Reaction-Partner-Dependent Switching between Single-Valley and Bifurcating Reactivity in 3-Azacyclopentadienone...

The cycloaddition behavior of 3-azacyclopentadienone depends strongly on reaction-partner identity, but how this dependence reshapes post-transition-state dynamics remains unclear. Herein, we investigate the endocycloadditions of 3-azacyclopentadienone with furan and cyclopentadiene using density functional theory, DLPNO-CCSD(T) single-point calculations, quasi-classical molecular dynamics trajectories, and complementary orbital analyses. The results show that reaction-partner identity reshapes both the transition-state region and the downstream reaction landscape. In the furan system, one endo pathway proceeds through a highly asynchronous ambimodal transition structure but exhibits a strong dynamic preference for one product channel, whereas the competing pathway follows a conventional single-valley route. In contrast, both endo pathways with cyclopentadiene access bifurcating post-transition-state surfaces and display substantial two-product dynamic competition. Solvation-model-based SMD calculations in dichloromethane, tetrahydrofuran, and acetonitrile indicate that solvation produces only modest changes in barrier heights and does not alter the qualitative kinetic ordering. Intrinsic bond orbital and electron density of delocalized-bond analyses further support these contrasting behaviors in terms of post-transition-state bond reorganization and cyclic electron delocalization. These results demonstrate that reaction-partner identity can switch 3-azacyclopentadienone cycloadditions among single-valley descent, biased ambimodal branching, and persistent post-transition-state bifurcation.