Computational mechanistic study on N-nitrosation reaction of secondary amines

The presence of potentially carcinogenic nitrosamines in drugs has been a worldwide concern, driving strategies to control or mitigate their formation to protect patient health. Understanding the critical factors for N-nitrosation, such as mechanisms and energy barriers, enhances the risk assessment process to understand potential nitrosamine formation. Evaluation of the structural impact of amines on the N-nitrosation rate in the presence of nitrites and acidic media is of great interest to pharmaceutical companies assessing the risk of nitrosamine drug substance-related impurities. A range of secondary amines was explored using DFT calculations to assess the impact of electronic and steric effects on activation energy. Asym-N₂O₃ was selected as the nitrosating agent since its reaction was shown to be favorable following screening of pathways employing nitrosyl chloride, nitrous acid, asym-N₂O₃, sym-N₂O₃, and trans-cis-N₂O₃. The relatively low activation energies obtained for all amines indicate the reaction is very likely to occur if the reactive components encounter, even for amines with sterically hindered and electron-withdrawing groups. Understanding the interaction between the amine and nitrosating agent is therefore the defining factor in the risk of formation of more complex nitrosamines within drugs.

Calculations were performed using the Gaussian-16 program. The B3LYP-D3/def2-TZVP level of theory was applied for structure optimizations. The IEF-PCM implicit model was used for the solvent effect. Intrinsic reaction coordinate calculations were carried out to connect the transition state with the associated minimum.