Metal-Free Intermolecular Azide-Alkyne Cycloaddition Promoted by Glycerol

Metal-free intermolecular Huisgen cycloadditions using nonactivated internal alkynes have been successfully performed in neat glycerol, both under thermal and microwave dielectric heating. In sharp contrast, no reaction occurs in other protic solvents, such as water, ethanol, or diols. DFT calculations have shown that the BnN3/glycerol adduct promotes a more important stabilization of the corresponding LUMO than that produced in the analogous BnN3/alcohol adducts, favoring the reactivity with the alkyne in the first case. The presence of copper salts in the medium did not change the reaction pathway (Cu(I) acts as spectator), except for disubstituted silylalkynes, for which desilylation takes place in contrast to the metal-free system.