Sui Generis Helicene-Based Supramolecular Chirogenic System

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Sui Generis Helicene-Based Supramolecular Chirogenic System: Enantioselective Sensing, Solvent Control, and Application in Chiral Group Transfer Reaction Mohammed Hasan,†,‡ Vaibhav N. Khose,† Tadashi Mori,§ Victor Borovkov,*,‡ and Anil V. Karnik*,† †

Department of Chemistry, University of Mumbai, Vidayanagari, Santacruz, Mumbai 400098, India Department of Chemistry and Biotechnology, Tallinn University of Technology, Akadeemia tee 15, Tallinn 12618, Estonia § Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita 565-0871, Japan ‡

S Supporting Information *

ABSTRACT: A novel dioxa[6]helicene-based supramolecular chirogenic system (1) as a specific chiral recognition host for enantiopure trans-1,2-cyclohexanediamine (2) is reported. Host 1 with an inherent free phenolic group and a (1S)-camphanate chiral handle on the opposite terminal rings of the helicene chromophore acted as an efficient turn on fluorescent sensor for S,S-2 with an excellent enantioselective factor, α = KSS/KRR = 6.3 in benzene. This specific host−guest interaction phenomenon is found to be solvent-dependent, which leads to an enantioselective chiral (camphanate) group transfer to the diamine guest molecule. In the case of R,R-2, the de value is up to 68% even at room temperature. Intriguingly, the induced helicity in dioxa[6]helicene diol 6, upon supramolecular hydrogen-bonding interactions, is of opposite sense with positive helicity for S,S-2 and negative helicity for R,R-2, as shown by circular dichroism spectroscopy and in combination with theoretical calculations. This chiral supramolecular system is found to be an excellent host−guest pair for enantiomeric recognition of 2, based on their electronic and steric factors.

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INTRODUCTION

The significance of chirality is well-recognized in most natural systems, which are generally based on supramolecular host− guest interactions. Additionally, different industries such as food, agrochemical, and pharmaceuticals favor the use of enantiopure molecules. Besides the preparation of chiral molecules, analysis of their chiral purity with high accuracy and precision is crucial for various applications. Chiral sensors with fluorescence as a detection tool are of particular interest owing to their quick response time and high sensitivity with very low detection limits; thus, they are gaining considerable attention so far. Generally, sensors suitable for a wide range of compounds are most abundant in terms of applicability.1 However, the presence of more than one chiral guest may result in binding competition, generating inaccuracy or even mistakes. Therefore, a sensor with a stereospecific recognition ability2 toward a certain guest possessing less substrate scope and thus tolerating the presence of other substrates is in high demand. In the quest for such specific host−guest systems, we developed a novel nonresolvable, stereodynamically labile heterohelicene3 chromophore-based camphanate ester 13b as a suitable chirogenic host and tested it for the chiral sensing application (Figure 1). Despite their unique steric and electronic properties, helicenes have not been widely exploited as supramolecular systems for chiral recognition yet.4 To fill this gap, varied bidentate functionalities including 1,1′-bi-2naphthol, hydroxyl acids, amines, diamines, amino alcohols (see © 2017 American Chemical Society

Figure 1. Structure of the host and guest molecules studied.

Supporting Information) have been screened as potential guests. It turned out that specifically only trans-1,2-cyclohexanediamine, 2, gave an enantioselective recognizable turn on sensing response. Indeed, 2 has been often used as an effective guest for many supramolecular systems owing to its profound hydrogen bonding and coordination ability with metal ions.5 Additionally, Pu et al. reported its nucleophilic addition to an electron-deficient trifluromethyl ketone group in chiral biaryls Received: December 19, 2016 Accepted: February 3, 2017 Published: February 17, 2017 592

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Figure 2. Fluorescence spectra of 1 (6.9 × 10−6 M) in the presence of 1:1 (S,S)-2 and (R,R)-2 (a) in benzene (slit width 7.5, 7.5) and (b) in toluene (slit width 5.0, 5.0).

chromophore, circular dichroism (CD) measurements of host 1 were taken in different solvents (see Supporting Information). A rather weak CD signal with a negative Cotton effect (CE) in the range of 350−360 nm of helicene absorption was detected in nonpolar solvents such as benzene, toluene, cyclohexane, and chloroform. However, the CD response in more polar, aprotic solvents, such as acetonitrile and THF, gave positive CE in the same region (see Supporting Information). This solventdependent opposite trend in the CD spectrum can be attributed only to the conformational changes occurring in different solvents.6 Thus, it is concluded that this solventdependent switching to a specific stereodynamic conformational feature is responsible for the inversion of chiral recognition observed in fluorescence as helicity also plays a key role in controlling the enantiomer recognition of 2. In particular, it is most likely that the phenolic group of host 1 acts as a proton donor to one of the amino groups of 2, whereas it may further accept hydrogen bonding from another amino group via a lone electron pair of the phenolic oxygen. In addition, this amino group can also interact with the camphanate ester group of 1 via its lone pair under the overall steric crowding arrangement of the chiral camphanate handle to result in a rigid structure responsible for its turn on response (see Figure 3).7,8 With this hypothesis in mind, a bulkier 1,2-diamine, such as 1,2-diphenyl ethylenediamine, 3 was investigated. As expected, the host failed to exhibit any stereospecific recognition behavior in the case of 3, indicating that two bulky phenyl groups were

for use as a turn on sensor for the general class of diamines and amino alcohols.5a−c

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RESULTS AND DISCUSSION Host 1 exhibits absorption and emission maxima at 347 and 395 nm, respectively, in benzene and toluene (see Supporting Information). Upon interaction with 2, there is a formation of the corresponding 1:1 complex, as indicated by a Job’s plot analysis (see Supporting Information). Host 1 behaves as an efficient enantioselective turn on sensor with a stereospecific difference ratio, ef = ΔFS/ΔFR, of 6.3 in benzene and 1.93 in toluene (at 1:1 host−guest ratio), where ΔFS and ΔFR are changes in the fluorescence intensity of 1 in the presence of S,S2 and R,R-2 (see Figure 2). The association constants and Gibbs free energies (ΔG0) at 298 K for the corresponding diastereomeric supramolecular complexes are summarized in Tables S3 and S6. For both enantiomers, the ΔG0 values are negative, indicating energetically favorable host−guest interactions. However, the energy in the case of the S,S-enantiomer is 1.1 kcal mol−1 being lower than that of R,R-2, reflecting its higher thermodynamic stability in benzene. Benzene and toluene, both being nonpolar and non-hydrogen-bonded aromatic solvents, preferentially recognize only the S,S-2 enantiomer. However, the distinctive behavior of fluorescence responses of 1 to 2 in these solvents is attributed to different solvent-effect parameters including the extent of favorable hydrogen bonding and solute−solvent π−π stacking interactions.6 Comparatively, more polar halogenated solvents such as chloroform gave a rather weak recognition for R,R-2 with an ef value of only 1.29, whereas in highly polar protic solvents such as methanol, no detectable chiral recognition was observed. By contrast, polar aprotic solvents, such as acetonitrile, showed a moderate sensing ability with an ef of 1.55 for R,R-2 with emission shifted at a longer wavelength of 405 nm. Furthermore, polar-coordinating solvent tetrahydrofuran (THF) showed a fluorescence quenching phenomenon, favoring R,R-2. This remarkable solvent dependence is apparently due to the existence of supramolecular hydrogenbonding interactions between host 1 and guest 2 followed by a possible unidirectional switch in the helical conformation of the helicene chromophore governed by the polarity and type of solvent used. Indeed, there are several reports on the solvent control of supramolecular chirality recognition.6 In turn, this can also be responsible for the stereoselectivity inversion of 2 enantiomers. To confirm the conformational switch in helicene

Figure 3. Proposed supramolecular complex between host 1 and R,R-2 enantiomer in benzene. (a) Schematic view and (b) 3D ball and stick model (only selected hydrogen atoms are displayed for better clarity). 593

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Figure 4. Time-dependent changes observed in 1H NMR spectra of host 1 with 1:1 (R,R)-2 in CDCl3.

Scheme 1. Chiral Camphanate Group Transfer Reaction between Host 1 and Guest 2 in Chloroform

unable to fit into the cavity provided by the chiral camphanate handle. Thus, it is evident that the intramolecular spatial arrangement of 1, particularly the distance between phenolic hydroxyl and camphanate ester and the size of the formed cavity, was exclusively suitable to fit 2 to ensure efficient hydrogen-bonding interactions. The inference was further evaluated with model host 4, which has a similar free phenolic group and attached chiral camphanate handle. Hosts 1 and 4 differ in shape and size of the chiral pockets provided by the camphanate ester group owing to varying distance and spatial arrangement toward the phenolic hydroxyl group required for hydrogen bonding. As expected, the model compound 4 failed to exhibit any enantiodiscrimination fluorescence response for 2. This result further supported the existence of co-operative binding between the phenolic hydroxyl and the camphanate chiral handle of 1 from one side and two amino groups of 2 from another side, which is an essential prerequisite for the observed chiral recognition. Similar studies with the helicene camphorsulfonate, 5, in which the camphor moiety is attached via a longer spacer, were not successful owing to an increase in the distance between two interaction points, thus indicating cooperative origin of the chiral recognition mechanism in our supramolecular system. To further understand the mechanistic details of this host− guest assembly, corresponding nuclear magnetic resonance (NMR) studies were performed (see Experimental Section). The C1 proton of the host initially appearing at 8.8 ppm exhibited a distinct downfield chemical shift (Δδ) of 0.1 ppm at the 1:1 host−guest ratio (see Supporting Information) owing

to the hydrogen-bonding interactions in the host−guest complex. Furthermore, surprisingly, the initial clean solution obtained upon mixing the two components started to precipitate after 1−2 h at room temperature. The corresponding 1H NMR spectra recorded at different time intervals of 0, 1, 2, 3, and 4 h after mixing revealed that a chiral camphanate group transfer reaction takes place (see Figure 4).9 In particular, host 1 being dissymmetric in nature showed all nine aromatic proton signals spreading over the range of 7.1− 8.8 ppm. Slowly with time, the intensity of C1 proton starts to decrease with the simultaneous appearance of a new peak at 8.5 ppm. After 3 h, most of the original signals of the host completely disappeared, and a new set of symmetrical signals appeared. At the end of the reaction, the C1 proton was considerably shifted upfield to 8.3 ppm (Δδ = 0.5 ppm). Furthermore, the two different protons at C3 and C10, appearing as double doublets (dd) with ortho and meta couplings, at 7.5 and 7.8 ppm merged together to give rise to one dd signal at 7.25 ppm with the integration of two protons. Similarly, other signals were also merged together to generate a simplified, symmetric 1H NMR pattern (see Figure 4). The newly generated NMR spectrum has only five signals in the aromatic region (between 7.2 and 8.3 ppm), each for two protons. This is a result of the formation of C2-symmetric dioxa[6]helicene diol 6 via the chiral camphanate group transfer reaction to guest R,R-2 (see Scheme 1). The same reaction with S,S-2 enantiomer takes more than 4 h, longer time than with R,R-2 enantiomer owing to the highly efficient chiral recognition ability of this supramolecular system. On the 594

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Figure 5. CD spectra (a) calculated for the optimized P-conformation of 6. (b) Induced CD studies between host 6 and enantiomers of 2 in chloroform (c = 1 × 10−4 M). (c) DFT-based optimized structure of the P-conformation of 6. (d) Proposed conformational dynamic equilibrium of 6 in the presence of R,R-2 in chloroform.

2 has been successfully exploited in developing fluorescence and NMR sensors using C2-symmetric biaryl hosts.5a−c This system forms the corresponding 1:2 adduct with the excess of 2, and the mechanism involves several stepwise addition and elimination steps, which are very distinguishable over the course of the reaction. In comparison, in our case, the reaction is completed within 4 h, which is comparatively fast with the regenerated 6 parent helicene diol. Similar NMR studies with with meso-3 show no noticeable stereoselectivity with the same hosts, further indicating the high specificity of host 1 exclusively toward guest 2. Additionally, it appears that especially chloroform as a solvent assists this group transfer reaction by generating achiral helicene diol 6, yet it does so with quenched fluorescence without any recognition abilities in fluorescence. However, host 6 is found to be ∼20 times more fluorescent than host 1 (see Supporting Information). Thus, it is most likely that the fluorescence turn on response of 1 for S,S-2 in benzene and toluene is due to the formation of a thermodynamically stable rigid supramolecular complex. In nonpolar solvents, the noncovalent interaction would be preferred involving a hydroxyl group and carbonyl group of 1, whereas in polar solvents, the primary interaction could be an approach of nucleophilic nitrogen toward electrophilic carbonyl carbon, involving partial separation of charges stabilized by polar solvents. Both of these situations would necessitate different helical conformations of 1

basis of these NMR studies, the rate of the reaction was calculated to be 5.83 × 10−6 and 5.97 × 10−7 mol dm−3 s−1 for R,R-2 and S,S-2, respectively. The difference in the rate constants encouraged us to apply this enantioselective group transfer reaction for kinetic resolution of racemic 2. For this purpose, host 1 was separately mixed with R,R-2 (in a 1:1 ratio) and with rac-2 (in 1:2 ratio) (see Supporting Information) followed by NMR recording. The product of the reaction of R,R-2 with 1 gave a distinct signal at 6.38 ppm, which corresponds to the amidic proton of 7. However, a similar reaction for kinetic resolution of rac-2 with 1 resulted in peaks at 6.68 ppm (16%, S,S-enantiomer) and 6.39 ppm (84%, R,Renantiomer) with a de value of 68% for R,R-enantiomer. It can be noted that for kinetic resolution R,R-2 is preferred over S,S2, whereas S,S-2 enantiomer is preferably recognized with enhanced fluorescence in nonpolar solvents. Thus, kinetic resolution is a kinetically controlled phenomenon, whereas diastereomeric supramolecular complex formation responsible for fluorescence recognition is thermodynamically preferred. The expected mechanism of this group transfer reaction involves a nucleophilic attack on the carbonyl group of phenolic ester of host 1, as a consequence of the enhanced nucleophilicity of amino groups in 2 owing to the presence of the adjacent β-amino group. This further supports our proposed model (see Figure 3) where the second amino group interacts with the phenolic ester group and not with the lactone group of the camphanate handle. This nucleophilic property of 595

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for a steric fit, resulting in the recognition of opposite enantiomers of 2 depending on the solvent polarity. Hence, host 1 is a sui generis example for specific chiral recognition, as a result of the suitable distance between the free phenolic and phenolic ester groups with limited space under camphanate cavity to fit 2 exclusively. The proposed model for the supramolecular interaction between the host and the guest in nonpolar aromatic solvents can assist the chiral group transfer reaction observed in chloroform. To support the validity of this model, we anticipated that in the parent helicene diol 6, the two phenolic groups have suitable distance to interact with 2 via phenol−amine hydrogen bonding.8 This supramolecular interaction will induce specific helicity into the achiral stereodynamically labile helicene chromophore 6 that in turn can be easily monitored by CD spectroscopy.7 Host 6 exists as two fast interconvertible helical conformations, which cannot be resolved, yielding a CD silent spectrum. To our delight, the S,S-2 enantiomer induced specific helical conformation in 6 to show positive CE at 345 nm, whereas the R,R-2 enantiomer resulted in a mirror image negative CE. The CD intensity, Δε, is up to ±0.42 cm−1 M−1 in the region of helicene chromophore absorption (Figure 5b). Upon interactions with the enantiomers of 2, the conformational dynamic equilibrium of 6 is shifted to one of the two possible helicities, subsequently inducing the CD activity (Figure 5d).10 This confirms that the opposite enantiomers induce opposite helicities in the parent helicene chromophore. A Job’s plot analysis unambiguously confirmed the 1:1 stoichiometry between 6 and 2 on complexation, as studied by UV−vis spectroscopy (see Supporting Information). Intriguingly, the P-conformer of the parent helicene diol 6 (Figure 5c) exhibits negative CE at the low-energy absorption band corresponding to 1La electronic transition, as determined by the theoretical calculation at the RI-CC2/def2-TZVP// DFT-D3(BJ)-TPSS/def2-TZVP level (Figure 5a).11

99.5% ee, and were used without any further purification. Spectral grade solvents were used for sensing studies and purchased from S. D. Fine Chemicals. The melting points were recorded and were uncorrected. The graphs were plotted using Orgin-06 software. All glassware was cleaned and dried in an oven before use. Silica gel 60 (230−400 mesh) was used for column chromatography. 1H, 13C, and 2D NMR spectra were recorded on a Bruker Advance II, 300/75 MHz. Chemical shift values are expressed in δ (ppm) relative to tetramethylsilane. Fluorescence spectra were recorded on a Perkin-Elmer L55 spectrofluorometer with a scan range of 1000 nm/s. UV−vis spectra were recorded on a Shimadzu spectrophotometer, model no. UV-2450. Electronic CD (ECD) studies were carried out on a Jasco J-815 CD spectrophotometer. Synthesis of Helicenes 1, 5, and 6. Hosts 1, 5, and 6 were synthesized and characterized using reported procedures.3b Synthesis of 3-Hydroxy-naphthalene-2-camphanate (4). (1S)-Camphinic chloride (1.6 g, 7.5 mmol, 1.2 equiv) dissolved in 10 mL of acetone was added to a solution of 2,3dihydroxynaphthalene (1.0 g, 6.25 mmol) and anhydrous K2CO3 (2.6 g, 18.8 mmol, 3 equiv) in 25 mL of acetone, dropwise at 0 °C. The solid started to precipitate as a monocamphanate derivative. The reaction mixture was further stirred for half an hour, and the progress was monitored using thin-layer chromatography. On completion, the reaction was quenched by pouring on to crushed ice having a 5% HCl solution. The solid obtained was filtered off, washed with water, and air-dried. The product was purified using silica gel column chromatography with a 95% yield. Mp: 180 °C, specific rotation: [α]58925 = +11.49° (c = 0.174 g/100 mL) in THF solvent. IR (KBr, cm−1): 3458, 2969, 1778, 1754, 1744, 1633, 1273, 1015, 487. 1H NMR (300 MHz, CDCl3): 1.16 (s, 3H), 1.18 (s, 3H), 1.19 (s, 3H), 1.81 (m, 1H), 2.05 (m, 1H), 2.25 (m, 1H), 2.67 (m, 1H), 5.97 (s, 1H), 7.25 (s, 1H), 7.34 (m, 2H), 7.61 (m, 2H), 7.71 (d, J = 7.5 Hz, 1H). 13C NMR (75 MHz, CDCl3): 9.7, 16.7, 29.0, 31.0, 54.9, 55.0, 77.2, 91.2, 112.2, 120.1, 124.4, 126.3, 126.4, 127.4, 128.4, 132.7, 138.7, 145.7, 165.9, 178.7. ESI-MS: m/z = 339.79, elemental analysis for C20H20O5: analytical calculated: C, 70.57; H, 5.92. Found: C, 70.60; H, 5.89. Methods. Fluorescence Sensing. Before recording the fluorescence, the hosts were crystallized or passed through the short alumina column each time. The freshly purified hosts were used for fluorescence studies. The stock solution of the host (6.9 × 10−5 M) was prepared by dissolving 0.9 mg of 1 in 25 mL of solvent. From this stock solution, 1 mL was pipetted out and transferred to various 10 mL standard flasks containing 0.0 (blank), 0.2, 0.4, 0.6, 0.8, and 1.0 equiv of (1R,2R)-2 and (1S,2S)-2 in two separate sets of experiments. The flasks were kept at room temperature for 4 h before recording their fluorescence responses. NMR Sensing. Experiment A. In an NMR sample, 10.4 mg of 1 in 0.5 mL of CDCl3 was mixed with 5.6 mg of (1R,2R)-2 in a vial to obtain a 1:1 host−guest ratio. The clean solution obtained upon mixing 1 and 2 started to precipitate on standing in the NMR tube at 25 °C. 1H NMR of the mixture was recorded at different time intervals after mixing, which confirmed the chiral group transfer reaction. The similar experiment with racemic 2 showed the enantioselective nature of this group transfer reaction. Experiment B. In a different set of experiments, 10.4 mg of 1 was dissolved in 0.5 mL of CDCl3 containing 5.6 mg of (1S,2S)-2 to obtain a 1:1 host−guest ratio. The 1H NMR

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CONCLUSIONS In conclusion, we have successfully developed dioxa[6]helicene-based host 1 as a turn on fluorescence sensor for specific chiral recognition of trans-2. This solvent-dependent fluorescence detection is a thermodynamically favored phenomenon, as evidenced by the negative Gibbs free energy. The 1H NMR studies showed that the chiral camphanate group transfer reaction occurring in chloroform is 10 times faster for R,R-2, indicating a kinetically controlled mechanism. Besides, the potential host 1 has been applied for kinetic resolution of rac-2 with 68% de. Furthermore, the parent helicene diol 6 showed induced CD spectra with the corresponding enantiomers of 2, which in combination with the theoretical studies further confirm the proposed host−guest interaction mechanism. This supramolecular system is a unique example of specific chirality sensing. Further extension of these studies and application for other supramolecular systems are currently under progress and will be reported in due course. We believe that the phenomenon of controlling helical conformation via chiral auxiliary and solvent polarity will undoubtedly give a new dimension to the area of development of chirality sensors using stereodynamic probes.

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EXPERIMENTAL SECTION Materials. All chiral guests (both enantiomers) were purchased from Sigma-Aldrich or Alfa Aesar, were of 99.0− 596

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37, 449−459. (f) Pandey, A. D.; Mohammed, H.; Pissurlenkar, R. R. S.; Karnik, A. V. Size-Induced Chiral Discrimination Switching by (S)(−)-2(α-Hydroxyethyl)benzimidazole-Derived Azacrowns. ChemPlusChem 2015, 80, 475−479. (2) (a) Wei, G.; Zhang, S.; Dai, C.; Quan, Y.; Cheng, Y.; Zhu, C. A New Chiral Binaphthalene-Based Fluorescence Polymer Sensor for the Highly Enantioselective Recognition of Phenylalaninol. Chem. Eur.J. 2013, 19, 16066−16071. (b) Lin, J.; Hu, Q.-S.; Xu, M.-H.; Pu, L. A Practical Enantioselective Fluorescent Sensor for Mandelic Acid. J. Am. Chem. Soc. 2002, 124, 2088−2089. (3) (a) Eskildsen, J.; Krebs, F. C.; Faldt, A.; Sommer-Larsen, P.; Bechgaard, K. Preparation and structural properties of 7,8-dioxa[6]helicenes and 7a,14c-dihydro-7,8-dioxa[6]helicenes. J. Org. Chem. 2001, 66, 200−205. (b) Hasan, M.; Pandey, A. D.; Khose, V. N.; Mirgane, N. A.; Karnik, A. V. Sterically Congested Chiral 7,8Dioxa[6]helicene and Its Dihydro Analogues: Synthesis, Regioselective Functionalization, and Unexpected Domino Prins Reaction. Eur. J. Org. Chem. 2015, 3702−3712. (4) (a) Reetz, M. T.; Sostmann, S. 2,15-Dihydroxy-hexahelicene (HELIXOL): Synthesis and use as an enantioselective fluorescent sensor. Tetrahedron 2001, 57, 2515−2520. (b) Wang, D. Z.; Katz, T. J. A [5]HELOL Analogue that Senses Remote Chirality in Alcohols, Phenols, Amines, and Carboxylic Acids. J. Org. Chem. 2005, 70, 8497− 8502. (c) Tran, H. T. N.; Falk, H. Concerning the Chiral Discrimination and Helix Inversion Barrier in Hypericinates and Hypericin Derivatives. Monatsh. Chem. 2002, 133, 1231−1237. (d) Ernst, K.-H. Stereochemical Recognition of Helicenes on Metal Surfaces. Acc. Chem. Res. 2016, 49, 1182−1190. (e) Huang, Q.; Jiang, L.; Liang, W.; Gui, J.; Xu, D.; Wu, W.; Nakai, Y.; Nishijima, M.; Fukuhara, G.; Mori, T.; Inoue, Y.; Yang, C. Inherently Chiral Azonia[6]helicene-Modified β-Cyclodextrin: Synthesis, Characterization, and Chirality Sensing of Underivatized Amino Acids in Water. J. Org. Chem. 2016, 81, 3430−3434. (f) Anger, E.; Iida, H.; Yamaguchi, T.; Hayashi, K.; Kumano, D.; Crassous, J.; Vanthuyne, N.; Roussel, C.; Yashima, E. Synthesis and chiral recognition ability of helical polyacetylenes bearing helicene pendants. Polym. Chem. 2014, 5, 4909−4914. (g) Shen, Y.; Chen, C.-F. Helicenes: Synthesis and applications. Chem. Rev. 2012, 112, 1463−1535. (5) Selected examples of cyclohexanediamine sensing, see (a) Yu, S.; Plunkett, W.; Kim, M.; Pu, L. Simultaneous Determination of Both the Enantiomeric Composition and Concentration of a Chiral Substrate with One Fluorescent Sensor. J. Am. Chem. Soc. 2012, 134, 20282− 20285. (b) Wang, C.; Wu, E.; Wu, X.; Xu, X.; Zhang, G.; Pu, L. Enantioselective Fluorescent Recognition in the Fluorous Phase: Enhanced Reactivity and Expanded Chiral Recognition. J. Am. Chem. Soc. 2015, 137, 3747−3750. (c) Xu, Y.; Yu, S.; Chen, Q.; Chen, X.; Xiao, M.; Chen, L.; Yu, X.; Xu, Y.; Pu, L. Greatly Enhanced Fluorescence by Increasing the Structural Rigidity of an Imine: Enantioselective Recognition of 1,2-Cyclohexanediamine by a Chiral Aldehyde. Chem. Eur.J. 2016, 22, 5963−5968. (d) Wen, K.; Yu, S.; Huang, Z.; Chen, L.; Xiao, M.; Yu, X.; Pu, L. Rational Design of a Fluorescent Sensor to Simultaneously Determine Both the Enantiomeric Composition and the Concentration of Chiral Functional Amines. J. Am. Chem. Soc. 2015, 137, 4517−4524. (e) Tumambac, G. E.; Wolf, C. Enantioselective analysis of an asymmetric reaction using a chiral fluorosensor. Org. Lett. 2005, 7, 4045−4048. (f) Iwaniuk, D. P.; Yearick-Spangler, K.; Wolf, C. Stereoselective UV Sensing of 1,2Diaminocyclohexane Isomers Based on Ligand Displacement with a Diacridylnaphthalene N,N′-Dioxide Scandium Complex. J. Org. Chem. 2012, 77, 5203−5208. (g) He, X.; Zhang, Q.; Liu, X.; Lin, L.; Feng, X. Determination of concentration and enantiomeric excess of amines and amino alcohols with a chiral nickel(II) complex. Chem. Commun. 2011, 47, 11641−11643. (h) Zheng, Y.-S.; Hu, Y.-J. Chiral Recognition Based on Enantioselectively Aggregation-Induced Emission. J. Org. Chem. 2009, 74, 5660−5663. (i) Xiong, J.-B.; Xie, W.-Z.; Sun, J.-P.; Wang, J.-H.; Zhu, Z.-H.; Feng, H.-T.; Guo, D.; Zhang, H.; Zheng, Y.-S. Enantioselective recognition for many different kinds of chiral guests by one chiral receptor based on tetraphenylethylene cyclohexylbisurea. J. Org. Chem. 2016, 81, 3720−3726. (j) Xiong, J.-B.;

spectra of the mixture were recorded at different time intervals to study the kinetics of the group transfer reaction. A similar kinetic experiment was also performed with (1R,2R)-2 enantiomer. CD Studies. A 1 × 10−4 M solution of 1:1 host 6 and guest (1S,2S)-2 and (1R,2R)-2 was separately prepared using chloroform as a solvent. The induced CD spectra were measured for 400−280 nm with a 10 nm/min scan speed and 5 accumulations at 25 °C.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.6b00522. Experimental details, characterization data, fluorescence, NMR, UV−vis, and CD spectroscopic data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.V.K.). *E-mail: [email protected] (V.B.). ORCID

Tadashi Mori: 0000-0003-3918-0873 Anil V. Karnik: 0000-0002-3426-8705 Author Contributions

The manuscript was written through equal contributions of A.V.K., V.B., and M.H. Experimental work was carried out by M.H. and V.N.K. at the Department of Chemistry, University of Mumbai, India. Theoretical calculations were performed by T.M. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grant-in-Aid SR/S1/OC-02/2007 from DST-India. M.H. thanks DST-India and UGC-MANF for the JRF and SRF awards. M.H. and V.B. acknowledge funding from the European Union’s Seventh Framework Programme for Research, Technological Development, and Demonstration under Grant Agreement no. 621364 (TUTIC-Green). Financial support (to T.M.) by Grants-in-Aid for Scientific Research, Challenging Exploratory Research, and on Innovative Areas “Photosynergetics” (grant numbers JP15H03779, JP15K13642, and JP15H01087) from JSPS, the Matching Planner Program from JST (grant number MP27215667549) is greatly acknowledged. We thank the National Centre for Nanoscience and Nanotechnology, University of Mumbai, for providing CD machine facility.

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DOI: 10.1021/acsomega.6b00522 ACS Omega 2017, 2, 592−598

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DOI: 10.1021/acsomega.6b00522 ACS Omega 2017, 2, 592−598