How Racemic Secondary Alkyl Electrophiles Proceed to Enantioselective Products in Negishi Cross-Coupling Reactions
How Racemic Secondary Alkyl Electrophiles Proceed to Enantioselective Products in Negishi Cross-Coupling Reactions
复制标题
外消旋仲烷基亲电子试剂如何在根岸交叉偶联反应中生成对映选择性产物
DOI：
10.1021/om1012049
复制
发表时间：
2011-06-27
期刊：
影响因子：
2.8
通讯作者：
Lin, Delian
中科院分区：
文献类型：
--
作者：
Lin, Xufeng;Sun, Jian;Lin, Delian
采用密度泛函方法研究了镍-双恶唑啉吡啶配合物催化Negishi交叉偶联反应的Ni（0）-Ni（II）和Ni（I）-Ni（III）两种机理.基于能量跨度模型，Ni（I）-Ni（III）机理包含金属转移-氧化加成-还原消除的顺序步骤，比Ni（0）-Ni（II）机理更有利。外消旋仲烷基亲电试剂的偶联产物的对映选择性通过还原消除步骤的相对反应速率（r（s）/r（R））来计算，所述还原消除步骤形成S-对映异构体形式的偶联产物，而不是产生R-对映异构体。可以从Ni（I）-Ni（III）机理中这两种还原消除途径的过渡态的相对自由能计算r（s）/r（R）。模型反应的对映选择性计算值与实验结果一致。并讨论了催化剂配体的不对称位阻对还原消除反应的影响。
Two mechanisms, namely, the Ni(0)-Ni(II) and Ni(I) -Ni(III) mechanisms, for nickel-bis(oxazolinyl)pyridine complex catalyzed Negishi cross-coupling reaction were investigated with density functional calculations. The Ni(I)-Ni(III) mechanism, containing sequential steps of transmetalation-oxidative addition-reductive elimination, is more favorable than the Ni(0)-Ni(II) mechanism, based on the energetic span model. The enantioselectivity of the coupled product from a racemic secondary alkyl electrophile was calculated by the relative reaction rate (r(s)/r(R)) of the reductive elimination step that forms the coupled product in the S-enantiomer over that leading to the R-enantiomer. The r(s)/r(R) can be calculated from the relative free energy of the transition states for these two reductive elimination pathways in the Ni(I)-Ni(III) mechanism. The calculated enantioselectivity for the model reaction is consistent with the experimental report. The influence of the asymmetric steric hindrance of the catalyst ligand on the reductive elimination step is also discussed.