Weakly Nucleophilic Conjugate Bases of Superacids as Powerful Nucleophiles in Vinylic Bimolecular Nucleophilic Substitutions of Simple β-Alkylvinyl(aryl)-λ~3-bromanes

摘要

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_2CH_2, Tf_3CH, Tf_2NH, etc.) function as nucleophiles toward the vinyl-λ~3-bromanes. For instance, the vinylic S_N2 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_N2 reaction with these conjugate bases of superacids. The differences between the nudeofugalities of aryi-λ~3 -iodanyl and aryl-λ~3 -bromanyl groups (the latter being greater) probably play a pivotal role in these unique reactions.