Visible-Light-Mediated Photocatalytic Aerobic Dehydrogenation of N-heterocycles by Surface-Grafted TiO₂ and 4-amino-TEMPO

Abstract

Herein, the visible-light-induced dehydrogenation of N-heterocycles such as tetrahydroquinolines, tetrahydroisoquinolines, and indolines in O2-containing suspensions of a commercially available titanium dioxide photocatalyst yielding the corresponding heteroarenes is presented. 4-Amino-2,2,6,6-tetramethylpipiridinyloxyl (4-amino-TEMPO) was found to exhibit a beneficial role, as it increased the yield and improved the selectivity of the dehydrogenation reaction. Both the selectivity and the yield are further enhanced by grafting 0.1 wt % of Ni(II) ions onto the TiO2 surface. It is proposed that the basic reactant adsorbs at Lewis acid sites present at the TiO2 surface. The dehydrogenation reaction is initiated by visible-light excitation of the resulting surface complex and a subsequent single-electron transfer from the excited N-heterocycle via the conduction band of TiO2 to O2. Ni(II) ions possibly serve as an electron transfer bridge between the conduction band of TiO2 and O2, while the TEMPO derivative is assumed to act as a selective redox mediator involved in reactions of the generated reactive oxygen species.