Photochemical Rearrangement of Diarylethenes: Reaction Efficiency

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Photochemical Rearrangement of Diarylethenes: Reaction Efficiency and Substituents Effect Alexey V. Zakharov, Elena B. Gaeva, Andrey G. Lvov, Anatoly V. Metelitsa, and Valerii Z. Shirinian J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b01587 • Publication Date (Web): 31 Jul 2017 Downloaded from http://pubs.acs.org on August 1, 2017

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The Journal of Organic Chemistry

PHOTOCHEMICAL REARRANGEMENT OF DIARYLETHENES: REACTION EFFICIENCY AND SUBSTITUENTS EFFECT Alexey V. Zakharov,1, Elena B. Gaeva,2 Andrey G. Lvov, 1 Anatoly V. Metelitsa,2 Valerii Z. Shirinian1* 1

N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47, Leninsky prosp., 119991 Moscow, Russian Federation

2

Institute of Physical and Organic Chemistry, Southern Federal University, 194/2 Stachka Avenue, Rostov on Don 344090, Russian Federation.

Abstract In recent years, a great synthetic potential of the photorearrangement of diarylethenes leading to naphthalene derivatives via a cascade process of photocyclization/[1, n]-H shift / cycloreversion has been demonstrated. In this work first a multifaceted study of the influence of various factors on the efficiency of the photorearrangement of diarylethenes of furanone series containing benzene and oxazole derivatives as aryl residues has been carried out. The efficiency of this phototransformation (quantum yields) and the effect of methoxy substituents in the phenyl moiety have been studied. Despite the multistage process, the quantum yields of the photorearrangement are rather high (0.34-0.49). It has been found that the efficiency of photocyclization of diarylethenes increases with the introduction of electron-donating methoxy groups in the phenyl moiety. Using the DFT calculations we have been able to estimate in the photoinduced isomer the distance between hydrogen atom and carbon atom to which it migrates in the result of the sigmatropic shift. For all studied diarylethenes, this value was 2.67-2.73 Å, which is less than the sum of van der Waals radii of carbon and hydrogen atoms (2.9 Å).

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Introduction The photochemical reactions of diarylethenes (stilbenes) with the participation of the hexatriene system are widely studied both from the point of view of theoretical and fundamental aspects, and in terms of practical application.1-4 The photochromic transformations of diarylethenes (reversible cyclization) attract researchers to design light-controlled materials and devices.5-7 The most known reaction of the diarylethenes in synthetic chemistry is the photocyclization of hexatriene system followed by the oxidation (Mallory reaction) or elimination.1-3 These reactions find application in the synthesis of phenanthrene derivatives and are also extensively studied to develop effective methods of the synthesis of polyaromatic systems from simple compounds. Finally, in the last decade a new reaction of diarylethene - a photocyclization with subsequent rearrangement, resulting in the opening of one of the aromatic (in most cases, heterocyclic) rings has been developed.8 This phototransformation with styrylfurans and styrylthiophenes9 has been found for the first time by T.-I. Ho et al. who later studied this process in detail.10 Also the similar reactions were described for terarylenes.11 We have found that this reaction carried out for the diarylethene of the oxazole series (Scheme 1A).12 It was shown that the photocyclization of diarylethenes is accompanied by a [1,9]-sigmatropic shift of the hydrogen atom, followed by the opening of the oxazole ring and the formation of naphthalene derivatives (Scheme 1C). Later we have shown that this reaction is a general method and the diarylethenes with different heterocycle moieties (thiophene, benzo[b]thiophene, furan, indole, imidazole, thiazole, pyrazole) can be involved into this process (Scheme 1B). 13 It was found that the reaction is easily scaled and the desired polyaromatic (polyheteroaromatic) compounds could be successfully obtained with good yields. Scheme 1.

The literature data14 and our own studies8,12,13,15 indicate the promise of this phototransformation. The reaction can be used to synthesize a wide variety of polyaromatic


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(heteroaromatic) compounds with different functional groups.13 The mechanism of the photoreaction includes tandem transformation of three basic processes: the first stage is the photocyclization of the hexatriene system of diarylethenes (stilbenes), resulting in the formation of metastable cyclohexadiene structure II (Scheme 1C). In the second stage, there is a [1,9]sigmatropic rearrangement II, which is accompanied by a rearomatization of the benzene ring and leads to the formation of a dihydronaphthalene derivative III. At the last stage, there are opening of the heterocyclic ring and the rearomatization of the naphthalene system. The aim of this work is a multifaceted study of the effect of various factors on the efficiency of the preparative photoinduced rearrangement of the diarylethenes of furanone series containing benzene and oxazole derivatives as aryl moieties. Among such factors, the variations of the substituents in the phenyl residue of diarylethenes have been studied. In particular, the introduction of methoxy substituent is of interest, since its effect on the efficiency of photoinitiated processes has been reported.16 In addition, the introduction of a methoxy group to the diarylethene framework can lead to new reactions, for example, to hydrolysis followed by rearrangement17 or elimination.18 The efficiency of this multistage process has also been studied by comparing the total quantum yields (it should be noted that the efficiency of the rearrangement process of this type was not studied before) and the thermal stability of metastable cyclohexadiene structure II (Scheme 1C). The latter potentially has a significant effect on the next stage of rearrangement.

Results and Discussion Synthesis. A more suitable object for these studies served the diarylethenes of the furanone series. Such choice was due, first of all, to the ease of their synthesis from commercially available starting compounds,19 as well as good photochromic characteristics.20 The synthesis of diarylethenes involves two main steps: the reaction of the corresponding phenylacetic acids 1a-f with oxazolyl bromoketone 2a and following intramolecular condensation of the forming ester leading to the desired diarylethenes of furanone series (Scheme 2, Table 1).18,21 The reaction is carried out under an inert atmosphere, and the yields range from 45 to 68%. Scheme 2.



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Table 1. Structures of starting diarylethenes 3, photoproducts 4 and their isolated yields.* Yield 3, %

No.

3a

60

4a

77

3b

56

4b

78

3c

58

4c’/4c”

74

3d

68

4d

92

3e

63

4e

59

3f

45

4f

31

No.

Diarylethene

Photoproduct

Yield 4, %

* Photoreaction was performed in dichloromethane solutions (0.2 g DAE in 10 ml CH2Cl2). A preparative irradiation reaction of diarylethenes 3 has been carried out in methylene chloride at room temperature in a conventional glass reactor with a volume of 10 ml, similar to the conditions developed by us in the work.13 The yields of the isolated photoreaction products 4 are given in Table 1 (the loading is 0.2 g of diarylethene in 10 ml of solvent; the purification was carried out by column chromatography). Yields on average range from good to high, but relatively low yields have been obtained for ortho-substituted diarylethenes (59% for 4e and 31% for 4f). The high efficiency of the photorearrangemet process was demonstrated by 1H NMR monitoring.

1

H-NMR monitoring of the photorearrangement of 3-methoxysubstituted

diarylethene 3c is given in Figure 1 (NMR monitoring for other diarylethenes is given in SI). The ACS Paragon Plus Environment

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formation

of

putative

by-products

associated

with

photocyclization/elimination

or

photorearrangement/cycloreversion of the benzene ring is practically not observed, which is clearly seen from the change of the position of the methoxy group signal. In the case of 3methoxy-substituted diarylethene 3c, two possible structural isomers are formed: the photocyclization can proceed at both orto- and para-positions to the methoxy group (Table 1, Figure 1). As can be seen from Figure 1, as the signal of the methoxy group of the starting diarylethene disappears (3.82 ppm), two signals at 3.91 and 3.96 ppm are formed. The ratio of ortho- to para-isomer is about 1: 1.6. Figure 1. 1H NMR monitoring of diarylethene 3c photoreaction under UV irradiation (λ = 365 nm) in CDCl3 solution (C = 6.0 x 10-2 M): before irradiation (A) and after irradiation (B-E).

We tested four solvents for the photoreaction: acetonitrile, chloroform, methylene chloride and ethanol. The most suitable among these solvents is methylene chloride. The starting diarylethene is readily soluble in methylene chloride, while the solubility of the reaction product is low, and as the reaction proceeds, the product formed precipitates, which simplifies its further purification. In acetonitrile and ethanol, the initial diarylethene is soluble poorly. These solvents can be used for the photochemical studies at low concentrations (1 x 10-5), but they are not suitable for the preparative reactions. In chloroform, the substances are soluble well, but upon irradiation, a partial decomposition of chloroform is observed, for this reason, its use in the preparative photoreaction is not also advisable. Absorption spectra. Electronic absorption spectra of diarylethenes 3a-f in acetonitrile are characterized by the long-wavelength absorption bands with maxima in the region of 282290 nm. Molar absorption coefficient values at the maximum wavelengths have been calculated to be 18800-31200 M-1·cm-1 (Table 2). The structure of the diarylethenes 3a-f, and in particular ACS Paragon Plus Environment


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the R-substituent on the phenyl moiety, does not significantly affect their spectral absorption properties. The products of the irreversible photoinduced rearrangement 4a-f absorb in longer wavelength spectral region than initial diarylethenes. They possess maxima of S1←S0 transition long-wavelength absorption bands at the 300-346 nm and much lower values of molar absorption coefficients – 3500-9200 M-1·cm-1 (Table 2). The maxima of the bands corresponding to S2←S0 transitions lie within the 220-260 nm wavelength interval and are characterized by the molar absorption coefficient values of 28900-38500 M-1·cm-1. It should be noted that the absorption band of products 4 possess pronounced vibrational structure that is indicative for the condensed molecular systems. Table 2. Absorption and photochemical properties of investigated diarylethenes in acetonitrile (T=298). λ, nm (ε·10-3, mol·L-1·cm-1)

Entry 1

3a

218 (24.4); 290 (22.8)

4a

225 (44.3); 240 (38.5); 300 (9.2)

3b

282 (18.8)

2 4b

3

4

5

6

222 (26.1); 241 (28.9); 297 (shoulder) (6.6);

0.34

0.49

333 (3.7); 346 (3.5)

3c

288 (19.8)

4c

229 (34.3); 260 (36.53); 329 (5.9)

3d

289 (21.1)

4d

φAB (313nm)

226 (31.4); 255 (32.0); 294 (5.3); 307 (5.2);

0.43

0.39

341 (5.6)

3e

289 (21.4)

4e

223 (34.7); 250 (37.3); 311 (7.5)

3f

224 (28.4); 292 (31.2)

4f

225 (31.8); 259 (26.3); 319 (5.36)

-

-

It has been established that the introduction of electron-donating methoxy group into the phenyl fragment of the diarylethenes leads to a significant bathochromic shift of the longwavelength absorption band maxima of products 4. For instance, in the case of methoxy substituted product 4b the long-wavelength shift of the absorption band maximum reaches 46 nm comparing to the unsubstituted compound 4a. In the case of methyl substituted 4e, the shift is 35 nm compared to the naphthalene 4a.


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A typical absorption spectrum of the diarylethene 3a and the rearrangement product 4a is presented in Figure 2 (the absorption spectra of the other investigated compounds are given in SI). Figure 2. Absorption spectra of 3a and 4a in acetonitrile (T=298 K). 50000

3a 4a

-1

40000

. -1.

, mol L cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30000

20000

10000

0 250 300 350 400 450 500 550 600 650 700 750 800

wavelength, nm

Photokinetic studies. The investigation of the 3→4 photochemical reaction has been carried out in an acetonitrile solution under UV irradiation (λ = 313 nm). UV light exposure activates transformation of diarylethenes into the corresponding structural isomers 4a-f which are products of photoinduced rearrangements. The latter is evidenced by the comparison of absorption spectra of solutions obtained under irradiation and absorption spectra of the preparative obtained compounds 4. An absorption spectrum of 3b during UV irradiation is given in Figure 3. The dynamics of spectral alterations under irradiation of diarylethenes 3 solutions are characterized by the presence of isosbestic points: one isosbestic point at 263 nm for compound 3a, two isosbestic points at 267 and 346 nm for compound 3b (Figure 3), three isosbestic points at 212, 269 and 333 nm and at 218, 273, 339 nm for compounds 3c and 3d, respectively (Figures of the absorption spectra changes of diarylethenes 3a, c-f are given in SI). It is seen that there is an isosbestic point at the 263-273 nm in all the cases (see insets in Figure 3). The existence of the isosbestic points in the absorption spectra of the compounds 3a-d upon UV irradiation allowing to conclude that the rates of 1,9-sigmatropic rearrangement and of the subsequent process of oxazole cycloreversion are high. This in turn means that the photocyclization products (structure II, Scheme 1C) and dihydronaphthalene derivatives (structure III, Scheme 1C) have a low thermal stability which is manifested in the absence of their contribution in the recorded steady-state absorption spectra. Therefore, the absorption spectra obtained under irradiation of diarylethenes 3a-d are superpositions of solely two forms – ACS Paragon Plus Environment


the initial form 3 and the final form 4. Although in the case of diarylethene 3c two structural isomers 4c’ and 4c” are formed, but there is a distinct isosbestic point in the absorption spectra obtained after irradiation. The isosbestic point corresponds to the transformation of 3c into 4c. This fact can be explained by the proximity of the molar extinction coefficient values of the two isomers at the given wavelength. Figure 3. Absorption spectra of an acetonitrile solution of 3b obtained under irradiation with UV light (λirr = 313 nm); C = 2.5 x 10-5 M; T = 298 K; time interval between spectra is 120 s. (inset: an enlarged fragment of the figure demonstrating the presence of an isosbestic point (i.p.)). 0,8

i.p.267nm

0,8

h=313nm

absorbance

0,6

absorbance

0,6

0,4

0,2

0,0 250

0,4

255

260

265

270

275

280

wavelength, nm

0,2

0,0 225

250

275

300

325

350

375

400

wavelength, nm

At the same time, for diarylethenes 3e and 3f a set of absorption spectra obtained upon irradiation demonstrates not only the absence of the isosbestic point but also presents an additional absorption band characterized by the maximum at 508 nm. The intensity of this band rises during the initial time period and then slowly decreases until zero level (Figure 4). Figure 4. Absorption spectra of an acetonitrile solution of 3e obtained under irradiation with UV light (λirr= 313 nm); C = 2.6 x 10-5 M; T = 298 K; time interval between spectra is 120 s. 0,04

0,6

0,03

0,5

0,02

0,8

0,01

0,00

0,4

-0,01 400

450

500

550

600

wavelength, nm

absorbance

absorbance

0,6

absorbance

1,0

0,8

absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0,4

0,3

0,6

0,2 255

0,4

260

265

270

wavelength, nm

0,2

0,2

0,0

0,0 250

300

350

400

450

500

550

600

650

225

250

wavelength, nm


275

300

325

wavelength, nm

350

375

400

275

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It has been established that disappearance of the long-wavelength absorption band is caused by a photochemical reaction but not a thermal process. Upon switching off the irradiation source when the long-wavelength absorption exists, the intensity of spectral distribution remains constant during the observation period exceeding 3.6 x 103 s. Upon irradiation into the band characterized by the maximum at 546 nm there is its fading and a full disappearance. Such behavior is typical for thermally stable products of diarylethenes photocyclization. 5,6,22 Appearance of the additional long-wavelength absorption band upon irradiation and therefore absence of a distinct isosbestic point for diarylfuranones 3e is related to the presence of the ortho-substituent in the benzene ring. The photocyclization of diarylethene 3e proceeds by two possible ways: by the substituted carbon atom (path A) and by the unsubsituted one (path B, Scheme 3). In the first case the formed cyclic form 3eB possessing the long-wavelength absorption maximum at 508 nm undergoes photochemical cycloreversion reaction due to 313 nm light excitation into the band of its S2←S0 transition. As a result, the form 3eB transforms into the initial form 3e (photochromic process). In the second case the cyclic form 3eC undergoes irreversible sigmatropic rearrangement forming naphthalene derivative 4e. Thus, compounds for which two parallel photoinitiated processes are realized: one of which is reversible (3e 3eB) and the other irreversible (3e → 4e), the reaction is completed by complete conversion of the starting diarylethene to the naphthalene derivative. Scheme 3.

Relatively low product yields of the preparative photoreaction of ortho-substituted diarylethenes (31 and 59 %, Table 1) can be associated with an alternative photochromic transformation process. Similar competition between two photocyclization reactive cites has been observed earlier for a set of compounds.23 For the additional confirmation that photo-initiated rearrangement proceeds via the formation of cyclic form II (Scheme 1C), the isomeric diarylethene 3g has been synthesized (Scheme 4). We have previously found that for diarylethenes that do not contain the carbonyl ACS Paragon Plus Environment


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group in the geminal position of phenyl moiety, the rearrangement process slows down and the yields of the final reaction products decrease.12 Indeed, a colored photorearrangement intermediate 3gB (Figure 5) was recorded for diarylethene 3g, which undergoes fast decoloration due to the thermal process of [1,9]-sigmatropic shift, characterized by a half-time period of about 100 s. In contrast to diarylethene 3g, in the case of 3a-f (as it has been demonstrated above) the photocyclization product II (Scheme 1C) is extremely unstable, thus, it is impossible to detect the colored intermediate. This fact can be explained by the high rate of the 1,9-sigmatropic shift process (time constant has been estimated to be less than 1 s). Scheme 4.

Figure 5. UV-Vis spectra of diarylethene 3g before and after irradiation with UV light (λ = 365 nm) for 10 s in acetonitrile (c = 1.1 x 10−3 M) and thermal decomposition of colored intermediate 3gB observed at 525 nm (on inset).

To confirm the data of electron spectroscopy, we carried out an optimization of the structure of the intermediates 3eB and 3eC (Scheme 3) by density functional theory (DFT) method on B3LYP/6-31G(d)24 and CAM-B3LYP/6-31G(d)25 levels of theory. The solvent effect (acetonitrile) was simulated using the Polarizable Continuum Model (PCM).26 The absorption spectra were calculated by linear-response time-dependent DFT (TD-DFT)27 on the TD-CAMB3LYP level of theory, by broadening vertical stick spectra with Gaussians of width 1500 cm -1. ACS Paragon Plus Environment

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All results were obtained using the GAUSSIAN 09 program package.28 The calculated absorption spectra of the intermediates 3eB and 3eC are given in Figure 6. Figure 6. The calculated absorption spectra of the intermediates 3eB (left) and 3eC (right).

As can be seen, they have maxima in the visible region (505 and 478 nm, respectively) and in the UV part of the spectrum (310 and 308 nm), but in the case of 3eB the value agrees well with the given experiment (Figure 6), so we confirmed proposed in the Scheme 3 mechanism. Obviously, the difference in the spectra of these intermediates with an identical chromophore system is due to the steric repulsion of the methyl group and carbonyl in 3eC, which leads to a rather significant difference (λmax = 27 nm). To evaluate the efficiency of the photoreaction 3→4, we have calculated the quantum yields for diarylethenes 3a-d by means of kinetic modelling technique29 (numerical integration using a Runge–Kutta semiimplicit procedure with minimization algorithm of the Powell type) of the absorbance vs time curves obtained under continuous irradiation (λirr=313 nm) of the initial compounds 3a-d in acetonitrile. The obtained values of the quantum yields of the photoinitiated rearrangement for diarylethenes 3a-d are rather high and have close values lying in the interval 0.34-0.49 (Table 2). The highest value (0.49) showed the 4-methoxyphenyl derivative 3b and the smallest value (0.34) was obtained for the unsubstituted derivative 3a. The high rates of the [1.9]-sigmatropic shift and the subsequent process of the opening of oxazole cycle, the absence of reverse thermal processes, as well as the resistance to photodegradation of the rearrangement products 4 allow us to use the obtained values of quantum yields to evaluate the efficiency of the 3 → 3C photocyclization reaction (Scheme 3). Thus, it can be concluded that the introduction of the electron-donating methoxy group in the phenyl fragment leads to an increase in the efficiency of photocyclization process of diarylethenes 3 which is probably due to an increase in the electron density at the reaction centers.



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DFT calculations. To obtain additional information on the mechanism of photoreaction as well as the structures of the initial diarylethenes 3 and the primary intermediates of the photorearrangement, we have optimized the structures of diarylethenes by the DFT method on B3LYP/6-31G (d) level of theory. It was previously reported that this method quite accurately describes the structure of photoactive terarylenes11,30 and diarylethenes.31 Calculations have shown that for all diarylethenes 3, the antiparallel photoactive conformation is stable and the distance between the reaction centers should not exceed 4.2 Å (3.49 Å for 3a, Figure 7).32 In this work, we were first able to estimate the distance between the hydrogen and carbon atoms to which it migrates as a result of the sigmatropic shift in the photoinduced isomer. For all compounds 3 this value was 2.67-2.73 Å, which is less than the sum of van der Waals radii of carbon and hydrogen (2.9 Å). Figure 7. The structure of diarylethene 3a and its primary photoisomer (DFT).

Another potential parameter capable of influencing the efficiency of the light-induced rearrangement is the thermal stability of the photoinduced form II (Scheme 1C), a form preceding the [1.9]-sigmatropic rearrangement. The thermal stability of the photoinduced form II depends on the energy barrier, characterized by the activation energy EA (Figure 8). To find this value, it is necessary to calculate the energy of the transition state TS, which requires a large expenditure of computer time. Figure 8. Energetic diagram of thermal cycloreversion reaction.

Irie et al. proposed a simplified approach to predicting the thermal stability of the photoinduced form of diarylethenes based on the correlation of the EA value with the difference ACS Paragon Plus Environment

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in the ground state energies of the initial and cyclic forms.33-35 The higher the value of the difference in the energy of the isomers, the lower the activation energy of the reverse dark reaction EA and the diarylethene less thermally stable. High thermal stability is ensured by low values of ΔE. We also recently found that the thermal stability of the photoinduced form of diarylethenes correlates well with this parameter.36 The thermal stability of photoinduced forms of diarylethenes 3a-f has been evaluated by on the basis of quantum chemical calculations of the difference in the total energy of the ground state of the initial and cyclic forms of diarylethenes using the DFT method on B3LYP/6-31G (d) level of theory (Table 3). The calculations were carried out for all possible photoinduced forms of synthesized diarylethenes. As can be seen from the table, all the compounds studied have a sufficiently low thermal stability. For comparison, the values of the total energy difference and the half-lives for structurally similar previously studied diarylethenes are given in Table 3.36c Table 3. Calculated values of ground state’s energy differences of photoinduced isomer. entry

DAE

1

3a

Photoinduced isomer

ΔEAB, kcal/ mol

entry

27.55

6

DAE

Photoinduced isomer

ΔEAB, kcal/ mol 31.66

3e 2

3b

3

30.43

7

29.80

22.49

8

32.88

3c 4

5

3d

3f 24.72

9

21.45

10

E = 16.92 kcal/mol τ1/2 = 30.5 h


34.19

3g

29.73

E = 7.71 kcal/mol τ1/2 = 3200 h


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Despite the relatively low thermal stability of the investigated diarylethenes the analysis of quantum chemical calculations data and yields of photoproducts indicate that the effect of the thermal stability of the photoinduced form on the photorearrangement process is negligible. This fact is an indirect confirmation that the rate of the sigmatropic rearrangement is significantly higher in comparison with the thermal cycloreversion reaction. However, despite this, some correlations were revealed. In particular, the relatively high thermal stability of the photoinduced form for the 3d compound (21.45 kcal·mol-1) correlates well with the yield of the photoproduct (92%). It can also be noted that the low thermal stability of the cyclic form of ortho-substituted diarylethenes

3e,f

promotes

the

complete

conversion

of

the

diarylethene

to

the

photorearrangement product.

Conclusion Thus, a multifaceted study of the effect of various factors on the efficiency of the preparative photoinduced rearrangement of diarylethenes of the furanone series was carried out. The efficiency of this phototransformation (quantum yields) and the effect of methoxy substituents in the phenyl moiety have been studied. Despite the multistage process, the quantum yields of the photorearrangement are rather high (0.34-0.49). It has been established that the efficiency of photocyclization of diarylethenes obtained increases with the introduction of electron-donating methoxy groups in the phenyl fragment. The presence of ortho-substituents in the phenyl moiety leads to the appearance of a thermally stable cyclization product of diarylethenes capable to the photochemical cycloreversion reaction giving the starting compounds. The proceeding the photochromic cyclization/cycloreversion reactions along with the photoinduced rearrangement of the diarylethenes leads to a decrease in the yields of the photorearrangement products. In the photoinduced isomer using the quantum chemical method (DFT on B3LYP/6-31G (d)) it has been estimated that the distance between hydrogen atom and carbon atom to which its migrates in the result of the sigmatropic shift. For all the studied diarylethenes, this value was 2.67-2.73 Å, which is less than the sum of van der Waals radii of carbon and hydrogen atoms (2.9 Å).

EXPERIMENTAL SECTION AND METHODS General information. Proton nuclear magnetic resonance spectra (1H NMR) and carbon nuclear magnetic resonance spectra (13C NMR) were recorded in deuterated solvents on a spectrometers working at 300 MHz for 1H, 75 MHz for

13

C. Data are represented as follows:

chemical shift, multiplicity (s, singlet; d, doublet; m, multiplet; br, broad), coupling constant in hertz (Hz), integration, and assignment. Melting points (Mp) were recorded using an apparatus and not corrected. Mass spectra were obtained on a mass spectrometer (70 eV) with direct ACS Paragon Plus Environment

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sample injection into the ion source. High resolution mass spectra were obtained from a TOF mass spectrometer with an ESI source. All chemicals and solvents were purchased from commercial sources and used without further purification. Silica column chromatography was performed using silica gel 60 (70−230 mesh); TLC analysis was conducted on silica gel 60 F254 plates. Photochemical studies. Electronic absorption spectra and kinetic curves of photoinduced rearrangements of the investigated compounds were recorded on a Varian Cary 50 UV-Vis spectrophotometer equipped with a thermostatic cell. The solution (2 mL) was stirred continuously with a magnetic bar driven by a variable-speed stepper motor in the 1 cm×1 cm quartz cell closed with a Teflon bung. The irradiation of solutions was derived from a 300 W Xe lamp performed on a research arc source “Newport 66902” equipped with 1/8 m manual monochromator (“Newport 77250”) for allocation of irradiation wavelength. The photon flux power was measured on a "Newport Power Meter 2903-C" (I0313= 6.8 x 10-8 mol·L-1·s-1). Acetonitrile of the spectroscopic grade ("Aldrich") was used to prepare solutions. Photokinetic experiments. The calculated Abscalc(λ) versus t curves were obtained by numerical integration using a Runge–Kutta semiimplicit procedure. In order to check the validity of the model and to estimate the required parameters, the residual error (RE): RE = ƩpƩj[Abscalc(j) − Absexp(j)]2/pj, where p is the number of plots fitted simultaneously and j is the number of points in each plot, was minimized until a good fit was reached. The minimization algorithm is of the Powell type. The set of optimized parameters was checked to be unique. Quantum yields of photorearrangement for diarylethenes 3a-d were calculated by means of kinetic modelling technique29 (numerical integration using a Runge–Kutta semiimplicit procedure with minimization algorithm of the Powell type) of the absorbance vs time curves obtained under continuous irradiation (λirr=313 nm) of the initial compounds A (3a-d) in acetonitrile. As photoproducts B (4a-d) are photochemically stable and there is no thermal reversible transformation towards the initial isomer, kinetic equation of the observed photoreaction is described by follows: -d[A]/dt = I’0FφABε’A[A], where I’0 – light intensity at the irradiation wavelength, moles of photons·L-1·s-1; F = (1 – 10Abs’

)/Abs’ – photokinetic factor (Abs – absorbance); ε’A – molar absorption coefficient of the

initial form A, L·mol-1· cm-1; φAB – photoreaction quantum yield; [A] – concentration of the initial compound. The kinetic curves obtained by the above method for compound 3a-f are given in Figure S9-S12 (SI).



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Synthesis and characterization of starting diarylethenes. Diarylethenes 3a-f were synthesized by the reaction of phenylacetic acids 1a-f with bromoketone 2a according to literature procedures.19 Diarylethene 3g was obtained from oxazolacetic acid 1g and bromoketone 2b. Diarylethene 3a and photoproduct 4a were described previously.13 General procedure. Acid 1 (3 mmol) and potassium carbonate (4.5 mmol) were suspended in DMF (10 ml) and argon was passed through this mixture for 20 min. Then bromketone 2 (3 mmol) was added and the suspension was stirred under argon at room temperature for 0.5 h and at 80°C for 2 h. The mixture was cooled, poured into water (200 mL), and extracted with ethyl acetate (3 × 50 mL). The combined organic phases were washed with water (2 × 100 mL), dried with magnesium sulfate, and evaporated in vacuum. The resulting crude product was suspended in cold ethanol (5 mL) and resulting precipitate was filtered to give target product. 3-(4-Methoxyphenyl)-4-(5-methyl-2-phenyl-1,3-oxazol-4-yl)furan-2(5H)-one (3b). Yellow powder, 56% yield (0.58 g); mp 130–133 oC; IR (KBr), cm-1: 3464, 2970, 1739; 1H NMR (300 MHz, CDCl3) δ = 1.89 (s, 3H, CH3), 3.86 (s, 3H, CH3), 5.22 (s, 2H, CH2), 6.96 (d, J = 8.6 Hz, 2H, Harom), 7.50-7.52 (m, 5H, Harom), 8.01 (m, 2H, Harom); 13C NMR (75 MHz, CDCl3) δ = 12.2, 55.3, 70.8, 114.1 (2C), 122.9, 124.9, 126.2 (2C), 126.7, 128.9 (2C), 129.0, 130.4 (2C), 130.7, 147.9, 148.6, 159.9, 160.9, 173.6. HRMS (ESI-TOF) m/z: [M + H]+ calcd for C21H18NO4: 348.1230; found: 348.1227; MS (EI) m/z (%) = 347 (18) [M], 187 (25), 43 (100). 3-(3-Methoxyphenyl)-4-(5-methyl-2-phenyl-1,3-oxazol-4-yl)furan-2(5H)-one (3c). Yellow powder, 58% yield (0.60 g); mp 139–141 oC; IR (KBr), cm-1: 3476, 3424, 2952, 1750; 1H NMR (300 MHz, CDCl3) δ = 1.85 (s, 3H, CH3), 3.82 (s, 3H, CH3), 5.25 (s, 2H, CH2), 6.94 (d, J = 7.6 Hz, 1H, Harom), 7.11 (m, 2H, Harom), 7.34 (t, J = 8.0 Hz, 1H, Harom), 7.49 (m, 3H, Harom), 8.01 (m, 2H, Harom);

13

C NMR (75 MHz, CDCl3) δ = 12.1, 55.4, 70.8, 114.6, 114.6, 121.5, 125.1, 126.3

(2C), 126.7, 128.8, 128.9 (2C), 129.7, 130.8, 132.0, 149.1, 149.5, 159.8, 160.9, 173.2. HRMS (ESI-TOF) m/z: [M + H]+ calcd for C21H18NO4: 348.1230; found: 348.1221; MS (EI) m/z (%) = 347 (100) [M], 105 (60), 43 (25). 3-(3,4,5-Trimethoxyphenyl)-4-(5-methyl-2-phenyl-1,3-oxazol-4-yl)furan-2(5H)-one o

(3d).

-1

Yellow powder, 68% yield (0.83 g); mp 167–169 C. IR (KBr), cm : 3465, 2942, 1748, 1127; 1

H NMR (300 MHz, CDCl3) δ = 1.94 (s, 3H, CH3), 3.83 (s, 6H, CH3), 3.89 (s, 3H, CH3), 5.25 (s,

2H, CH2), 6.79 (s, 2H, Harom), 7.49-7.51 (m, 3H, Harom), 8.00-8.03 (m, 2H, Harom); 13C NMR (75 MHz, CDCl3) δ = 12.2, 56.3 (2C), 60.9, 70.8, 106.5 (2C), 125.1, 125.9, 126.2 (2C), 126.5, 128.8 (2C), 128.9, 130.8, 138.5, 148.9, 149.1, 153.4 (2C), 160.9, 173.2; HRMS (ESI-TOF) m/z: [M + Na]+ calcd for C23H21NO6Na: 430.1261; found: 430.1251; MS (EI) m/z (%) = 407 (100) [M], 247 (40), 105 (45).


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3-(2-Methylphenyl)-4-(5-methyl-2-phenyl-1,3-oxazol-4-yl)furan-2(5H)-one

(3e).

Yellow

powder, 63% yield (0.63 g); mp 182–184 oC; IR (KBr), cm-1: 3424, 2930, 1754; 1H NMR (300 MHz, CDCl3) δ = 1.67 (s, 3H, CH3), 2.26 (s, 3H, CH3), 5.32-5.35 (m, 2H, CH2), 7.23-7.25 (m, 2H, Harom), 7.31-7.33 (m, 2H, Harom), 7.47-7.49 (m, 3H, Harom), 7.97-8.00 (m, 2H, Harom);

13

C

NMR (75 MHz, CDCl3) δ = 11.3, 19.9, 71.1, 125.6, 126.0, 126.2 (2C), 126.6, 128.9 (2C), 129.0, 129.1, 130.3, 130.5, 130.7, 131.0, 137.5, 149.9, 150.9, 160.5, 173.3; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C21H18NO3: 332.1281; found: 332.1273; MS (EI) m/z (%) = 331 (18) [M], 105 (100), 43 (25). 3-(2-Methoxyphenyl)-4-(5-methyl-2-phenyl-1,3-oxazol-4-yl)furan-2(5H)-one

(3f).

Yellow

powder, 45% yield (0.47 g); mp 177–179 oC; IR (KBr), cm-1: 3433, 2940, 1744; 1H NMR (300 MHz, CDCl3) δ = 1.79 (s, 3H, CH3), 3.77 (s, 3H, CH3), 5.31(s, 2H, CH2), 6.98-7.05 (m, 2H, Harom), 7.30-7.32 (m, 1H, Harom), 7.37-7.43 (m, 1H, Harom), 7.47-7.49 (m, 3H, Harom), 7.98-8.00 (m, 2H, Harom);

13

C NMR (75 MHz, CDCl3) δ = 11.6, 55.7, 71.0, 111.3, 120.3, 120.7, 123.0,

126.2 (2C), 126.8, 128.9 (2C), 129.5, 130.5, 130.6, 131.2, 149.4, 150.6, 157.8, 160.3, 173.4; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C21H18NO4: 348.1230; found: 348.1222; MS (EI) m/z (%) = 347 (25) [M], 316 (100), 43 (100). 3-(5-Methyl-2-phenyl-1,3-oxazol-4-yl)-4-phenylfuran-2(5H)-one (3g). Yellow powder, 31% yield (0.29 g); mp 142–144 oC; IR (KBr), cm-1: 3446, 3060, 2954, 2921, 2854, 1738, 1460, 1063, 682; 1H NMR (300 MHz, CDCl3) δ = 2.39 (s, 3H, CH3), 5.30 (s, 2H, CH2), 7.41 (s, 1H, Harom), 7.44-7.46 (m, 5H, Harom), 7.56-7.59 (m, 2H, Harom), 7.99-8.02 (m, 2H, Harom);

13

C NMR (75

MHz, CDCl3) δ = 11.6, 29.7, 71.0, 118.2, 126.2 (2C), 127.3, 127.9 (2C), 128.7 (2C), 128.9 (2C), 130.2, 130.3, 131.2, 148.6, 158.7, 160.4, 172.7; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C20H16NO3: 318.1125; found: 318.1114; MS (EI) m/z (%) = 317 (100) [M], 272 (33), 105 (67), 77 (33), 43 (46). Synthesis and characterization of photoproducts. General procedure. The irradiation was carried out by 6W Vilber Lourmat (France) UV-lamp model VL-6.LC (365 nm light). Diarylethene 3 (0.6 mmol) was dissolved in dichloromethane (8 mL) and the resulting solution was irradiated (UV, λ = 365 nm, 6W) with stirring in 10 ml flatbottomed glass vessel. After completion of the reaction (TLC control) photoproduct 4 was isolated by filtration (compounds 4a-d) or by column chromatography eluting by light petroleum - ethyl acetate (4:1) (compounds 4e,f). N-(7-methoxy-5-methyl-1-oxo-1,3-dihydronaphtho[1,2-c]furan-4-yl)benzamide (4b). White powder, 78% yield (0.16 g); mp 173–174 oC; 1H NMR (300 MHz, DMSO-d6) δ = 2.66 (s, 3H, CH3), 3.98 (s, 3H, CH3), 5.37 (s, 2H, CH2), 7.47 (d, J = 9.1 Hz, 1H, Harom), 7.56-7.68 (m, 4H, Harom), 8.07 (d, J = 7.2 Hz, 2H, Harom), 8.79 (d, J = 9.0 Hz, 1H, Harom), 10.43 (s, 1H, NH); ACS Paragon Plus Environment

13

C


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NMR (75 MHz, CDCl3) δ = 15.1, 55.8, 68.9, 105.6, 117.9, 120.5, 122.6, 124.5, 128.3 (2C), 128.8, 129.0 (2C), 132.4, 134.1, 135.1, 137.9, 145.5, 158.6, 166.1, 171.4; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C21H18NO4: 348.1230; found: 348.1225; MS (EI) m/z (%) = 347 (38) [M], 105 (100), 77 (54). N-(6-methoxy-5-methyl-1-oxo-1,3-dihydronaphtho[1,2-c]furan-4-yl)benzamide and N-(8methoxy-5-methyl-1-oxo-1,3-dihydronaphtho[1,2-c]furan-4-yl)benzamide

(4c’/4c’’).

Diarylethene 3c, solvent = dichloromethane, irradiation time = 2 h 50 min; isolated mixture of two isomers; light brown powder (total yield 74%, 0.15 g); mp 167–170 oC. 4c’ isomer (62%): 1

H NMR (300 MHz, DMSO-d6) δ = 2.65 (s, 3H, CH3), 3.96 (s, 3H, CH3), 5.39 (s, 2H, CH2), 7.39

(dd, J = 9.3, 2.7, 1H, Harom), 7.64 (m, 3H, Harom), 8.06 (d, J = 7.7 Hz, 2H, Harom), 8.23 (d, J = 9.4 Hz, 1H, Harom), 8.28 (d, J = 2.6 Hz, 1H, Harom), 10.36 (s, 1H, NH); 4c” isomer (38%): 1H NMR (300 MHz, DMSO-d6) δ = 2.83 (s, 3H, CH3), 3.98 (s, 3H, CH3), 5.37 (s, 2H, CH2), 7.21 (d, J = 7.7 Hz, 1H, Harom), 7.64 (m, 4H, Harom), 8.06 (d, J = 7.7 Hz, 2H, Harom), 8.54 (d, J = 8.2 Hz, 1H, Harom), 10.36 (s, 1H, NH).

13

C NMR (75 MHz, CDCl3) δ = 14.9, 19.2, 55.8, 56.3, 68.5, 68.9,

102.1, 108.5, 115.1, 116.7, 117.5, 119.5, 125.4, 126.3, 127.9, 128.3, 128.4, 128.5, 128.9, 129.6, 129.8, 130.2, 132.4, 134.2, 139.5, 140.6, 148.6, 148.7, 158.7, 159.6, 166.0, 166.1, 171.3, 171.5; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C21H18NO4: 348.1230; found: 348.1231; MS (EI) m/z (%) = 347 (58) [M], 242 (13), 105 (100), 77 (54). N-(7,8,9-trimethoxy-5-methyl-1-oxo-1,3-dihydronaphtho[1,2-c]furan-4-yl)benzamide (4d). Red powder, 92% yield (0.22 g); mp 176–178 oC; IR (KBr), cm-1: 3183, 935, 1753, 1475, 1271; 1

H NMR (300 MHz, CDCl3) δ = 2.84 (s, 3H, CH3), 3.94 (s, 3H, CH3), 4.01 (s, 3H, CH3), 4.03 (s,

3H, CH3), 5.10 (s, 2H, CH2), 7.50-7.54 (m, 2H, Harom), 7.59-7.64 (m, 1H, Harom), 7.99 (d, J = 7.1 Hz, 2H, Harom), 8.09 (s, 1H, NH), 8.18 (s, 1H, Harom); 13C NMR (75 MHz, DMSO d6) δ = 17.3, 56.2, 61.2, 61.9, 68.5, 98.7, 116.5, 124.2, 126.5, 127.4, 128.2 (2C), 128.9 (2C), 132.4, 134.1, 139.1, 143.5, 147.6, 151.7, 154.9, 166.0, 171.5; HRMS (ESI-TOF) m/z: [M + Na]+ calcd for C23H21NO6Na: 430.1261; found: 430.1240; MS (EI) m/z (%) = 407 (25) [M], 105 (100), 77 (29). N-(5,9-dimethyl-1-oxo-1,3-dihydronaphtho[1,2-c]furan-4-yl)benzamide (4e). White powder, 59% yield (0.12 g); mp 241–244 oC; IR (KBr), cm-1: 3232, 1767, 1648; 1H NMR (300 MHz, DMSO-d6) δ = 2.66 (s, 3H, CH3), 3.03 (s, 3H, CH3), 5.37 (s, 2H, CH2), 7.59-7.66 (m, 5H, Harom), 8.06-8.09 (m, 2H, Harom), 8.13-8.15 (m, 1H, Harom), 10.42 (s, 1H, NH);

13

C NMR (75 MHz,

DMSO-d6) δ = 15.5, 25.2, 67.8, 119.6, 124.3, 127.3, 128.1, 128.2, 128.3 (2C), 129.0 (2C), 131.8, 132.5, 134.1, 135.0, 135.5, 140.4, 150.1, 166.3, 170.3; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C21H18NO3: 332.1281; found: 332.1276; MS (EI) m/z (%) = 331 (42) [M], 302 (8), 226 (13), 105 (100), 77 (54).


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N-(9-methoxy-5-methyl-1-oxo-1,3-dihydronaphtho[1,2-c]furan-4-yl)benzamide

(4f).

Reddish-brown powder, 31% yield (0.06 g); mp 183–185 oC; IR (KBr), cm-1: 3277, 2926, 1765, 1741, 1648, 1508, 1485, 1273; 1H NMR (300 MHz, DMSO-d6) δ = 2.64 (s, 3H, CH3), 3.98 (s, 3H, CH3), 5.32 (s, 2H, CH2), 7.26 (d, J = 7.5 Hz, 1H, Harom), 7.56-7.69 (m, 4H, Harom), 7.82 (d, J = 8.3 Hz, 1H, Harom), 8.06 (d, J= 6.6 Hz, 2H, Harom), 10,39 (s, 1H, NH);

13

C NMR (75 MHz,

DMSO-d6) δ = 15.4, 56.2, 67.3, 109.3, 118.0, 127.9, 128.3 (2C), 128.6, 128.8, 128.9, 129.0 (2C), 131.3, 132.4, 134.1, 135.7, 139.3, 149.3, 157.0, 166.2; HRMS (ESI-TOF) m/z: [M + H]+ calcd for C21H18NO4: 348.1230; found: 348.1229; MS (EI) m/z (%) = 347 (54) [M], 318 (46), 242 (33), 105 (100), 77 (92), 51 (21).

ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. 1

H, 13C NMR and HRMS spectra for all new compounds; 1H NMR monitoring of photoreaction

and absorption spectra for diarylethenes 3; data of DFT calculations (PDF). AUTHOR INFORMATION Corresponding Author: Prof. Valerii Shirinian *E-mail: [email protected] *E-mail: [email protected]

Acknowledgements This work was supported by Russian Foundation for Basic Research (RFBR Grants 1503-05546 and 16-33-60013). E. B. Gaeva would like to acknowledge financial support for the spectral and photochemical studies from the Ministry of Education and Science of the Russian Federation within the framework of the State Assignment for Research (project No. 4.9645.2017/8.9).

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