Ligand-Accelerated C−H Activation Reactions: Evidence for a Switch of Mechanism
作者:Keary M. Engle、Dong-Hui Wang、Jin-Quan Yu
DOI:10.1021/ja105044s
日期:2010.10.13
Initial rate studies have revealed dramatic acceleration in aerobic Pd(II)-catalyzed C-H olefination reactions of phenylacetic acids when mono-N-protected amino acids are used as ligands. In light of these findings, systematic ligand tuning was undertaken, which has resulted in drastic improvements in substrate scope, reaction rate, and catalyst turnover. We present evidence from intermolecular competition studies and kinetic isotope effect experiments that implies that the observed rate increases are a result of acceleration in the C-H cleavage step. Furthermore, these studies suggest that the origin of this phenomenon is a change in the mechanism of C-H cleavage from electrophilic palladation to proton abstraction.
Hydrogen‐Bond‐Enabled Dynamic Kinetic Resolution of Axially Chiral Amides Mediated by a Chiral Counterion
作者:Alison J. Fugard、Antti S. K. Lahdenperä、Jaqueline S. J. Tan、Aroonroj Mekareeya、Robert S. Paton、Martin D. Smith
DOI:10.1002/anie.201814362
日期:2019.2.25
their enantioselective synthesis remains a challenge. Herein, a counterion‐mediated O‐alkylation method for the generation of atropisomeric amides with an er up to 99:1 is outlined. This dynamickineticresolution is enabled by the observation that the rate of racemization of atropisomeric naphthamides is significantly increased by the presence of an intramolecular O−H⋅⋅⋅NCO hydrogen bond. Upon O‐alkylation
非联芳基阻转异构体在医学、材料和催化方面具有重要价值,但其对映选择性合成仍然是一个挑战。本文概述了一种抗衡离子介导的 O-烷基化方法,用于生成 er 高达 99:1 的阻转异构酰胺。这种动态动力学分辨率是通过观察到由于分子内 OH⋅⋅⋅NCO 氢键的存在而显着增加了阻转异构萘酰胺的外消旋化速率而实现的。当氢键供体发生 O 烷基化时,旋转势垒显着增加。量子计算表明,分子内氢键减少了芳基酰胺键的旋转势垒,使外消旋化的平面过渡态稳定约 40 kJ mol -1,从而促进了观察到的动态动力学分辨率。