Liu Yuhua, Chen Jipei, Zhan Kangsheng, Shen Yiqiang, Gao Hui, Yao Lingmin
School of Physics and Electronic Engineering, Guangzhou University Guangzhou 510006 China
Key Laboratory of Molecular Target & Clinical Pharmacology, School of Pharmaceutical Sciences & the Fifth Affiliated Hospital, Guangzhou Medical University Guangzhou 511436 China
RSC Adv. 2018 Oct 16;8(62):35453-35460. doi: 10.1039/c8ra07886f. eCollection 2018 Oct 15.
As a major challenge in C-H borylation, how to control the selectivity has attracted lots of attention, however, the related mechanistic information still needs to be uncovered. Herein, density functional theory (DFT) has been used to study the mechanism for the ligand controlled regioselectivity in the iridium-catalyzed C-H borylation of aromatic imines, which is inspired by experimental observations (R. Bisht, B. Chattopadhyay, , 2016, , 84-87). The proposed Ir(i)-Ir(iii) catalytic cycle includes (i) the oxidative addition of the C-H bond to iridium(i); (ii) the reductive elimination of a C-B bond; (iii) the oxidative addition of Bpin to an iridium(i) hydride complex; and (iv) the reductive elimination of a B-H bond. The oxidative addition of a C-H bond to the iridium center is the determining step. For the ligand AQ, -selectivity is proposed to be attributed to the decreased steric hindrance and increased electron donating effect of AQ (8-aminoquinoline) which promotes proton-transfer in the -transition state of C-H activation. While, for the TMP ligand, the steric repulsion between the TMP (4,5,7,8-tetramethyl-1, 10-phenanthroline) ligand and the -substituted imine hinders the C-H activation and favors borylation. Our calculations provide insights into further ligand design to achieve different regioselective borylation of aromatics. Guided by the results, the regioselectivity in the borylation of aromatics may be achieved by accordingly modifying the electronic and steric substituents of the ligand.
作为C-H硼化反应中的一个主要挑战，如何控制选择性已引起了广泛关注，然而，相关的机理信息仍有待揭示。在此，受实验观察结果（R. Bisht, B. Chattopadhyay, 2016, 84 - 87）启发，采用密度泛函理论（DFT）研究了铱催化芳香族亚胺的C-H硼化反应中配体控制区域选择性的机理。所提出的Ir(i)-Ir(iii)催化循环包括：（i）C-H键向铱(i)的氧化加成；（ii）C-B键的还原消除；（iii）Bpin向铱(i)氢化物配合物的氧化加成；以及（iv）B-H键的还原消除。C-H键向铱中心的氧化加成是决定步骤。对于配体AQ，区域选择性被认为归因于AQ（8-氨基喹啉）空间位阻的降低和给电子效应的增强，这促进了C-H活化的α-过渡态中的质子转移。而对于TMP配体，TMP（4,5,7,8-四甲基-1,10-菲咯啉）配体与α-取代亚胺之间的空间排斥阻碍了C-H活化并有利于β-硼化反应。我们的计算为进一步设计配体以实现芳烃的不同区域选择性硼化反应提供了见解。受这些结果的指导，芳烃硼化反应中的区域选择性可以通过相应地修饰配体的电子和空间取代基来实现。
