Aluminum oxide for photocatalytic organic transformations
Leow, Wan Ru
Date of Issue2017
School of Materials Science and Engineering
The use of sunlight to drive organic reactions constitutes a green and sustainable strategy for organic synthesis. In such reactions, the photoredox requirements of organic reactions are typically fulfilled through using wide band gap semiconductors or redesigning the photocatalyst. Herein, we demonstrated the use of a reverse strategy; instead of redesigning the dye as per convention, the reactant is chemisorbed on the Brønsted base sites of Al2O3 to form a surface complex, causing an upshift in its HOMO to a level accessible for electron abstraction by the dye. This enables the highly selective oxidation of benzylic alcohols to aldehydes by a large variety of dyes, even though negligible reaction occurred in the absence of Al2O3 or with other metal oxides. The charge-transfer surface complex formed between the dye and Al2O3 is also essential in facilitating the transport of electrons from BnOH to O2. Next, we further extended the use of Al2O3 complexation in conjunction with photocatalysis to drive the selective aerobic oxidation of phenylboronic acids. It was discovered that all metal oxides with Brønsted basicity can also enable high yields of phenols. The proximity and strength of the Brønsted base sites appear to be crucial towards the reaction; there seems to be a positive relationship between the yields of alcohol and the quantity of strong Brønsted base sites, rather than with the quantity of weak or all Brønsted base sites. Lastly, we explored the idea of rendering Al2O3 photocatalytically active through carbon-modification, which would eliminate the need for a dye. The resultant carbon doped Al2O3 enables the selective oxidation of benzylic amines to form imines under visible light irradiation. We believe that our aforementioned discoveries may subvert our understanding of the role of Al2O3 in photocatalytic reactions. It may also bring forth a new methodology of utilizing surface complexation mechanisms between the reactants and earth-abundant materials to effectively achieve a wider range of photoredox reactions.