Unveiling the Hidden σ‐Dimerization of a Kinetically Protected Olympicenyl Radical
Qin Xiang, Jun Xu, Jing Guo, Yanfeng Dang, Zhanqiang Xu, Zebing Zeng, Zhe Sun
Abstract
Abstract The σ‐dimer of a kinetically protected olympicenyl radical, which evaded the experimental detection, was revealed by conversion into biolympicenylidene with E ‐configuration in a regioselective manner. The complicated stereochemistry and energetics of the σ‐dimers derived from C 2 v symmetry and uneven spin distribution of the olympicenyl radical were revealed by the theoretical calculations, and the energetic preference of π‐dimer over σ‐dimer by a minute gap was disclosed. The E‐ biolympicenylidene, a polycyclic ene structure previously considered as reactive intermediate in the phenalenyl radical system, exhibited exceptional stability, which allowed for a detailed investigation on its singlet diradical character and physical properties by means of X‐ray crystallography, UV‐vis‐NIR absorption/emission spectroscopy and cyclic voltammetry, and assisted by theoretical calculations. The E‐ biolympicenylidene showed high resistance towards both thermal and photochemical ring‐cyclization reactions, which was attributed to high activation energies for the rate‐determining electrocyclization operated on both disrotatory and conrotatory mode, as well as a small spin density at the bonding sites for the radical‐radical coupling process.