Molecular-Level Interface Research Center and Department of Chemistry, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea.
J Am Chem Soc. 2012 Oct 24;134(42):17778-88. doi: 10.1021/ja308205d. Epub 2012 Oct 9.
Organometallic chelates are readily obtained upon coordination of metal species to multidentate ligands. Because of the robust structural nature, chelation frequently serves as a driving force in the molecular assembly and chemical architecture, and they are used also as an efficient catalyst in numerous reactions. Described herein is the development of a Rh(NHC) catalytic system for the hydroarylation of alkenes and alkynes with 2,2'-bipyridines (bipy) and 2,2'-biquinolines; the most representative chelating molecules. Initially generated (bipy)Rh(NHC) chelates become labile because of the strong trans-effect of N-heterocyclic carbenes, thus weakening a rhodium-pyridyl bond, which is trans to the bound NHC. Subsequent rollover cyclometalation leads to the C-H bond activation, eventually giving rise to double functionalization of chelate molecules. Density functional calculations are in good agreement with our mechanistic proposal based on the experimental data. The present study elucidated for the first time the dramatic NHC effects on the rollover cyclometalation pathway enabling highly efficient and selective bisfunctionalization of 2,2'-bipyridines and 2,2'-biquinolines.
金属有机螯合物很容易通过金属物种与多齿配体的配位得到。由于其结构稳定，螯合作用常常是分子组装和化学结构的驱动力，并且它们也被用作许多反应中的高效催化剂。本文描述了一种 Rh(NHC)催化体系的发展，用于烯烃和炔烃与 2,2'-联吡啶（bipy）和 2,2'-联喹啉的氢芳基化；这些是最具代表性的螯合分子。最初生成的（bipy）Rh(NHC)螯合物由于 N-杂环卡宾的强反式效应而变得不稳定，从而削弱了与 NHC 键合的铑-吡啶键，该键位于反式位置。随后的翻转环金属化导致 C-H 键活化，最终导致螯合物分子的双重官能化。密度泛函计算与基于实验数据的我们的机理假设很好地一致。本研究首次阐明了 NHC 对翻转环金属化途径的显著影响，从而能够实现 2,2'-联吡啶和 2,2'-联喹啉的高效和选择性双官能化。
