Chinese Chemical Letters  2018, Vol. 29 Issue (1): 87-90   PDF    
Switched enantioselectivity by solvent components and temperature in photocyclodimerization of 2-anthracenecarboxylate with 6A, 6X-diguanidio-γ-cyclodextrins
Jigao Yia, Wenting Liangb, Xueqin Weia, Jiabin Yaoa, Zhiqiang Yana, Dan Sua, Zhihui Zhonga, Guowei Gaoa, Wanhua Wua, Cheng Yanga    
a Key Laboratory of Green Chemistry & Technology, College of Chemistry and State Key Laboratory of Biotherapy, West China Medical Center, Sichuan University, Chengdu 610064, China;
b Institute of Environmental Sciences, Department of Chemistry, Shanxi University, Taiyuan 030006, China
Abstract: A series of 6A, 6X-diguanidio-γ-cyclodextrins (CDs) were used as chiral hosts for mediating the enantiodifferentiating[4 + 4] photocyclodimerization of 2-anthracenecarboxylic acid (AC). These γ-CD derivatives form stable 1:2 ternary complexes with AC in aqueous ammonia solutions and the head-tohead photodimers 3 and 4 were greatly enhanced in the presence of these diguanidio-γ-CDs. The enantioselectivity of chiral photodimers 2 and 3 is a critical function of the temperature and the ammonia contents, showing inverted product chirality by changing these external factors.
Key words: Cyclodextrins     Photocyclodimerization     Anthracenecarboxylic acid     Supramolecular complexation     Solvent effect    

Stereochemical control of photoreactions is an intriguing, though highly challenging, topic of current photochemistry [1-3]. Efficient chirality delivery in photoreactions is extremely difficult on account of the high reactivity and short lifetime of photosubstrates at the electronic excited state. Supramolecular photochirogenesis has recently emerged as a promising alternative to chiral photochemistry [4, 5]. Chiral host molecules provide relatively strong non-covalent interaction and a confined chiral environment to make energy and chirality transfer efficient [6]. A variety of natural and synthetic chiral hosts, including chirally-modified supercages of zeolites, chiral templates, native and modified cyclodextrins (CDs), and bio-macromolecules, have been exploited as chiral inductors for conducting enantioselective photoreactions [7-23]. However, unlike conventional chiral photoreactions, in which antipodal photoproducts are often feasible by applying antipodal chiral sources, it is usually difficult to obtain both antipodes of supramolecular hosts that are from natural sources [24]. Chemists usually need to resort to other strategies to achieve antipodal photoproducts in supramolecular photochirogenesis.

Recently, the enantiodifferentiating photocyclodimerization of 2-anthracecarboxylic acid (AC) has attracted extensive attention and has been studied as a model chiral photoreaction [25]. Photocyclodimerization of AC leads to photodimers 1-4, in which the syn-head-to-tail (HT) 2and anti-head-to-head (HH) 3 are chiral. A variety of chiral hosts have been exploited for conducting the photocyclodimerization of AC, among which γ-CD derivatives are the most extensively investigated. CDs are naturally occurring, inherently chiral, and UV-transparent host molecules that can accommodate a wide range of organic guests in aqueous solutions [26-28]. γ-CDs have a large cavity that simultaneously includes two AC molecules predominantly through hydrophobic interaction. Grafting cationic groups onto the primary rim of γ-CD is effective in enhancing the complexation and manipulating the stacking model of AC pairs. We have found that the HH photodimers could be significantly improved by using dicationic γ-CDs 5a-d and 6a-d (Scheme 1) as chiral hosts [20, 29]. Interestingly, the reaction enantioselectivity of photocyclodimerization of AC mediated by 6A, 6E-diguanidio-γ-CD 6d is a critical function of the ammonia components and reaction temperature [29]. Changing the ammonia contents and the temperature even led to a switching of product chirality. Such an unusual behaviour found with 6d motivated us to extend the investigation to 6a-c for the purpose of better understanding the role of these environmental factors.

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Scheme 1. Photocyclodimerization of AC and modified γ-CDs.

6A, 6X-diguanidio-γ-CDs 6a-c were synthesized according to the literature procedure previously reported [29]. The complexation behaviour of AC with γ-CD derivatives were studied by UV–vis and circular dichroism spectral titrations. As shown in Table 1, 6a-d show much stronger association constants than native γ-CD in 25% ammonia, especially for the K1 values, which are higher than that of native γ-CD by factors of up to 4. Adding ammonia to water will decrease the polarity of the solvent, and, consequently, the hydrophobic interaction is weakened while the electrostatic and hydrogen-bonding interactions are, in turn, enhanced with the ammonia contents. The significantly higher K1 values could thus be ascribed to the improved dual electrostatic/hydrogen-bonding interactions between guanidinium and carboxylate. The improved complexation validated that the guanidinium groups play an important role in the formation of the 1:2 host-guest ternary complexes, and should therefore significantly influence the orientation of AC pairs in the cavity of γ-CDs.

Table 1
Association constants of AC with native and modified γ-CDs assuming a stepwise formation of 1:1 and 1:2 complexes.a

Photolyses of AC in the presence of a diguanidio-γ-CD were carried out with an LED lamp at 365 nm. Table 2 displays the results of AC photodimerizations in the presence of γ-CD derivatives with ammonia contents at -40 ℃. For photodimerization of AC in the absence of a γ-CD derivative, the HT photodimers 1 and 2 are dominant, showing a HH/HT ratio of 0.26 in 30% ammonia. This could be accounted for by the electrostatic repulsion between anionic carboxylates, which inhibits the formation of HH photodimers. The same is true when the photocyclodimerization was carried out in the presence of native γ-CD. Increasing the ammonia contents does not drastically alter the product distribution of γ-CD-mediated photocyclodimerization, affording HH/HT ratios in the range from 0.26-0.30. On the contrary, the HH photodimers were significantly improved with diguanidio-γ-CDs 6a-6c in aqueous ammonia solutions. The HH/HT ratios were improved to 0.93 for 6a, 0.86 for 6b, and 1.13 for 6c. This could be reasonably rationalized by the electrostatic attraction between guanidium groups of γ-CDs and AC's carboxylate groups. Positive ee's for 2 and negative ee's for 3 were observed with native γ-CD. Interestingly, negative ee's of 2 were observed with 6a, which is contrary to that obtained with native γ-CD. On the other hand, the ee's of 3 were significantly improved with 6a, offering up to -25% ee in 50% ammonia. These results clearly demonstrate that the arrangement of AC pairs in the cavity of γ-CD is significantly affected by the guanidium-carboxylate interaction. Chirality inversions from 11.7% ee to -18.5% ee for 2 and 9.7% ee to -6.9% ee for 3 were observed with 6b when the ammonia contents were increased from 30% to 85%; by simply changing the ammonia contents, the inversion of product chirality can be realized. Chirality switching was also observed with 3 when using 6c as a chiral source (Table 2). This is consistent with that observed with 6d[29], demonstrating the generality of the solvent composition-induced chirality inversion with diguanidio-γ-CDs.

Table 2
Enantiodifferentiating photocyclodimerization of AC in aqueous ammonia in the absence and presence of 6a-c and γ-CDa.

The temperature effects on the photocyclodimerization of AC with 6A, 6X-diguanidino-γ-CDs 6a-c were investigated in 30% ammonia. We have recently demonstrated significant unimolecular chirality switching with temperature variation [30]. When γ-CD was used as the host, reducing the temperature did not cause significant change of product distribution. On the other hand, the enantioselectivity of both 2 and 3 is apparently increased with the decrease of temperature (Table 3). With diguanidio-γ-CDs, the HH photodimers is apparently improved when reducing the temperature, to give HH/HT ratios of up to 1.13 with 6c at -40 ℃. The yields of syn- and anti-HH photodimers 3 and 4 are highly sensitive to the relative distance between two guanidium groups. Under the same temperature, the yields of 3 gradually increase at the expense of 4 with increasing distance between the two guanidium groups in 6ac. The 3/4 ratio exhibits a progressive increase from 0.81 for 6a to 2.23 for 6c in 30% ammonia at -40 ℃. This result is in line with the results observed with 5a-d in aqueous methanol solution [20], further demonstrating an excellent structure–function relationship. Contrary to the temperature effects observed with native γ-CD, the ee values of 2 and 3 decreased/inverted upon reducing the temperature with host 6a. Chirality inversion was also observed with 6b and 6c, further indicating the critical role of the environmental factors in the photocyclodimerization of AC mediated by diguanidino-γ-CDs.

Table 3
Photocyclodimerization of AC mediated by native γ-CD and 6A, 6X-diguanidino-γ-CDs 6a-c in NH3-H2O (3:7 w/w) at different temperatures.a

To quantitatively analyze the temperature effect on the product enantioselectivity, the natural logarithm of the relative enantiomer ratios of 2 and 3 were plotted as a function of the reciprocal temperature according to the Eyring equations. As shown in Fig. 1, all hosts show satisfactory straight lines over the temperature range examined. The differential enthalpy (ΔΔH‡) and entropy changes (ΔΔS‡) were derived from the slope and intercept of the regression lines. Relatively large negative ΔΔS‡ values were seen with diguanidino-γ-CDs (Table 4), suggesting that the entropy term plays an important role in determining the enantioselectivity of photoproducts. These results indicate that the introduction of guanidium groups onto CD significantly switched the complexation entropy of AC and diguanidino-γ-CDs in aqueous ammonia solutions.

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Fig. 1. Plots of the relative rate constants for the formation of enantiomers of (a) 2 and (b) 3 against the reciprocal temperature upon photocyclodimerization of AC catalyzed by γ-CD (square), 6a (empty circle), 6b (solid circle), and 6c (triangle).

Table 4
The differential activation parameters for the formation of enantiomeric 2 and 3 upon photocyclodimerization of AC mediated by γ-CD and 6a–c.

In conclusion, 6A, 6X-diguanidino-γ-CDs 6ac were prepared as chiral hosts for the enantiodifferentiating [4 + 4] photocyclodimerization of AC. The yields of HH photodimers 3 and 4 were greatly improved as a result of the electrostatic interactions between the guanidium groups of 6ac and the carboxylate group of AC. The relative yields of photodimers 3 and 4express a tight correlation with the distance of the two guanidium groups in 6ac. By varying the reaction temperature and the ammonia contents, the ee of the product was affected, and even the sense of product chirality was dramatically switched. The temperature-dependent inversion of the ee of the product was assigned to the relatively large differential entropy upon formation of the diastereomeric complexes. This study verified the switching of product chirality by the variation of solvent composition and temperature when 6A, 6X-diguanidino-γ-CDs were used for the photocyclodimerization. It also demonstrates that the entropic contribution in the present supramolecular photochirogenesis system is controllable through the manipulation of environmental factors, and provides a new approach for the multidimensional entropy control towards photochirogenesis.

Acknowledgments

This work was supported by the grants from the National Natural Science Foundation of China (Nos. 21572142, 21372165 and 21402129) and State Key Laboratory of Polymer Materials Engineering (No. sklpme2014-2-06). We thank Prof. Ye Tao and Dr. Yan Huang of BSRF for their assistance during the use of UV lightsources.

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