Zhao Haitao, Lin Zhenyang, Marder Todd B
Department of Chemistry and Open Laboratory of Chirotechnology of the Institute of Molecular Technology for Drug Discovery and Synthesis, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong.
J Am Chem Soc. 2006 Dec 13;128(49):15637-43. doi: 10.1021/ja063671r.
The detailed reaction mechanism for the reduction of CO2 to CO catalyzed by (NHC)Cu(boryl) complexes (NHC = N-heterocyclic carbene) was studied with the aid of DFT by calculating the relevant intermediates and transition state structures. Our DFT calculations show that the reaction occurs through CO2 insertion into the Cu-B bond to give a Cu-OC(=O)-boryl species (i.e., containing Cu-O and C-B bonds), and subsequent boryl migration from C to O, followed by alpha-bond metathesis between pinB-Bpin (B2pin2, pin = pinacolate = OCMe2CMe2O) and (NHC)Cu(OBpin). The overall reaction is exergonic by 38.0 kcal/mol. It is the nucleophilicity of the Cu-B bond, a function of the very strong alpha-donor properties of the boryl ligand, rather than the oxophilicity of boron, which determines the direction of the CO2 insertion process. The boryl migration from C to O, which releases the product CO, is the rate-determining step and involves the "vacant" orbital orbital on boron. The (NHC)Cu(boryl) complexes show unique activity in the catalytic process. For the analogous (NHC)Cu(alkyl) complexes, the CO2 insertion into the Cu-C bond giving a copper acetate intermediate occurs with a readily achievable barrier. However, the elimination of CO from the acetate intermediate through a methyl migration from C to O is energetically inaccessible.
借助密度泛函理论（DFT），通过计算相关中间体和过渡态结构，研究了（N - 杂环卡宾）铜（硼基）配合物（NHC = N - 杂环卡宾）催化二氧化碳还原为一氧化碳的详细反应机理。我们的DFT计算表明，该反应通过二氧化碳插入到铜 - 硼键中生成一个铜 - 氧代羰基 - 硼基物种（即含有铜 - 氧键和碳 - 硼键），随后硼基从碳迁移到氧，接着频哪醇硼烷（B2pin2，pin = 频哪醇 = OCMe2CMe2O）与（NHC）铜（OBpin）之间发生α - 键复分解反应。整个反应的放热量为3千卡/摩尔。决定二氧化碳插入过程方向的是铜 - 硼键的亲核性，它是硼基配体极强α - 供体性质的函数，而非硼的亲氧性。硼基从碳迁移到氧从而释放出产物一氧化碳，这一步是速率决定步骤，且涉及硼上的“空”轨道。（NHC）铜（硼基）配合物在催化过程中表现出独特的活性。对于类似的（NHC）铜（烷基）配合物，二氧化碳插入到铜 - 碳键中生成醋酸铜中间体的反应具有易于实现的能垒。然而，通过甲基从碳迁移到氧来从醋酸盐中间体中消除一氧化碳在能量上是不可行的。
