Unveiling the Hidden σ-Dimerization of a Kinetically Protected Olympicenyl Radical.

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 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.