Gold-catalyzed aerobic oxidation of dibenzylamine

Au(OAc)3 is applied as an effective catalyst of the selective oxidation of Dibenzylamine to dibenzylimine using molecular oxygen as the only oxidant. When Au(OAc)3 was preadsorbed onto CeO2, the supported catalyst was more active than any homogeneous or heterogeneous catalyst known for this reaction. Although, some fascinating color changes in the early stage of the reaction indicated the formation of an amine complex, conventional filtration experiments proved the heterogeneity Dibenzylamine of the system. The fate of the active gold component was studied by in situ X-ray absorption spectroscopy (XANES) using a specially designed cell. These investigations revealed that in the early stage of the reaction Au(OAc)3 is dissolved and subsequently reduced by the amine and the in situ formed gold nanoparticles are the real active species of the reaction. Formation of gold nanoparticles during Dibenzylamine oxidation was proved independently by transmission electron microscopy. Our findings lead to a simple synthetic procedure using a commercially available gold salt, which upon interaction with the amine forms highly active and selective gold nanoparticles.

Gold nanoparticles formed in situ from Au(OAc)3 show excellent activity Dibenzylamine in the oxidation of dibenzylamine to imine with dioxygen. Evolution of the active gold species is followed by in situ XANES and confirmed by electron microscopy.

Here, we report the oxidation of dibenzylamine to dibenzylimine with molecular oxygen as the only oxidant. Imines are important intermediates in organic chemistry, and their formation by the oxidation of amines has been well applied in natural product synthesis. The best known catalysts for this reaction are Ru/Al2O3, Ru2(OAc)4Cl, Ru-hydroxyapatite, and CuCl. Recently, even gold powder was suggested for this transformation but the turn-over frequency (TOF) was only about 0.001 h−1 at 100 °C. From a practical point of view, gold powder cannot be considered as a catalyst, since an Au/dibenzylamine mass ratio of higher than 25 was necessary to achieve 64% conversion in 1 day. We have found that gold acetate, poorly soluble in water and organic solvents, is far more efficient in the oxidative dehydrogenation of Dibenzylamine. Beside the catalytic study, we have also elucidated the probable nature of the active species by XANES.