Origin of a Kinetically\nSelective Route for Thermoreversible\nValence Tautomerism-Based Photoswitching in <i>O</i>‑Heterocycles\nContaining Conjugated Dienes

The thermal electrocyclic ring opening of fused <i>cis</i>-cyclobutene to <i>cis</i>,<i>cis</i>-diene is\nprohibited according to the Woodward–Hoffmann (WH) rules; nonetheless,\nexperiments provide firm evidence for their formation. However, the\nmechanism, electronic structure, and behavior during the reaction\nare ambiguous. Herein, we attempt to gain insights into the mechanism\nof thermal ring opening in four <i>O</i>-heterocycles containing\na conjugated diene as the core skeleton. The results emphasize that\nthe reaction initiates with a conrotatory ring opening to produce\na <i>cis</i>,<i>trans</i>-diene as a strained\ntransient intermediate following the WH rules but ultimately leads\nto a forbidden product. The conversion of <i>cis</i>,<i>trans</i>-diene to <i>cis</i>,<i>cis</i>-diene is kinetically preferred through double-bond rotation, involving\nthe formation of a singlet biradicaloid. The expected H-shift is unfavorable\nbecause the requirement of suprafacial H transfer incorporates additional\nstrain, while the transfer of other available H is symmetry-forbidden.\nThe increased electrophilicity at O promotes C–O dissociation\nin the 8-membered cyclic <i>cis</i>,<i>trans</i>-dienes with fused benzene. This enables thermoreversible photoswitching\nvia a two-step mechanism and kinetically refutes the direct disrotatory\npathway. These systems can outperform energy storage, and critical\ninsight into their mechanism with the aid of comprehensive electronic\nstructure analysis would assist in their rational design in the future.