meta-Selective C–H Borylation of Benzamides and Pyridines by an Iridium–Lewis Acid Bifunctional Catalyst
作者:Lichen Yang、Nao Uemura、Yoshiaki Nakao
DOI:10.1021/jacs.9b03138
日期:2019.5.15
We report herein the iridium-catalyzed meta-selective C-H borylation of benzamides by using a newly designed 2,2'-bipyridine (bpy) ligand bearing an alkylaluminum biphenoxide moiety. We also demonstrate the iridium-catalyzed C3-selective C-H borylation of pyridine with a 1,10-phenanthroline (Phen) ligand bearing an alkylborane moiety. It is proposed that the Lewisacid-base interaction between the
Regioselective CâH bond transformations are potentially the most efficient method for the synthesis of organic molecules. However, the presence of many CâH bonds in organic molecules and the high activation barrier for these reactions make these transformations difficult. Directing groups in the reaction substrate are often used to control regioselectivity, which has been especially successful for the ortho-selective functionalization of aromatic substrates. Here, we describe an iridium-catalysed meta-selective CâH borylation of aromatic compounds using a newly designed catalytic system. The bipyridine-derived ligand that binds iridium contains a pendant urea moiety. A secondary interaction between this urea and a hydrogen-bond acceptor in the substrate places the iridium in close proximity to the meta-CâH bond and thus controls the regioselectivity. 1H NMR studies and control experiments support the participation of hydrogen bonds in inducing regioselectivity. Reversible direction of the catalyst through hydrogen bonds is a versatile concept for regioselective CâH transformations. Directing groups in a substrate are frequently used to direct the regioselectivity of CâH activation reactions. Now it has been shown that regioselectivity can be directed by a ligand, which binds to both the catalysing metal centre and a distal hydrogen-bond acceptor in the substrate. This secondary interaction places the metal in close proximity to the reacting CâH bond.