N-alkylation of amines with alcohols over alumina-entrapped Ag catalysts using the “borrowing hydrogen” methodology

Ag/Al2O3 catalysts were found to be highly active and selective in the N-alkylation of aniline with a variety of aromatic and linear alcohols. In the presence of a base, for example, Cs2CO3 or K3PO4 as promoter, high yields of secondary amines were obtained. The highest activity was found with 2.4 wt.% Ag/Al2O3. The reaction involves the transient catalytic dehydrogenation of the alkylating alcohol to the corresponding carbonyl compound which then reacts spontaneously with the amine. In the final step, the hydrogen from the alcohol dehydrogenation step is transferred back to the imine, giving the secondary amine as the final product. With secondary amines, piperidine and pyrrolidine, the hemiaminal intermediate can undergo dehydrogenation as well as dehydration to give amides and amines, respectively. The heterogeneous nature of Ag/Al2O3 allows for easy recovery and regeneration, which makes it an attractive system in the context of green and sustainable synthesis.