(678b) Mechanistic Insights into the Influence of the Electrode Surface in the Selective Alcohol Oxidation of Primary Benzyl Alcohols

The selective oxidation of alcohols to their corresponding aldehydes and ketones is a critical reaction in the synthesis of fine chemicals and has been intensely studied over the past two decades. Few efforts, however, have examined the selective oxidative competition between different primary alcohols when two or more substrates are oxidized simultaneously. The present work specifically examines the competing selectivity between 4-methoxybenzyl alcohol (MBA) and 4-nitrobenzyl alcohol (NBA) towards oxidation to their corresponding aldehydes using electrochemical methods. The MBA:NBA selectivity at the Pt anode was found to be 1:1 but significantly improved to 19:1 when reticulated vitreous carbon (RVC) was used as the anode.

Potential-dependent AIMD and DFT simulations with explicit solvents were carried out to examine the complex solid-liquid interface and elucidate the mechanistic features that control observed selectivity differences. Our results indicate the different selectivity originates from the changes in the adsorption energies on different electrode surfaces as well as the changes in intrinsic oxidation barriers. The Pt electrode strongly bind benzyl alcohols and induce significant structural changes in adsorbed aromatic, forming covalent Pt-C and Pt-O bonds between the Pt surface and alkoxy intermediates. The difference in the apparent activation barrier between the MBA and NBA is in the range of -5 to +3 kJ/mol, thus leading to the 1:1 selectivity. On the contrary, benzyl alcohols bind weakly to the RVC electrode surface via π−π interactions. This preserves the molecular structures and the electronic difference between two substrates induced by the electron-donating MeO and electron-withdrawing NO₂ substituents. The substituent effect has little influence on intrinsic reaction barriers but show significant differences in the adsorption energies (~30 kJ/mol) induced by the positively charged anode, resulting in the 19:1 selectivity. Mechanistic findings in this work can provide fundamental insights into electrochemical reactions that occur at metal and carbon electrode surfaces.