Metal- and reagent-free highly selective anodic cross-coupling reaction of phenols

Abstract

The direct oxidative cross-coupling of phenols is a very challenging transformation, as homo-coupling is usually strongly preferred. Electrochemical methods circumvent the use of oxidizing reagents or metal catalysts and are therefore highly attractive. Employing electrolytes with a high capacity for hydrogen bonding, such as methanol with formic acid or 1,1,1,3,3,3-hexafluoro-2- propanol, a direct electrolysis in an undivided cell provides mixed 2,2'-biphenols with high selectivity. This mild method tolerates a variety of moieties, for example, tert-butyl groups, which are not compatible with other strong electrophilic media but vital for later catalytic applications of the formed products. Current synthesis: In a highly sustainable access to mixed biphenols, electric current makes the use of reagents, catalysts, and leaving groups obsolete. Solvents with a strong tendency to act as hydrogen-bond donors lead to a unique selectivity for the cross-coupling product. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.