Visible light-initiated aerobic oxidation of amines to imines over TiO2 microspheres with TEMPO+PF6−

Abstract

Semiconductor photocatalysis holds great promise to drive vital chemical reactions utilizing sunlight. Amongst semiconductors, TiO₂-related materials are one of the most viable to achieve enhanced photocatalytic performances because of their intrinsic merits. Here TiO₂ microspheres assembled from nanocrystals with a distinct hierarchical architecture and a high specific surface area were fabricated using a simple template-free solvothermal process. Assembling amines on TiO₂ microspheres initiated by visible light can lead to a surface complex that captures visible light for further oxidation of amines. Moreover, the selective oxidation of amines could be boosted by fully exploring the surface polarity of TiO₂ microspheres with more polar 2,2,6,6-tetramethylpiperidine-1-oxoammonium hexafluorophosphate (TEMPO⁺PF₆⁻) instead of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) as the redox mediator. As such, cooperative photocatalysis with TEMPO⁺PF₆⁻ was framed over TiO₂ microspheres to initiate the efficient and selective aerobic oxidation of benzyl amines into imines. Significantly, the activity of TEMPO⁺PF₆⁻ surpassed that of TEMPO in aiding the visible light-initiated selective oxidation of amines over TiO₂ microspheres, reaching more than about 3 times in some cases. This work suggests that the surface properties of a semiconductor could be maneuvered to enable coupling with a suitable redox mediator to ameliorate selective organic conversions in an unprecedented manner.