6a-Thiathiophthenes

Gerson and co-workers" " " some years ago reported an analysis of the ESR spectrum of the anion-radical 233 of 2,5-diphenyl[l,2,4]dithiazolo-[l,5-fe][l,2,4]dithiazole. Some of the work reported at the same time concerning anion-radicals of analogous 6a-thiathiophthenes was subsequently reinterpreted." " However, the inference from footnote 7 in Gerson et al. is that the results for 233 are not superseded. (Hyperfine splittings indicated for 233 are in gauss.)... 

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. 

As in Volume 1, structure (1), with the numbering shown, will be adopted as the most convenient representation of 6a-thiathiophthen. 

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. 

Triphenyl-6a-thiathiophthen (2) has been compared structurally with 3,4-diphenyl- and 2,4-diphenyl-6a-thiathiophthen (3). The difference between the S—S distances in the triphenyl compound is less than in the two disubstituted derivatives. 

An example of a molecule with a completely unsymmetrical substitution pattern, and yet almost exactly equal S—S bond lengths, is provided by 2-p-dimethylaminophenyl-4-phenyl-6a-thiathiophthen (4). Comparison with structure (3) indicates that the effect of the dimethylamino-group is to shorten considerably the nearer S—S bond. The shortening effect is discussed in terms of the reduced electronegativity of one of the terminal... 

Full details have appeared of X-iay studies on 2,5-diphenyl- and 2-methyl-4-phenyl-6a-thiathiophthen, previously reported briefly. It is noteworthy that the replacement of the 2-phenyl group in the 2,4-diphenyl compound (3) by a methyl group has only minor effects on the S—S distances and other bond lengths. 

These theoretical results could explain the similarity in dimensions between 2,4-diphenyl-6a-thiathiophthen and 2-methyl-4-phenyl-6a-thia-thiophthen which has been noted above. The inequality of the S—S lengths in the symmetrically substituted 2,5-diphenyl compound may be due, at least in part, to the differing angles of twist of the two phenyl groups (3° and 45°). 

The total energy calculated for 6a-thiathiophthen by Hordvik and his co-workers is in good agreement with the value obtained earlier by Clark and Kilcast. These authors have estimated ir-populations, and hence charge densities, for 6a-thiathiophthen and its methyl derivatives since their results are related to the chemical reactivity of 6a-thiathiophthens, they are discussed later in this chapter. 

All-valence-electron calculations on 2,S-dimethyl-6a-thiathiophthen and the corresponding dithiolylidene ketone have been reported by Japanese workers, but the calculated electronic transition energies do not appear to match observed values particularly well. 

Using the stretched film technique, the electronic spectrum of 2,5-dimethyl-6a-thiathiophthen has been measured with polarized light parallel and perpendicular to the long axis of the molecule. The resulting data are compared with those predicted (by SCF-PPP calculations) for different models of the system. It is claimed that the experimental results are best understood in terms of an unsymmetrical model (6), although this conclusion seems to conflict with the crystallographic data. 

Numerous routes to 1,2-dithiolylidene ketones (see p. 507), and hence to thiathiophthens, are described in a paper which is chiefly concerned with comparisons, particularly in relation to their electronic spectra, of 6a-thiathiophthens, the corresponding dithiolylidene ketones, and certain other structurally similar systems, for example (15 X = O or S), (16 X = O or S), and (17 X = O or S). From their survey of an extensive collection of spectroscopic data, the authors conclude that varying degrees... 

In the same paper, the conversion of 6a-thiathiophthens into 1,2-dithiolylidene ketones is discussed. The use of sulphuric acid as a reagent for this purpose, with unsymmetrically substituted compounds, may lead to only one or both of the possible isomeric ketones. With 2-methyl-5-phenyl-6a-thiathiophthen, treatment with sulphuric acid gives the ketone (18), confirming an earlier report which had been questioned. 

Further and more definitive examples of electrophilic substitution reactions in the thiathiophthen series have been described. " In the parent compound, in which both 2- and 3-positions are available for attack, formylation has been proved to occur at the 3-position, as would be anticipated from earlier experimental and theoretical work. 2-t-Butyl-6a-thiathiophthen formylates in the 4-position. The thiathiophthen aldehydes show characteristic absorption bands in the i.r. at ca. 1670 cm analogous to that shown by thiophen-2-aldehyde (1673 cm ), implying electronreleasing properties at the 3- (or 4-) position. 

Vilsmeier-Haack formylation of 2,5-diaryl-6a-thiathiophthens leads to the 3-formyl compounds, which are also obtainable from 3-aryl-l,2-dithiolium salts by treatment with triethylamine, and details have been given for the bromination of 2,5-disubstituted thiathiophthens. Nitration of 2-methylthio-5-phenyl-6a-thiathiophthen gives the 3-nitro-compound in poor yield, and attempted nitrosation experiments lead, in several examples, to rearranged structures of type (19), although the first stage of these reactions is presumed to be electrophilic attack on the 3-position. 

As noted earlier in this chapter, charge densities have been calculated for 6a-thiathiophthen and some of its derivatives localization energies for Wheland-type intermediates in substitution reactions have also been estimated. The results indicate, as has been inferred previously, that the 3-position should be attacked by electrophiles. The localization energies for attack at sulphur are very similar to those for attack at C(3) thus, attack at sulphur may become competitive, and indeed examples of this mode of reaction are known. The theoretical treatment further predicts that attack by nucleophiles should occur at the 2- (or 5-) position, in agreement with the experimental evidence. 

The formation of carbanions is involved in the rearrangement of 2-isopropyl-6a-thiathiophthens (23) to reduced thiophen derivatives (24 R = H, Me, or Ph), isolated as the methylated compounds (25), ... 

The conversion of isothiathiophthens into 6a-thiathiophthens has been investigated. The rearrangement of compound (33 = R = Ph,... 

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