Graduate School of Pharmaceutical Sciences, University of Tokushima, 1-78 Shomachi, Tokushima 770-8505, Japan.
J Am Chem Soc. 2011 Mar 16;133(10):3342-4. doi: 10.1021/ja200479p. Epub 2011 Feb 22.
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.
我们首次报道了硼-λ(3)-溴烷交换反应立体选择性合成简单的(E)-β-烷基乙烯基（芳基）-λ(3)-溴烷，以及在温和条件下它们在乙烯基位碳原子上的独特双分子亲核取代反应。有趣的是，即使是较弱的亲核阴离子，如超强酸的共轭碱（HBF(4)、TfOH、Tf(2)CH(2)、Tf(3)CH、Tf(2)NH 等），也可作为亲核试剂与乙烯基-λ(3)-溴烷反应。例如，(E)-乙烯基-λ(3)-溴烷与双（三氟甲磺酰基）甲烷钾的 vinylic S(N)2 反应立体选择性地通过氧进攻生成（Z）-乙烯氧基氧代锍叶立德，构型完全反转，而与双（三氟甲磺酰基）酰亚胺钾反应则主要生成（Z）-乙烯氧基亚磺酰胺。相比之下，这些超强酸的共轭碱与(E)-β-烷基乙烯基-λ(3)-碘烷不能发生 vinylic S(N)2 反应。芳基-λ(3)-碘烷和芳基-λ(3)-溴烷基团的离去能力（后者较大）的差异可能在这些独特反应中起着关键作用。
