www.asiachem.news December 2021 | 39 transition state trans-TS giving trans-32, indicating that cis-32 is kinetically favored. These calculations indicate that cis-32 was formed exclusively in the triplet-sensitized denitrogenation of 31 because of the intersystem crossing (ISC) of the planar triplet diradical 36 to the more stable 6, which selectively leads to cis-32. Experimentally, the activation energy barrier of the radical coupling reaction in 6 was found to be 41.4 kJ mol–1, which is similar to the activation energy barrier for the transformation from 6 to cis-32 determined by quantum chemical calculations. When we started the IRC calculation from trans-TS, it was suggested that there is a third intermediate 33 between trans-32 and trans-TS, which had not been observed in the bonding homolysis process. Quantum chemical calculations (BS-UB3LYP/6-31G(d)) showed that the energy of 33 was approximately 36.8 kJ mol–1 higher than that of 6. It was also found that there was almost no energy barrier to trans32, and trans-32 was quickly obtained when 33 was generated (Figure 10). In other words, in the direct excitation of 31, 134 generated in the stepwise denitration process preferentially gives trans-32 via the puckered-type intermediate 33 (Scheme 7). By contrast, in the triplet sensitization reaction, cis-32 is exclusively generated from the triplet intermediate 334, which has a long lifetime and can change its conformation to the most stable planar diradical 36. After intersystem crossing to 6, cis-32 was selectively formed in the ring-closing reaction. As shown in scheme 7, the mystery of the remarkable spin multiplicity effect on stereoselectivity observed in the photo-denitration reaction of 31 seems to be solved by assuming the intervention of a previously unobserved puckered-type singlet diradical. However, there is no direct experimental evidence for the existence of such a diradical, and the analogy is based on DFT calculations, which are inherently unable to treat open-shell singlet diradicals with theoretical precision. The present author aimed to experimentally capture the third structure 33 intervening in the bond homolysis process. Therefore, we attempted to stabilize the puckered structure by introducing a bulky substituent, as is usual, and directly observed the derivative of 33 (λcald = 480 nm), which is expected to appear at wavelengths shorter than those of 6 (λcald = 580 nm) in our calculations (Figure 11). Azo compound 35 was synthesized by introducing –OCH2Ph as a bulky substituent in the alkoxy moiety and a meta-dimethoxyphenyl group (–C6H3(OMe)2) as an aryl group. The photodenitrogenation reaction was conducted in a low-temperature organic glass matrix using 2-methyltetrahydrofuran (MTHF, melting point 137 K). At 110–120 K, where a soft matrix was obtained, a strong absorption around 580 nm corresponding to a planar singlet diradical was observed and attributed N N Ph Ph MeO OMe MeO OMe Ph Ph Ph Ph MeO OMe N2 31 trans-32 77 K fluorescence ~300 nm + 515 m MeO OMe Ph Ph cis-32 tripletsensitization direct (singlet) photolysis puckered diradical 33 cis-32 trans-32 Ph Ph O O CH3 CH3 H H O O Ph H H CH3 H3C 0.0 43.9 –13.3 65.4 trans-TS cis-TS Hrel in kJ mol-1 Ph OMe OMe planar singlet diradical 6 at (U)B3LYP/6-31G(d) Ph Ph MeO OMe MeO OMe Ph Ph –28.8 36 ISC MeO OMe Ph Ph 36.8 33 Scheme 6. Puckered diradical 33 as a new intermediate of bond homolysis. Figure 10 Computational study on reactivity of planar singlet diradical 6. to 37. At temperatures below 98 K, where the matrix is hard, no absorption near 580 nmwas observed, and a chemical species with electronic absorption near a shorter wavelength of 450 nm was observed instead of 37. Because of the good agreement with the calculated absorption wavelength of 36 and the silence in the electron paramagnetic resonance (EPR) spectrum, the absorption near 450 nm was assigned to the third minimum-energy structure, a puckered-type singlet diradical 36. Anomalous long wavelength emission from trans-32 The puckered-type diradical 33 is the third minimal-energy structure found in the groundstate bond homolysis process and is an important intermediate that controls the stereoselectivity of radical coupling reactions. To determine whether the puckered-type structure exists in the bond dissociation process in an electronically excited state, we measured the emission spectrum of trans-32 (Figure 12).47
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