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. 2018 Oct 16;8(62):35453-35460.
doi: 10.1039/c8ra07886f. eCollection 2018 Oct 15.

Mechanistic study of the ligand controlled regioselectivity in iridium catalyzed C-H borylation of aromatic imines

Yuhua Liu  1 , Jipei Chen  1 , Kangsheng Zhan  1 , Yiqiang Shen  1 , Hui Gao  2   3 , Lingmin Yao  1
Affiliations

Mechanistic study of the ligand controlled regioselectivity in iridium catalyzed C-H borylation of aromatic imines

Yuhua Liu et al. RSC Adv. .

Abstract

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, J. Am. Chem. Soc., 2016, 138, 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 B2pin2 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, ortho-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 ortho-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 ortho-substituted imine hinders the ortho C-H activation and favors meta 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.

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Conflict of interest statement

There are no conflicts to declare.

Figures

Fig. 1
Fig. 1. Potential mechanisms of iridium(iii)/iridium(v) and iridium(i)/iridium(iii) catalytic cycles for aromatic C–H borylation.
Scheme 1
Scheme 1. Computational model for aromatic C–H borylation.
Scheme 2
Scheme 2. Three types of potential mechanisms for C–H borylation of aromatic imine.
Fig. 2
Fig. 2. Energy profile of path I in the borylation of aromatic imine catalyzed by AQ/Ir system.
Scheme 3
Scheme 3. Energy change in the second step of B–H reductive elimination for path II in AQ/Ir(i)-assisted C–H borylation of aromatic imine.
Fig. 3
Fig. 3. Key transition states and intermediates in Ir(i)/AQ catalyzed C–H activation in path I.
Fig. 4
Fig. 4. Energy profile of path III in the borylation of aromatic imine catalyzed by TMP/Ir(i) system.
Fig. 5
Fig. 5. Energy profile of path I′ in the borylation of aromatic imine catalyzed by TMP/Ir(i) system.
Scheme 4
Scheme 4. Energy change in the second step of B–H reductive elimination for path II′ in TMP/Ir(i) assisted C–H borylation of aromatic imine.
Fig. 6
Fig. 6. Energy profile of path III′ in the borylation of aromatic imine catalyzed by TMP/Ir(iii) system.
Fig. 7
Fig. 7. Key transition states and intermediates in Ir(i)/TMP system catalyzed C–H activation in path I′.
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