Oxygen atom transfer in the photocatalytic oxidation of alcohols by TiO2: oxygen isotope studies.

The selective oxidation of alcohols into carbonyl compounds using dioxygen in lieu of toxic or corrosive stoichiometric oxidants such as ClO , Cr, and Cl2, is one of the most challenging functional group transformations. The oxidation of alcohols using dioxygen as the oxidant has been successfully realized by using noble-metal and transitionmetal complexes for catalysis. TiO2 photocatalysis has also attracted much attention as a potential and promising strategy for this aim, because of its high oxidation ability, environmentally friendly properties, and the benefit of using O2 as an oxidant and light as the driving force. A few successful cases involving TiO2 photocatalysis in acetonitrile, water, or solvent-free systems have recently been reported. Molecular oxygen plays a vital role in the aerobic oxidation of alcohols in all these systems. Therefore, it is significant and necessary to reveal how the dioxygen participates in the reaction process. In the noble-metal catalysis system, the role of dioxygen has been proven to oxidize the reduced noble-metal center (for example, M or M hydride species) without an O-atom transfer from dioxygen to the products. In the aerobic oxidation of alcohols in the cytochrome P450 system, a gem-diol intermediate is formed, in which one hydroxyl group comes from the alcohol substrate (ca. 100 % O abundance) and the other from O2 (98% O labeled). Such a gem-diol intermediate leads to approximately 50% of the carbonyl product having incoorporated O atoms. Unlike these thermal catalytic systems, the essential role of dioxygen in the oxidation of alcohols by TiO2 photocatalysis has not been completely clarified yet. Herein we disclose an unexpected phenomenon: when the photocatalytically oxidative transformation of isotopelabeled alcohols was performed over pure anatase TiO2 in organic solvents, such as benzotrifluoride (BTF), the oxygen atom in the substrate alcohol is completely replaced by one of the oxygen atoms of dioxygen, that is, the photocatalytic process involves a selective cleavage of the C O bond of the alcohol with concomitant formation of a new C=O bond in the product aldehyde in which the O atom comes from dioxygen. This finding adds fundamental insight to the oxidation process of alcohols on the TiO2 surface, which is of importance for both the TiO2 photocatalysis and the selective oxidation of alcohols. O-enriched benzyl alcohol and cyclohexanol were used for the TiO2 photocatalytic oxidation (Table 1). The original abundance of O in the O-enriched benzyl alcohol was 65% (Table 1, entries 1–2) and 90% (Table 1, entries 4–7), respec-