Mechanistic Aspects of Photoinduced Rearrangement of 2,3

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A: Kinetics, Dynamics, Photochemistry, and Excited States

Mechanistic Aspects of Photoinduced Rearrangement of 2,3Diarylcyclopentenone Bearing Benzene and Oxazole Moieties Evgeni M. Glebov, Natalia V. Ruban, Ivan P. Pozdnyakov, Vjacheslav P. Grivin, Victor Fedorovich Plyusnin, Andrey G. Lvov, Alexey V. Zakharov, and Valerii Z Shirinian J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 21 Aug 2018 Downloaded from http://pubs.acs.org on August 21, 2018

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The Journal of Physical Chemistry

Mechanistic Aspects of Photoinduced Rearrangement of 2,3Diarylcyclopentenone Bearing Benzene and Oxazole Moieties Evgeni M. Glebova,b*, Natalia V. Rubanb,c, Ivan P. Pozdnyakova,b, Vjacheslav P. Grivina, Victor F. Plyusnina,b, Andrey G. Lvov,d* Alexey V. Zakharov,d Valerii Z. Shiriniand a

V. V. Voevodsky Institute of Chemical Kinetics and Combustion, 3, Institutskaya Str., Novosibirsk, 630090, Russia e-mail: [email protected] b

c

Novosibirsk State University, 2, Pirogova str., Novosibirsk, 630090, Russia.

A. V. Nikolaev Institute of Inorganic Chemistry, 3 Lavrentyev prosp., Novosibirsk, 630090, Russia

d

N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47, Leninsky prosp., 119991 Moscow, Russia e-mail: [email protected]

Abstract Photoinduced rearrangement of diarylethenes leading to the formation of naphthalenes or isoelectronic benzoannulated heterocycles is a novel reaction in preparative organic photochemistry. Recently it was shown that unsymmetrical diarylethenes containing benzene and oxazole derivatives efficiently undergo this transformation leading to amide derivatives of naphthalene. Mechanistic study of skeletal rearrangement for a typical representative of these compounds, namely 3-(5methyl-2-phenyl-1,3-oxazol-4-yl)-2-phenylcyclopent-2-en-1-one, was performed by stationary and laser flash photolysis as well as DFT calculations. The mechanism of the rearrangement was found to comprise several thermal stages. Both singlet and triplet states of the initial compound can be transformed to the reaction product, which results in the dependence of the quantum yield vs. concentration of dissolved oxygen. Three reactive intermediate were registered in the laser flash ACS Paragon Plus Environment

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photolysis experiment; the predicted structures were in accordance with DFT calculations of the electronic absorption spectra. In addition to previously proposed mechanism of skeletal rearrangement based on a sigmatropic shift of hydrogen two new parallel pathways based on formation of a carbanion/carbocation were determined. 1. Introduction Photoinduced 6π-electronic cyclization of a hexatriene system with the formation of a metastable colored intermediate is a key stage of numerous transformations used in organic synthesis (Scheme 1A).1,2,3,4,5 Detecting and study of properties of these intermediates is an important part of mechanistic investigations. In some cases, e.g. in the stilbene photocyclization to phenanthrene in the presence of dissolved oxygen, the colored primary photoisomer could be detected under stationary photolysis conditions.6 However, for some substrates this intermediate is so unstable that it could be detected only under special conditions. For example, cooling to 77 K allowed to stabilize the colored intermediate of the photoreaction of 2-vinyl-1,3-terphenyl, which undergoes fast [1,5]-H or double [1,2]-H shifts (Scheme 1B, top).7,8 Laser flash photolysis allowed to register the intermediate of stirylfuran photoreaction, which further undergoes [1,9] hydrogen shift and the disclosure of the benzene ring (Scheme 1B, bottom reaction).9 Recently we have described the photorearrangement of the diarylethene 1; under the UV irradiation this compound is practically quantitatively converted into naphthalene 4 (Scheme 1C).10 The reaction mechanism includes three stages: the photocyclization, hydrogen shift and disclosure of a heterocycle. However, the key intermediate 2, which, by analogy with the photochromic diarylethenes of the cyclopentenone series,11,12,13,14 had to exhibit an absorption band in the visible spectral region, was not registered either by UV or by 1H NMR spectroscopy.10

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Scheme 1. Photoinduced 6π-electrocyclization in organic synthesis.

In this work the photochemistry of compound 1 was studied by the stationary photolysis, laser flash photolysis and DFT calculations. The goal was to examine tentative Scheme 1C by direct time-resolved measurements. It should be noted that the photoreaction 1 → 4 is an example of the common photoreaction of diarylethenes occurring via photocyclization / [1,n]-H shift / cycloreversion mechanism, which is actively developed in recent studies.5 Compounds of different structures demonstrate a similar transformation,15,16,17,18,19 which provides large synthetic potential of this reaction.


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2. Experimental The initial compound 1, 3-(5-methyl-2-phenyl-1,3-oxazol-4-yl)-2-phenylcyclopent-2-en-1one, and the product of its skeletal rearrangement compound 4, N-(5-Methyl-1-oxo-2,3-dihydro-1Hcyclopenta[a]naphthalen-4-yl)benzamide (Scheme 1C) were synthesized as described previously.10 Solutions for photochemical experiments were prepared using spectrophotometrical grade acetonitrile (Cryokhrom, Russia). If necessary, solutions were deoxygenated by means of 20 min bubbling with argon. The value of O2 concentration in air-saturated acetonitrile is 2.4×10-3 M.20 The residual oxygen content of the solution after bubbling with high purity argon typically does not exceed 1%21 (note that the estimation performed in original paper [9] should be increased in an order of magnitude22). UV absorption spectra were recorded using Agilent 8453 (Agilent Technologies) and Varian Cary 50 spectrophotometers. Stationary photolysis was performed using the irradiation of a high-pressure mercury lamp with a set of glass filters for isolation of the 313 nm line. In several experiments, the excimer XeCl lamp (excilamp) was used as a quasicontinuous source of UVirradiation at 308 nm (half width of light pulse, 5 nm; pulse duration, 1 µs; frequency, 200 kHz; incident light flux, 8×1015 photons⋅cm-2 s-1).23 To measure the radiation intensity for the quantum yield calculations the IrCl62- complex in methanol solutions was used as a chemical actinometer (quantum yield of photoreduction in air-saturated solutions is 0.1 when initial concentration is less than 10-3 M24). The nanosecond laser flash photolysis experiments were performed using excitation by irradiation of the 4th harmonics of a Nd:YAG laser (Lotis TII, Belarus, 266 nm, 5 ns pulse duration). The illumination spot area was ca. 0.07 cm2 and energy per pulse was up to 20 mJ; the setup was similar to that described in detail.25 Light power meter SOLO 2 (Gentec, Canada) was used to measure the laser pulse energy.


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For the extraction of reaction rate constants from the experimental data, two calculation procedures were used. (i) A system of differential equations corresponding to the selected reaction scheme was solved numerically by a fourth-order Runge-Kutta method, and the results were compared with the experimental kinetic curves. (ii) The experimental data at different wavelengths were globally fitted by a three-exponential model. DFT calculations. All results were obtained using the GAUSSIAN 09 program package.26 Density functional theory (DFT) calculations were performed using the B3LYP27 and CAMB3LYP28 functionals with the 6-31G(d) and 6-31G(d,p) basis sets. Minimum structures were obtained from ground state optimizations. Calculation of vibrational frequencies was performed to prove that the optimized structure corresponds to a true minimum on the potential energy surface. The solvent influence was modeled using the Polarizable Continuum Model (PCM) in acetonitrile.29 The absorption spectra were calculated by linear-response time-dependent DFT (TD-DFT)30 on the TD-CAM-B3LYP level of theory by broadening vertical stick spectra with Gaussians of width 1500 cm−1. The QST2 formalism was used to calculate the transient state for [1,9]-H shift. Thermodynamic calculations we calculated at the ωB97xD/6-31G(d,p) level of theory.31 3. Results and Discussion 3.1. Stationary photolysis of 1 and quantum yield of skeletal rearrangement Irradiation (308 nm) of 1 in acetonitrile solution results in irreversible reaction of skeletal rearrangement.10 The changes in the UV spectra for the air-saturated solution are shown in Fig. 1a. The appearance of an isosbestic point at 265 nm indicates the presence of only one photochemical reaction. No backward dark reactions were observed. Further irradiation results in the very slow disappearance of the reaction product in the course of the secondary photochemical reaction. Curve 8 in Fig. 1A represents the spectrum of the product of skeletal rearrangement coinciding with that reported previously6, which allows us to determine the quantum yield of the reaction. Recently we


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used this assumption to calculate the quantum yields of photorearrangement of diarylethene 1 analogues - 2,3-diarylfuran-2(5H)-ones bearing different substituted benzene.18 Figure 1. A - Changes in the UV spectrum caused by stationary photolysis (308 nm) of 1 in CH3CN. 1 cm cell, initial concentration 4.35×10-5 M, air-saturated solution. Curves 1-8 correspond to 0; 6; 10; 15; 20; 28; 360; 46 s of irradiation. B - dependence of quantum yield of skeletal rearrangement vs. concentration of dissolved oxygen in acetonitrile solution. Experimental points and fit using Eq. III with parameters: P1 = 2.1×1011 s-2, P2 = 4.9×108 s-2; P3 = 1.9×1012 M-1s-2; P4 = 2.0×109 s-2.

Performing experiments in argon-saturated or in oxygen-saturated solutions results in the same changes in the UV spectrum as shown in Fig. 1A. The quantum yield of photolysis sufficiently depends on the concentration of dissolved oxygen (Fig. 1B). The increase of the quantum yield


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caused by the removal of oxygen from the solutions could be explained by opening a new channel of the product formation. We have proposed that both singlet and triplet excited states of 1 can lead to the reaction product. This explanation was supported by laser flash photolysis experiments (vide infra). 3.2. Laser flash photolysis of 1 in CH3CN Experiments on the laser flash photolysis of 1 in CH3CN were performed with excitation at 266 nm. Intermediate absorption was observed (Fig. 2), which spectrum and kinetic behavior depend on the concentration of dissolved oxygen. Figure 2. Results of the laser flash photolysis (266 nm) experiment with 1 (2.0×10-5 M) in CH3CN (air-saturated solutions, 1 cm cell). A - typical kinetic curves. Red lines are the results of global fit using the biexponential function (I). B – intermediate differential absorption spectra; curves 1-3 correspond to time delay 0; 0.8; 40 µs.


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Typical kinetic curves in air-saturated solutions are shown in Fig. 2A. Intermediate absorption spectra corresponding to different time delays from the exciting laser pulse are shown in Fig. 2B. Typical kinetic curves and intermediate absorption spectra in argon-saturated solutions in the microsecond time region are shown in Fig. 3. The dependence of the intermediate absorption amplitude vs. laser pulse energy is linear at low energies with the trend to saturation at higher energies (see Fig. S1 in Supporting Information), which is typical for one-photon processes.32 Figure 3. Laser flash photolysis (266 nm) of 1 (2.6×10-5 M) in CH3CN (argon-saturated solutions, 1 cm cell). Microsecond time domain. A – typical kinetic curves; B – intermediate absorption spectra. Curves 1-4 correspond to time delays 0.4; 1.6, 6; 46 µs after the laser pulse.

Figures 2 and 3 show that at least two intermediates are involved in the photolysis process. The first intermediate decays faster than the second one, and the characteristic time of its disappearance depends of the concentration of dissolved oxygen. The kinetic curves in Fig. 2A


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demonstrate fast parts with the decay time less than 1 µs, which is typical for the quenching of the triplet state of an organic molecule by dissolved oxygen.33,34 Therefore, it is important to assume that the first intermediate is the triplet state of the initial compound, 1(T). In air-saturated solutions the triplet state is presumably quenched by oxygen, while in argon-saturated solutions one can expect the reactions of T-T annihilation and different first-order processes. After the termination of the fast process in air-saturated solutions (Fig. 2) the residual absorption should belong to the initial intermediate of the photorearrangement 2 (Scheme 1C). The shapes of the spectra of intermediates were determined from analysis of spectral changes in airsaturated solutions (Fig. 2B). Curve 1 in Fig. 2B corresponding to zero time delay from the laser pulse is the superposition of the differential absorption spectra of 1(T) and 2. Curve 2 in Fig. 2B corresponding to time delay 0.8 µs from the laser pulse corresponds to the situation when the fast first process of the triplet state quenching is completed (see Fig. 2A). Therefore, curve 2 in Fig. 2B corresponds to the differential spectrum of intermediate 2, and the difference between curves 1 and 2 in Fig. 2B corresponds to the differential spectrum of the triplet state 1(T). The initial compound 1 has no absorption in the visible spectral region; therefore, differential absorption spectra are equal to the absorption spectra of intermediates. The shapes of the spectra are presented in Fig. 4. Note that the spectra are matched in amplitudes; the ratio between the maximal absorption coefficients was not determined. The spectrum of intermediate 2 (curve 1 in Fig. 4) contains the absorption band with the maximum in the region of 520 nm. The origin of this band is attributed to the system of conjugated π-electronic bands in 2 (Scheme 1C), similar to photoinduced isomers of the related photochromic diarylethenes.11-14 Further transformation of 2 to 3 violates this conjugation and results in disappearance of the absorption band in the visible region. The assignment of spectrum represented by curve 1 in Fig. 4 to intermediate 2 was supported by DFT calculations. The calculated spectrum of 2 is shown in Fig. 5 and it possesses absorption


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maxima at 510 nm. Note that both final product 4 (experimental data10) and intermediate 3 (DFT calculations, see SI) do not absorb in the visible spectral region (Scheme 1). Figure 4. Shapes of the UV-Vis spectra of intermediate 2 (curve 1) and triplet state 1(T) (curve 2). The curves are matched in maximal amplitudes.

Figure 5. Calculated absorption spectrum of intermediate 2.

The characteristic time of the triplet state 1(T) decay in air-saturated solutions is about 1 µs (Fig. 2A). In turn, the absorption of the remaining intermediate 2 is changed in the microsecond time domain, which is evident from the origin of the kinetic curves (Fig. 2A). In the case of argon-


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saturated solutions (Fig. 3) the kinetic curves at the whole time region are the superposition of those for 1(T) and 2 (and/or products of reactions of intermediate 2). The complete decay of the intermediate absorption occurs in the time domain of tens of milliseconds without formation of a new colored species. The typical characteristic kinetic curves are shown in Fig. 6A. The dissolved oxygen has no effect on the kinetic curves in this time domain. Figure 6. Laser flash photolysis (266 nm) of 1 (4.2×10-5 M) in CH3CN (air-saturated solutions, 1 cm cell). Millisecond time domain. A – typical kinetic curves (blue lines). Red lines are the results of biexponential global fit (Eq. I) with the characteristic lifetimes (0.75 ± 0.39) ms and (174 ± 21) ms. B – intermediate absorption spectra at different times. Curve 1 – zero time (sum of amplitudes A1(λ) + A2(λ)); curve 2 – after the end of the first process (amplitude A2(λ)).

The set of the kinetic curves (some of which are shown in Fig. 6A) was globally fitted using the biexponential model (I) with the zero residual. The residual corresponding to the


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differential absorption spectrum of the final product was taken to zero because in our case the final product (compound 4, Scheme 1C) had no absorption in the visible spectral region (see Fig. 1A). The results of the global fit are shown in Fig. 6A as the red lines. The reciprocal characteristic lifetimes 1/τ1 and 1/τ2 extracted from the global fit (Eq. 1) were (1.3 ± 0.7)×103 s-1 and (5.7 ± 0.8) s-1.

∆A( λ ,t ) = A1 ( λ )e − t / τ1 + A2 ( λ )e − t / τ 2 + A3 ( λ )

(I)

Fitting function (I) could be used for the reaction mechanism A→B→C. In this case, if the characteristic lifetimes are very different, the differential absorption spectra of the intermediates A and B, namely SA and SB (Species Associated Differential Spectra, SADS) are the sum of amplitudes A1(λ) + A2(λ) and the amplitude A2(λ) correspondingly35 (for the calculation of the SADS in the general case see SI). The SADS SA and SB (Fig. 6B) extracted from the experimental data in fact have the same shapes. This fact could be interpreted in three ways: (1) intermediates A and B really have similar absorption spectra; (2) biexponential decay of a colored intermediate; (3) parallel disappearance of two intermediates with the similar spectra in the visible region. In summary, the mechanism of photochemical skeletal rearrangement has to explain at least three peculiarities of the process: (1) dependence of the quantum yield on the concentration of dissolved oxygen (Fig. 1B); (2) nature of intermediate 2 reaction observed in air-saturated solutions in the microsecond time domain (Fig. 2); (3) biexponential decay of intermediate absorption in the millisecond time domain without changes of the shape of the spectrum (Fig. 6).


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3.3. Quantum yield of Skeletal Rearrangement vs. Dissolved Oxygen Concentration The explanation of the quantum yield dependence vs. dissolved oxygen concentration (Fig. 1B) could be given using Scheme 1C without details of transition from intermediate 2 to final product 4. The simplified mechanism of skeletal rearrangement based on assumption of complete transition 2 → 4 is described by equations 1-8. The behavior of the intermediate absorption in the microsecond time domain (Figs. 2, 3) and the dependence of the quantum yield vs. dissolved oxygen concentration (Fig. 1B) testify that there exist at least two channels towards the final product (4 in Scheme 1A) formation. The first channel (reactions 2-4) corresponds to the direct transition from the singlet excited state of the initial compound (further marked as 1(S*), while the ground singlet state is further marked as 1(S)) to intermediate 2 (followed by its transition to 4; here we do not discuss the details of the process). These reactions are fast; their characteristic lifetimes are less than the time resolution of the laser flash photolysis setup (5 ns).36 The second channel is the transition of the triplet state of 1 (further marked as 1(T)) to intermediate 2 (reaction 7), again followed by transformation of 2 to 4. The reaction mechanism also includes quenching of the triplet state by dissolved oxygen (5), all the possible pseudo-first order channels of intercombination conversion (6) and T-T annihilation (8). hν 1(S ) → 1(S*)

(1)

k2 1(S*) → 1(S )

(2)

k3 1(S*) →

(3)

2

k4 1(S*) → 1(T )

1(T ) +

(4)

k5 O2 → 1(S ) + 1O2

3

k6 1(T ) → 1(S )

(5)

(6) ACS Paragon Plus Environment

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k7 1(T ) →

2

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(7)

k8 1(T ) + 1(T ) → 1(S*) + 1(S )

(8)

Let us assume that all molecules of 2 transit to the final reaction product 4. In the conditions of the steady-state photolysis one can neglecting T-T annihilation (8), apply the method of stationary concentrations to the mechanism (1-7) and derive Equation II for the quantum yield of the product formation (for details see section I.2. in SI):

ϕ =

d[ 2 ] / dt 1 k4 k7 ( k3 + ) = Iabs / V ( k2 + k3 + k4 ) k5 [O2 ] + ( k6 + k7 )

(II)

where Iabs is the absorbed light intensity (mole quanta per second) and V is the sample volume. In this case, we have a three-parametric dependence of the quantum yield vs. the dissolved oxygen concentration:

ϕ = P1 +

P2 [O2 ] + P3

(III)

where P1, P2, P3 are parameters related to the rate constants k2 – k7. The experimental dependence of the quantum yield vs. concentration of dissolved oxygen (Fig. 1B) could be fitted using Eq. III (an example of a fit with P1 = 0.11, P2 = 1.4×10-4 and P3 = 10-3 is represented by the solid curve in Fig. 1B). Here P1 is a quantum yield (k3/(k2+k3+k4)) of 2 in reaction (3) only. It is clear that including Eq. (8) into the reaction scheme, one can also fit the experimental dependence (though the final fitting function will not be as easy as Eq. III). Therefore, the dependence of quantum yield vs. dissolved oxygen concentration in principle can be explained by two routes of the product formation (both from singlet and triplet excited states).


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3.4. Detailed Mechanism of Skeletal Rearrangement The detailed mechanism of photoinduced rearrangement explaining all the experimental facts is represented by Scheme 2. The rate constants determined in this study are collected in Table 1. Scheme 2. Detailed mechanism of photochemical skeletal rearrangement of compound 1.

Table 1. Reaction rate constants determined and estimated in this work. k5

k6 + k 7

2k8/εε *

**k9,

**k10

**k11

k14

k15

M-1s-1

s-1

s-1

s-1

M-1s-1

M-1s-1

s-1

s-1

(1.7 ± 0.8)×

(7.9 ± 0.2)×

(4.6 ± 0.1)

109

104

×106

1×105

7×109

7×109

5.7 ± 0.8

(1.3 ± 0.7)× 103

*ε is the molar absorption of 1(T) at 420 nm. **Estimated values.

3.4.1. Fast reactions in air-saturated solutions The rate constant of 1(T) quenching by oxygen (reaction 5) was determined by the global fit of the set of the kinetic curves (examples are shown in Fig. 2A) using the biexponential model with the residual (I). The shapes of the second component A2(λ) and residual A3(λ) were similar.


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Note that A3(λ) is also intermediate absorption; it decays in the millisecond time domain (Fig. 6). The examples of the best fits are shown in Fig. 2A as the red lines. One can see that the first characteristic time is much larger than the second one. The first characteristic time corresponds to the quenching of the triplet state by dissolved oxygen (reaction 5), which is definitely the pseudofirst order reaction (the value of O2 concentration in air-saturated acetonitrile is much higher than the concentration of the initial compound 1). The appropriate first-order rate constant of quenching determined as the reciprocal characteristic time τ1 is equal to (4.0 ± 1.9) ×106 s-1, which gives the rate constant of the triplet state quenching equal to k5 = (1.7 ± 0.8)×109 M-1s-1. This value is an order of magnitude less that the rate constant of diffusion-controlled reaction in acetonitrile at room temperature (kD = 1.9×1010 M-1s-1).21 The rate constants of ca. 0.1kD are typical for quenching of the triplet states of aromatic molecules by oxygen.37 The second characteristic time extracted from the global fit procedure (Fig. 2A) is τ2 = 7.3 ± 1.1 µs. This lifetime could not be assigned to the transition of intermediate 2 to intermediate 3 (Scheme 1C), because intermediate 3 has no absorption in the visible spectral region. It means that the reaction pathway 2 → 3 → 4 (Scheme 1C)10 could not explain residual absorption decaying in the millisecond time domain. We propose that, in addition to the pathway 2 → 3 → 4, two another possibilities of naphthalene 4 formation could be realized. The correspondent pathways are shown in Scheme 2; they are the self-reaction of intermediate 2 and its reaction with the initial compound 1. These reactions represent the alternative pathway of skeletal rearrangement without the sigmatropic shift. The alternative mechanism is based of the proton detachment from intermediate 2 with the formation of carbanion 5. Both initial compound 1 and intermediate 2 can act as bases in these reactions.38 One can propose that two species formed in these reactions, namely carbanion 5 and carbocation 6, can exhibit absorption in the visible spectral region, which spectrum could be similar


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to that of intermediate 2. This suggestion was supported by DFT calculations of their absorption spectra. According to calculations, carbanion 5 and carbocation 6 exhibit absorption maxima at 468 nm (Figure 7) and 485 nm (Figure 8), respectively. These values are close to observed absorbance of colored intermediate (Figure 6). Transformation of intermediates 5 and 6 to the final reaction product 4 (Scheme 2) can occur in the millisecond time domain explaining the results of laser flash photolysis experiments (Fig. 6). Figure 7. Calculated absorption spectrum of carbanion 5.

Now let us discuss the rate constant of intermediate 2 decay in air-containing solutions. Its characteristic lifetime is τ2 = 7.3 ± 1.1 µs, therefore, the effective first-order rate constant is keff = (1.4 ± 0.2)×105 s-1. According to Scheme 2 there are three reactions (9-11) giving a contribution to the effective rate constant. k9 2 →

3

(9)

k10 2 + 1  →

5 + 7

(10)

k11 2 + 2  →

5 + 6

(11)


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Figure 8. Calculated absorption spectrum of cation 6.

The experimental kinetic curves could be easily fitted in the framework of reaction scheme (1, 3-5, 9-11). An example of a fit could be found in SI (section I.3). Because of large number of variable parameters (rate constants of reactions 9-11, molar absorption coefficients of intermediates 1(T), 2, 5, 6, and initial concentrations of 1(T) and 2) the set of fitting parameters is not unique. Therefore, only brief estimation of rate constants is possible. The value of monomolecular rate constant k9 was estimated as 105 s-1. The rate constants k10 and k11 were estimated by the values of 7×109 M-1s-1; so they are close to the rate constant of diffusion-controlled reaction. There exists an alternative explanation of the process in air-saturated solutions based on the reactions of singlet oxygen (1O2) formed in reaction 5. Both initial compound 1 and intermediate 2 could react with 1O2. The possible structure of the reaction product 8 is shown in Scheme 3. The [4+2]-cycloaddition of oxazole derivatives with singlet oxygen was described earlier39,40 and recently we studied the competition of this reaction with photorearrangement of diarylethenes bearing oxazole and benzene.41 According to recently works of Alabugin et al., the ozonide substructure of 8 is strongly stabilized by stereoelectronic interactions between peroxide lone pairs and the vicinal σ*C-O and σ*C-N orbitals.42,43,44


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Scheme 3. Possible product of reaction of 1 with singlet oxygen.

In this case, the second process (Fig. 2A) could reflect possible difference in molar absorption coefficients of intermediate 2 and product 8. One can expect that 8 could not absorb in the visible spectral region. Again, the rate could be estimated from the comparison of the experimental kinetic curves and solutions of the differential equations corresponding to the proposed mechanism (1-5, 12-13). The lifetime of singlet oxygen in neat acetonitrile is 54 µs.45 k12 O2 + 1  → 8

(12)

k13  → CH3 CN

(13)

1

1

O2

Quenching

The estimated value of k12 is 7×109 M-1s-1. This value seems to be rather high. Typical values of rate constants of singlet oxygen reactions with aromatic molecules are 2-3 orders of magnitude less than the rate constant of diffusion-controlled reaction; e.g. for oxazoles the value of 3×107 M-1s-1 is typical.46 Therefore, the possibility of explanation of the experimental data by the reactions of singlet oxygen seems less probable than the first explanation based on the formation of carbanion 5. 3.4.2. Reactions of 1(T) in argon-saturated solutions The kinetic curves in argon-saturated solutions (Fig. 3A) are contributed by both triplet state 1(T) and intermediates 2, 5 and 6. The changes of absorption caused by the reactions of intermediates 2, 5 and 6 in the time interval 1 – 10 µs should be the same as in the case of airsaturated solutions (Fig. 2A). These changes are small in comparison with the changes in absorption


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caused by reactions of triplets 1(T). Therefore, in the following analysis only the reactions of 1(T) were taken into account. The decay of the triplet state in argon-saturated solutions could be caused both by second– order reaction of T-T annihilation (8) and first-order reactions (6, 7). The dependence of the observed effective first-order rate constant determined at the initial (10-20% of total amplitude) part of the kinetic curves, Eq. IV vs. initial differential absorption of 1(T) is linear (Fig. 9). For the case of mixed reactions of second order (T-T annihilation in our case) and first (or pseudo-first) order the dependence kobs vs. (∆D)0 is determined by Eq. V. Figure 9. Laser flash photolysis (266 nm) of 1 (1.4×10-5 M) in CH3CN (argon-saturated solutions, 1 cm cell). Dependence of effective first order rate constant kobs at 420 nm vs. initial absorption of the triplet state 1(T).

kobs = −

kobs = (

( d( ∆D ) / dt )t = 0 ( ∆D )t = 0

2ksec ond

ε

(IV)

)( ∆D )0 + kfirst

(V)


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where ∆D and ε are the differential intermediate absorption and molar absorption coefficient of the intermediate (1(T) in our case), kfirst and ksecond are the rate constants of first and second order reactions. The best-fit parameters of the linear dependence (Fig. 9) were 2ksecond/ε = (4.6 ± 0.06)×106 s-1 and kfirst = (7.9 ± 0.2)×104 s-1. In the framework of the reaction scheme (1-8) ksecond = k8 is the rate constant of T-T annihilation and kfirst = k6 + k7 is the sum of rate constants of intersystem crossing and transition of 1(T) to intermediate 2. The intersection kfirst does not depend on the concentration of the initial compound 1, therefore the possibilities of concentration quenching of triplets (both physical and reactive) could be ruled out. 3.4.3. Decay of intermediates 5, 6 In the framework of reaction mechanism presented at Scheme 2 the kinetic curves observed in the millisecond time domain (Fig. 6) represent two independent reactions. These reactions are the transitions of intermediates 5 and 6 (the shapes of their absorption spectra are similar) to the final reaction product 4 (Eqs. 14, 15). k14 5  → H+ k15 6  → −H +

4

(14)

4

(15)

Curve 1 in Fig. 6B is the sum of the spectra of intermediates 5 and 6. Curve 2 in Fig. 6B represents the spectrum of the intermediate with the longer lifetime. The reciprocal characteristic lifetimes 1/τ1 and 1/τ2 extracted from the global fit (Eq. I), which are (1.3 ± 0.7)×103 s-1 and (5.7 ± 0.8) s-1, are the rate constants k14 and k15. However, we have no way to establish the correspondence between rate constants and intermediates (e.g. the reciprocal characteristic lifetime 1/τ1 could be either k14 or k15).


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3.5. Theoretical insight into the proposed mechanism We performed DFT calculations to support the mechanism proposed at the Scheme 2. The first pathway for the key intermediate 2 contains thermal [1,9]-H shift towards 3 at the first step. Fig. 10 represents the energy profile of transformation 1→4. Ground states energies for 1-4 were calculated at the B3LYP/6-31G(d,p) theory level, the transition state of thermal cycloreversion 2→1 (TS1) was performed according Masunov et al47 (UB3LYP/6-31G(d)), the transition state of thermal [1,9]-H shift 2→3 (TS2) was performed according Alabugin et al48 (B3LYP/6-31G(d,p)). Figure 10. Energy profile of the photorearrangement of 1 via cyclization/[1,n]-H shift sequence.

The photocyclization of 1 leads to 2, which is 26.2 kcal/mol less stable due to loss of aromaticity of benzene and oxazole rings. Thermal [1,9]-H shift towards 3 restores 6π-electron system in one benzene ring, for which reason 3 is 15.3 kcal/mol more stable than 2. We have optimized the transition state of [1,9]-H shift (Figure 11). In this structure, the hydrogen atom is in the "half-position" between the initial and final carbon atoms, approximately at an equal distance from them. This results in a partial alignment of the bonds in the cyclohexadiene fragment, which is aromatized during the migration of hydrogen. ACS Paragon Plus Environment

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Figure 11. Calculated transition state (TS2) for thermal [1,9]-H shift.

The activation energy for this process amounted to 27.7 kcal/mol, that is in good correlation with results48 previously obtained by Alabugin et al. for [1,5]-H shift in 8a,10-dihydrophenanthrene to give 9,10-dihydrophenanthrene (Scheme 1B, top). It should be noted, that the thermal cycloreversion of intermediate 2 towards 1, which was observed for some photochromic diarylethenes bearing phenyl rings,49 has a higher value of Ea = 40.7 kcal/mol (transition state TS1), which makes this process unfavorable. In general, this fully complies with Kawai et al. for photochemical reactions of terarylenes with thiazole ethene bridge.50 It is important to compare thermodynamic parameters for three proposed pathways for intermediate 2 (Scheme 4). We performed thermodynamic calculations at the ωB97xD 6-31G(d,p) level of theory. This method provide acceptable results for thermochemistry,51 and it was used previously for theoretical insight into diarylethene transformations.50,52 According these calculations, the first stage of three proposed pathways ([1,9]-H shift, deprotonation, protonation) are thermodynamically favorable in gas phase (Scheme 4). Scheme 4. Calculated thermodynamic parameters for proposed reaction pathways.


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3.6. Role of the base in the reaction mechanism We have introduced the step of the colored photoisomer 2 deprotonation into the revised mechanism of diarylethene 1 photorearrangement (Scheme 2). The similar heterolytic cleavage of a substituent on the reactive carbon of the photoinduced isomer was considered recently for baseinduced reactions of 2-(fur-3-yl)ethenylarenes53 and nucleophile-induced rearrangements of photochromic diarylethenes.54 For additional confirmation of the proposed mechanism, we studied influence of bases on the stability of photoisomer 10 of diarylethene 9 (Scheme 5). Scheme 5. Mechanism of photochemical skeletal rearrangement of compound 9. O N Me O 9

Ph

Solutions of 9 in MeCN (1.3*10-3 M) before irradiation...

UV ...after irradiation with UV 365 nm (2x8W) for 1 min...

O H

N

Me O 10 colored

Ph ...after addition of pyridine (II) and DABCO (III)...

O ...after 30 min in dark.

NH Ph Me O 11

Diarylethene 9 is an isomer of compound 1, which undergoes the similar photorearrangement towards naphthalene 11.10 The migrating hydrogen in 10 is less acidic because it is free from the influence of the carbonyl group, unlike 2. This allowed us to observe the bleaching of 10 in stationary experiments.10 Previously we have assigned this fast thermal process to allowed [1,9]-H shift, but this process evidently is more complicated.


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We assumed that addition of bases should lead to the boost of bleaching caused by further transformation of intermediate 10. To verify this hypothesis, we irradiated colorless solutions of diarylethene 9 in CH3CN (C = 1.3×10-3 M, photo A at Scheme 5) for 1 min by UV light at 365 nm (two 8W lamps). This resulted in the formation of purple solutions (photo B in Scheme 5), proving formation of intermediate 10. We immediately added one drop of pyridine (sample II) or few crystals of DABCO (sample III), and in the both cases we observed an instantaneous bleaching (photo C). This result was interpreted as the deprotonation of photoisomer 10 that restored benzene aromaticity (similarly to carboanion 5, Scheme 2) and simultaneously disturbed the conjugated πsystem, responding for deep color of 10. The acceleration of bleaching was explained by the fact, that pyridine (pKa = 5.2) and DABCO (pKa = 8.9 for mono-protonated molecule) are stronger bases than diarylethene 9 (for model unsubstituted oxazole pKa = 0.8) and its photoisomer 10 (for model 2-phenyl-2-oxazoline pKa = 4.455). In this way, we have demonstrated the possibility of deprotonation of photoinduced isomers for diarylethenes bearing oxazole and benzene rings. Note that acidity of hydrogen on the reactive carbon of the photoisomer for diarylethene 1 is higher than for diarylethene 9. The conjugated carbonyl in the structure of 2 and external base in the case of structure 10 have the similar impact on stability of these intermediates. 4. Conclusion Thus, we have studied the some mechanistic aspects of photoinduced rearrangement of diarylethenes comprising derivatives of oxazol and benzene as aryl moieties. This process occurs both via singlet and triplet pathways. Reaction mechanism is rather complicated and three pathways of the formation of the final naphthalene 4 are possible (Scheme 2). In addition to the literature pathway via intermediate 2 the reaction can occur via the formation of the carbanion 5 or carbocation 6. These colored reactive intermediates were registered by laser flash photolysis technique; their identification was supported by DFT calculations and additional experiment with ACS Paragon Plus Environment

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the base addition. Future study the mechanism of this new photoinduced rearrangement will focus on more details evaluation of the role of singlet oxygen in reactions performed in air-saturated solutions and the determination of molar absorption coefficients of the triplet state 1(T) and intermediates. Acknowledgement The work was supported by the Russian Foundation of Basic Research (grants № 18-0300161_a and № 16-33-60013_mol_a_dk).

Supporting Information File of Supporting Information collects some additional experimental raw data and details of DFT calculations.

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TOC Graphic


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Graphical abstract 586x266mm (96 x 96 DPI)



Scheme 1. Photoinduced 6π-electrocyclization in organic synthesis. 122x104mm (300 x 300 DPI)


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Scheme 2. Detailed mechanism of photochemical skeletal rearrangement of compound 1. 95x49mm (300 x 300 DPI)



Scheme 3. Possible products of reaction of 1 with singlet oxygen. 31x10mm (300 x 300 DPI)


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Scheme 4. Calculated thermodynamic parameters for proposed reaction pathways. 29x5mm (300 x 300 DPI)



Scheme 5. Mechanism of photochemical skeletal rearrangement of compound 9. 207x182mm (96 x 96 DPI)


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Figure 1. A - Changes in the UV spectrum caused by stationary photolysis (308 nm) of 1 in CH3CN. 1 cm cell, initial concentration 4.35×10-5 M, air-saturated solution. Curves 1-8 correspond to 0; 6; 10; 15; 20; 28; 360; 46 s of irradiation. B - dependence of quantum yield of skeletal rearrangement vs. concentration of dissolved oxygen in acetonitrile solution. Experimental points and fit using Eq. III with parameters: P1 = 2.1×1011 s-2, P2 = 4.9×108 s-2; P3 = 1.9×1012 M-1s-2; P4 = 2.0×109 s-2. 288x416mm (300 x 300 DPI)



Figure 2. Results of the laser flash photolysis (266 nm) experiment with 1 (2.0×10-5 M) in CH3CN (airsaturated solutions, 1 cm cell). A - typical kinetic curves. Red lines are the results of global fit using the biexponential function (I). B – intermediate differential absorption spectra; curves 1-3 correspond to time delay 0; 0.8; 40 µs. 288x412mm (300 x 300 DPI)


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Figure 3. Laser flash photolysis (266 nm) of 1 (2.6×10-5 M) in CH3CN (argon-saturated solutions, 1 cm cell). Microsecond time domain. A – typical kinetic curves; B – intermediate absorption spectra. Curves 1-4 correspond to time delays 0.4; 1.6, 6; 46 µs after the laser pulse. 288x416mm (300 x 300 DPI)



Figure 4. Shapes of the UV-Vis spectra of intermediate 2 (curve 1) and triplet state 1(T) (curve 2). The curves are matched in maximal amplitudes. 288x412mm (300 x 300 DPI)


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Figure 5. Calculated absorption spectrum of intermediate 2. 545x306mm (96 x 96 DPI)



Figure 6. Laser flash photolysis (266 nm) of 1 (4.2×10-5 M) in CH3CN (air-saturated solutions, 1 cm cell). Millisecond time domain. A – typical kinetic curves (blue lines). Red lines are the results of biexponential global fit (Eq. I) with the characteristic lifetimes (0.75 ± 0.39) ms and (174 ± 21) ms. B – intermediate absorption spectra at different times. Curve 1 – zero time (sum of amplitudes A1(λ) + A2(λ)); curve 2 – after the end of the first process (amplitude A2(λ)). 288x412mm (300 x 300 DPI)


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Figure 7. Calculated absorption spectrum of carbanion 5. 540x305mm (96 x 96 DPI)



Figure 8. Calculated absorption spectrum of cation 6. 541x304mm (96 x 96 DPI)


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Figure 9. Laser flash photolysis (266 nm) of 1 (1.4×10-5 M) in CH3CN (argon-saturated solutions, 1 cm cell). Dependence of effective first order rate constant kobs at 420 nm vs. initial absorption of the triplet state 1(T). 288x412mm (300 x 300 DPI)



Figure 10. Energy profile of the photorearrangement of 1 via cyclization/[1,n]-H shift sequence. 908x459mm (96 x 96 DPI)


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Figure 11. Calculated transition state (TS2) for thermal [1,9]-H shift. 768x522mm (96 x 96 DPI)


Mechanistic Aspects of Photoinduced Rearrangement of 2,3

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