www.asiachem.news December 2021 | 33 By Manabu Abe, Zhe Wang, and Rikuo Akisaka https://doi.org/10.51167/acm00021 F F RO OR 1 2 3 4 triplet ground state singlet ground state Ar Ar RO OR max ~600 nm ( ~5000 M–1cm–1) X X X X DR Ar Ar RO OR CP Scheme 2 Localized 1,3-diyl diradicals in cyclic structures. New Insights into Bond Homolysis Process and Discovery of Novel Bonding System (C–π–C) by Generating Long-lived Singlet Diradicals Chemistry of Localized Singlet Diradicals (singlet radical pair) It has recently been discovered that in some cases, three energy minima may be observed in the bond homolysis process; nevertheless, it is generally believed that there are two energy minima in this process. For example, homolytic cleavage of the carbon–carbon σ-bond of the ethane molecule (CH3–CH3) generates a singlet radical pair of two methyl radicals (·CH3) (Scheme 1) whose energy difference is approximately equal to the bond dissociation energy of the σ bond, approximately 85 kcal mol–1. If we can directly observe the thermal equilibrium process between these two states, we can examine the solvent and substituent effects on the equilibrium process and clarify the details of the bond homolysis process experimentally. However, since the activation energy of the σ-bond formation reaction between methyl radicals is almost zero, it is extremely difficult to directly observe the singlet radical pair in a simple manner. To achieve the direct observation of singlet radical pairs based on kinetic stabilization, the authors focused on singlet diradicals in a cyclic framework (Scheme 2). Prior to the study on singlet diradicals by the present authors, Kistiakowsky1, Hoffmann2,3, Cl oss4,5, Schaefe r6, Be r son6, Adam7, Dougherty8,9, and Borden10 reported on the reactivity and ground state of 1,3-diradicals in the 1930s, 1960s, 1970s, 1980s, and 1990s, respectively. The role of 1,3-diradicals in the thermal isomerization of cyclopropane to propene was first proposed by Kistiakowsly et al. (Scheme 2), and theoretical and experimental studies were performed by Hoffmann and Berson et al. The triplet ground states of cyclobutane-1,3-diyl-diradical 1 and cyclopentane-1,3-diyl-diradical 2 were discovered by Closs, Adam, and Dougherty, and the ground state of 2,2-difluoro-1,3-diradical 3 was found to be singlet by Borden et al. To study the reactivity of singlet 1,3-diradicals experimentally and to clarify the bonding homolysis process, the present authors deemed it essential to extend the lifetime of the singlet diradicals by stabilizing them kinetically. In the latter half of the 1990s, we began to study 1,3-diradicals 4 with oxygen functional groups at the 2-position, keeping in mind that the molecular design of long-lived singlet diradicals requires (1) a singlet ground state and (2) the easy synthesis of a variety of derivatives. As a result, we succeeded in generating localized singlet diradicals DR that can be easily observed by time-resolved spectroscopy owing to their strong absorption peak around 600 nm in the visible region, which quantitatively gives the σ-bonded product CP.11–13 Ground state spin multiplicity of 1,3-diradicals Closs et al. found that when the substituents at the C2 position of 1,3-diradicals are hydrogen atoms (X = H), the most stable spin multiplicity of the diradical is a triplet (Table 1).4,5 This is because the energy difference between the two orbitals ψS and ψA, which the two electrons of the 1,3-diradical occupy, becomes small due to the orbital interaction between ψS and the pseudo-π orbital σCH at the second position.3 The triplet state becomes energetically stable when one electron occupies ψS,H and ψA,H each according to Hund’s rule (Figure 1). In other words, by increasing the energy
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