36 | December 2021 www.facs.website Kinetic Stabilization of π-Single Bonded Chemical Species Using Macrocyclic Stretching Effect32 We further aimed to determine whether the longevity of π-single-bonded chemical species can be achieved by kinetic stabilization through a novel molecular design. In this context, inspired by recent synthetic studies on cyclic paraphenylene,33–36 we designed a molecule with a diradical skeleton in the macrocyclic ring (Scheme 3)37. Specifically, the diradical skeleton has a planar structure and a larger macrocyclic ring than the corresponding ring-closing structure; therefore, we expected it to be less molecularly distorted and thus more kinetically stabilized in the reaction yielding the ring-closed product. The distorted macrocyclic skeleton was expected to have a stretching effect, pulling the C1–C3 bonds of the ring closure outward. As a model molecule, we designed a ring skeleton at the meta-position of the phenyl group in the radical moiety to minimize thermodynamic stabilization. Further, the macrocyclic effect Scheme 3 Concept of “stretch effect” for the kinetic stabilization of singlet diradical. macrocycle stretch 1 2 1 3 2 3 R R R R 12 13 R E13-12 (kcal mol–1) H –9.6 –7.0 –1.0 at (U)B3LYP/6-31G(d) 1 3 2 C1–C3 (pm) 158 162 161 concept of stretch effect (a) (b) (c) macrocycle stretch on the kinetic stabilization of diradical 12 was investigated using quantum chemical calculations. In the case of 12a (R = H), which has a biphenyl group as a substituent, the ringclosed compound 13a was found to be more stable than 12a by 9.6 kcal mol–1. In the case of 13b, in which a benzene ring was introduced between the biphenyl groups, the energy difference was slightly smaller, estimated to be 7.0 kcal mol–1. When a naphthyl group with a more planar π-system was introduced, the energy difference between the diradical structure 12c and the ring-closed structure 13c was found to be much smaller, 1.0 kcal mol–1, indicating that the kinetic stabilization of the diradical 12c was expected. The stretching effect of the ring-closing structure was suggested by the fact that the C1–C3 bond distance of 13c was calculated to be 162 pm, which was longer than that of 13a (158 pm). Furthermore, the transition state energy of the ring-closing reaction was estimated to be 5.8 kcal mol–1 for the transformation from 12a to 13a, but it was 9.2 kcal mol–1 for the cyclization from 12c to 13c. It is clear that the introduction of the macrocyclic ring kinetically stabilized the diradical. Recently, we succeeded in synthesizing azo compounds 14a37and 14b,38 which can experimentally verify the macrocyclic effect on the kinetic stabilization of diradicals, and examined its reaction behavior (Scheme 4). When we attempted to generate diradicals 15 from 14 by the lase-flash-photolysis (LFP) method, we observed an absorption maximum around 600 nm, which is characteristic of singlet diradical species. Further, transient species with an absorption maximum around 600 nm, characteristic of singlet diradical species, were successfully observed (Figure 6). The lifetimes of the transient species 15a and 15b were determined to be 14.0 and 156 μs at 293 K, respectively, which are two-three orders of magnitude longer than that of 6. The transient species near 600 nm was identified as the singlet diradical 15; this is because the photodenitration of 14 produced a quantitative ring-closed product 16 and the signal near 600 nm was not quenched by molecular oxygen. The macrocyclic stretching effect could not be directly estimated from the reaction energy because the thermal equilibrium process between 15 and its ring-closed form 15b could not be observed, as in the case of diradical 28 (Figure 9). A close examination of the reactivity of ring-closed compound 15b showed that its chemical reactivity was strongly influenced by the stretching effect of the macrocyclic ring. Thus, 15b was stable at room temperature under degassed conditions but gradually transformed into oxidation products 18-20 under an oxygen atmosphere. Under the same conditions, 21 was stable, suggesting that the high reactivity of 15b was due to the macrocyclic effect. The dynamic solvent effect (viscosity effect) has also been confirmed for the case of macrocyclic singlet diradicaloild 16. Diradicaloid 16 lasts up to 400 μs in the high-viscosity solvent triacetin (GTA, π* = 0.63 kcal mol-1, η = 23.0 cP), which is 2.5 times longer than in benzene (π* = 0.55 kcal mol-1, η = 0.65 cP) at 293 K. On the other hand, in low-viscosity acetone (π* = 0.62 kcal mol-1, η = 0.32 cP), whose polarity is very close to that of GTA, the lifetime 16 was as short as 27.9 μs. This indicates that the viscosity of the solvent plays an important role in determining the lifetime of the singlet diradical. During the ring closure reaction, the movement of the macrocyclic skeleton is inhibited in the viscous solvent, resulting in a longer lifetime of the singlet diradicaloid 16. Nitrogen Atom Effect on the Reactivity of Singlet Diradicals As typified by N-heterocyclic carbenes (NHCs), the reaction behavior of carbenes is known to change dramatically with the
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