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

摘要

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.