Stable Yet Strongly Lewis‐Acidic Anions Enabling Cooperative Catalysis with Cationic Transition‐Metal Complexes

Abstract

Abstract Ionic transition‐metal complexes play a crucial role as catalysts in organic transformations. Their counteranions often stand aside from the catalytic cycle or occasionally participate in the catalytic cycle as a Brønsted base due to their nucleophilic character. Herein, we developed a stable yet Lewis‐acidic anion, BBcat , based on tetrakis(pentafluorophenyl)borate and featuring Lewis‐acidic catechol borane moieties and applied it to transition‐metal catalysis to recognize Lewis‐basic substrates. Upon admixture with Bu 4 N – BBcat , 31 P NMR chemical shift of O═PEt 3 significantly lower‐shifted, implying the strong Lewis acidity of BBcat despite being an anion. In an Ir complex supported by a bidentate phosphine ligand, BBcat resides in the second coordination sphere as a noncoordinating counteranion. Ir/PHOX‐BBcat exhibited an 8.2‐fold higher reaction rate than Ir/PHOX‐BArF consisting of the non‐coordinating anion in the hydrogen isotope exchange of acetophenone derivatives bearing additional Lewis‐basic functionalities. The acceleration effect depends on the steric hindrance and basicity of the additional Lewis‐basic sites, located at remote positions to the C─H bond to be deuterated. These results clearly indicate that the interaction of BBcat with the Lewis‐basic sites plays a crucial role, facilitating cooperative catalysis of the cationic transition‐metal center and the counteranion.