Energy Transfer (EnT) Catalysed [4+2] Cycloaddition of Non-Symmetrical Dienes: From Mechanistic Anomaly to Fully Functional Reaction Pathway

Harnessing the reactivity of triplet biradicals to selectively access unconventional cycloaddition pathways remains a fundamental challenge in energy transfer (EnT) catalysis. Herein, we demonstrate that systematic variation of diene substitution can redirect the intrinsic [2+2] reactivity of non-symmetrical dienes towards highly selective [4+2] cycloaddition. Notably, incorporation of a traceless vicinal BPin group invokes a non-conventional [4+2] pathway providing polysubstituted cyclohexenes with excellent regio- and diastereoselectivity. Mechanistic and computational investigations reveal that A-1,3-strain and geometric isomerisation shape the conformational landscape of the triplet biradical, accounting for the observed divergent reactivity. These findings establish substitution-controlled excited-state conformational dynamics as a key parameter for directing EnT-mediated cycloadditions beyond conventional [2+2] reactivity.