(201g) Promoter Effect of Alklyamine Functionalized Silica on Gold Nanoparticle Catalyzed Hydroamination Reactions
The hydroamination of alkynes is widely used to produce imines for the pharmaceutical industry. The reaction is catalyzed by gold nanoparticles and the reputed heterogeneous hydroamination mechanism necessitates distinct protonation and deprotonation events. We hypothesize that the colocalization of labile hydrogen donors and the reactive surface would improve catalyst performance. We synthesized alkylamine-functionalized, hollow, mesoporous silica shells containing encapsulated gold nanoparticles to use as hydroamination nanoreactors. The chemical structure of the grafted alkylamines was characterized with solid-state, magic angle spinning, nuclear magnetic resonance spectroscopy (MAS-NMR) and Fourier-transform infra-red spectroscopy. The alkylamine loading was quantified with thermogravimetric analysis and suggested that the heterogeneous catalyst was approximately 20 % alkylamine by weight. We calcined an aliquot of the catalysts to remove the alkyamine functionalization. Finally, the hydroamination of 1-octyne with aniline was monitored with quantitative nuclear magnetic resonance spectroscopy at 360 K in a series of batch reactions. The catalytic results suggest that the colocalization of grafted alkylamines significantly improves catalyst performance when compared to the calcined nanoparticles. Future work is underway to derivatize the alkylamines to functional groups of greater basicity. Further studies will investigate the catalytic properties of gold nanoparticles colocalized with silica-grafted alkylguanidiniums and alkylimidazoles, which mimic highly-active amino acids, histidine and arginine respectively. The successful colocalization of grafted organic promoters and the active surface has the potential to reduce the loss of homogeneous cocatalysts and the concurrent cost of product purification. The grafting of organic promoters offers additional strategies to tailor the surface properties of catalytic materials for specific reactions of interest.