A Novel Photochromic System of
4,5-Dialkenylthiophenes Constructed by
the Samarium Diiodide Promoted
Coupling Reactions of
Thiophene-2-carboxylate with Aryl
Ketones
Jiun-Jie Shie, Shyh-Ming Yang, Chao-Tsen Chen, and Jim-Min Fang*
Department of Chemistry, National Taiwan UniVersity, Taipei 106, Taiwan, Republic of China
[email protected] Received December 20, 2001
ABSTRACT
The SmI2-promoted coupling reaction of ethyl thiophene-2-carboxylate with aryl ketones (2 equiv), followed by acid-catalyzed dehydration and oxidative aromatization, gave dialkenylthiophenes 1b−d, which underwent electrocyclizations upon irradiation with 300-nm light in CH3CN solution to give the corresponding closed-ring species with absorption λmax≈ 425 nm. The interconversion between dialkenylthiophenes and their corresponding closed-ring species constitutes a novel photochromic system bearing an ester group for potential uses in linkage and wavelength tuning.
Photochromism is a light-induced reversible isomerization between two forms having different absorption spectra. The photochromic system based on the interconversion of 1,3,5-hexatriene to cyclohexadiene has been extensively investi-gated,1 partly as a result of their potential applications to
optical memories and switches. The photocyclization of stilbene to dihydrophenanthrene is a well-known photochro-mic system.2According to the Woodward-Hoffmann rule,3
the double bond of stilbene should have (Z)-configuration
to conform a concerted conrotatory cyclization in the photochemical conditions. Dihydrophenanthrene can return rapidly to stilbene in the dark; however, it can also undergo oxidative aromatization in the presence of oxygen.2 By
replacing the phenyl rings with thiophene rings, 1,2-di(3-thienyl)ethene undergoes a photocyclization reaction to give the closed-ring species with an improved stability.4 This
(1) For reviews, see: (a) Laarhoven, W. H. In Photochromism: Mol-ecules and Systems; Du¨rr, H., Bouas-Laurent, H., Eds.; Elsevier: Amster-dam, 1990; Chapter 7, pp 270-313. (b) Irie, M.; Uchida, K. Bull. Chem. Soc. Jpn. 1998, 71, 985. (c) Organic Photochromic and Thermochromic Compounds; Crano, J. C., Guglielmetti, R. J., Eds.; Plenum Press: New York, 1999. (d) Irie, M. Chem. ReV. 2000, 100, 1685.
(2) Muszkat, K. A.; Fischer, E. J. Chem. Soc. C 1967, 662. (e) Repinec, S. T.; Sension, R. J.; Szarka, A. Z.; Hochstrasser, R. M. J. Phys. Chem. 1991, 95, 10380.
(3) (a) Dewar, M. J. S. Angew. Chem., Int. Ed. Engl. 1971, 10, 761. (b) Zimmerman, H. E. Acc. Chem. Res. 1971, 4, 272. (c) Nakamura, S.; Irie, M. J. Org. Chem. 1988, 53, 6136.
(4) Kellogg, R. M.; Groen, M. B.; Wynberg, H. J. Org. Chem. 1967, 32, 3093.
ORGANIC
LETTERS
2002
Vol. 4, No. 7
1099-1102
10.1021/ol0172830 CCC: $22.00 © 2002 American Chemical Society
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result is attributed to the low aromatic stabilization energy difference between the open- and closed-ring isomers of thiophene by comparison with that of benzene.1b,3cIrie and
co-workers further demonstrate that the light-induced cy-clization of 1,2-bis(2,4-dimethylthien-4-yl)perfluorocyclo-pentene gives a stable colored species.1bThe cyclopentene
ring incorporates the requisite double bond of the (Z)-configuration, and the methyl groups at the 2- and 2′ -positions prevent the closed-ring species from oxidative aromatization.
We report herein a novel photochromic system (eq 1) based on the interconversion between 4,5-dialkenyl-thiophenes 1a-d and their closed-ring isomers 2a-d. Dialkenylthiophene 1a-d were readily prepared by a three-step sequence (Scheme 1): (i) coupling of ethyl
thiophene-2-carboxylate (3) with aryl ketones 4-7 (2 equiv) by the promotion of SmI2, giving diols 8-11,5(ii) acid-catalyzed
dehydration, giving dienes 12-15, and (iii) oxidation by DDQ (1 equiv), giving dialkenylthiophenes 1a-d. The
central double bonds of 1a-d were confined in the thiophene ring to adopt a (Z)-configuration required by the concerted electrocyclizations.
The UV-vis spectrum of dialkenylthiophene 1a6exhibited λmaxat 242 nm ( ) 38800) and 289 nm ( ) 13800). Upon
irradiation of 1a (colorless solution) with 300-nm light, an orange color immediately appeared, indicating the existence of new species. The reaction course of 1a in CDCl3(ca. 0.1
M) was recorded by the1H NMR spectra.7After irradiation
for 10 min, closed-ring species 2a (ca. 30%) was produced as inferred from a new set of proton signals, in which a singlet atδ 2.99 was ascribed to the methylene protons of
tetrahydrobenzothiophene moiety. The ratio of 2a increased as the irradiation progressed to 60 min. However, several other compounds including benzothiophene 168 and
6,7-dihydrobenzothiophene 179 were also produced at the
expense of 2a (Scheme 2).
Compound 16 was likely obtained by an oxidative aro-matization of 2a similar to that proposed in the conversion
(5) (a) Yang, S.-M.; Fang, J.-M. Tetrahedron Lett. 1997, 38, 1589. (b) Yang, S.-M.; Nandy, S. K.; Selvakumar, A. R.; Fang, J.-M. Org. Lett. 2000, 2, 3719.
(6) Compounds 1a-d and 8-18 were fully characterized by spectral methods (IR, UV, MS, HR-MS,1H and13C NMR). The details are reported in Supporting Information, whereas some pertinent physical and spectral properties are herein listed. Compound 1a: solid, mp 109-111°C; UV (CHCl3)λmax() 242 nm (38800), 289 nm (13800); FL (CHCl3, c ) 2× 10-4M)λem415 nm on excitation at 364 nm;1H NMR (CDCl3, 300 MHz) δ 7.77 (1 H, s), 7.73-7.70 (2 H, m), 7.58-7.54 (4 H, m), 7.44-7.36 (6 H, m), 7.16 (1 H, dd, J ) 8.5, 1.5 Hz), 7.10 (1 H, dd, J ) 8.5, 1.5 Hz), 5.57 (1 H, s), 5.50 (1 H, s), 5.41 (1 H, s), 5.35 (1 H, s), 4.38 (2 H, q, J ) 7.3 Hz), 1.39 (3 H, t, J ) 7.3 Hz). Compound 1b: UV (CHCl3)λmax() 290 nm (35400). Compound 1c: UV (CH3CN)λmax() 249 nm (34400), 300 nm (16800). Compound 1d: UV (CH3CN)λmax() 251 nm (34900), 300 nm (13700).
(7) The photochemical reactions were conducted in a Rayonet photo-chemical reactor using 300 or 450-nm lamps. The corresponding NMR and UV-vis time course spectra are included in Supporting Information. Scheme 1a
aReagents and conditions: (i) SmI
2, THF, HMPA, 25°C,
12-24 h. 8, 50% yield; 9, 43%; 10, 32%; 11, 37%. (ii) Cat. p-TsOH, PhH, reflux, 4-12 h. 12, 95% yield; 13, 87%; 14, 67%; 15, 73%. (iii) For 1a, DDQ, PhH, 60°C, 12 h; 83% yield. For 1b-d, DDQ, PhCH3, reflux, 24 h; 1b, 69% yield; 1c, 80%; 1d, 84%.
Scheme 2
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of dihydrophenanthrene to phenanthrene. When the photo-chemical reaction of 1a was conducted in deoxygenated CD3
-CN solution, the formation of 16 was inhibited. Compound
17 might be derived from 2a by an H-shift. Attempts to
isolate 2a failed, presumably because of its instability. Attempts to trap the transient closed-ring species 2a, as an
o-thiophenequinodimethane, by N-phenylmaleimide in the
photochemical reaction of dialkenylthiophene 1a (300-nm, CH3CN, 25°C, 48 h) also failed, even though the existence
of 2a could be inferred from a newly occurring NMR signal atδ 2.99 (s). However, 2a was alternatively generated by
the thermal electrocyclization of 1a in refluxing toluene and successfully trapped by N-phenylmaleimide to give a [4 + 2] cycloaddition product 18.10Although the trapping
experi-ment under thermal conditions should not be strictly cor-related to the photochemical reaction of 1a, this result lent a clue to intermediate 2a in support of the newly NMR signal atδ 2.99 found under photochemical conditions.
The drawbacks of oxidative aromatization and H-shift would be avoided in the photochemical cyclization of trienes
1b-d as they have C-5 and C-6 positions substituted by
methyl groups. When a colorless acetonitrile solution (1.1× 10-5M) of 1b was irradiated with 300-nm light at room temperature, the 290-nm absorption of 1b decreased along with the growth of a new 429-nm absorption attributed to the formation of the yellow closed-ring species 2b (Figure 1). The corresponding1H NMR spectra7also showed a newly
occurring singlet (δ 6.64) ascribed to the H-3 of 2b.
According to the NMR study, the maximal content of the
closed-ring species 2b was estimated to be 15% in the reaction mixture under the photochemical conditions (30 min at -20°C in CD3CN solution).
The photochemical properties of 1c (Ar ) Ph) and 1d (Ar ) p-MeOC6H4) were similar to that of 1b (Ar )
2-naphthyl), even though they had different aryl groups. Irradiation of 1c and 1d with 300-nm light in CH3CN solution
afforded the closed-ring species 2c and 2d, which showed absorption maxima at 422 and 425 nm, respectively.7 On
the basis of these results, the moiety of o-thiophenequino-dimethine carboxylate in 2b-d was likely the main con-tributing chromophore. The absorption change in 2b-d was trivial, as their aryl substituents might not be coplanar to the chromophore.
We also demonstrate that the closed-ring species 2b-d are photochemically reversible.11When a mixture of 1b/2b
and 1d/2d was irradiated with 450-nm light, respectively,
2b and 2d disappeared and returned rapidly (within 1 min)
to the open-ring species (1b and 1d). The interconversion between 1c and 2c was realized by alternate irradiation with UV (300-nm) and visible (419-nm) lights at 10-s intervals. In a preliminary experiment, the absorption intensity at 424 nm showed a significant change after irradiation of 1c/2c for several cycles (see Supporting Information).
The ring-closure and ring-opening reactions as shown in eq 1 should be both photochemically and thermally allowed. The closed-ring species were unstable on standing at room temperature, and they returned to the open-ring species in the dark. For example, a mixture of 1b/2b (ca. 85:15) obtained by irradiation of 1b with 300-nm light for 40 s in CH3CN was removed from the light source and allowed to
stand at room temperature in the dark. According to the absorption intensity changes at 290 and 429 nm, 2b returned to 1b to a considerable degree (∼20%) in the first minute and 2b completely disappeared in 40 min.7 A mixture of 1c/2c (ca. 82:18) also behaved similarly to give exclusively 1c after standing at room temperature for 24 h in the dark.
This interconversion was monitored by1H NMR spectra, in
which the characteristic H-3 signal of 1c (singlet atδ 7.52)
increased as that of 2c (singlet atδ 6.22) disappeared.7
In summary, the interconversion between dialkenylthio-phenes 1b-d and their corresponding closed-ring species constitutes a novel photochromic system. The preparation of 1b-d is simple by using readily available starting materials via a three-step sequence (Scheme 1), though the yield is not yet optimized. The ester group in trienes 1b-d may serve as a linker to polymeric supports or undergo functional group transformation to tune the photochemical properties. The closed-ring species 2b-d are unstable and return to 1b-d in the dark even at room temperature. The photochemical and thermal stability of these substrates must be improved in order to reach a practical application of this novel dialkenylthiophene/dihydrobenzothiophene system.
The elegant diarylethene photochromic system developed
(8) For comparison, an authentic sample of 16 was also prepared by treatment of triene 1a with DDQ in refluxing toluene. Compound 16 exhibited a characteristic H-3 atδ 8.27 (s).
(9) Compound 17 was isolated by chromatography of the reaction mixture at the interval of 90-min irradiation of 1a. The structure of 17 was tentatively assigned according to the1H NMR, HR-MS, and NOE analyses. There is no enhancement of H-3 atδ 7.68 (s) on irradiation of H-7 at δ 4.53 (t, J ) 9 Hz).
(10) Compound 18 had the endo-configuration as its NOESY spectrum showed a correlation of H-4a/H-7a (δ 4.34-4.33, m) with the ethylene-bridge protons (δ 2.15-2.11, m).
(11) Miyasaka, H.; Murakami, M.; Itaya, A.; Guillaumont, D.; Nakamura, S.; Irie, M. J. Am. Chem. Soc. 2001, 123, 753.
Figure 1. The UV-vis spectral change of compound 1b on irradiation with 300-nm light at 25°C in CH3CN solution.
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by Irie and co-workers1b,dhas demonstrated high quantum
yield, thermal stability, and fatigue resistance. This system incorporates a central ring of perfluorocyclopentene with substitution of two aromatic rings. When the open isomer changes to the closed isomer upon irradiation, the two aromatic rings must change simultaneously to form another conjugated chromophore. In contrast, our current photochro-mic system utilizes a thiophenyl ring as the central moiety. The synthesis of the photo-/thermochromic substrates of dialkenylthiophenes is relatively easy by using the com-mercially available starting materials of thiophene-2-car-boxylate and aryl ketones. Furthermore these two aryl groups are not incorporated into the contributing o-thiophenequino-dimethane chromophore of the closed-ring species. This unique structure bearing two independent chromophores thus provides a good opportunity to build the photochromic system having a fluorescent sensing property. For example,
one can replace the naphthyl ring in 1b by an appropriate coumarin as the acceptor chromophore for the light emitted from the closed-ring species, which is generated by UV-light irradiation. Modification of the ketone moieties in our current system would not be difficult. We are currently engaged in this endeavor to demonstrate such energy-transfer mechanism in a photochromic system.
Acknowledgment. We thank the National Science
Coun-cil for financial support and Professor Pi-Tai Chou (National Taiwan University) for helpful discussion.
Supporting Information Available: Detailed
experi-mental procedures, NMR and UV-vis spectra. This material is available free of charge via the Internet at http://pubs.acs.org.
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