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 × 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.
Highlights
The insight into fundamental processes and innate reactivity o en triggers innovation
The effect of the macrocycle on the structure and reactivity of SDR3b was evaluated by the complete active space self-consistent eld (CASSCF)[69] and broken-symmetry (BS)[70] density functional theory (BS-DFT) methods at the (U)uB97X-D/6-31G(d)[71,72,73] level of theory within the Gaussian 16 74 package
In the past two decades, continuous efforts were made to extend the lifetime of putative intermediates and experimentally elucidate the bond homolysis process
Summary
The insight into fundamental processes and innate reactivity o en triggers innovation. The isolation allowed a detailed investigation of the ground-state spin multiplicity and reactivity of this species, resulting in the elucidation of its triplet ground state and its heavy-atom tunnelling reaction.[45,46,47] carbon–carbon singlet diradical S-DR2a (s293 1⁄4 80 ns in npentane), in which electron-withdrawing groups (EWGs) lower the energy of the singlet state with regard to that of the triplet state, was rst detected in 1998,48 whereas the longer-lived SDR2b (s293 1⁄4 209 ns in benzene) featuring exible alkoxy groups has been studied in our laboratory.[49,50,51,52]. ALifetime values s293 were determined in benzene These scaffolds, structural rigidity precludes the s bond formation between the radical centres, a phenomenon termed “stretch effect” (Scheme 2a). This effect was studied using macrocyclic singlet diradicaloid S-DR3a.
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