Larsen Matthew A, Oeschger Raphael J, Hartwig John F
Department of Chemistry, University of California, Berkeley, California 94720, United States.
ACS Catal. 2020 Mar 6;10(5):3415-3424. doi: 10.1021/acscatal.0c00152. Epub 2020 Feb 11.
An in-depth study of iridium catalysts for the borylation of alkyl C-H bonds is reported. Although the borylation of aryl C-H bonds can be catalyzed by iridium complexes containing phen or bpy ligands at mild temperatures and with limiting arene, the borylation of alkyl C-H bonds remains underdeveloped. We prepared a library of phenanthrolines that contain varying substitution patterns. The corresponding phen-Ir trisboryl carbon monoxide complexes were synthesized to determine the electron-donating ability of these ligands, and the initial rates for the borylation of the C-H bonds in THF and diethoxyethane to oxygen catalyzed by Ir complexes containing these ligands were measured. For some subsets of these ligands, the donor ability correlated positively with the rate of C-H borylation catalyzed by the complexes containing ligands within a given subset. However, across subsets, ligands possessing similar donor properties to one another form catalysts for the borylation of alkyl C-H bonds with widely varying activity. This phenomenon was investigated computationally, and it was discovered that the stabilizing interactions between the phenanthroline ligand and the boryl ligands attached to Ir in the transition state for C-H oxidative addition could account for the differences in the activity of the catalysts that possess similar electron densities at Ir. The effect of these interactions on the borylation of secondary alkyl C-H bonds is larger than it is on the borylation of primary alkyl C-H bonds.
报道了对用于烷基C-H键硼化的铱催化剂的深入研究。尽管在温和温度下且芳烃限量时，含phen或bpy配体的铱配合物可催化芳基C-H键的硼化，但烷基C-H键的硼化仍未得到充分发展。我们制备了一系列具有不同取代模式的菲咯啉。合成了相应的phen-Ir三硼基一氧化碳配合物，以确定这些配体的给电子能力，并测量了含这些配体的Ir配合物催化THF和二乙氧基乙烷中C-H键与氧硼化的初始速率。对于这些配体的某些子集，给体能力与给定子集中含配体的配合物催化C-H硼化的速率呈正相关。然而，在各个子集之间，彼此具有相似给体性质的配体形成的用于烷基C-H键硼化的催化剂活性差异很大。通过计算对这一现象进行了研究，发现菲咯啉配体与C-H氧化加成过渡态中与Ir相连的硼基配体之间的稳定相互作用可以解释Ir处具有相似电子密度的催化剂活性差异。这些相互作用对仲烷基C-H键硼化的影响大于对伯烷基C-H键硼化的影响。
