Comparison of the reaction of benzylammonium N-methylides with that of benzylsulfonium S-methylides

Allylbenzylsulfonium S-methylides 8S and dibenzylsulfonium S-methylides 18S have been generated by the fluoride ion-induced desilylation of S- benzyl-S-[(trimethylsilyl)methyl](alk-2-enyl)sulfonium salts 4S and S- benzyl-S-[(trimethylsilyl)methyl](4-substituted benzyl)sulfonium salts 7S, and the isomerized products are compared with those of the corresponding N- methylides. S-Methylides 8S selectively rearrange toward the allyl groups (path a in Chart 2), whereas rearrangement to the benzyl groups (path b) competitively occurs in N-methylides 8N. Isomerization of S-methylides 18S to S-benzylides 19S and 20S competes with sigmatropic rearrangement to the benzyl groups (paths a and b in Chart 3), whereas the isomerization of N- methylides 18N is not observed.