Asking Ligands to Lend a Hand

Abstract

In classical transition-metal-catalysts, the ligand is a spectator, while all key transformations such as oxidative addition, β-hydride elimination and reductive elimination occur at the metal center. Herein, the ligand plays no direct role in the catalytic cycle and its function is limited to stabilizing the metal center and tuning its coordination environment. In recent years, participating ligands, which play an active role during the course of reaction, have received greater attention. The use of such ligands has allowed for the development of more active/selective catalysts and opened up new reaction pathways. There are many ways in which a ligand can influence the metal center to accelerate the catalytic transformation. For example, hemilabile ligands allow reversible dissociation of a donor unit to adapt to the stereoelectronic requirements of the reaction intermediates.Redox-non-innocent ligands participate in transferring electrons to/from the metal. There is a class of ligands that directly participate in bond making/breaking and undergo reversible chemical transformation during the catalytic cycle.These modes of ligand cooperativity have been discussed in many excellent reviews including those by Caulton,Milsteinand Grützmacher. Herein, we aim to address two distinct strategies to achieve ‘ligand-driven-chemistry’ – the use of (a) electronically asymmetric ligand, and (b) protic ligands. A brief discussion on the role of ligands in water activation for functionalization of organic compounds is given at the end.