Zhao Haitao, Dang Li, Marder Todd B, Lin Zhenyang
Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China.
J Am Chem Soc. 2008 Apr 23;130(16):5586-94. doi: 10.1021/ja710659y. Epub 2008 Mar 29.
The detailed mechanism for the diboration of aldehydes catalyzed by (NHC)Cu(boryl) complexes (NHC = N-heterocyclic carbene) was studied with the aid of DFT by calculating the relevant intermediates and transition states. The results show that the catalyzed diboration occurs through aldehyde insertion into Cu-B to give a Cu-O-C(boryl) species followed by sigma-bond metathesis with a diboron reagent. It is the "electron-richness", that is, the nucleophilicity of the Cu-boryl bond, which gives rise to a small insertion barrier and determines the direction of insertion. The results of our calculations also explain the formation of the product, observed experimentally, from the stoichiometric reaction of (IPr)Cu-Bpin (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) with mesitylaldehyde. In the absence of a diboron reagent, the insertion intermediate having a Cu-O-C(boryl) linkage isomerizes to the thermodynamically preferred Cu-C-O(boryl) isomer via a boryl migration to the metal-bonded oxygen through an S(E)2-like transition state. We have also studied the catalyzed diboration of 2-pyridinecarboxaldehyde, which gives the unexpected reductive coupling product 1,2-di-2-pyridyl-1,2-bis(pinacolboroxy)ethane. The insertion intermediate, which contains a coordinated pyridyl group, isomerizes easily to a 1,2-dihydropyridine form, preventing its metathesis with a diboron reagent to give the expected diboration product as observed for other aldehyde substrates.
借助密度泛函理论（DFT），通过计算相关中间体和过渡态，研究了（NHC）Cu（硼基）配合物（NHC = N - 杂环卡宾）催化醛的双硼化详细机理。结果表明，催化双硼化反应通过醛插入Cu - B生成Cu - O - C（硼基）物种，然后与双硼试剂进行σ键复分解反应。正是“富电子性”，即Cu - 硼基键的亲核性，导致了较小的插入势垒并决定了插入方向。我们的计算结果还解释了通过（IPr）Cu - Bpin（IPr = 1,3 - 双（2,6 - 二异丙基苯基）咪唑 - 2 - 亚基）与均三甲苯甲醛的化学计量反应实验观察到的产物形成。在没有双硼试剂的情况下，具有Cu - O - C（硼基）键的插入中间体通过硼基向与金属键合的氧迁移，经类似S（E）2的过渡态异构化为热力学上更稳定的Cu - C - O（硼基）异构体。我们还研究了2 - 吡啶甲醛的催化双硼化反应，该反应得到了意外的还原偶联产物1,2 - 二 - 2 - 吡啶基 - 1,2 - 双（频哪醇硼氧基）乙烷。含有配位吡啶基的插入中间体很容易异构化为1,2 - 二氢吡啶形式，从而阻止其与双硼试剂进行复分解反应生成如其他醛底物所观察到的预期双硼化产物。
