Thiophene 1,1-dioxides, decomposition

Benzyne and substituted benzynes, generated by thermal decomposition of 2-carboxybenzenediazonium chloride or by aprotic diazotization of appropriate anthranic acids, reacted with a variety of thiophene dioxides to produce naphthalene derivatives generally in moderate yields (Scheme 56) [34,159]. In two cases, the naphthalenes so produced further reacted with benzyne to give benzobarrelene derivatives. 

The ketocarbene, generated by Cu(acac)2-assisted decomposition of the iodonium ylide (476), is trapped by phenylethyne and carbon disulfide to give thiophene dioxides (477) and (478), respectively, in moderate yields (Scheme 99) <89TL6673>. 

Several unsuccessful attempts have been made to prepare the parent 1,4-dithiocin by valence isomerization approaches. The diepisulfide (2S2) was obtained from benzene dioxide, but decomposition at 20 °C ( i/2 30 min) gave only benzene by stepwise sulfur extrusion (74AG(E)737). 2,5-Dithiabicyclo[4.2.0]octadiene (2S3) on pyrolysis at 200 °C gave benzene (50%) and a trace of thiophene a non-concerted ring opening to dithiocin (254) followed by immediate destruction of the latter was suggested (71JA4627). 

Cyclizations can occur with heteroatoms present in the tether as long as the groups are not strongly nucleophilic. Decomposition of a-diazo- 3-arylmethanesulfonyl esters (173) resulted in the formation of 1,3-dihydrobenzo[c]thiophene 2,2-dioxides (174 equation 38),143 which are valuable precursors to o-quinodimethanes. Reaction with /V-aryldiazoamides (175) has been shown to be a useful method for preparing 2(3//)-indolinones (176 equation 39),22a while reaction of a-diazo-(3-keto esters (177) has been developed as a process to synthesize 3-acetylbenzofiiran-2(3//)-ones (178 equation 40).144... 

Thus, the ease of desulfonylation of a l,3-dihydrobenzo[c]thiophene 2,2-dioxide and therefore the products obtained is dependent on the stability of the intermediate o-quinodimethane. The photochemical decomposition of l,3-dihydrobenzo[c]thiophene 2,2-dioxide probably involves an excitation energy of >74 kcal/mole and is not possible under the conditions already mentioned, whereas its 1,3-diphenyl derivative may be photochemically desulfonylated under these conditions because the intermediate o-quinodimethane derivative (96) is stabilized relative to o-quinodimethane itself (see also Cava and Shirley96).101... 

Thermal decomposition of 3-azido-2-formylfuran, -thiophene, and -selenoles has also been used to prepare furo[3,2-c]isoxazole (97), thieno[3,2-c]isoxazole (4), and selenolo[3,2-c]isoxazoles (98) (Equation (27)) <76CS165>. In contrast, some azido ketones eliminate nitrogen at ambient temperature and cyclize as exemplified in the reactions of 2-chloro-3-acetylindole (99) (Equation (28)) and 3-chloro-2-benzoylbenzothiophene dioxide (101) (Equation (29)) with sodium azide (78CB1521, 92JOC2127). 

Selenophene 1,1-dioxides have been found to be more thermally labile than the corresponding thiophene 1,1-dioxides. While dimerization is often observed in the thermal decomposition of the latter, the neat thermolysis of selenophene 1,1-dioxides leads primarily to ring-opened products. For example, thermolysis of 2,3,4,5-tetraphenyl-selenophene 1,1-dioxide 5 gave a variety of a ring-opened products (Equation 1) <1998H(48)61>, whereas thermolysis of compound 5 in toluene leads to the formation of 2,3,4,5-tetraphenylfuran. 

Anion-radicals from dithienothiophene dioxides liave been studied (e.g., 121) for which ESR and polarographic results have been presented. Similar results were obtained for certain isomers of 121 but not for others. During electrolytic reduction of deuterated dithienothiophene dioxides, it was found that H-D exchange occurred in the anion-radicals. The source of protons was either impurity from solvent decomposition (DMF) or an adventitious unknown present in acetonitrile. Janssen has also correlated electronic absorption spectra and polarographic reduction potentials with calculated molecular quantities for a wide range of thiophene S,S-diox-ides. ... 

Thermal decomposition of thiophene 1,1-dioxides has already been partly described in connection with their dimerization (Sect. 3.1.2). Pyrolysis products of thiophene 1,1-dioxides are much dependent on reaction conditions. Thus, pyrolysis of tetraphenylthiophene 1,1-dioxide 185 produced compounds 186-192 depending upon the reaction conditions (Scheme 103) [248]. 

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