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

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 Bu₄N-BBcat, ³¹P NMR chemical shift of O═PEt₃ 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.