www.asiachem.news December 2021 | 37 N N MeO OMe h (355 nm) –N2 MeO OMe 14 15 MeO OMe Ph Ph trans-21 O O MeO OMe O O O CO2Me MeO H O O H OMe OMe NOE 17 18 19 20 MeO OMe 16 + + a 14.0 μs b at 293 K in benzene 155.9 μs macrocycle macrocycle 15a 16a Scheme 4. Generation of 15 with a macrocyclic structure. Figure 6. LFP study on 15a at 293 K in benzene. adjacent nitrogen functional groups.40–44 Considering this, we investigated the effect of nitrogen atoms on the reactivity of localized 1,3-diradicals (Figure 7).45,46 As mentioned above, diradical 2 is a triplet ground-state molecule, but the ground state of 22 changes to a singlet when nitrogen atoms are introduced in the five-membered ring, as in carbene. However, the ground state of carbene is a closed-shell singlet, while the ground state of 22 (ΔEST = ES – ET = –2.7 kcal mol –1 in C 2) is an open-shell singlet. Diradicals 23 and 24, with highly electronegative F or OH groups introduced on the carbon sandwiched between the radical moieties of 22, show larger singlet–triplet energy differences, ΔEST = –19.7 and –14.9 kcal mol–1 in C 2, respectively; moreover, an increase in these energy differences suggests that the singlet becomes more stable upon introducing the electron-withdrawing groups. N N H H N N X X 22: X = H 23: X = F 24: X = OH singlet ground state O N O H X X 25: X = H triplet ground state 26: X = OH singlet ground state Figure 7. Nitrogen-atom effect on the ground state spin-multiplicity. Figure 7. Nitrogen-atom effect on the gro nd state spi multiplicity. A S’ N N nN nN’ S A’ N N H H 22 Figure 8. Nitrogen-atom effect on the singlet ground state Figure 8. Nitrogen-atom effect on the singlet ground state The nitrogen-atom effect can be understood by the fact that the interaction between the orbitals nN and ψA increases the energy difference of ψs and ψA occupied by the two radical electrons, as shown in Figure 8. This is supported by the shorter nitrogen–nitrogen bond distance of diradical 22 (1.355 Å) than that of pyrazolidine (1.521 Å). The orbital interaction shown in Figure 8 is also proved by the fact that the triplet becomes the ground state for diradical 25, in which the electron-withdrawing group is introduced on the nitrogen atom. Interestingly, diradical 26, in which the electron-withdrawing substituent is introduced at the C2 position of the 1,3-diradical, changes to the singlet-ground-state diradical. Surprisingly, the 1,3-diradical with an electron-withdrawing substituent at the C2-position was found to be more energetically stable than the corresponding closed-ring form. To experimentally investigate the effect of nitrogen on the reactivity of 1,3-diradicals, we attempted to synthesize azo compounds 27 that can clean the diradical (Scheme 5). The cycloaddition of pyrazole with PTAD afforded azo compound 27 in a quantitative yield, and the denitrogenation of 27 led to the generation of derivative 28 of diradical 26. The reaction dynamics of diradical 28 were investigated by the laser flash photolysis (LFP) of 27 (Figure 9). As a result, we observed a chemical species exhibiting a strong absorption peak near 650 nm in the visible region, which is characteristic of singlet diradical species (Figure 9a). Interestingly, the species with absorption around 650 nm entailed two decay processes: a fast decay process (microseconds) and a slow decay process (milliseconds), as shown in Figure 9b. The species was determined to be the singlet diradical 28 because it was not quenched by molecular oxygen and
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