Synthesis of polycyclic aromatic quinones by continuous flow electrochemical oxidation: anodic methoxylation of polycyclic aromatic phenols (PAPs)

Abstract

Electrochemical oxidation of polycyclic aromatic phenols (PAPs) has been developed in a microfluidic cell to synthesize polycyclic aromatic quinones (PAQs). Methanol was used as nucleophile to trap the phenoxonium cation formed in the oxidation as an acetal, that later were hydrolysed to the quinone. Formation of hydrogen gas as the cathode reaction caused challenges in the flow cell and were overcome by recycling the reaction mixture through the cell at increased flow rate several times. The specific quinone formed were guided by the position of an initial hydroxygroup on the polycyclic aromatic hydrocarbon. An available para-position in the PAPs gave para-quinones, while hydroxy-groups in 2- or 3-position led to ortho-quinones. The substrates were analysed by cyclic voltammetry to estimate HOMO/LUMO energies to shed more light on this transformation. Easy separation of the supporting electrolyte from the product will allow recycling and make this a green transformation.