Physical chemistry chemical physics : PCCP ·10 Nov 2026 ·Added 10 Nov 2026

Pathway-selective solvent control of post-transition-state dynamics in the cycloaddition of 2-azacyclopentadienone with furan.

Rama Krishna Kadiyam, Savya Vijay Masarkar, Ravinder Pawar

DOI ↗

Abstract

The cycloaddition of 2-azacyclopentadienone (2-AzaCPD) with furan was investigated computationally to clarify the mechanistic behavior of the competing Endo-1 and Endo-2 pathways in the gas phase and in acetonitrile. Stationary points were characterized at the ωB97X-D/6-311++G** level, and refined ZPE-corrected relative energies were obtained from DLPNO-CCSD(T)/def2-QZVPP/TightPNO single-point calculations. Intrinsic reaction coordinate calculations, quasi-classical molecular dynamics simulations, and intrinsic bond orbital (IBO) analyses were employed to examine reaction-path connectivity, dynamical outcomes, and electronic reorganization. In the gas phase, both endo pathways are strongly directed toward single cycloadduct-forming manifolds, although Endo-1 proceeds through a multibonding and highly asynchronous transition structure. In acetonitrile, Endo-1 exhibits reduced transition-state geometrical asymmetry, accompanied by qualitative differences in orbital reorganization and a broader distribution of post-transition-state dynamical outcomes. The ACN-phase Endo-1 trajectories exhibit direct formation of cycloadduct 1a, access to the IM-A region, and reactant recrossing, while a separate subset exhibits paired-direction outcomes involving cycloadduct 1a formation in one propagation direction and IM-A access in the opposite direction. However, no direct formation of the alternative cycloadduct 1a' is observed. In contrast, the Endo-2 pathway remains predominantly product-directed in acetonitrile. These findings demonstrate that solvation can selectively influence post-transition-state dynamics and give rise to pathway-dependent dynamical branching, although the observed trajectory outcomes do not, by themselves, establish a formal post-transition-state bifurcation of the multidimensional potential-energy surface.