Conformationally Dictated Divergent Triplet Reactivity of Conjugated Dienes: Visible-Light-Induced Electrocyclization, 1,5-HAT, and Cycloaddition

Abstract The precise control of reaction trajectories remains a formidable challenge in photochemical synthesis, particularly for conjugated dienes whose consecutive π-systems give rise to multiple competing reaction pathways. Herein, we report a divergent synthetic platform enabled by visible-light-induced energy transfer (EnT) catalysis. By exploiting how cyclic conjugated dienes accommodate or restrict the requisite torsional distortion of the cyclic backbone to form a trans-cycloalkene intermediate, this strategy dictates their photochemical fate, elegantly channeling reactivity into three distinct manifolds through ring-dependent kinetic control. Through strategic manipulation of skeletal parameters─such as ring size, geometric rigidity, and steric hindrance─we achieved selective access to highly strained cyclobutene-fused scaffolds via intramolecular 4π electrocyclization, functionalized azetidines via an intramolecular 1,5-hydrogen atom transfer (1,5-HAT), and tricyclic adducts via an intermolecular [4 + 2] Diels–Alder cycloaddition. Comprehensive experimental and density functional theory (DFT) studies reveal that the precise pathway divergence of these photogenerated transient intermediates is strictly dictated by conformationally controlled kinetic barriers. These geometric parameters govern not only the accessibility of the highly reactive, twisted trans-cycloalkene intermediate but also the subsequent kinetic competition among divergent reaction channels. Ultimately, this work demonstrates that visible-light sensitization enables the generation and productive exploitation of such high-energy transient species, providing a modular blueprint for the programmed construction of complex architectures from simple diene precursors.