Computational Mechanistic Study on N-Nitrosation Reaction of Secondary Amines

Context 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 confidence in decision-making for the risk assessment of nitrosamine formation. Evaluation of 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 were explored using DFT calculations to assess the impact of electronic and steric effects on activation energy. Asym -N ₂ O ₃ was selected as the nitrosating agent since its reaction was shown to be favourable following screening of pathways employing nitrosyl chloride, nitric acid, asym -N ₂ O ₃ , sym -N ₂ O ₃ and trans-cis -N ₂ O ₃ . 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 of amine and nitrosating agent is therefore the defining factor in the risk of formation of more complex nitrosamines within drugs. Furthermore, the amine pK _a plays an important role in the relative reaction rates and should also be considered in risk evaluations. Methods Calculations were performed using Gaussian-16 program. The B3LYP-D3/def2-TZVP level of theory was applied for structure optimizations. The IEF-PCM implicit model was used for solvent effect. Intrinsic reaction coordinate calculations were carried out to connect the transition state with the associated minimum.