Metal-Free Catalytic Boration at the β-Position of α,β-Unsaturated Compounds: A Challenging Asymmetric Induction
Metal-Free Catalytic Boration at the β-Position of α,β-Unsaturated Compounds: A Challenging Asymmetric Induction
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DOI：
10.1002/anie.201001198
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发表时间：
2010-01-01
影响因子：
16.6
通讯作者：
Fernandez, Elena
中科院分区：
文献类型：
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作者：
Bonet, Amadeu;Gulyas, Henrik;Fernandez, Elena
由手性膦配合物催化前手性烯烃的氢硼化反应首次得到了手性富含α的手性硼化合物。[1]在没有金属的情况下，金属介导的不对称诱导C±B键的形成比现有的涉及底物和手性硼烷试剂相互作用的方法更成功的原因有三：1)非手性硼烷试剂的低成本/可获得性，2)较温和的反应条件，以及最重要的是，通过筛选手性配体来优化和最大化不对称诱导的可能性。自那以后，已经取得了相当大的进展，特别是在缺电子烯烃的对映选择性金属介导的硼氢化、[3]二硼、[4]和β硼化[5]方面。然而，仍然有一个挑战需要克服：与非手性硼试剂的无金属不对称硼加成反应的发展。我们的团队最近研究了在手性膦或卡宾配体修饰的铜、[5c，d]钯、[5g]镍、[5h]和铁[5J]配合物存在下的金属介导的不对称共轭硼加成反应。Oestreich和他的同事最近对这一领域进行了优雅的审查，[6]，作者得出结论，不对称的无金属方法可能是向前迈进的下一个开创性的步骤。Hoveyda和他的同事最近报道了一种由N-杂环卡宾(NHC)促进的环和非环β-α，β-不饱和羰基的高效的无金属Pinacolato-Bin2反应。[7]机理研究表明，仅10mol%的卡宾就可以通过对其中一个硼原子的亲核攻击来激活双(Pinacolato)二硼试剂(B2pin2)(方案1)。
Enantioenriched α-chiral boron compounds were first obtained using chiral rhodium–phosphine complexes from the catalytic hydroboration of prochiral alkenes.[1] There are three reasons why metal-mediated asymmetric induction in CÀB bond formation is more successful than existing methods involving interactions between the substrate and a chiral borane reagent [2] in the absence of a metal: 1) the low cost/availability of the achiral borane reagent, 2) the milder reaction conditions, and, most importantly, 3) the possibility for optimization and maximization of the asymmetric induction by screening the chiral ligands. Considerable progress has since been made, particularly in relation to the enantioselective metal-mediated hydroboration,[3] diboration,[4] and βboration [5] of electron-deficient olefins. However, one challenge still remains to be overcome: the development of a metal-free asymmetric boron-addition reaction with achiral boron reagents.Our group has recently studied metal-mediated asymmetric conjugate borylation reactions in the presence of copper,[5c, d] palladium,[5g] nickel,[5h] and iron [5j] complexes that were modified with either chiral phosphine or carbene ligands. This field has recently been elegantly reviewed by Oestreich and co-workers,[6] and the authors conclude that asymmetric metal-free approaches to conjugate borylation might be the next pioneering step forwards. Hoveyda and coworkers recently reported an efficient metal-free β-boration of cyclic and acyclic α, β-unsaturated carbonyl groups promoted by N-heterocyclic carbenes (NHCs).[7] Mechanistic studies revealed that 10 mol% of carbene alone can activate the diboron reagent, bis (pinacolato) diboron (B2pin2), by nucleophilic attack at one of the boron atoms (Scheme 1).