Development of Bifunctional Ligands for Efficient Selective Activation of Simple Arenes

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In this research, a novel “roof” ligand Bpin-SpiroBpy was developed for the meta-selective C-H activation of simple arenes. This concept was proved by iridium-catalyzed C-H borylation reaction of various kinds of mono-substituted arenes, in which the “roof” sterically protects the remotest para site besides the ortho site to achieve meta-selective C-H activation in the iridium-catalyzed borylation. The strategy proved general, and a variety of monosubstituted arenes including alkylbenzenes, anilines, protected-phenols, and drug molecules could be selectively borylated at the meta position in both high yields and selectivity. The choice of “roof” also appeared to be important. It was found that the rigidness of Bpin group on the three-dimensionally expanded SpiroBpy ligand effectively created a molecular pocket that accommodates the substrate approaching the catalytic center only in the meta orientation.In the following investigations, it was found that the iridium C-H borylation of electron-rich arenes was largely boosted by employing SpiroBpy as the ligand. When electron-rich arenes such as 1,3-dimethoxybenzene and 1,3-di-tert-butylbenzene were used as the substrate for the iridium-catalyzed C-H borylation with HBpin, the borylated product was obtained in high yields by using SpiroBpy as the ligand. In contrast, the borylation proceeded in lower yields with the commonly used ligands (dtbpy or tmphen) under similar conditions. This method is expected to expand the toolbox of this reaction to more diverse applications.