The azide–alkyne cycloaddition catalysed by transition metal oxide nanoparticles†
Colloidal nanoparticles of Earth-abundant, first-row transition metal oxides and sulfide, namely magnetite (Fe3O4), manganese and cobalt ferrite, (MnFe2O4, CoFe2O4), manganese(II) oxide (MnO) and sulfide (α-MnS), were used as catalysts in the cycloaddition between azides and methyl propiolate. The presence of these nanoparticles allowed us to carry out the cycloadditions under milder conditions and with a regioselectivity comparable to the classic “metal-free” thermal processes. Ferrite nanoparticles gave higher conversion than MnO and α-MnS nanoparticles. The feasibility of the cycloaddition onto 1,2-disubstituted acetylenes was also proved. Ferrite nanocatalysts could be magnetically recovered and reused without significant loss of catalytic activity. Density functional theory (DFT) calculations support a mechanistic hypothesis that attributes the increased cycloaddition rate to the adsorption of the azide onto to the nanocatalyst surface.