Dynamical insights into denitrogenation of 1-pyrazoline: exploring pathways via transition states and a second-order saddle†

The mechanism of 1-pyrazoline denitrogenation has garnered significant attention due to its remarkable stereoselectivity. In this study, the thermal denitrogenation mechanism of 1-pyrazoline was investigated using ab initio classical trajectory simulations to elucidate post-transition state and post-second-order saddle dynamics. Trajectories initiated from the synchronous transition state region predominantly followed the minimum energy pathway, forming the trimethylene diradical intermediate, which subsequently yielded cyclopropane with a preference for single inversion of the configuration. Additionally, the post-second-order saddle dynamics revealed that most trajectories followed the minimum energy path, offering alternative pathways for cyclopropane formation with retention of the configuration. In contrast, trajectories initiated from asynchronous transition state regions mostly deviated from the minimum energy path, leading to longer-lived diazenyl diradicals while still favoring single inversion in the final products. Despite significant diradical lifetimes, trajectories from all six transition-state regions exhibited a preference for single inversion cyclopropane formation, suggesting that product selectivity is dictated by dynamical effects rather than the reaction pathway.