A Bicyclic N-Heterocyclic Carbene as a Bulky but Accessible Ligand: Application to the Copper-Catalyzed Borylations of Aryl Halides

The Journal of Organic Chemistry
Shin AndoTadao Ishizuka

Abstract

Imidazol-2-ylidene and imidazolin-2-ylidene are frequently employed as the core structures of N-heterocyclic carbene (NHC) ligands, because these have two nitrogen sites adjacent to carbenic carbons. The electronic and steric effects of N-substituents to a bound metal have been aptly studied, and bulky substituents have commonly been introduced on nitrogen(s). However, too much bulkiness can sometimes result in an ineffectiveness of the hindered substrates. In this study, we thoroughly compared our original bicyclic NHC, which has bulky substituents on its noncarbenic carbons, with ubiquitous NHC ligands to probe its steric properties. A well-defined bicyclic NHC-CuCl complex was successfully synthesized and applied to the copper-catalyzed borylations of aryl halides at ambient temperature. A bicyclic NHC-CuCl proved to be a better catalyst than commonly used NHC ligand-Cu complexes, particularly for sterically hindered substrates, which suggested that the bicyclic NHC ligand offered a bulky but accessible environment to the bound copper.

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Citations

Jul 20, 2016·Chemical Reviews·Emily C NeeveTodd B Marder
Apr 28, 2016·Chemical Communications : Chem Comm·Zhi-Wei YangYao Fu
Sep 13, 2018·Chemical Society Reviews·David HemmingPatrick G Steel
Jun 22, 2019·Organic & Biomolecular Chemistry·Laura KuehnTodd B Marder
Sep 1, 2020·Bioscience, Biotechnology, and Biochemistry·Laksana Anugerah AdhiYukiharu Fukushi
Oct 9, 2019·The Journal of Organic Chemistry·Toshiyuki KameiToyoshi Shimada
Mar 8, 2019·Chemical Reviews·Andreas A DanopoulosPierre Braunstein
Dec 5, 2018·The Journal of Organic Chemistry·Shin AndoTadao Ishizuka
Oct 8, 2020·Journal of the American Chemical Society·Ya-Ming TianTodd B Marder

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