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

Abstract

The σ-dimer of a kinetically protected olympicenyl radical, which evaded the experimental detection, was revealed by conversion into biolympicenylidene with Econfiguration in a regioselective manner. The complicated stereochemistry and energetics of the σ-dimers derived from C 2v 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.

Steric Hindrance Favors σ Dimerization over π Dimerization for Julolidine Dicyanomethyl Radicals

“…6H) 13. C{ 1 H} NMR (101 MHz, CDCl 3 ): δ 153.1, 143.7, 124.9, 124.1, 116.6, 105.0, 47.6, 47.3, 40.6, 37.5, 32.4, 32.2, 29.2.…”

“…Free radicals that exist in equilibrium with dimers are useful building blocks for stimuli-responsive materials, − spin crossover materials, , and dynamic covalent assemblies. ,,− Such metastable radicals dimerize to form either weak σ dimers or π dimers (pimers). − See Figure . While σ dimerization is a standard bonding motif, the radical–radical bond within π dimers features a multicenter covalent bonding pattern that results from head-on π orbital overlap that results in the atoms being closer than van der Waals distance but much longer than a traditional two-atom bond. − …”

Doublet Open‐Shell Graphene Fragments

“…Upon addition of p-chloranil to the equilibrating system, the hitherto hidden σ-dimer of 5 a could be transformed into a stable singlet diradical species, Ebiolympicenylidene (Figure 7c). [53]…”

“…Therefore, the σ‐dimer of 5 a existed as a “hidden” minor species that evaded detection. Upon addition of p ‐chloranil to the equilibrating system, the hitherto hidden σ‐dimer of 5 a could be transformed into a stable singlet diradical species, E ‐biolympicenylidene (Figure 7c) [53] …”

π-Extended Doublet Open-Shell Graphene Fragments Exhibiting One-Dimensional Chain Stacking

“…In addition, the fact that BR2 was more stable than BR1 manifested the importance of kinetic blocking for the enhancement of the stability, since the 13-position bears no spin density. It is noteworthy that the treatment of the TIPSe substituted OR derivative with p -chloranil gave the dimerized singlet diradicaloid biolympicenylidene; however, no such products were identified with BR1 and BR2 .…”