Sulfur walk

JOC1537). The mechanisms of these transformations may involve homolytic or heterolytic C —S bond fission. A sulfur-walk mechanism has been proposed to account for isomerization or automerization of Dewar thiophenes and their 5-oxides e.g. 31 in Scheme 17) (76JA4325). Calculations show that a symmetrical pyramidal intermediate with the sulfur atom centered over the plane of the four carbon atoms is unlikely <79JOU140l). Reactions which may be mechanistically similar to that shown in Scheme 18 are the thermal isomerization of thiirane (32 Scheme 19) (70CB949) and the rearrangement of (6) to a benzothio-phene (80JOC4366). 

The rearrangement (automerization) of Dewar thiophene 5-oxide (61), observed by NMR, occurs so much more rapidly than that of the corresponding episulfide that special mechanisms have been invoked. The one which involves a zwitterionic intermediate (Scheme 108) is favored over a pseudopericyclic sulfur-walk mechanism in which the electrons of the carbon-sulfur o--bond and the pair of electrons on sulfur exchange places as the sulfur atom migrates around the ring (80JA2861). 

In contrast to (14), several examples of Dewar thiophenes are known. The first example was (17), prepared by irradiation of (16) (72CJC2721, 75CPB2773). The degenerate Sulfur walk to (17 ) occurs at a rate of 55 s at 157 "C (77JA629), appreciably faster than thermal... 

Peroxy acid oxidation of (17) gave sulfoxide (18) whose F NMR spectrum showed equivalent CF3 groups even at —95 °C (76JA4325). Tlie rate ratio for the sulfur walk in (18/17) is an astounding 10 ° at 25 "C theoretical reasons for the difference have been discussed (80JA286i). 

Dewar thiophenes i.e. 22 and 23) are intermediates in the photoisomerization of cyanothiophenes. Their presence has been demonstrated by trapping and by direct NMR observation (79CC881, 79CC966). The rapid sulfur walk i.e. 22- 23) fully explains the substituent scrambling in the room temperature irradiations (i.e. 21 - 24). 

The initially formed singlet excited state 1 can convert either to the corresponding triplet state 2 by intersystem crossing, or to the Dewar isomer 4. In the former case, homolytic cleavage of the S-C(5) bond in 2 can lead to the biradical 3 and ultimately result in ring-opened or ring-contracted products. The Dewar isomer 4 is responsible for the formation of the isomeric thiophene 6 via 5 obtained by a sulfur walk . 

Photolysis of thiophenes is known to result in formation of Dewar thiophenes. The transposition of functional groups upon irradiation of thiophenes has been explained by the familiar sulfur-walk mechanism via the initial formation of a Dewar thiophene as illustrated for 2-cyanothiophene in Scheme 17 <79CC881>. The presence of trifluoromethyl groups imparts special stability on the Dewar thiophene, and this feature has been used to advantage by several organic chemists. 

Direct evidence of a thermally induced sulfur walk was also obtained in Dewar thiophene 14, which rearranges to 15 with a half-life of 2 min at -35°C (8e). 

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