126: Mild process for selective pyridine production from biomass

Abstract: Pyridine is a commodity chemical that is used as a solvent and precursor for value-added chemicals in the agricultural and pharmaceutical industries. Current methods to produce pyridine such as the Chichibabin synthesis require temperatures exceeding 400 °C and produce alkylated pyridines as side products, decreasing yield to pyridine itself. Pyridine can also be produced by the Bonnemann cyclization, but that requires the use of hazardous hydrogen cyanide. The two key reactions to produce pyridine with ammonia and aldehydes are the formation of imines and acceptorless dehydrogenation. The formation of imines occurs at room temperature. The dehydrogenation to release hydrogen gas is what requires high temperature. Oxidative dehydrogenation can produce the same pyridine product at much lower temperatures. Glutaraldehyde can be obtained from processing hemicellulose via furfural. Imine formation with ammonia and cyclization can lead to dihydropyridine, which would leave pyridine after dehydrogenation. The reaction of glutaraldehyde to the pyridine moiety has been demonstrated before, but the main product is alkylated pyridine polymers using oxygen as terminal oxidant. Selective conversion to unalkylated pyridine has not been demonstrated. Here, we implement copper salts as oxidants and as catalysts for the aerobic dehydrogenation of glutaraldehyde and ammonia to pyridine in 99% reaction yield below 100 °C in water. The reaction is susceptible to forming various side products, but careful design of the reaction conditions can push selectivity to the unalkylated pyridine ring especially at higher working concentrations. When optimized for high yield, the only byproducts of the reaction are water. This metal-catalyzed oxidative dehydrogenation may prove useful for forming derivatives of pyridine at mild temperatures as well.