Unraveling Ambimodal Periselectivity in Pentafulvene–Radialene Cycloadditions: Interplay of Geometry, Charge Delocalization, and Reaction Dynamics

The ambimodal cycloaddition between pentafulvene and a homologous series of [<i>n</i>]radialenes (<i>n</i> = 3-6) was systematically investigated using density functional theory, DLPNO-CCSD(T) calculations, and quasi-classical molecular dynamics (QCMD) simulations to elucidate the geometric and electronic origins of periselectivity. Despite their intrinsic instability, these π-rich systems enable the formation of multiple products through bifurcated potential energy surfaces (PESs). Two distinct ambimodal transition states were identified, corresponding to bispericyclic (exo) and trispericyclic (endo) pathways. The reaction revealing [5]-radialene exhibits the most pronounced ambimodal character, driven by favorable exocyclic π-overlap, extensive charge delocalization, and the lowest activation barrier. Intrinsic bond orbital and interaction region indicator analyses reveal asynchronous σ-bond formation and progressive orbital reorganization that govern dynamic branching behavior. QCMD simulations further confirm that product distributions are dictated by PES topography rather than static transition-state energetics, underscoring the significance of dynamic effects in determining selectivity. This comprehensive study establishes a unified mechanistic framework linking geometry and electronic delocalization to dynamic selectivity control in cross-conjugated π-systems, offering predictive insights into ambimodal bis- and trispericyclic reactivity with potential implications for synthetic and biosynthetic design.