = Ph, R = H) heated with thioacetamide. 

Structural data for dithiazole derivatives (35 R = Me) and (35 R = / -N02-C6H4) indicate molecular dimensions similar to those of systems studied previously. Some closely related structures (36 X = O or NR) have been described, and an interesting series of papers deals with various compounds derived from 2-iminodithiazoles which could be formulated as bicyclic systems, for example (37) and (38 X = S or NR). In the last of the four papers, the methylation reactions of these and other compounds are discussed, in relation to earlier work on the methylation of thiathiophthens. 

The question of symmetry in 6a-thiathiophthens, in relation to the interpretation of various physical studies previously reported and to theoretical studies, is reviewed in a paper dealing with CNDO calculations of ionization and excitation energies, for the parent compound and its 2,S-dimethyl derivative. 6a-Thiathiophthen is included in a further theoretical study which covers a range of heterocyclic sulphur compounds. 

Most closely related to the 6a-thiathiophthens, and to dithiolylidene ketcmes, are the l,6a-dithia-6-azapentalenes, which have been discussed previously. A new X-ray study in this series provides data for the triphenyl compound (55) and its S-methyl derivative (56). Comparison of the S—S and S—distances in the two compounds is instructive. 

The closely related ethano- and propano-6a-thiathiophthens (6) and (7) have been compared, and significant features (notably the unusually short C-3a—S-6a bond) in the structure of the ethano-compound have been attributed to strain produced by the ethylene bridge between C-3 and C-4. In the much more complex 3,4-propano-6a-thiathiophthen (8), the S—S... 

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