Identification of Ti(salen) Complexes for Efficient Catalysis in Single-Electron Steps by Cyclic Voltammetry

We describe the identification of an active Ti(salen) catalyst for the radical arylation of epoxides by a cyclic voltammetry study of mechanism-based predictors, such as the redox potentials of the complexes and their E_qC_r-equilibria, for the success of catalysis. Surprisingly, by far the most active catalyst features an uncommon tetrasubstituted ligand backbone, which renders chloride binding to the active Ti(III) species less favorable, thereby increasing catalyst activity due to improved substrate binding. Catalysis is most efficient in the "green" solvent ethyl acetate and can be initiated using base metals as well as electrochemical methods for the reduction of the Ti(salen)-precatalyst. Compared to the commonly employed titanocene catalysis, the use of the newly developed Ti(salen) catalyst allows for the use of milder and more sustainable reactions conditions, a broader substrate scope, and facile modification of the catalyst's electronic and steric properties.