Unveiling the Hidden σ‐Dimerization of a Kinetically Protected Olympicenyl Radical

• Published in: Chemistry : a European journal Vol. 27; no. 31; pp. 8203 - 8213
• Main Authors: Xiang, Qin, Xu, Jun, Guo, Jing, Dang, Yanfeng, Xu, Zhanqiang, Zeng, Zebing, Sun, Zhe
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 01.06.2021
• Subjects: Biolympicenylidene; Chemical reactions; Chemistry; Crystallography; dehydrogenation; Dimerization; Dimers; Emission spectroscopy; High resistance; Mathematical analysis; olympicenyl; Photochemical reactions; Photochemicals; Physical properties; singlet diradical; Spectroscopy; Stereochemistry; Thermal resistance; dimerization; singlet diradical; olympicenyl; dehydrogenation; Biolympicenylidene
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.202100631

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 C2v 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. The hitherto undetected σ‐dimerization pathway in an olympicenyl system was revealed from both theoretical and experimental perspectives. Theoretical calculations gave clear structural and energetic profiles for the σ‐dimerization, and regioselective oxidation of the olympicenyl radical afforded biolympicenylidene with significant singlet diradical character and high resistance towards both thermal and photochemical ring‐cyclization.