Greener photocatalytic route to azide-alkyne cycloaddition reactions: Role of hole/oxygen in air

Abstract

Unifying both the properties of CuI and CuII, we represent a unique and smart nano-photocatalytic system for serving as a catalyst for base-free click reaction in the green solvent with high efficiency, selectivity and fast reaction kinetics. Thus, the synthesized heterogeneous photocatalyst based on Cu(II) ion-doped TiO2 nanocomposite [CuII@TiO2] is found to be highly efficient for carrying out azide-alkyne 1,3-dipolar cycloaddition reaction (CuAAC) in an aqueous environment without needing a base. The reaction features high product selectivity, mild reaction conditions in green solvent, broad substrate scope, excellent turnover number, high yields, fast reaction kinetics, and good functional-group compatibility. Mechanistically, upon light irradiation (λ = 400 nm), the TiO2-nanocore releases electron from its valence band to the conduction band which in turn photochemically reduces Cu(II) to Cu(I) achieving spatial and temporal control over the CuAAC reaction. On the other hand, the generated hole and/or oxygen in the air as a green oxidant maintains the Cu(I) to Cu(II) catalytic cycle. Thus, the cooperative effect of both the generated electron and the hole continually maintains the catalytic cycle achieving spatial and temporal control over the Click reaction. Our catalyst is advantageous over other reported catalytic systems and can be treated as a complement to existing methods. We also address the role of the generated hole on the TiO2-nanocore and oxygen in the air as a green oxidant, which possibly maintains the Cu(I) to Cu(II) and vice versa catalytic cycle as feed for the photocatalyst. The smart catalytic system is tolerant to air, highly efficient, has a wide range of substrates/functional groups, uses green solvent, and is thus advantageous over other reported catalytic systems.