HETEROCYCLES

Paper | Special issue | Vol. 86, No. 2, 2012, pp. 1129-1134
Received, 26th June, 2012, Accepted, 25th October, 2012, Published online, 12th November, 2012.
DOI: 10.3987/COM-12-S(N)62

■ Hydrogen Bonding Induced Highly Selective Isomerization of an Olefin Having Heteroaromatic Rings

Takuya Kobayashi and Tatsuo Arai*

Department of Chemistry, Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Ten-nodai, Tsukuba-shi, Ibaraki 305-0006, Japan

Abstract

Heteroaromatic compounds having two different isomerization parts in the molecule containing nitrogen at the aromatic ring have been synthesized and their photoisomerization and fluorescence properties have been examined.

INTRODUCTION
Much attention has been focused on the nitrogen containing aromatic compounds from the fundamental point of view as well as application to optical materials and medicinal compounds. 1,2 The reason for the increase of active interest and importance for the research in these compounds might be the characteristic nature and reactivity of these compounds. Especially, nitrogen containing aromatic compound exhibited characteristic properties in the excited state such as intramolecular hydrogen atom transfer induced highly selective trans-to-cis isomerization and unusual fluorescence emission at considerably longer wavelength region. For example, in the case of 1,3 only cis-isomer forms intramolecular hydrogen bonding between pyrrole ring and pyridine ring which is confirmed by the highly down field shift of the NH proton of the pyrrole ring. Due to this intramolecular hydrogen bonding, 1 exhibited one-way trans-to-cis photoisomerization in the excited singlet state. That is, while trans-1 underwent photoisomerization to cis-1, cis-1 did not give trans-1. However, cis-1 underwent intramolecular hydrogen atom transfer to give the tautomer (cis-1 tautomer), which is revealed by the observation of the fluorescence at unexpectedly longer wavelength region peaking at 570nm. Compound 2 exhibited similar photochemical properties.

In the course of our study on the effect of hydrogen bonding on photochemical behavior of olefinic compounds, we are interested in the compounds with pyrrole ring and two hetroaromatic rings capable to form intramolecular hydrogen bonding with pyrrole NH, to know the factor to control the photoisomerization direction among the two different isomerization parts. We wish to report here the photochemical and spectroscopic properties of potentially hydrogen bonding compounds by introduction of two different N-containing aromatic ring, pyridine and quinoxaline ring connecting by C=C double bond at both ends of pyrrole ring, 3.

RESULTS AND DISCUSSION
Figure 1 shows absorption spectra of trans,trans-3 having absorption maximum at 456 nm and trans,cis-3 having absorption maximun at 483 nm. As is usual for the hydrogen bonded compounds, but is unusual for aromatic olefins such as stilbene, the trans,cis-3 shows absorption at longer wavelength probably due to the intramolecular hydrogen bonding influencing some kind of increase of conjugation. These experimental results are in accordance with the previous findings for 1, and it is cleared that 3 shows absorption at the lowest energy region among some pyrrolylethenes reported previously.3-8
On irradiation of trans,trans-3 in benzene at 498 nm, which is the isosbestic point, the absorption maximum of trans,trans-3 decreased and that of the isomer increased. (Figure 2).

In order to confirm conformation of the isomer, the 1H NMR analyses have been performed. As shown in Figure 3 proton signals changed with irradiation time at 365 nm and this peak change indicates the production of the cis isomer at quinoxaline side instead of pyridine side. Furthermore, no more isomerization to the cis isomer at the pyridine side took place. Thus, in compound 3 having two possible cis conformation, only quinoxaline side isomerized to the cis conformation to give trans,cis-3.
The isomer ratio of ([trans,trans-3]/[trans,cis-3]) at the photostationary state exciting at the isosbestic point 498 nm was determined to be ([trans,trans-3]/[trans,cis-3])pss = 3/97. This result indicates that the photoisomerization from trans,trans-3 to trans,cis-3 is much more efficient than that of the reverse process: the quantum yield of trans,trans-3-to-trans,cis-3 photoisomeriation is more than 30 times more efficient than that of trans,cis-3-to-trans,trans-3 photoisomerization.
The results of the preferential photoisomerization to give trans,cis-3 may indicate that the isomerization reactivity is controlled by the size of the aromatic ring, which may be related to the energetic situation of the isomerization processes to the quinoxaline side or pyridine side; because of the larger aromatic ring for quinoxaline ring than that of pyridine ring, the energy curve favors to the isomerization reaction to the quinoxaline side. These discussions are supported by the energy in the singlet excited state. That is, if one considers trans,trans-3 composed of two olefinic chromophore of 1 and 2, the excited state energy of 2 has the lower energy than that of 1 (Es = 76 kcal/mol, Es = 65 kcal/mol for 1 and 2, respectively).3,8 Therefore, the reactivity of trans,trans-3 in the singlet excited state (Es =57 kcal/mol for 3) may be dominated at the olefinic part of 2 containing quinoxaline ring, resulting in the selective photoisomerization mentioned　above.

Figure 4 shows the fluorescence spectra of trans,trans-3 in various solvents. As clearly shown in this figure, the fluorescence spectra shifted to the longer wavelength region and the vibrational structure disappeared with increasing solvent polarity. This results can be explained by the charge transfer character of trans,trans-3 in the excited singlet state by the electron rich pyrrole ring to the electron deficient quinoxaline ring. The fluorescence quantum yield of trans,trans-3 is relatively high in non-polar and polar solvent from 0.14 to 0.27.

In conclusion, heteroaromatic compounds having two different isomerization parts in the molecule containing nitrogen at the aromatic ring have been synthesized and their photoisomerization and fluorescence properties have been examined. Among the obtained results, specific photoisomerization from the trans,trans isomer to the trans,cis isomer with larger conjugation moiety exclusively took place. Furthermore, the intramolecular hydrogen bonding highly inhibited the isomerization from trans,cis isomer to trans,trans isomer. Moreover, the trans,trans isomer exhibited considerably high fluorescence quantum yield in various solvent with different maximum wavelength sifting to longer wavelength with increasing solvent polarity.

EXPERIMENTAL
Materials

5-((E)-2-(Quinoxalin-3-yl)vinyl)-1H-pyrrole-2-carbaldehyde
A mixture of 2-bromomethyl quinoxaline (2.51 g, 11.3 mmol) and triethylphosphite (8.72 g, 52.5 mmol) was refluxed for 12 h. After cooling, the excess triethylphosphite was removed in vacuo to leave red residue. This crude product diethyl (quinoxalin-2-ylmethyl)phosphonate was used in the next step without further purification.
To a mixture of sodium hydride (0.665 g, 27.7 mmol) and 2,5-diformylpyrrole (1.35 g, 11.0 mmol) in dry THF (60 mL) was added diethyl (quinoxaline-2-ylmethyl)phosphonate dissolved in dry THF (10 mL), and the mixture was stirred at 80 °C for 1.5 h, after which time the reaction was quenched by additional of water (5 mL). The solvent was evaporated and the residue was dissolved in CHCl3 (200 mL), washed with brine, dried over sodium sulfate, filtered and evaporated. The residue was purified by silica gel column chromatography [eluent: CHCl3/EtOAc (4:1)] to give the desired product as a red solid (1.15 g, 42 %).
1H NMR (400 MHz, CDCl3): δ 10.23 (s, 1H), 9.57 (s, 1H), 8.97 (s, 1H), 8.09-8.04 (m, 2H), 7.83-7.96 (m, 3H), 7.35 (d, J = 16.2 Hz, 1H), 7.04-7.03 (m, 1H), 6.70-6.68 (m, 1H).

2-((E)-2-(Pyridin-3-yl)vinyl)-5-((E)-2-(quinoxalin-3-yl)vinyl)-1H-pyrrole (3)
n-BuLi (0.30 ml of a 2.6 M hexane solution, 0.78 mmol) was added slowly to a suspension of 2-methylpyridinephosphonium bromide (289 mg, 0.667 mmol) in dry THF (15 mL) at -78 °C under nitrogen atomosphere. After complete addition, the reaction mixture was brought to room temperature and was stirred for 0.5 h. Then, the reaction mixture was cooled to -78 °C and added 5-((E)-2-(quinoxalin-3-yl)vinyl)-1H-pyrrole-2-carbaldehyde (254 mg, 1.02 mmol) dissolved in dry THF (10 mL), and the reaction mixture was stirred for 2 h, after which time the reaction was quenched by additional of EtOAc (10 mL) and water (10 mL). The solvent was evaporated and the residue was dissolved in EtOAc (50 mL × 2), washed with brine, dried over sodium sulfate, filtered and evaporated. The residue was purified by silica gel column chromatography [eluent: Hexane/EtOAc (2:1)] to give the desired product as a red solid (208 mg, 63 %).
1H NMR (400MHz, DMSO): δ 11.75 (brs, 1H), 9.11 (s, 1H), 8.54 (d, J = 4 .0 Hz, 1H), 7.96-8.03 (m, 2H), 7.66-7.83 (dm, J = 7.1 Hz, 3H), 7.83 (d, J = 16.1 Hz, 1H), 7.53 (d, J = 16.3 Hz, 1H), 7.43 (d, J = 7.1 Hz, 1H), 7.34 (d, J = 16.3 Hz, 1H), 7.17-7.20 (m, 1H), 7.18 (d, J = 16.1 Hz, 1H), 6.61-6.63 (m, 1H), 6.54-6.56 (m, 1H). Anal. Caled for C21H17N3: C, 81.00; H, 5.50; N, 13.49. Found: C, 80.77; H, 5.68; N, 13.22. IR (KBr disk): 3427, 3346, 1638 cm-1. Mp: 152 °C.

Apparatus
Absorption and fluorescence spectra were measured on a Shimadzu UV-1600 and on a Hitachi F-4500 fluorescence spectrometer, respectively. All solvents of spectral grade for spectroscopy were purchased and used without further purification. All measurements were carried out at room temperature under Ar. The concentration of solution for spectroscopy was adjusted so that the absorption maximum at the excitation wavelength was 0.1 for each sample. Fluorescence quantum yields were determined relative to anthracene (Φf ＝ 0.27 in ethanol). 1H NMR spectra in CDCl3 or DMSO-d6 with TMS as an internal standard were measured on a 400 MHz NMR spectrometer, AV-400 (Bruker).

ACKNOWLEDGEMENTS
This work was supported by a Grant-in-Aid for Scientific Research in a Priority Area “New Frontiers in Photochromism” (No. 471) and a Grant-in-Aid for Scientific Research (B) (No. 23350075) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan.

References

1. S. J. Evenson, M. J. Mumm, K. I. Pokhodnya, and S. C. Rasmussen, Macromolecules, 2011, 44, 835. CrossRef
2. A. Gangjee, O. Adair, and S. F. Queener, J. Med. Chem., 1999, 42, 2447. CrossRef
3. M. Obi, H. Sakuragi, and T. Arai, Chem. Lett., 1998, 169. CrossRef
4. F. D. Lewis, B. A. Yoon, T. Arai, T. Iwasaki, and K. Tokumaru, J. Am. Chem. Soc., 1995, 117, 3029. CrossRef
5. Y. Yang and T. Arai, Tetrahedron Lett., 1998, 39, 2617. CrossRef
6. T. Arai, M. Moriyama, and K. Tokumaru, J. Am. Chem. Soc., 1994, 116, 3171. CrossRef
7. M. Ikegami and T. Arai, J. Chem. Soc., Perkin Trans. 2, 2002, 342. CrossRef
8. K. Kudo, A. Momotake, Y. Kanna, Y. Nishimura, and T. Arai, Chem. Commun., 2011, 47, 3867. CrossRef