Photocatalytic Oxidative Dimerization of Electronically Diverse Phenols Using Borate to Prevent Overoxidation.

Phenol overoxidation has severely hindered the advancement and synthetic utility of oxidative phenol coupling for over two decades, preventing the development of general catalytic methods. Electron-deficient phenols resist selective coupling due to their high oxidation potential, while monosubstituted phenols undergo uncontrolled overoxidation, making their selective transformation highly challenging. We present a strategy that harnesses biphenol-boron complexation to suppress overoxidation, unlocking the selective catalytic oxidative coupling of both electron-deficient and monosubstituted phenols. This method is broadly applicable to both heterogeneous and homogeneous photocatalytic systems, each operating via distinct mechanisms. By overcoming a fundamental barrier in oxidative phenol coupling, this work not only redefines the scope of catalytic oxidative transformations but also paves the way for new advances in oxidative couplings with implications in pharmaceuticals and materials science.