Zhong Rong-Lin, Sakaki Shigeyoshi
Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, College of Chemistry, Jilin University, Changchun 130023, P. R. China.
Element Strategy Initiative for Catalysts and Batteries, Kyoto University, Goryo-Ohara 1-30, Nishikyo-ku, Kyoto 615-8245, Japan.
J Am Chem Soc. 2020 Sep 30;142(39):16732-16747. doi: 10.1021/jacs.0c07239. Epub 2020 Sep 22.
Methane borylation catalyzed by CpM(Bpin) (M = Ru or Rh; HBpin = pinacolborane; = 2 or 3) and (TMPhen)Ir(Bpin) (TMPhen = 3,4,7,8-tetramethyl-1,10-phenanthroline) was investigated by DFT in comparison with cyclohexane borylation. Because Ru-catalyzed borylation has not been theoretically investigated yet, its reaction mechanism was first elucidated; CpRu(Bpin) is an active species, and CpRu(Bpin)(H)(CH) is a key intermediate. In , the Ru is understood to have an ambiguous oxidation state between +IV and +VI because it has a H··Bpin bonding interaction with a bond order of about 0.5. Methane borylation occurs through oxidative addition of methane C-H bond followed by reductive elimination of borylmethane in all of these catalysts. The catalytic activity for methane borylation increases following the order CpRu(Bpin) < (TMPhen)Ir(Bpin) < CpRh(Bpin). Cyclohexane borylation occurs in the same mechanism except for the presence of isomerization of a key intermediate. Chemoselectivity of methane over cyclohexane increases following the order Ir < Ru < Rh. In all of these catalysts, the rate-determining step (RDS) of cyclohexane borylation needs a larger Δ° than the RDS of methane borylation because the more bulky cyclohexyl group induces larger steric repulsion with the ligand than methyl. One reason for the worse chemoselectivity of the Ir catalyst is its less congested transition state of the reductive elimination of borylcyclohexane. Herein, use of a strongly electron-donating ligand consisting of pyridine and N-heterocyclic carbene with bulky substituents is computationally proposed as a good ligand for the Ir catalyst; actually, the Ir complex of this ligand is calculated to be more active and more chemoselective than CpRh(Bpin) for methane borylation.
通过密度泛函理论（DFT）研究了由CpM(Bpin)（M = Ru或Rh；HBpin = 频哪醇硼烷； = 2或3）和(TMPhen)Ir(Bpin)（TMPhen = 3,4,7,8 - 四甲基 - 1,10 - 菲咯啉）催化的甲烷硼化反应，并与环己烷硼化反应进行了比较。由于Ru催化的硼化反应尚未进行理论研究，因此首先阐明了其反应机理；CpRu(Bpin)是活性物种，而CpRu(Bpin)(H)(CH)是关键中间体。在 中，Ru被认为具有介于+IV和+VI之间的模糊氧化态，因为它具有键级约为0.5的H··Bpin键合相互作用。在所有这些催化剂中，甲烷硼化反应通过甲烷C - H键的氧化加成，然后是硼甲烷的还原消除而发生。甲烷硼化反应的催化活性按CpRu(Bpin) < (TMPhen)Ir(Bpin) < CpRh(Bpin)的顺序增加。除了关键中间体存在异构化外，环己烷硼化反应以相同的机理发生。甲烷相对于环己烷的化学选择性按Ir < Ru < Rh的顺序增加。在所有这些催化剂中，环己烷硼化反应的速率决定步骤（RDS）比甲烷硼化反应的RDS需要更大的Δ°，因为体积更大的环己基与配体之间的空间排斥比甲基更大。Ir催化剂化学选择性较差的一个原因是其硼化环己烷还原消除的过渡态拥挤程度较低。在此，通过计算提出使用由吡啶和带有庞大取代基的N - 杂环卡宾组成的强供电子配体作为Ir催化剂的良好配体；实际上，计算得出该配体的Ir配合物在甲烷硼化反应中比CpRh(Bpin)更具活性和化学选择性。
