Photoinduced Skeletal Rearrangement of Diarylethenes Comprising

Letter pubs.acs.org/OrgLett

Photoinduced Skeletal Rearrangement of Diarylethenes Comprising Oxazole and Phenyl Rings Andrey G. Lvov, Valerii Z. Shirinian,* Vadim V. Kachala, Alexey M. Kavun, Igor V. Zavarzin, and Mikhail M. Krayushkin N. D. Zelinsky Institute of Organic Chemistry, RAS, 47, Leninsky prosp., 119991 Moscow, Russian Federation S Supporting Information *

ABSTRACT: A novel photochemical rearrangement of diarylethenes bearing oxazole and benzene derivatives as aryl moieties that results in the formation of polyaromatic systems was investigated. The mechanism of the transformation includes photocyclization, sequential [1,9] and [1,3]-hydrogen shifts, as well as a lateral oxazole ring-opening process. It was shown that this reaction can be an effective synthetically preparative method for the preparation of naphthalene (polyaromatic) derivatives.

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tion/rearrangement; in the latter case, it most often encountered a sigmatropic rearrangement reaction.10 It should also be mentioned here about a photochromic transformation of diarylethenes that includes the photocyclization and cycloreversion reactions and these processes can be repeated many times. These diarylethene features were first found by Kellogg et al. in the late 1960s,11 but they have rapidly developed over the past 20 years due to considerable interest in data storage and molecular switch applications.12−15 The diarylethenes comprising five-membered heterocyclic substituents as aryl moieties belong to the thermally irreversible (P-type) photochromic compounds because the photoinduced isomer undergoes the cycloreversion reaction only upon irradiation by visible light.12,16 The first type of tandem transformation of diarylethenes includes two reactions covering a photocyclization process and subsequent oxidation leading to phenanthrenes or phenanthrenoids III.17,18 These transformations contribute in a significant way to carbon−carbon bond formation by allowing access to exceptional molecular structures that cannot be obtained by conventional means.6,19,20 It is the preferred method for the preparation of many polynuclear aromatic (heteroaromatic) compounds.21 A second tandem reaction is a cyclization/elimination transformation that also leads to the polycyclic aromatic systems. The intermediate forming on the photocyclization stage is transformed into phenanthrene by elimination of a suitable leaving substituent in the ortho-position on one of the aromatics22 or in the central double bond.23 The presence of methoxy, halogen, or tosylate substituents facilitates to the elimination reaction. Finally, a third transformation is a cyclization/rearrangement tandem reaction that results in the opening of one of the aromatic

iarylethenes (stilbenes) are one of the most reactive systems in photochemistry.1,2 Their cis/trans isomerization,3 [2 + 2] cycloaddition,4 and photocyclization5 reactions are widely known. Among them, the photocyclization of diarylethenes is a subject of long-standing interest. The reaction was widely studied in the 1970−80s to understand the mechanistic processes and also for use in various syntheses.6−8 This type of photocyclization occurs with a wide range of 1,2diaryl-substituted ethenes and related compounds. In addition, 1,2-diarylethenes comprising heterocycles such as thiophene, furan, pyrrole, and pyridine as aryl moieties also undergo a photocyclization reaction resulting in various final products. This process proceeds in accordance with the Woodward−Hoffmann rule9 by a conrotatory 6π-electrocyclization mechanism of the cisisomer of stilbenes I leading to the formation of the thermodynamically less stable isomer, 4a,4b-dihydrophenanthrene II, which is able to transform into different reaction products depending on the structure nature and on the reaction conditions (Scheme 1). All these transformations can be classified into three types of tandem reactions: photocyclization/oxidation, photocyclization/elimination, and photocyclizaScheme 1. Photoreaction of 6π-Electron System

Received: July 15, 2014

© XXXX American Chemical Society

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amido-substituted polyarene including the phenanthrene derivatives 2a−h (Table 1).

rings. The formation of the unstable photoinduced isomer contributes further rearrangement reactions. The various rearrangement reactions resulting in the opening of aromatic cycle have been described in several recent publications.24−27 In a continuation of our studies on the photochromic properties of diarylethenes,28 we have found a new skeletal rearrangement resulting in the formation of polycyclic aromatic systems. Recently, we have synthesized a new class of photochromic diarylethenes: 2,3-diarylcyclopent-2-ene-1-ones comprising thiophene and oxazole derivatives as aryl moieties and their photochromic properties have been studied.29 These compounds exhibit a typical photochromic transformation, i.e., under irradiation by UV light to form a thermally stable colored photoinduced form able to reversibly isomerize to the initial form under illumination by visible light. However, it turned out that replacement of the thiophene ring with benzene leads to a loss of the photochromic properties. For a further explanation of this feature, we have studied the photoreaction in more detail, and the diarylethene 1a was chosen as test compound. It was found that upon irradiation of the acetonitrile solution of the diarylethene 1a by UV light (λmax = 365 nm) the formation of the colored form is not actually observed (there is no absorption in a visible region), but the changes in UV absorption spectrum with a clear isosbestic point at 266 nm are observed, indicating formation of only one new product (Figure 1, Scheme 2).

Table 1. Structures of Diarylethenes 1 and Photoproducts 2

NMR monitoring of the process showed that the reaction proceeds without significant side processes, and it is especially noticeable for diarylethenes where the carbonyl group and the phenyl moiety are attached geminally to a carbon atom of double bond (ex. 1a,c−e,g; Table 1).30 These compounds compared with other diarylethenes have relatively a short time of reaction and high yields of the reaction that also can be explained by the influence of a carbonyl group. The photocyclization of noncyclic stilbenes that can be readily prepared from commercially available aldehydes and easily synthesized oxazole acetic acid is an added benefit of this reaction (entry 8, Table 1). The method can be considered as an alternative convenient approach for the construction of polycyclic aromatic compounds that have potential applications in various areas of organic chemistry, medicinal chemistry, and material sciences.31 The structures of the synthesized compounds have been proven by UV, IR, and 1H and 13C NMR spectroscopy and mass spectrometry methods. The 1H NMR signal of the peri-proton of the naphthalene system (compounds 2a,c−e,g) is shifted to low field (9.07−9.41 ppm), which is probably due to the deshielding effect of the near located carbonyl group.32 Conclusive evidence of the presence of an amide group in the photoproducts has been obtained by IR (characteristic broad signals at 3200−3350 cm−1) and 1H NMR spectroscopy (singlet signal of amide group proton at the 10.05−10.54 ppm) as well as by its chemical trans-

Figure 1. Changes of absorption spectrum of compound 1a under irradiation with UV light (365 nm) in acetonitrile (c = 3.2 × 10−5 M).

Scheme 2. Photoreaction of Diarylethene 1a

The reaction was scaled up and carried out in a chloroform solution (c = 0.1 M) under irradiation by UV (λ = 365 nm) in the presence of atmospheric oxygen, and naphthalene 2a was isolated in 80% yield. The absorption spectrum of the obtained product coincides with that of the observed in the photoreaction and has an absorption maximum at 310 nm. In order to investigate the reaction scope, a series of experiments were performed with different diarylethenes comprising benzene and oxazole derivatives as aryl moieties. It was found that the reaction is quite general, and the different 1,2diarylethenes can be cyclized under these conditions to yield an B

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confirmed by the photoreaction of compounds 1b and 1f, where the formation of a new absorption band in the visible region with absorption maxima at 535 and 470 nm, respectively, was registered.36 The difference for these compounds can be explained by the effect of the carbonyl group, which promotes proton transfer from the carbon atom of the hexatriene system 4a to oxygen atom due to keto−enol tautomerism (Scheme 5). Further, the reaction proceeds in a similar way, where the key steps are sequential sigmatropic rearrangements (compound 7) and oxazole (compound 8) ring-opening process.

formation into a benzylamine fragment by reduction with lithium aluminum hydride (Scheme 3). The structure of the obtained compound 3 bearing a benzylamine fragment was proved by spectral methods as well as by comparison with the structures of other known analogues.33 Scheme 3. Reduction of Naphthalene 2a

Scheme 5. Alternative Mechanism of the Diarylethene Rearrangement The structures of the naphthalenes annulated with cyclopentene and cyclopentenone (compounds 2a,b,f) have been additionally proven by two-dimensional NMR experiments.34 Figure 2 shows the structurally valuable NOESY correlations observed for these naphthalene systems.

In conclusion, we have studied a novel photochemical rearrangement of diarylethenes comprising oxazole and benzene derivatives as aryl moieties which leads to naphthalene (phenanthrene) derivatives. The mechanism of this transformation includes photochemically allowed six-electron conrotatory photocyclization, sequential [1,9] and [1,3]-hydrogen shifts, as well as a lateral oxazole ring opening process. It was found that the presence of the carbonyl group in the geminal position with respect to the phenyl fragment promotes the photorearrangement reaction and increases the yields of the final products. It has been shown that this photocyclization can be an ample and useful synthetic method for the preparation of polyaromatic systems, primarily naphthalenes.

Figure 2. Selected key 2D-NOESY correlations of compounds 2a,b,f.

The proposed mechanism of this transformation is illustrated in Scheme 4. The first step is likely a photocyclization reaction Scheme 4. Proposed Mechanism of Skeletal Rearrangement of Diarylethenes

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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures and full spectroscopic data for all new compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

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leading to the formation of the photoinduced form 4 (the direct reaction of the photochromic process). Further, there is a [1,9]sigmatropic rearrangement to form an intermediate 5, which in turn undergoes the following [1,3]-sigmatropic rearrangement, resulting in the formation of dihydronaphthalene 6. In the final step of the process, there is the opening of oxazoline ring to form an amide group. The driving force of this rearrangement is initially given by the restoration of the aromaticity of the benzene ring and then of the naphthalene system to form a thermodynamically more stable structure. It should be noted that the occurrence of such skeletal rearrangement is dictated by the structure of the diarylethene, namely the presence of a hydrogen atom at the ortho-position of the benzene ring and the methyl group at position 5 of the oxazole ring, which prevents oxidation leading to the formation of phenanthrene system. Another an important factor is an ability to seamlessly undergo opening of the oxazoline ring (compound 6) formed upon irradiation.35 A direct confirmation that the reaction proceeds through formation of the photoinduced cyclic form is the appearance of color upon irradiation of solutions of the compounds 1b,f,h (unlike other diarylethenes) that quickly disappears. It was

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the Russian Foundation for Basic Research (RFBR grant 14-03-31871) is gratefully acknowledged. REFERENCES

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