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

We report herein, for the first time, the stereoselective synthesis of simple (<i>E</i>)-β-alkylvinyl(aryl)-λ³-bromanes via a boron-λ³-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 (HBF₄, TfOH, Tf₂CH₂, Tf₃CH, Tf₂NH, etc.) function as nucleophiles toward the vinyl-λ³-bromanes. For instance, the vinylic S_N2 reaction of (<i>E</i>)-vinyl-λ³-bromanes with potassium bis(triflyl)methanide stereoselectively produced (<i>Z</i>)-vinyloxy oxosulfonium ylides with exclusive inversion of configuration via oxygen attack, while that with potassium bis(triflyl)imide afforded predominantly (<i>Z</i>)-vinyloxysulfoximines. In marked contrast, (<i>E</i>)-β-alkylvinyl-λ³-iodanes do not undergo the vinylic S_N2 reaction with these conjugate bases of superacids. The differences between the nucleofugalities of aryl-λ³-iodanyl and aryl-λ³-bromanyl groups (the latter being greater) probably play a pivotal role in these unique reactions.