Visible light photocatalysis of dye-sensitized TiO2: The selective aerobic oxidation of amines to imines

Herein, we report an efficient protocol for the selective oxidation of amines with the sole involvement of conduction band electrons (e(cb)(-)) of TiO2 for both the activation of O-2 and the ensuing oxidation reaction. Visible light was harvested by dye molecules on the surface of TiO2, which injects electrons into its conduction band, generating dye radical cations. Charge transfers from dye radical cations to amines are relayed by the catalytic cycle of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) to TEMPO+, which is an oxidant that can conduct the direct two-electron oxidation of amines to imines. TEMPO was regenerated by the oxidation of TEMPOH with superoxide generated by the interaction of e(cb)(-) and O-2. This work suggests that one can exploit the strategy of cooperative photocatalysis with each component performing in its optimal role: TiO2 for O-2 activation, dye for visible light harvesting, and TEMPO for oxidative transformations.