Electrocatalytic Hydrogen Atom Transfer for the Activation of Alkenes: Mechanism of Formation and Reactivity of Cobalt Hydride

Metal hydride species have proven to be a crucial chemical motif across chemical disciplines. As the key intermediate in electrocatalytic hydrogen evolution by molecular catalysts, understanding how metal hydrides are formed and how they react has allowed the design of more efficient electrocatalysts. Independently to the field electrocatalysis, metal hydride species have been critical for organometallic catalysis as reagents for hydrogen atom transfer (HAT), in particular the activation of alkenes to a diverse array of hydrofunctionalization products. Recently, these two fields have been explicitly linked via the emergence of electrocatalytic hydrogen atom transfer (e-HAT); utilizing primarily Cobalt based catalysts (salen and bipyridine) members of the NSF Center for Synthetic Organic Electrochemistry (CSOE) have shown that in situ electro-generated cobalt hydride species can catalyze highly sought-after organic transformations of alkenes, such as alkene isomerizations and enantioselective hydrocyanation reactions. However, from an electrochemical perspective these reactions remain poorly understood, severely limiting the design of new hydrofunctionalization reactions. Here, cyclic voltametric studies of Co(salen) provide a sorely needed mechanistic framework to understand these multi-step electrocatalytic reactions. Using model homolytic reactions we establish rate constants for cobalt hydride formation as well as a relationship between hydride donor ability and alkene activation. Hammett analysis of a series of modified salen ligands shows changes in the rate determing step, suggesting tunability in future organic electrosynthetic reactions. Finally, we detect key off-cycle intermediates that inhibit catalytic turnover and suggest further optimization in yield and enantioselectivity selectivity are possible. In summary, we contend that these mechanistic studies provide an important template for studying complex, multistep organic reactions from the perspective of traditional molecular electrocatalysis as developed by Jean-Michel Savéant.