Direct Introduction of OH Group to sp2-Carbon on Platinum-Loaded Titanium Oxide Photocatalyst

Photocatalytic oxidation of various organic compounds by using molecular oxygen has been broadly investigated as a valuable synthetic route, but the application has been still limited because the active oxygen species from the molecular oxygen are usually difficult to be controlled. In this short review, we describe two kinds of photocatalytic oxygenation with H₂O molecule as a source of OH group; i.e., photocatalytic hydroxylation of aromatic ring and photocatalytic hydration of alkene, on Pt/TiO₂ photocatalyst. In both reactions, a photoformed active oxygen species attacks to the sp²-carbon to form a reaction intermediate. In the aromatic ring hydroxylation, the elimination of the H atom from the intermediate proceeds while the conjugate double bond of the aromatic ring maintains. In the alkene hydration, the addition of the H radical to the intermediate results in forming an alcohols molecule with the saturation of the double bond. In the former case, the electrophilic property of the active oxygen species predominantly determines the orientation of the OH group in the produced phenols. On the other hand, in the latter case, the thermodynamic stability of the intermediate radical species leads the active oxygen species preferentially to the terminal sp²-carbon, which makes it possible to provide the unique regioselectivity of alcohol according to the anti-Markovnikov rule.