Base-metal and Metal-free Hydride Donor Reagents – From Fundamentals to Applications in Electrocatalysis

Base-metal and Metal-free Hydride Donor Reagents – From Fundamentals to Applications in Electrocatalysis

Prof. Dr. Inke Siewert
Georg-August-Universität Göttingen

Hydride transfer reactions are fundamental functional group transformations in chemical synthesis, especially for the reduction of polar double bonds. Hydride donors proceed a two-electron, proton-coupled reduction in a concerted fashion thereby circumventing the formation of high energy intermediates. Noble metalhydrides, particularly Rh, Pd and Pt, are generally excellent hydride donors and can often be used in catalytic amounts through in thermal hydrogenation reaction protocols. However, due to the low abundance of noble metal-based hydrides, recent scientific efforts have investigated base-metal and metal-free hydride donor and their utilization in (electro)catalytic reduction processes.

In an electrocatalytic application, the conjugated reagent must be regenerated by consecutive single electron transfer steps and protonation, which is usually challenging. Electrochemical reduction often produces highly reactive radical species as intermediates, which have a strong tendency to, for example, dimerize instead of undergoing the desired 1H⁺/1e⁻ follow-up reactivity.

I will present the electrochemical formation of a base-metal and metal-free hydride donor reagents for storing multiple electrons and protons focusing of the distinct differences and similarities in their formation and reactivity.^[1,2] Furthermore, I will show the application of such hydride transfer reagents in the electrochemical hydrogenation of C=O bonds in ketones, aldehydes using electrons and protons.^[2] A large variety of substrates was accessible by this method and in-depth mechanistic studies gave valuable insights for catalyst improvement.