Thiophene dioxides rearrangement

Phenanthro[4,5-b,c,d]thiophene-4,4-dioxide (20) fragments mainly via the cyclic sulfinate ester15 (Section II.D). Naphthothiete sulfone (65) rearranges also to a cyclic sulfinate (66) prior to fragmentation, as shown by the similarities in their El mass spectra30. The parent SO ion at m/z 308 which was much less pronounced for 65 than for 66 (10 vs. 54%) indicated, however, that the former does not decompose exclusively via 66. 

Phenanthro[4,5-b, c, d]thiophene 4-oxide (19) and 4,4-dioxide (20) undergo first the well-known sulfenate (22) or sulfinate ester (23) rearrangements , respectively (equation 6). The sulfoxide loses an oxygen atom and enters the fragmentation pathway of 18 or loses HCO (more likely in two steps) from 22. Both sulfenate and sulfinate ions can fragment further via 21 after losing a sulfur atom or eliminating SO , respectively. 

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 . 

If a diene unit is located at an appropriate distance from the developing zwitterionic intermediate in such reactions (especially in good ionizing solvents), an intramolecular cycloaddition may intervene. This is what happens with the tetrahydrobenzo[c]thiophene 2,2-dioxide derivative (588), which leads to the bis-homobenzene (589) rapid valence isomerization of this gives the product (590) (770R(25)i). This process has been termed bis-homoconjugative rearrangement (Scheme 251). 

When the 3-(l-chloroalkyl)benzo[6]thiophene-l,1-dioxides (338 R = H or Me) are treated with piperidine, they readily form the enamines (339 R = H or Me).717b The mechanism of these reactions involves an Ss2 nucleophilic displacement followed by a rearrangement. 

When 1,3-diphenyl-l,3-dihydrobenzo[c]thiophene 2,2-dioxide is heated at 250° in diethyl phthalate, 1,2-diphenylbenzocyclobutene is not formed. Instead, rearrangement [Eq. (6)] of the initially formed o-quinodimethane derivative (96) occurs to give 9-phenyl-9,10-... 

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