Visible Light Mediated Cyclization of Tertiary Anilines with Maleimides Using Nickel(II) Oxide Surface-Modified Titanium Dioxide Catalyst

Surface-modified titanium dioxides
by highly dispersed NiO particles
have an extended absorption in the visible light region and a reduced
hole–electron pair recombination than unmodified TiO2. They have now been successfully applied as highly active heterogeneous
photocatalysts in the visible light mediated direct cyclization of
tertiary anilines with maleimides to give tetrahydroquinoline products
in moderate to high yields at ambient temperature. In contrast with
unmodified titanium dioxide catalysts that are conventionally used
in a stoichiometric amount in combination with UVA light, only a catalytic
amount (1 mol %) of the surface-modified TiO2 catalyst
is needed along with visible light to efficiently catalyze the reaction.
Compared with transition-metal complexes such as Ru­(bpy)3Cl2 or Ir­(ppy)2(dtbbpy)­PF6, advantages
of these surface-modified titanium dioxides as photocatalyst include
high catalytic activity, low cost, ease of recovering, and being able
to be used for at least nine times without significant decay of catalytic
activity.