Progress In The Development of an Electrochemically-Reversible Hydride Transfer Mediator for Organic Synthesis

Abstract

Hantzsch’s ester or amide is commonly used as a stoichiometric hydride transfer reagent in organic synthesis. The goal of this research is to see if Hantzsch’s ester or amide can be converted into an electrochemically-reversible hydride transfer mediator. The first step in achieving this objective is to realize the reversible 2 electron, 1 proton reduction of the corresponding pyridinium, 1, as shown below. Cyclic voltammetry studies of 1 in CH3CN and CH2Cl2 indicate that the reduction of 1 by itself is totally irreversible due to the rapid dimerization of the initially formed uncharged radical. We believe the key to avoid this pathway is to add a H-donor that can H-bond to the carbonyl O’s of 1 in such a way that slows dimerization and also properly positions an acidic H to H-bond to the radical, facilitating a second electron-transfer, proton-transfer step to generate the hydride donor. Urea derivatives are currently being explored for this purpose. This presentation will describe the results of these efforts to achieve reversibility in this system. Figure 1