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Stepwise Photochemical-Chiral Delivery in γ-Cyclodextrin-Directed Enantioselective Photocyclodimerization of Methyl 3-Methoxyl-2-Naphthoate in Aqueous Solution Lin Luo, Su-Fang Cheng, Bin Chen, Chen-Ho Tung, and Li-Zhu Wu* Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry and Graduate University, The Chinese Academy of Sciences, Beijing 100190, People’s Republic of China Received June 17, 2009. Revised Manuscript Received July 31, 2009 Irradiation of methyl 3-methoxyl-2-naphthoate (2,3-NA) with λ > 280 nm results in photocyclodimerization to produce cubane-like photocyclodimer 1 and the [4 þ 4] intermediate 2. The optically pure enantiomers of the intermediate 2 have been achieved by high-performance liquid chromatography (HPLC) resolution on a chiralcel OJRH column. Comparison of the enantiomeric excess (ee) values for photocyclodimer 1 and the intermediate 2 obtained in γ-CD aqueous solution reveals the stepwise photochemical-chiral delivery for the first time, which is recognized to be a consequence of an in situ increase in the ee value from 39% for the [4 þ 4] intermediate 2 to 48% for photocyclodimer 1 upon irradiation of 2,3-NA in the presence of γ-CD.
Introduction One of the challenging research subjects in chemistry is to perform highly stereoselective reactions. In contrast to the great advance in asymmetric synthesis using thermal reactions during the last few decades, asymmetric photochemistry still lies in the area of basic research.1-7 The reason is due to the long held belief that, in the excited states, chiral interactions are too weak and short-lived to achieve high stereochemical recognition and differentiation. In recent years, however, the use of supramolecular chiral systems offers an important breakthrough of chiral phototransformation.1,4,8-18 By virtue of multiple intermolecular interactions, supramolecular chiral systems are able to associate with the substrate in both the ground and excited states and, thus, photochemically deliver chiral information from a chiral entity to a prochiral substrate. There have been many established systems of asymmetric photoreactions reported thus far; however, direct observation of the transient species is still difficult to achieve. *To whom correspondence should be addressed. E-mail:
[email protected]. ac.cn. (1) Chiral Photochemistry; Inoue, Y., Ramamurthy, V., Eds.; Marcel Dekker: New York, 2004. (2) Molecular and Supramolecular Photochemistry; Ramamurthy, V., Schanze, K. S., Eds.; Marcel Dekker: New York, 2001. (3) Advances in Photochemistry; Neckers, D. C., Volman, D. H., Von B€unau, G., Eds.; John Wiley and Sons: New York, 1996. (4) Wu, L.-Z.; Chen, B.; Luo, L.; Xu, H.-X.; Liao, G.-H. Asymmetric Catalysis: New Concept and Methods; Ding, K., Fan, Q.-H., Eds.; Chemical Industry Press of China: Beijing, China, 2008; pp 197-249 (in Chinese). (5) Rau, H. Chem. Rev. 1983, 83, 535. (6) Kim, J.-I.; Schuster, G. B. J. Am. Chem. Soc. 1990, 112, 9635. (7) Inoue, Y. Chem. Rev. 1992, 92, 741. (8) Green, B. S.; Lahav, M.; Rabinovich, D. Acc. Chem. Res. 1979, 12, 191. (9) Ramamurthy, V.; Venkatesan, K. Chem. Rev. 1987, 87, 433. (10) Toda, F. Acc. Chem. Res. 1995, 28, 480. (11) Scheffer, J. R. Can. J. Chem. 2001, 79, 349. (12) Takahashi, K. Chem. Rev. 1998, 98, 2013. (13) Nakamura, A.; Inoue, Y. J. Am. Chem. Soc. 2003, 125, 966. (14) Bauer, A.; Westkaemper, F.; Grimme, S.; Bach, T. Nature 2005, 436, 1139. (15) Sivaguru, J.; Natarajan, A.; Kaanumalle, L. S.; Shailaja, J.; Uppili, S.; Joy, A.; Ramamurthy, V. Acc. Chem. Res. 2003, 36, 509. (16) Lv, F. F.; Chen, B.; Wu, L.-Z.; Zhang, L.-P.; Tung, C.-H. Org. Lett. 2008, 10, 3473. (17) Ishida, Y.; Kai, Y.; Kato, S.; Misawa, A.; Amano, S.; Matsuoka, Y.; Saigo, K. Angew. Chem., Int. Ed. 2008, 47, 8241. (18) Nishijima, M.; Wada, T.; Mori, T.; Pace, T. C. S.; Bohne, C.; Inoue, Y. J. Am. Chem. Soc. 2007, 129, 3478.
782 DOI: 10.1021/la902176e
Very recently, we found that the use of native γ-CD is an effective and versatile strategy for the enantioselective photocyclodimerization of naphthalene analogue: methyl 3-methoxyl2-naphthoate (2,3-NA).19 During the course of irradiation with light λ > 280 nm, 2,3-NA undergoes photocyclodimerization to produce anti-head-to-head cubane-like photocyclodimer 1 and [4 þ 4] intermediate 2. This result confirms that the formation of the cubane-like photocyclodimer of naphthalene analogue is a two-photon process. That means 2,3-NA absorbs the first photon to give a [4 þ 4] cycloaddition product 2, which subsequently undergoes either dissociation to give 2,3-NA or photochemical transformation to produce cubane-like photocyclodimer 1 through absorption of the second photon (Scheme 1). It is generally accepted that the structure of the photocyclodimer reflects the preferred orientation of the precursor,1-4 and the enantiomeric excess (ee) value of the chiral product is a direct function of the relative stability of the diastereomeric precursors with γ-CD in the ground state. This photochemical reaction takes place by a two-photon process. Here arises a question: the high enantioselectivity (48% ee) of anti-head-to-head photocyclodimer 1 induced by native γ-CD in aqueous solution is determined by one of the steps or both? To answer this question, the resolution of 2 became imperative under the situation. In the present work, we report that the [4 þ 4] intermediate 2 could be successfully resolved by high-performance liquid chromatography (HPLC) analysis on a chiralcel OJ-RH column. A comparison of the ee values for photocyclodimer 1 and the intermediate 2 obtained in γ-CD aqueous solution reveals the stepwise photochemical-chiral delivery, which is recognized to be a consequence of an in situ increase in the ee value upon irradiation of 2,3-NA in the presence of γ-CD.
Results and Discussion The photochemical reaction was carried out in a Pyrex tube with a 500 W high-pressure mercury lamp as a light source. After irradiation of 2,3-NA@γ-CD, prepared by sonication of 2,3-NA and 2 equiv of γ-CD in aqueous solution, with light λ > 280 nm in (19) Luo, L.; Liao, G.-H.; Wu, X.-L.; Lei, L.; Tung, C.-H.; Wu, L.-Z. J. Org. Chem. 2009, 74, 3506 and references therein.
Published on Web 08/19/2009
Langmuir 2010, 26(2), 782–785
Luo et al.
Article
Scheme 1. The Photocyclodimerization Process of 2,3-NA
aqueous solution for 3 h, the reaction mixture was extracted with chloroform and then subject to HPLC separation with an achiral Intersil ODS-3 column. The product analyses show that in the presence of γ-CD 2,3-NA undergoes photocyclodimerization to afford cubane-like photocyclodimer 1, the [4 þ 4] intermediate 2, and unknown compound 3 (Figure 1a). On the basis of the consumption of the starting material 2,3-NA, the yield of 1, 2, and 3 was determined by HPLC separation being 85:12: 280 nm, 2,3-NA undergoes photocyclodimerization to produce anti-head-to-head cubanelike photocyclodimer 1 and the [4 þ 4] intermediate 2. The optically pure enantiomers of the intermediate 2a and 2b have been, for the first time, obtained by HPLC resolution. A comparison of the ee values for photocyclodimer 1 and the intermediate 2 obtained in γ-CD aqueous solution provides a snapshot on the stepwise photochemical-chiral delivery. The use of the Langmuir 2010, 26(2), 782–785
supramolecular chiral system, γ-CD studied in this work, renders the possibility to directly obtain the transient species that is rather challenging in asymmetric photoreactions, thus providing an intriguing access to monitoring the photochemically chiral delivery from a chiral entity to an included substrate. Extension of the present study to other substrates should prove to be of value in delineating more precisely the factors that control the reaction mechanism operating in the chiral supramolecular systems.
Experimental Section HPLC Column. The HPLC columns used in this work: Intersil ODS-3 semi-preparative column, GL Sciences, Inc., 5 μm, 250 10 mm; OJ-RH analytic column, Daicel Chemical Industries, Ltd., 5 μm, 150 4.6 mm; and OJ-H analytic column, Daicel Chemical Industries, Ltd., 5 μm, 250 4.6 mm. HPLC Separation Conditions. ODS-3 column: UV detection at 260 nm; eluent, 6:4 acetonitrile/water, flow rate, 2 mL/min. OJ-RH column: UV detection at 220 nm; eluent, 8:2 methanol/ water; flow rate, 0.5 mL/min. OJ-H column: UV detection at 220 nm; eluent, 7:3 n-hexane/isopropanol, flow rate, 0.8 mL/min. Resolution of the [4 þ 4] Intermediate 2. HPLC resolution of the [4 þ 4] intermediate 2 was achieved with an Intersil ODS-3 and then a chiralcel OJ-RH column. The racemic [4 þ 4] intermediate 2 generated from irradiation of 2,3-NA in methanol with light λ > 280 nm must be purified by HPLC with an ODS-3 column. The obtained fractions (in 6:4 acetonitrile/water) were immediately analyzed on a chiralcel OJ-RH analytic column without evaporation of solvents to protect it from any thermal decomposition. ee Determination for the [4 þ 4] Intermediate 2. The ee value of the [4 þ 4] intermediate 2 was determined by HPLC with an Intersil ODS-3 and then a chiralcel OJ-RH column. The [4 þ 4] intermediate 2 generated from photochemical transformation of 2,3-NA@γ-CD must be purified by HPLC with an ODS-3 column. The obtained fractions (in 6:4 acetonitrile/water) were immediately analyzed on a chiralcel OJ-RH analytic column without evaporation of solvents to guarantee the exact ee measurement. Acknowledgment. We are grateful for financial support from the National Natural Science Foundation of China (20732007 and 20672122), the Ministry of Science and Technology of China (2006CB806105, G2007CB808004, 2007CB936001, and 2009CB220008), and the Bureau for Basic Research of the Chinese Academy of Sciences. Supporting Information Available: Details on HPLC separation, stability evaluation of the [4 þ 4] intermediate 2, and HPLC-MS (ESI) of the intermediate 2 separated by a chiralcel OJ-RH column. This material is available free of charge via the Internet at http://pubs.acs.org. DOI: 10.1021/la902176e
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