W. Schroth et al., BIS(BENZO[4,5]THIENO)[3,2-C-2',3'-E][1,2] DITHIINE, A VALENCE ISOMER OF DITHIOXO-THIOINDIGO, Chemische Berichte, 127(2), 1994, pp. 401-408
3-Mercaptobenzo[b]thiophene (1) is transformed with assistance of base
s, especially of amines, into the benzo[b]-thieno-annellated 1,2-dithi
ine 6 and not into the originally claimed ''Dithioxo-thioindigo'' tran
s-3. Primarily, the formation of 6 from 1 involves oxidation to the di
sulfide 4 which may also be used separately. Conceivable rationalizati
ons are discussed. The same compound is accessible (in lower yield) by
thionation of thioindigo with the aid of the Lawesson reagent. X-ray
elucidation of 6 reveals a non-planar structure of the cyclic disulfid
e with a dihedral C-S-S-C-angle of 53-degrees. Despite of the absence
of any established chromophoric moiety, 6 is deeply red in the crystal
and in solution, as it is representative for the ''undisturbed'' 1,2-
dithiine system. Contrary to the usual behaviour of the non-annellated
1,2-dithiines, 6 displays no spontaneous sulfur extrusion under mild
conditions, but only at high temperatures and in the presence of sulfu
r-binding agents, leading to the benzo[b]thieno-annellated thiophene 2
2, Moreover, 6 may be reduced to the dithiol 14 (or 13, respectively)
and regenerated exclusively from this by oxidation. Further characteri
stic reactions of 6 are described (e.g. to the sulfoxide 18, to the py
ridazine 19, and to the thiophosphoric ester 21).