碱土金属催化碳二亚胺硼氢化反应机理的理论研究

Mechanism of Alkaline Earth Metal Catalyzed Hydroboration of Carbodiimides:A Theoretical Study

本文运用密度泛函理论(Density functional theory,DFT)算法,研究了β-二亚胺氢化碱土金属复合物催化碳二亚胺硼氢化反应的机理.计算结果表明,当使用镁复合物作为催化剂时,该反应催化循环中的活性催化物种为镁氢复合物.从镁氢复合物的生成开始,该反应经过碳氮双键插入镁-氢共价键,硼氮偶联成键,以及从硼到镁的氢负离子转移等步骤,从而再生镁氢复合物并释放产物.该反应的决速步为负氢原子从硼到镁的转移过程.此外,通过理论计算拓展预测了钙、锶等其他碱土金属复合物催化剂的反应机理.研究结果表明,和镁催化氢化反应不同,钙、锶参与反应的活性催化物种是碳二亚胺插入相应氢化金属复合物后所生成的对应的氨基金属络合物.以此化合物作为催化循环的开始,反应经历硼氮偶联成键后,可直接与碳二亚胺发生硼碳之间的氢转移,得到产物并再生活性催化物种.钙、锶复合物作为催化剂时经历不同反应机理的原因是,它们的原子半径远远大于镁,因此可以与额外的碳二亚胺结合,并发生协同氢转移反应.理论计算表明,当使用钙、锶复合物作为催化剂时,反应决速步活化能低于镁催化的反应途径.因此,如果使用钙、锶复合物作为催化剂将会获得更温和的反应条件.

Density functional theory （DFT） calculations are employed to study the mechanism of alkaline earth metal cata- lyzed hydroboration of carbodiimides. Our theoretical study revealed that the active catalytic species is a hydridemagnesium complex when magnesium is used as catalyst. The catalytic cycle starts with a C=N bond insertion into Mg--H bond fol- lowed by a B--N bond formation. A hydride transfer from boron to magnesium regenerates the active catalytic species and yields the hydroboration product. This process is considered to be the rate-determining step. Moreover, the mechanism of cal- cium or strontium catalyzed corresponding reactions was also studied theoretically. Alternatively, DFT calculations showed that the active catalytic species is amide-metal complex, which could be generated by the carbodiimide insertion into met- al-hydride bond. In this catalytic cycle, amide-metal complex reacts with borane to form a B--N bond. After the coordination of another molecular carbodiimide, a concerted hydride transfer takes place from boron to carbon, which yields the final prod- uct and regenerates the active species amide-metal. The different reaction pathway with calcium or strontium catalyzed corre- sponding reactions could be attributed to that the radius of calcium or strontium is larger than that of magnesium significantly. Thus, those two metals would be coordinated with an extra carbodiimide molecule, which is the precursor for the concerted hydride transfer. The DFT calculations showed that the activation free energy for the rate-determining step with calcium or strontium catalyst is much lower than that withmagnesium catalyst. Therefore, a mild reaction condition might be found with calcium or strontium as catalyst for corresponding reactions.