Rh/TiO2‑Photocatalyzed Acceptorless Dehydrogenation of N‑Heterocycles upon Visible-Light Illumination.pdf

TiO2 is an effective and extensively employed photocatalyst,

but its practical use in visible-light-mediated organic synthesis is mainly

hindered by its wide band gap energy. Herein, we have discovered that Rhphotodeposited

TiO2 nanoparticles selectively dehydrogenate N-heterocyclic

amines with the concomitant generation of molecular hydrogen gas

in an inert atmosphere under visible light (λmax = 453 nm) illumination at

room temperature. Initially, a visible-light-sensitive surface complex is

formed between the N-heterocycle and TiO2. The acceptorless

dehydrogenation of N-heterocycles is initiated by direct electron transfer

from the HOMO energy level of the amine via the conduction band of

TiO2 to the Rh nanoparticle. The reaction condition was optimized by

examining different photodeposited noble metals on the surface of TiO2

and solvents, finding that Rh0 is the most efficient cocatalyst, and 2-

propanol is the optimal solvent. Structurally diverse N-heterocycles such as tetrahydroquinolines, tetrahydroisoquinolines, indolines,

and others bearing electron-deficient as well as electron-rich substituents underwent the dehydrogenation in good to excellent yields.

The amount of released hydrogen gas evinces that only the N-heterocyclic amines are oxidized rather than the dispersant. This

developed method demonstrates how UV-active TiO2 can be employed in visible-light-induced synthetic dehydrogenation of amines

and simultaneous hydrogen storage applications.