Thiepins, sulfur elimination

Elimination of sulfur from methyl dibenzo[/),/]thiepin-10-carboxylatcs 15 (R2 = Me) can be achieved in moderate yields (39-55%) upon refluxing in diethyl phthalate in the presence of copper bronze.60 For the dibenzo[A,/]thiepin-10-carboxylic acids 15 (R2 = H), the loss of sulfur is accompanied by decarboxylation. Thus, treatment of these acids with copper bronze in refluxing quinoline for four hours gives the corresponding phenanthrenes 16 (R3 = H) in moderate yield (50%). However, the exposure time to high temperatures influences the product formation. Thus, the decarboxyiated dibenzothiepins are obtained after refluxing for only five minutes.60... 

Valence Isomerization of the 2-Thiabicyclo[3.2.0]heptadiene Moiety In principle, a valence isomerization of thiabicyclo[3.2.0]heptadiene skeleton would lead to a thiepin ring system. Wynberg et al. 23) reported that the photochemical adduct (28) from benzo[6]thiophene and dimethyl acetylenedicarboxylate was not thermally stable. When heated in diglyme, it loses sulfur to give dimethyl 1,2-naphthalenedicarboxylate. This reaction presumably proceeds via ring opening of 28 to 2,3-dimethoxycarbonylbenzo[6]thiepin (29) which readily eliminates sulfur. This synthetic route was successfully applied to the reaction of electron-deficient acetylenes with enamines of 2,3-dihydrobenzo[fe]thiophen-3-ones in which the enamine moiety constitutes part of a thiophene system. When 3-pyrrolidin-l-yl-benzo[6]thiophene (30) was allowed to react with dimethyl acetylenedicarboxylate... 

Thermal extrusion of a sulfur atom is the most common thermal reaction of a thiepin. The mechanism of this thermal process involves two orbital symmetry controlled reactions (69CC1167). The initial concerted step involving a reversible disrotatory electrocyclic rearrangement is followed by a concerted cheleotropic elimination of sulfur (Scheme 29). Similar aromatization reactions occur with thiepin 1-oxides and thiepin 1,1-dioxides, accompanied by the extrusion of sulfur monoxide and sulfur dioxide respectively. Since only a summary of the major factors influencing the thermal stability of thiepins was given in Section... 

Thiepines have been studied extensively due to theoretical and biological interest. The theoretical interest arises from whether thiepines with 8jt electrons are nonaromatic or antiaromatic. The subsequent interest is in thiepine-benzene sulfide valence isomerization. Because thiepines are generally thermally unstable and eliminate sulfur, the... 

Thiepins sometimes undergo sulfur extrusion even during their preparation. Thus, thiepins 55 accessible from thiophenes 52 and alkynes 53 either slowly eliminate sulfur to form the corresponding benzene derivatives 56 (in some cases, even at -30°C) or they can be transformed to 56 by heating... 

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