Cooperative Photocatalysis with 4‐Amino‐TEMPO for Selective Aerobic Oxidation of Amines over TiO₂ Nanotubes

Abstract

Attaching π‐conjugated molecules onto TiO2 can form surface complexes that could capture visible light. However, to make these TiO2 surface complexes durable, integrating 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO) or its analogues as a redox mediator with photocatalysis is the key to constructing selective chemical transformations. Herein, sodium 6,7‐dihydroxynaphthalene‐2‐sulfonate (DHNS) was obtained by extending the π‐conjugated system of catechol by adding a benzene ring and a substituent sodium sulfonate (−SO3−Na+). The DHNS−TiO2 showed the best photocatalytic activity towards the blue light‐induced selective aerobic oxidation of benzylamine. Compared to TEMPO, 4‐amino‐2,2,6,6‐tetramethylpiperidine‐1‐oxyl (4‐amino‐TEMPO) could rise above 70% in conversion of benzylamine over the DHNS−TiO2 photocatalyst. Eventually, a wide range of amines could be selectively oxidized into imines with atmospheric O2 by cooperative photocatalysis of DHNS−TiO2 with 4‐amino‐TEMPO. Notably, superoxide (O2•−) is crucial in coupling the photocatalytic cycle of DHNS−TiO2 and the redox cycle of 4‐amino‐TEMPO. This work underscores the design of surface ligands for semiconductors and the selection of a redox mediator in visible light photocatalysis for selective chemical transformations.

The Utility of Oxoammonium Species in Organic Synthesis: Beyond Alcohol Oxidation

“…78 In 2021, Lang et al reported a unique cooperative photocatalytic system for the aerobic oxidation of amines to imines, including the release of ammonia releasing using dye-TiO2 nanotubes with TEMPO + BF4or 4-NH2-TEMPO as the catalyst. 79,80 Scheme 6. Oxidation of primary and secondary amines to form imines of TEMPO + Clto give N-methylaniline and N-methylformanilide.…”

Hydroxynaphthalene–Nb2O5complex photocatalysis for selective aerobic oxidation of amines induced by blue light

“…These endeavors could lay the foundation for pursuing a Nb 2 O 5 -based alternative. For example, anchoring colorless hydroxynaphthalene ligands onto TiO 2 could implement either selective reduction64,65 or aerobic oxidation reactions [66][67][68]. Therefore, several representative hydroxynaphthalenes were selected for comparison (Fig.3).…”