Measuring Hydride Transfer Kinetics to Mhat Relevant Catalysts Using Cyclic Voltammetry

Abstract

The area of organic electrocatalysis has been growing in importance as many new transformations have been developed and previously known transformations are adapted to be accomplished electrochemically. As the field of synthetic organic electrochemistry emerges, it is becoming ever more important to study the mechanisms of these electrocatalytic reactions. Understanding the mechanics of the chemical processes and how they are coupled to the electrode is critical for continuing to expand the synthetic utility of electrocatalysis. Radical based hydrofunctionalization of alkenes through transition-metal catalyzed hydrogen atom transfer (MHAT) mechanisms is a proven, powerful mode of forming C−C, C−O, C−N and C−X bonds. In recent years, new asymmetric MHAT reactions have been enabled through electrocatalysis. The in situ generation of the key metal-hydride species plays a crucial role in both the kinetics and selectivity of the observed products. These high energy metal hydrides are too reactive and too short lived to be isolated, thus the oxidative mechanism leading to their formation is poorly understood. Here, we utilize a model catalytic cycle in which metal hydride formation is the rate determining step. Cyclic voltammetry enables us to extract the rate constant of hydride transfer. The differences in kinetic trends between catalysts and hydride donors indicate separate mechanisms of hydride transfer are operative for different classes of MHAT catalysts. Specific trends in ligand electronics, hydride donor sterics and hydricity for cobalt salen type catalysts allowed us to postulate a possible metal/ligand cooperative mechanism of hydride transfer. In summary, these studies represent a significant step towards understanding the homolytic reactivity of metal hydride species and will help enable the design of new electrosynthetic reactions involving radical intermediates.