A Supported Copper Hydroxide on Titanium Oxide as an Efficient Reusable Heterogeneous Catalyst for 1,3-Dipolar Cycloaddition of Organic Azides to Terminal Alkynes

An easily prepared supported copper hydroxide on titanium oxide (Cu(OH)(x)/TiO2) showed high catalytic performance for the 1,3-dipolar cycloaddition of organic azides to terminal alkynes in non-polar solvents under anaerobic conditions. The reactions of various combinations of organic azides (four examples, including aromatic and aliphatic ones) and terminal alkynes (eleven examples, including aromatic, aliphatic, and double bond-containing ones) exclusively proceeded to give the corresponding 1,4-disubstituted-1,2,3-triazole derivatives in a completely regioselective manner. For the transformation of benzyl azide and ethynylbenzene with 0.12 mol% of Cu(OH)(x)/TiO2, the turnover frequency was 505 h(-1) and the turnover number reached up to 800. These values were the highest among those with previously reported heterogeneous catalysts including Cu(OH)(x)/Al2O3. The observed catalysis was truly heterogeneous and the retrieved catalyst after the reaction could be reused at least three times with retention of its high catalytic performance. It was confirmed by the UV/Vis spectrum of Cu(OH)(x)/TiO2 and the amount of diyne formed that the Cu-II species in Cu(OH)(x)/TiO2 were reduced to Cu-I species by the alkyne-alkyne homocoupling at the initial stage of the reaction (during the pretreatment of Cu(OH)(x)/TiO2 with an alkyne). The catalytic reaction rate for the 1,3-dipolar cycloaddition linearly increased with an increase in the amount of in situ generated Cu-I species. Therefore, the in situ generated Cu-I species would be the catalytically active species for the present 1,3-dipolar cycloaddition.