Aqueous Phase Photocatalytic Hydroxylation of Arylboronic Acids Using Polyhedral Cu₂O Crystals

The high photocatalytic activity of {110}-bound Cu 2 O rhombic dodecahedra suggests that the photogenerated electrons, holes, and radicals can be utilized to catalyze some organic transformations. Compared to Cu 2 O cubes and octahedra, Cu 2 O rhombic dodecahedra give the best photocatalytic hydroxylation yield of 4-methoxyphenylboronic acid to 4-methoxyphenol at 95% in water and at room temperature in the presence of a hole acceptor, showing that surface control is also important to the catalyst performance. Cu 2 O rhombic dodecahedra also give generally excellent yields toward a variety of arylboronic acids. Arylboronic esters are less efficient choices for this reaction. Free radical scavenger test and electron paramagnetic resonance measurements indicate that photogenerated superoxide radicals are produced and added to arylboronic acids to form phenols. This work shows that ionic solids can find use in photo-assisted organic reactions.