38 | December 2021 www.facs.website was electron spin resonance (EPR)–silent, which is consistent with the prediction of quantum chemical calculations that 28 exhibits strong absorption at around 700 nm. Product analysis of the photo-denitrogenation reaction of 27 showed that the cyclized product 29 of 28 was not isolated, but the product 30 obtained by methoxy group rearrangement was identified as the main product. This experimental result is consistent with the fact that cyclized compound 29 is more energetically unstable than 28, as mentioned above. From the results of this product analysis and quantum chemical calculations, the reaction behavior of 28 observed by the LFP method can be understood by the reaction mechanism shown in Scheme 5. Diradical 28 generated by the LFP method from 27 first reaches thermal equilibriumwith the ring-closing compound 29 within a lifetime of approximately microseconds, and then gives the rearrangement product 30 in a reaction lasting for approximately milliseconds (Scheme 5). Therefore, nanosecond time-resolved infrared absorption (IR) spectra were measured to confirm the formation of ring-closing compound 29 in the reaction system. The fast decay process of 28 was experimentally identified as a radical recombination process to yield compound 29, and the activation parameters ΔH‡ and ΔS‡ of the ring-closing reaction were found to be 37.6 kJ mol–1 and –2.6 J K –1 mol–1 in toluene and 36.4 kJ mol–1 and –4.6 J K–1 mol–1 in acetonitrile, respectively. These results are in good agreement with the activation energy of 42.4 kJ mol–1 obtained by quantum chemical calculations. The thermal equilibrium process (K = k29/k28) between the singlet diradical and σ-bonded product was experimentally observed for the first time in this study. Although the solvent effect on the equilibrium constant K was small, it was found to be smaller for the polar solvent acetonitrile (ACN) than for the nonpolar solvent toluene (TOL); KTOL = 1.54, KACN = 0.72, and the zwitterionic nature of the singlet diradical was experimentally clarified (Scheme 5). Because the contribution of the zwitterionic structure of the singlet diradical was further enhanced by the nitrogen atom adjacent to the radical site, the energy difference between the singlet diradical and its ring-closed form became small, and the bonding homolysis process could be directly observed. Third Energy Minimum of the Bond Homolysis Process47 In general, there are two energy minima in the bond homolysis process: the singlet radical pair N N Ph Ph MeO OMe N N N O O Ph N N N N N O Ph O Ph Ph MeO OMe 27 N N O N O Ph MeO Ph Ph OMe –N2 h 28 N N N MeO OMe Ph Ph O O Ph 29 K = k13/k12 max ~650 nm N N N O OMe O Ph Ph Ph OMe 30 k14 ~μs ~ms N N O N O Ph MeO Ph Ph OMe k29 k28 N N O N O Ph MeO Ph Ph OMe !" at 273 K N N N N N O Ph O Ph Ph MeO OMe 27 –N2 355 nm N N O N O H MeO Ph Ph OMe 28 max ~650 nm kfast = k12 + k13 kslow = k14·k12/(k12 + k13) 0.2 μs 3.4 μs 70 μs K = k13/k12 11 12 [13] [12] Scheme 5 (above). Generation of singlet diradical 28 and its reactivity. Figure 9.(right) (a) LFP study on 28 at 293 K in toluene; (b) Time profile of 633 nm species after LFP of 27. and the sigma-bonded compound. Herein, we describe the recent discovery of a third energy-minimum structure in the bond homolysis process, a puckered-type intermediate 33. The discovery of this intermediate allows us to understand the spin-multiplicity effect on the stereoselectivity of the ring-closing product 32 produced in the denitrogenation of azo compound 31 and the emission observed from 32 at unusually long wavelengths (Scheme 6). As shown in Scheme 6, when the photo-denitrogenation of 31 was performed by the direct electronic excitation of the azo functional group (-N=N-), trans-32, which retains the stereochemistry of 32, is preferentially obtained (trans/ cis-28 = 85/15), whereas the denitration of the excited triplet of the azo moiety using a triplet sensitizer such as benzophenone yielded cis32 exclusively. To investigate the effect of spin multiplicity on the stereoselectivity of this radical coupling reaction, the reaction behavior of the previously known planar singlet diradical 6 was first investigated by quantum chemical calculations using the broken symmetry method (BS-DFT) (Figure 10). As a result, even though trans-32 is more stable thermodynamically than cis-32, the transition state cis-TS giving cis-32 is approximately 20 kJ mol–1 more stable than the
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