Dibenzothiophene-5,5-dioxide, pyrolysis

In contrast with the relatively facile thermal rearrangement of sulfinates to sulfones discussed in the preceding section, the reverse process is relatively, rarely encountered and is usually observed only at elevated temperatures. One of the first thermal sulfone to sulfinate isomerizations has been invoked by Fields and Meyerson to occur during the pyrolysis of dibenzothiophene S, S-dioxide (26) to dibenzofuran, through elimination of sulfur monoxide from the sultine intermediate 27 (equation 27). More recently, the flash vapor-phase pyrolysis of various 2,5-dialkyl and diaryl thiophene-S, S-dioxides has also been shown to involve SO extrusion and formation of the corresponding furans in good yields . 

The pyrolysis of dibenzothiophene 5,5-dioxide has been shown to occur via the ring-expanded sulfinate ester, which eliminates SO to give the strainless dibenzofuran (see Section III,C, 1). An expected product of... 

Dibenzothiophene has been isolated in low yield from the pyrolysis of benzene and sulfur dioxide in glass ampuls at between 400° and 540° and the reaction kinetics determined. 

Flash vacuum pyrolysis of dibenzothiophene 5,5-dioxide (244) affords a high yield of dibenzofuran. The sultine 245 (Scheme 63) was postulated as an intermediate, and this has been verified. Octafluorodibenzo-furan has been prepared by flash vacuum pyrolysis of the octafluoro analog of compound 244. The preparation of dibenzofuran from 2-phenoxyben-zenesulfonyl chloride by heating in the presence of a catalyst presumably involves the dioxide 244 as an intermediate. ... 

Pyrolysis of dibenzothiophene 5,5-dioxide at 690° gave a 95% yield of two products, dibenzofuran and dibenzothiophene in the ratio 6 1 (Fields and Meyerson, 1966a). The predominant reaction, therefore, involved overall loss of SO, presumably through an intermediate rearrangement... 

Pyrolysis of dibenzothiophene-5,5-dioxide, 63, which is closely related to fluorenone, results in the formation of dibenzofuran in direct analogy with the behavior of 63 under electron impact. 

It has been observed that sulfones do not decompose photochemically in the absence of sensitizer on irradiation above 3200 A so that the excited state is likely to be a triplet. It is possible that intersystem crossing to the vibrationally excited ground state occurs prior to bond breakage, but this is by no means certain . Occasionally rearrangement is observed in the decomposition of sulfones e.g., pyrolysis of dibenzothiophene-5,5-dioxide gives dibenzofuran instead of the de-sulfonylation product, biphenylene, viz. 

Pyrolysis of dibenzothiophene 5,5-dioxide and the derivatives affords mixtures of the corresponding dibenzofurans and dibenzothiophenes (Scheme 106) [214,252,253]. 

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