Amine-Functionalized Mesoporous Silica as a Support for on-Demand Release of Copper in the A3-Coupling Reaction: Ultralow Concentration Catalysis and Confinement Effect

The study of catalysts activity at ultralow concentration is of prime importance for the development of more sustainable catalytic processes. In this work, we designed a mesoporous MCM-41 silica with covalent functionalization with amine groups at different levels of coverage. This material was used as a support to immobilize small quantities of Cu(I) species to be used as a catalyst in the A(3) -coupling reaction. The support design allowed controlled release of the catalytically active species as well as its scavenging after reaction. This system achieved excellent catalytic performance, leading to 95% yield of the desired propargylamine within 2 h of reaction at 100 degrees C under microwave conditions, using only 0.02 mol % of catalyst, the lowest catalyst amount ever reported for this reaction and high TON (4750) and TOF (2375 h(-1)). An interesting effect was noticed, where the catalyst on the support yielded improved reaction rates as compared to unsupported catalysts in solution under the same concentration conditions. We discuss the possibility of a confinement of metal species inside the mesoporous structure of the support, which made the immobilized system more effective than its homogeneous counterpart. The present study pushes the limits of A(3)-coupling reaction conditions and the potential of supported metal catalysis.