Surface ligands enhance the catalytic activity of supported Au nanoparticles for the aerobic α-oxidation of amines to amides

The catalytic aerobic alpha-oxidation of amines in water is an atom economic and green alternative to current methods of amide synthesis. The reaction uses O-2 as terminal oxidant(,) avoids hazardous reactants and gives water as the only byproduct. Here we report that the catalytic activity of silica-supported Au nanoparticles for the aerobic alpha-oxidation of amines can be improved by tethering pyridyl ligands to the support. In contrast, immobilization of thiol groups on the material gives activities comparable to Au supported on bare silica. Our studies indicate that the ligands affect the electronic properties of the Au nanoparticles and thereby determine their ability to activate O-2 and mediate C-H cleavage in the amine substrate. The reaction likely proceeds via an Au catalyzed beta-hydride elimination enabled by backdonation from electron-rich metal to the sigma(C-H)* orbital. O-2, which is also activated on electron-rich Au, acts as a scavenger to remove H from the metal surface and regenerate the active sites. The mechanistic understanding of the catalytic conversion led to a new approach for forming C-C bonds alpha to the N atoms of amines.