Oxidative coupling revisited: solvent-free, heterogeneous and in water

Fe(III) treated K-10 montmorillonite and FeCl₃ (both hydrated and anhydrous) are compared as catalysts for oxidative coupling of phenol substrates under a range of conditions. While the commonly reported coupling of 2-naphthol proceeds under a range of conditions, other substrates are far less readily coupled. Counterintuitively, biphasic reactions of poorly water soluble substrates in contact with aqueous solutions of FeCl₃ are the most universally applicable conditions while many homogeneous reaction mixtures yield little or no coupling product. Fe(III) treated K-10 proved to be a poor catalyst for oxidative coupling of most substrates. Comparison of coupling conditions provides a framework for optimisation of green methodologies using oxidative coupling catalysts.