Vibrational circular dichroism study of optically pure cryptophane-A

Synthesis and VCD Spectroscopic Characterization of a Series of Azacryptands from a Chiral Valine-Based Derivative of Tris(2-aminoethyl)amine (TREN)

Utilizing experimental and computational vibrational circular dichroism (VCD) spectroscopy, we explored the conformational preferences of a series of chiral C3 -symmetric octaazacryptands with tris(2-aminoethyl)-amine head groups derived from valine. While the spectra of the smallest azacryptand with p-phenyl linkers and its elongated derivative with p-biphenyls linker were found to match well with the computed spectra, the computed conformational preferences of the m-biphenyl-based azacryptand did not seem to reflect the conformations dominating in chloroform solution. A detailed analysis revealed that structural changes resulting in a collapsed cage structure gave a notably better match with the experiment. It could subsequently be concluded from the VCD analysis, that the octaazacryptands prefer a collapsed structure, which is not predicted by density functional theory (DFT) calculations as the global minimum structures. These findings are expected to have consequences also for future studies on inclusion complexes of such azacryptands.

VCD spectroscopy reveals conformational changes of chiral crown ethers upon complexation of potassium and ammonium cations

Two chiral derivatives of 18-crown-6, namely the host molecules 2,3-diphenyl- and 2-phenyl-18c6, serve as model systems to investigate whether VCD spectroscopy can be used to monitor conformational changes occurring upon complexation of guests. Host-guest complexes of both crown ethers were prepared by addition of KNO₃. The more bulky 2,3-diphenyl-18c6 is found to undergo major conformational changes upon encapsulation of K⁺, which are revealed as characteristic changes of the VCD spectral signatures. In contrast, while 2-phenyl-18c6 also incorporates K⁺ into the macrocycle, strong conformational changes are not occurring and thus spectral changes are negligible. With an octyl ammonium cation as guest molecule, 2,3-diphenyl-18c6 shows the same conformational and spectral changes that were observed for K⁺-complexes. In addition, the asymmetric NH₃-deformation modes are found to gain VCD intensity through an induced VCD process. An analysis of the vibrational spectra enables a differentiation of VCD active and inactive guest modes: There appears to be a correlation between the symmetry of the vibrational mode and the induced VCD intensity. While this finding makes the host-guest complexes interesting systems for future theoretical studies on the origin of induced VCD signatures, the observations described in this study demonstrate that VCD spectroscopy is indeed a suitable technique for the characterization of supramolecular host-guest complexes.

Induced VCD and conformational chirality in host-guest complexes of a chiral ammonium salt with crown ethers

The hydrogen bonded complexes of the chiral ammonium salt α-methylbenzyl ammonium chloride (MBA-H⁺Cl^-) and the achiral crown ethers 18c6 and 15c5 serve as model systems to investigate the effect of host-guest complex formation on the conformational preferences of the macrocycles. We demonstrate that the intermolecular interactions result in new VCD signatures, that can be assigned to vibrational modes of the crown ethers. Based on a detailed conformational analysis, we investigate the origin of these signatures and discuss induced VCD (iVCD) and conformational chirality as possible sources of VCD intensity. The macrocycle in the MBA-H⁺/18c6 complex prefers either an achiral D_3d-symmetric conformation, which gives rise to iVCD, or chiral conformations, that feature individual contributions to the VCD spectrum. For the MBA-H⁺/15c5 complex, the contributions of the macrocycle to the VCD signatures are less pronounced and found to arise solely from conformational chirality. Therefore, analysis of the VCD signatures confirms that the small chiral guest molecule is able to affect the conformational preferences of a macrocyclic host. The study thus demonstrates the suitability of VCD spectroscopy for the characterization of analogous supramolecular host-guest complexes.

Enantiopure cryptophane derivatives: Synthesis and chiroptical properties

This review addresses the synthesis of enantiopure cryptophane and the study of their chiroptical properties. Cryptophane derivatives represent an important class of macrocyclic compounds that can bind a large range of species in solution under different conditions. The overwhelming majority of these host molecules is chiral, and their chiroptical properties have been thoroughly investigated. The first part of this review is dedicated to the optical resolution and the synthesis of enantiopure cryptophane derivatives. In a second part, the study of the chiroptical properties of these molecular hosts by different techniques such as electronic and vibrational circular dichroism and Raman optical activity is detailed. These techniques allow the determination of the absolute configuration of cryptophane derivatives and provide useful information about their conformation in different conditions.

Absolute configuration and host-guest binding of chiral porphyrin-cages by a combined chiroptical and theoretical approach

Porphyrin cage-compounds are used as biomimetic models and substrate-selective catalysts in supramolecular chemistry. In this work we present the resolution of planar-chiral porphyrin cages and the determination of their absolute configuration by vibrational circular dichroism in combination with density functional theory calculations. The chiral porphyrin-cages form complexes with achiral and chiral viologen-guests and upon binding one of the axial enantiomorphs of the guest is bound selectively, as is indicated by induced-electronic-dichroism-spectra in combination with calculations. This host-guest binding also leads to unusual enhanced vibrational circular dichroism, which is the result of a combination of phenomena, such as rigidification of the host and guest structures, charge transfer, and coupling of specific vibration modes of the host and guest. The results offer insights in how the porphyrin cage-compounds may be used to construct a future molecular Turing machine that can write chiral information onto polymer chains.

Vibrational circular dichroism (VCD) spectroscopy can be useful for determining the absolute configuration of chiral molecules, as long as the signal intensities are high enough. Here, the authors establish the absolute configurations of two large chiral porphyrin cages and, notably, discover that host-guest binding enhances their VCD intensities.

Chiroptical study of cryptophanes subjected to self-encapsulation

In 1,1,2,2-tetrachloroethane-d₂ , the ¹²⁹ Xe NMR spectrum of the Xe@cryptophane-223 complex bearing seven acetate groups (Xe@1 complex) shows an unusually broad signal compared with that of its congeners (Chapellet, LL. et al. J. Org. Chem. 2015;80:6143-6151). To interpret this unexpected behaviour, a ¹ H NMR analysis and a thorough study of the chiroptical properties of 1 as a function of the nature of the solvent have been performed. The ¹ H NMR spectra of 1 reveal that a self-encapsulation phenomenon takes place in DMSO-d₆ and 1,1,2,2-tetrachloroethane-d₂ solvents. Thanks to the separation of the two enantiomers of 1 by HPLC on chiral stationary phase, the two enantiomers of 1 have been studied in detail by polarimetry, electronic (ECD), and vibrational (VCD) circular dichroism spectroscopies. Except for ECD spectroscopy, these chiroptical techniques reveal spectroscopic changes as a function of the nature of the solvent. For instance, in DMSO and 1,1,2,2-tetrachloroethane, in which the self-encapsulation phenomenon takes place, the sign of the specific optical rotation of [CD(-)₂₅₄ ]-1 and [CD(+)₂₅₄ ]-1 is changed. These results have then been compared with those obtained with cryptophane-223 bearing only one acetate group on the propylenedioxy linker (compound 2) and with cryptophane-223 bearing six acetate groups (compound 3). A self-encapsulation phenomenon is also observed with compound 2. Finally, compounds 2 and 3 show different chiroptical properties compared with those obtained with the two enantiomers of compound 1.

Chirality Induction from a Chiral Guest to the Hydrogen-Bonding Network of Its Hexameric Resorcinarene Host Capsule

The hexameric capsule of resorcin[4]arene 1 is capable of encapsulating tertiary amines, which has recently been used in the application of [(1)₆ (H₂ O)₈ ] as (co-)catalyst in various asymmetric reactions. However, not much is known about the highly asymmetric but conformationally very dynamic structure of the capsule after uptake of chiral molecules. Therefore, in this contribution, we utilize electronic circular dichroism and vibrational circular dichroism spectroscopy to investigate how several chiral guest molecules affect the structural preferences of the capsule [(1)₆ (H₂ O)₈ ]. In particular, we show that one small chiral amine encapsulated in [(1)₆ (H₂ O)₈ ] is sufficient to control and dictate the stereochemical preferences of the entire capsule. Furthermore, neither strong π-π interactions nor a significant steric bulk are required for this induction. The observation of such a chiral imprint of the guest's stereochemistry onto its host molecule is expected to have implications also for other supramolecular capsules.

NMR investigation of chloromethane complexes of cryptophane-A and its analogue with butoxy groups

Host-guest complexes between cryptophane-A as host and dichloromethane and chloroform as guests are investigated using (1)H and (13)C NMR spectroscopy. Moreover, a related cryptophane, with the methoxy groups replaced by butoxy units (cryptophane-But), and its complexes with the same guests were also studied. Variable temperature spectra showed effects of chemical exchange between the free and bound guests, as well as of conformational exchange of the host. The guest exchange was studied quantitatively by exchange spectroscopy or line shape analysis. Extraction of kinetic and thermodynamic parameters led to the characterization of the affinity between guests and hosts. On the other hand, the host exchange was investigated by means of (13)C Carr-Purcell-Meiboom-Gill (CPMG) relaxation dispersion which aims at the determination of the transverse relaxation rate R2, the inverse of the transverse relaxation time T2, as a function of the repetition of the π pulses in a CPMG train. The variation of the measured transverse relaxation rate with the repetition rate νCPMG indicated conformational exchange occurring on the microsecond-millisecond time scale. Structural information was obtained through measurements of cross-relaxation rates, both within the host and between the host and the guest protons. The NMR results were supported by DFT calculations.

Host-guest complexes between cryptophane-C and chloromethanes revisited

Cryptophane-C is composed of two nonequivalent cyclotribenzylene caps, one of which contains methoxy group substituents on the phenyl rings. The two caps are connected by three OCH(2)CH(2)O linkers in an anti arrangement. Host-guest complexes of cryptophane-C with dichloromethane and chloroform in solution were investigated in detail by nuclear magnetic resonance techniques and density functional theory (DFT) calculations. Variable temperature proton and carbon-13 spectra show a variety of dynamic processes, such as guest exchange and host conformational transitions. The guest exchange was studied quantitatively by exchange spectroscopy measurements or by line-shape analysis. The conformational preferences of the guest-containing host were interpreted through cross-relaxation measurements, providing evidence of the gauche+2 and gauche-2 conformations of the linkers. In addition, the mobility of the chloroform guest inside the cavity was studied by carbon-13 relaxation experiments. Combining different types of evidence led to a detailed picture of molecular recognition, interpreted in terms of conformational selection.

Enantiopure supramolecular cages: synthesis and chiral recognition properties

Enantiopure compounds are ubiquitous in the chemical sciences and present a particular interest in the field of molecular recognition and host-guest systems. Indeed, chiral molecular receptors are at the basis of numerous biological recognition processes and have important implications in biochemistry or pharmacology. Chemists have been investigating this field for several decades, which has led to the development of the synthesis of chiral hosts, their enantiomeric differentiation, and the studies of their recognition properties towards important and bio-relevant chiral guest substrates. The design of molecular cages is a rather difficult task that is even more demanding when enantiopure molecules are required. In this review we chose to present the main families of synthetic organic supramolecular cages that have been developed, whose structures contain stereogenic centers or present an inherent chirality, giving rise to chiral supramolecular cages. Particular attention is given to obtaining enantiopure compounds. Their recognition properties are also underlined. A last important aspect of the review is to present how chiroptical spectroscopies can be used to characterize the recognition phenomena displayed by supramolecular cages.

Rhodium-catalyzed enantioselective addition of boronic acids to N-benzylnicotinate salts

The highly enantioselective catalytic asymmetric addition of aryl and alkenylboronic acids to N-benzylnicotinate salt 1 is described. The dihydropyridine 2 reaction products can be converted to synthetically useful piperidines. Application of the methodology to the preparation of enantioenriched quaternary chiral centers is also discussed.

Crystallographic observation of 'induced fit' in a cryptophane host-guest model system

Cryptophane-A, comprised of two cyclotriguaiacylenes joined by three ethylene linkers, is a prototypal organic host molecule that binds reversibly to neutral small molecules via London forces. Of note are trifunctionalized, water-soluble cryptophane-A derivatives, which exhibit exceptional affinity for xenon in aqueous solution. In this paper, we report high-resolution X-ray structures of cryptophane-A and trifunctionalized derivatives in crown–crown and crown–saddle conformations, as well as in complexes with water, methanol, xenon or chloroform. Cryptophane internal volume varied by more than 20% across this series, which exemplifies 'induced fit' in a model host–guest system.

Cryptophane-A is a prototypical organic host molecule that binds reversibly to neutral guest molecules. Taratula et al. report X-ray structures of cryptophane-A complexed with a range of host molecules to show that the cryptophane host–guest system exhibits ‘induced fit’.

Determination of the absolute configurations of bicyclo[3.1.0]hexane derivatives via electronic circular dichroism, optical rotation dispersion and vibrational circular dichroism spectroscopy and density functional theory calculations

The electronic circular dichroism (ECD), optical rotation dispersion (ORD), and vibrational circular dichroism (VCD) spectra of a pair of enantiomers, i.e. 3 and 4, of a bicyclo[3.1.0]hexane derivative have been measured in acetonitrile and acetonitrile-d(3), respectively. Extensive conformational searches at the B3LYP/6-311++G** level have been carried out for 3, which has four OH and one N(3) functional groups. For the bicyclo[3.1.0]hexane ring of 3, the boat-like conformers have been found to be much more stable than the chair-like conformers, while the number and the strength of the intramolecular hydrogen bonds have been identified as the dominant factors in the relative stability among the boat-like and among the chair-like conformers. DFT simulations of the ECD, ORD and VCD spectra have been performed for all low energy conformers at the B3LYP/6-311++G** and B3LYP/aug-cc-pVDZ level. Implicit continuum polarization model has been used to account for solvent effects in all these chiroptical measurements. Comparison of the DFT simulations with the experimental data shows that all three chiroptical properties yield the same absolute configuration assignment for . This work demonstrates that using multiple chiroptical spectroscopic methods in combination with DFT calculations allows one to determine absolute configurations with high confidence for chiral carbohydrates and their analogues, which possess a large number of rotatable bonds.

NMR investigation of guest-host complex between chloroform and cryptophane C

Guest-host complex between cryptophane C, possessing two non-equivalent caps, and chloroform is investigated by NMR spectroscopy. The kinetics of the chloroform exchange between the bound and free sites is determined by (1)H exchange spectroscopy. Moreover, the preferential orientation of chloroform molecule with respect to the cryptophane C frame is examined by the NOESY and ROESY experiments. The experimental findings are compared to the results of quantum chemical calculations.

Determination of the absolute configurations of synthetic daunorubicin analogues using vibrational circular dichroism spectroscopy and density functional theory

The absolute configurations of three synthesized anthracycline analogues have been determined using vibrational circular dichroism (VCD) spectroscopy and the density functional theory (DFT) calculations. The experimental VCD spectra of the three compounds have been measured for the first time in the film state, prepared from their CDCl(3) solutions. Conformational searches for the monomers and some dimers of the three compounds have been performed at the DFT level using the B3LYP functional and the 6-311G** and 6-311++G** basis sets. The corresponding vibrational absorption and VCD spectra have been calculated. The good agreement between the experimental and the calculated spectra allows one to assign the absolute configurations of the three compounds with high confidence. In addition, the dominant conformers of the three compounds have also been identified.

Vibrational circular dichroism spectroscopy of chiral molecules

In this chapter, new developments and main applications of vibrational circular dichroism (VCD) spectroscopy reported in the last 5 years are described. This includes the determinations of absolute configurations of chiral molecules, understanding solvent effects and modeling solvent-solute explicit hydrogen bonding networks using induced solvent chirality, studies of transition metal complexes and their peculiar and enormous intensity enhancements in VCD spectra, investigations of conformational preference of chiral ligands bound to gold nano particles, and two new advances in applying matrix isolation VCD spectroscopy to flexible, multi-conformational chiral molecules and complexes, and in development of femtosecond laser based VCD instruments for transient VCD monitoring. A brief review of the experimental techniques and theoretical methods is also given. The purpose of this chapter is to provide an up-to-date perspective on the capability of VCD to solve significant problems about chiral molecules in solution, in thin film states, or on surfaces.

Structure and absolute configuration of ginkgolide B characterized by IR- and VCD spectroscopy

Experimental and calculated (B3LYP/6-31G(d)) vibrational circular dichroism (VCD) and IR spectra are compared, illustrating that the structure and absolute configuration of ginkgolide B (GB) may be characterized directly in solution. A conformational search for GB using MacroModel and subsequent DFT optimizations (B3LYP/6-31G(d)) provides a structure for the lowest energy conformer which agrees well with the structure determined by X-ray diffraction. In addition, a conformer at an energy of 7 kJ mol(-1) (B3LYP/6-311+G(2d,2p)) with respect to the lowest energy conformer is predicted, displaying different intramolecular hydrogen bonding. Differences between measured and calculated IR and VCD spectra for GB at certain wavenumbers are rationalized in terms of interactions with solvent, intermolecular GB-GB interactions, and the potential presence of more than one conformer. This is the first detailed investigation of the spectroscopic fingerprint region (850-1300 cm(-1)) of the natural product GB employing infrared absorption and VCD spectroscopy.