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.

Application of EPR Spectroscopy in TiO2 and Nb2O5 Photocatalysis

“…Moreover, the spin trapping technique was not only used to detect and monitor the formed reactive species such as the oxygen and nitrogen reactive radicals [94,99] during the photocatalytic degradation process, but also is employed within the mechanistic pathway to monitor and/or to be involved in controlling the experimental conditions during the photocatalysis organic synthesis [82,[100][101][102]. The involvement of TEMPO derivative radical within a mechanistic pathway of the synthesis reaction is assumed to act as a selective redox mediator involved in reactions of the generated reactive oxygen species.…”

“…The involvement of TEMPO derivative radical within a mechanistic pathway of the synthesis reaction is assumed to act as a selective redox mediator involved in reactions of the generated reactive oxygen species. Balayeva et al [101] studied the visible-light-induced dehydrogenation of N-heterocycles such as tetra-hydroquinolines, tetrahydroisoquinolines, and indolines compounds on the surface of TiO 2 in an aerated system yielding the corresponding heteroarenes compounds. In this study, the authors found that 4-amino-TEMPO exhibits a beneficial role, as it improved the yield and increased the selectivity of the dehydrogenation reaction.…”

TiO2 Photocatalysis for the Transformation of Aromatic Water Pollutants into Fuels

“…Such an adsorbed organic substrate can react irreversibly with the photogenerated holes, minimizing the undesired electron/hole recombination [150]. Even though H2 can be formed simultaneously with other processes, e.g., the organic synthesis of organic compounds [160,161], however, such processes should not be considered as a dual function process [130]. The reforming process can be considered as a dual function photocatalysis process only when some requirements have been met: (i) H2 is mainly derived from the reduction of water and (ii) the targeted organic molecules are pollutants, or their oxidation aimed to synthesize value-added products, such as aldehyde, organic acid, and imine [130,162].…”

Visible‐Light‐Promoted Efficient Aerobic Dehydrogenation of N‐Heterocycles by a Tiny Organic Semiconductor Under Ambient Conditions

“…Perylene diimide (PDI) has been widely used as a basic structural unit for fabricating organic photo‐functional materials in recent years due to its unique optical and electronic properties . Compared to some semiconductor photocatalysts (TiO 2 and C 3 N 4 ), easy chemically modified PDIs can be used as functional photocatalysts in various photo‐promoted chemical transformation.…”