Weakly Nucleophilic Conjugate Bases of Superacids as Powerful Nucleophiles in Vinylic Bimolecular Nucleophilic Substitutions of Simple β-Alkylvinyl(aryl)-λ³-bromanes

Abstract

We report herein, for the first time, the stereoselective synthesis of simple (E)-β-alkylvinyl(aryl)-λ(3)-bromanes via a boron-λ(3)-bromane exchange reaction and their unique bimolecular nucleophilic substitutions at the vinylic ipso carbon atom under mild conditions. Interestingly, even weakly nucleophilic anions such as conjugate bases of superacids (HBF(4), TfOH, Tf(2)CH(2), Tf(3)CH, Tf(2)NH, etc.) function as nucleophiles toward the vinyl-λ(3)-bromanes. For instance, the vinylic S(N)2 reaction of (E)-vinyl-λ(3)-bromanes with potassium bis(triflyl)methanide stereoselectively produced (Z)-vinyloxy oxosulfonium ylides with exclusive inversion of configuration via oxygen attack, while that with potassium bis(triflyl)imide afforded predominantly (Z)-vinyloxysulfoximines. In marked contrast, (E)-β-alkylvinyl-λ(3)-iodanes do not undergo the vinylic S(N)2 reaction with these conjugate bases of superacids. The differences between the nucleofugalities of aryl-λ(3)-iodanyl and aryl-λ(3)-bromanyl groups (the latter being greater) probably play a pivotal role in these unique reactions.