Impact of the macrocyclic structure and dynamic solvent effect on the reactivity of a localised singlet diradicaloid with π-single bonding character. Issue 2 (16th November 2020)

• Record Type: Journal Article
• Authors: Wang, Zhe; Akisaka, Rikuo; Yabumoto, Sohshi; Nakagawa, Tatsuo; Hatano, Sayaka; Abe, Manabu
• Is Part Of: Chemical science. Volume 12:Issue 2(2021)
• Page Start: 613
• Page End: 625
• Publication Date: 2020-11-16
• Subjects: Chemistry -- Periodicals; 540.5
• DOI: 10.1039/d0sc05311b
• Languages: English
• ISSNs: 2041-6520

Abstract

Abstract : An extremely long-lived localised singlet diradical with π-single bonding character is found in a macrocyclic structure that retards the radical–radical coupling reaction by the "stretch and solvent-dynamic effects". Abstract : 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 × 10 3 s −1 (155.9 μs), approximately 11 and 1000 times slower than those of the first generation of macrocyclic system (7.0 × 10 4 s −1, 14.2 μs) and the parent system lacking the macrocycle (5 × 10 6 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.