C–H activation and C–C coupling of 4-methylpyridine using palladium supported on nanoparticle alumina

The C–H activation and C–C coupling of 4-methyl pyridine to 4,4′-dimethyl-2,2′-bipyridine was studied over palladium catalysts supported on nanoparticle alumina [nano-Al2O3(+)]. Even though Pd/Al2O3 catalysts are traditionally poor catalysts in this reaction, the Pd/nano-Al2O3(+) catalysts prepared via the precipitation method give the highest yield observed to date for this reaction system. The catalytic activity is very dependent on the catalyst preparation as Pd/nano-Al2O3(+) catalysts prepared via the wet impregnation method exhibit poor activities in the reaction. Additionally, a catalyst prepared using another nanoparticle alumina with larger particle sizes [nano-Al2O3(−)] does not have a significant activity. It was shown that using a commercial bimodal γ-Al2O3 support can result in an active catalyst if prepared via the precipitation method, but compared to the best performing catalyst, Pd/nano-Al2O3(+), the yields obtained from Pd/bimodal-γ-Al2O3 are lower and less reproducible. Pd surface area measurements indicate that the reaction is structure sensitive as there is no correlation between the Pd surface area and the catalytic activity. The reaction is also very sensitive to reactant quality and the 4-methyl pyridine must be distilled over KOH to ensure reproducible yields. Additional experiments indicate that this reaction requires a solid phase for catalysis.

Graphical abstractPalladium supported on nano-particle alumina is shown to be an efficient catalyst for the C–H activation and C–C coupling of 4-methylpyridine. The activity of this catalyst is greater than that obtained from the commonly used Pd/C catalyst and the efficiency of the reaction is strongly dependent upon support selection, method of palladium deposition and quality of precursors and reagents.Figure optionsDownload full-size imageDownload as PowerPoint slide