Origin of a Kinetically Selective Route for Thermoreversible Valence Tautomerism-Based Photoswitching in O-Heterocycles Containing Conjugated Dienes

Abstract

The thermal electrocyclic ring opening of fused cis-cyclobutene to cis,cis-diene is prohibited according to the Woodward−Hoffmann (WH) rules; nonetheless, experiments provide firm evidence for their formation. However, the mechanism, electronic structure, and behavior during the reaction are ambiguous. Herein, we attempt to gain insights into the mechanism of thermal ring opening in four O-heterocycles containing a conjugated diene as the core skeleton. The results emphasize that the reaction initiates with a conrotatory ring opening to produce a cis,trans-diene as a strained transient intermediate following the WH rules but ultimately leads to a forbidden product. The conversion of cis,trans-diene to cis,cisdiene is kinetically preferred through double-bond rotation, involving the formation of a singlet biradicaloid. The expected H-shift is unfavorable because the requirement of suprafacial H transfer incorporates additional strain, while the transfer of other available H is symmetry-forbidden. The increased electrophilicity at O promotes C−O dissociation in the 8membered cyclic cis,trans-dienes with fused benzene. This enables thermoreversible photoswitching via a two-step mechanism and kinetically refutes the direct disrotatory pathway. These systems can outperform energy storage, and critical insight into their mechanism with the aid of comprehensive electronic structure analysis would assist in their rational design in the future.