Transition metal catalysts are indispensable tools in the cutting-edge synthesis of fine chemicals, including pharmaceuticals and polymeric materials. Traditionally, the design of such catalysts has focused on the choice of the central metal and the supporting ligands. However, the role of counteranions in ion-separated transition metal catalysts has received relatively little attention.
Anions are inherently electron-rich species, and their involvement in catalysis has conventionally relied on their basicity. This narrow perspective has limited the functional versatility of anions, and in some cases, strongly basic anions have even inhibited catalytic activity by binding too tightly to the metal center.
A group with researchers at the University of Tokyo successfully designed and synthesized a thermally stable anion with strong Lewis acidity, achieved through strategic molecular design centered on a boron atom. By introducing this "acidic anion" as the counteranion in cationic transition metal catalysts, the group has developed a novel catalytic system that harnesses the acidity of the anion.
This is in stark contrast to conventional systems, where counteranions serve merely as bases. This approach endows catalysts with substrate recognition ability without lowering the activity, opening the door to highly selective transformations such as those required for pharmaceutical synthesis.
Papers
Journal: Angewandte Chemie International Edition
Title: Stable Yet Strongly Lewis-Acidic Anions Enabling Cooperative Catalysis with Cationic Transition-Metal Complexes
Authors: Ryo Mandai, Takanori Iwasaki*, Kyoko Nozaki*