Molecular dynamics simulation study of post-transition state bifurcation: A case study on the ambimodal transition state of dipolar/Diels–Alder cycloaddition

The potential energy surfaces for the reactions of 1,3-butadiene with 2-hydroxythioacrolein and 2-aminoacrolein exhibit ambimodal transition states leading to both dipolar (4 + 3) and Diels–Alder (4 + 2) cycloaddition products, thereby demonstrating a post transition state bifurcation feature. We have investigated the bifurcation dynamics of these reactions using three molecular dynamics (MD) methods: quasi-classical trajectory, classical MD, and ring-polymer MD simulations. The trajectory calculations were performed with the semiempirical GFN2-xTB method with the element-specific parameters optimized to reproduce the density-functional theory calculations. The effect of water solvation was examined using an implicit solvation model, revealing significant differences in bifurcation dynamic between gas-phase and solution-phase reactions. Nuclear quantum effects were found to play a crucial role in the proton-transfer process from the (4 + 3) intermediate to the (4 + 3) product in the case of the 2-aminoacrolein reaction.