(140e) Mild Process for Renewable Pyridine Production

Pyridine is a commodity chemical that finds uses as an industrial solvent and precursor for value-added chemicals. 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 requires the high temperature. Oxidative dehydrogenation can produce the same pyridine product at much lower temperatures.

Glutaraldehyde can be obtained from processing hemicellulose. 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 at mild temperatures. The reaction is susceptible to producing overalkylated pyridine rings, but careful design of the reaction conditions can push selectivity to the unalkylated pyridine ring. This metal-catalyzed oxidative dehydrogenation may prove useful for forming derivatives of pyridine at mild temperatures as well.