Impact of the macrocyclic structure and dynamic solvent effect on the reactivity of a localised singlet diradicaloid...

Localised singlet diradicals are key intermediates in bond homolysis processes. Generally, these highly reactive species undergo radical-radical coupling reaction immediately after their generation. Therefore, their short-lived character hampers experimental investigations of their nature. In this study, we implemented the new concept of "stretch effect" to access a kinetically stabilised singlet diradicaloid. To this end, a macrocyclic structure was computationally designed to enable the experimental examination of a singlet diradicaloid with π-single bonding character. The kinetically stabilised diradicaloid exhibited a low carbon-carbon coupling reaction rate of 6.4 × 10³ s^-1 (155.9 μs), approximately 11 and 1000 times slower than those of the first generation of macrocyclic system (7.0 × 10⁴ s^-1, 14.2 μs) and the parent system lacking the macrocycle (5 × 10⁶ s^-1, 200 ns) at 293 K in benzene, respectively. In addition, a significant dynamic solvent effect was observed for the first time in intramolecular radical-radical coupling reactions in viscous solvents such as glycerin triacetate. This theoretical and experimental study demonstrates that the stretch effect and solvent viscosity play important roles in retarding the σ-bond formation process, thus enabling a thorough examination of the nature of the singlet diradicaloid and paving the way toward a deeper understanding of reactive intermediates.