(129b) Aqueous-Phase Hydrogenation of Aromatic Rings and Carbonyl Compounds in the Presence of Applied Potential
Electrocatalytic hydrogenation (ECH) of series of phenolic compounds, di-aryl ethers and aromatic aldehydes, has been performed in C-supported metals as working electrodes (cathodes). H2 evolution (HER) is the prevalent side reaction on all metals, thus we define efficiency as the fraction of electrons used to reduce the organic compounds.
The rates of ECH of all tested compounds, as well as HER, increase with increasingly negative potentials. However, the efficiency is a strong function of the metal. Pt, for instance, strongly favors HER leading to low efficiencies, whereas Pd is highly efficient for reduction of carbonyl groups in aldehydes.
In the conversion of substituted phenols and diaryl ethers, the dominant reaction pathway is hydrogenation to cyclic alcohols and cycloalkyl ethers. Methoxy and benzyloxy groups undergo C-O bond cleavage via hydrolysis (minor route) and hydrogenolysis, which opens pathways to the formation of hydrocarbons. In stark contrast, aromatic aldehydes and ketones undergo only reduction of the carbonyl groups without exhibiting any hydrogenation of the aromatic ring. Interestingly, the presence of aromatic alcohol and acids during the conversion of aldehydes significantly increases the conversion rates of the carbonyl group, which is related to proton donation towards the carbonyl group during hydrogenation.
The results show that electrocatalysis drives the desired transformations at mild conditions and allow accurate control over the reaction pathways. Strategies to increase reaction rates at these conditions are increasing the concentration of hydronium ions and fine tuning the particle size of the metal. These effects are discussed in the light of competitive adsorption of hydrogen and the organic on the metal as key parameter determining the delivery of reducing equivalents, proton reduction and organic reduction.