A B3LYP/6-31+G(d) study of the reaction pathways and conformational preference in a model Chichibabin reaction

The mechanism of the sodium amide initiated amination of pyridine, known as the Chichibabin reaction, was investigated at the B3LYP/6-31+G(d) level of theory. Although this reaction has been known for more than one hundred years, it remains a very useful and sometimes unique method for the synthesis of a variety of heterocyclic nitrogen-containing compounds. A total of six SN(AE)-based mechanistic pathways were proposed to explain the formation of the major product, α-aminopyridine, and the minor γ-byproduct. Our investigations showed that reaction pathways leading to both products proceed similarly. Our results showed that the most likely first step is the addition of sodium amide to either the alpha or the gamma position, then hydrogen gas elimination occurs, and finally the desired product forms in the work-up step. We found that the temperature and especially the nature of the solvent are critical factors in the reaction energy barriers and thus in obtaining reasonable yield for the preferred product. The alpha aminopyridine pathway is preferable to the gamma substitution when modeled regardless of temperature. The influence of the studied experimental conditions provides a reasonable explanation for the experimentally observed gamma byproduct. Based on the experimental evidence and our calculations, it is proposed that the mechanism proceeds through the loss of hydrogen gas rather than the formation of sodium hydride in the second step.