Sulfenium salts

Finally Ebersbach s group succeeded in synthesizing the first (and, up to now, only known) authentic pyrido[l,2-a]azepine base 315 (Scheme 70) from their lactam 290 by thionation with Lawesson reagent (LR), methylation of thiolactam 313, and deprotonation of sulfenium salt 314 (91HCA1095). 

If the multiple bonds are between two C-atoms of the ring, as, for instance, in 3,4-dihydro-2//-pyran, the compounds react essentially like alkenes or alkynes. The heteroatom can also be involved in a double bond. In the case of X = O", the compounds behave like oxenium salts, in the case of X = S", like sulfenium salts, and in the case of X = N, like imines (azomethines). 

In contrast, azuleno[l,8-fcc]thiophene 179c is generated by the acid-catalyzed cyclodehydration of sulfine 179a and demethylation of the resulting sulfenium salt 179b (71JA2196). 

The same group also reported the first isolation and characterization of the corresponding sulfenium cation salt 173, as shown in Equation (47) <1998TL2593>. A single X-ray structure determination revealed that the two amino groups coordinate direct to the cationic sulfur atom. 

The process was shown to be more efficient when performed in the presence of some bromide salts whose oxidation allows lowering of the working potential and favors the S—S bond scission. Bromide mediated oxidation of bis(2-cyclopent-l-enylethane) and bis (2 -cyclohex-1 -enylethane) disulfide in AN/Et4NCl04/Et4NBr [130] undergoes the S—S bond cleavage followed by an intramolecular attack of the sulfenium cation on the double bond to give [3.3.0] and [4.4.0] bicyclic products. The anion Br also serves in the process as an... 

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