Vibrational circular dichroism spectroscopy for probing the expression of chirality in mechanically planar chiral rotaxanes

Active template synthesis

The active template synthesis of mechanically interlocked molecular architectures exploits the dual ability of various structural elements (metals or, in the case of metal-free active template synthesis, particular arrangements of functional groups) to serve as both a template for the organisation of building blocks and as a catalyst to facilitate the formation of covalent bonds between them. This enables the entwined or threaded intermediate structure to be covalently captured under kinetic control. Unlike classical passive template synthesis, the intercomponent interactions transiently used to promote the assembly typically do not 'live on' in the interlocked product, meaning that active template synthesis can be traceless and used for constructing mechanically interlocked molecules that do not feature strong binding interactions between the components. Since its introduction in 2006, active template synthesis has been used to prepare a variety of rotaxanes, catenanes and knots. Amongst the metal-ion-mediated versions of the strategy, the copper(I)-catalysed alkyne-azide cycloaddition (CuAAC) remains the most extensively used transformation, although a broad range of other catalytic reactions and transition metals also provide effective manifolds. In metal-free active template synthesis, the recent discovery of the acceleration of the reaction of primary amines with electrophiles through the cavity of crown ethers has proved effective for forming an array of rotaxanes without recognition elements, including compact rotaxane superbases, dissipatively assembled rotaxanes and molecular pumps. This Review details the active template concept, outlines its advantages and limitations for the synthesis of interlocked molecules, and charts the diverse set of reactions that have been used with this strategy to date. The application of active template synthesis in various domains is discussed, including molecular machinery, mechanical chirality, catalysis, molecular recognition and various aspects of materials science.

Vibrational Circular Dichroism of a Chiral Triplet Nitrene Investigated Under Matrix-Isolation Conditions in Parahydrogen

Vibrational circular dichroism (VCD) spectra of chiral high-spin organic radicals are expected to show a strong intensity enhancement and are thought to be difficult to predict using state-of-the-art theoretical methods. Herein we show that the chiral triplet nitrene obtained from photochemical cleavage of N2 from enantiopure 2-azido-9H-fluorenol does not feature extraordinarily strong intensities and that the experimental spectra match nicely with calculated ones. Thereby, this study demonstrates the general feasibility of studies on chiral high-spin organics by matrix-isolation VCD.

The End of the Beginning of Mechanical Stereochemistry

ConspectusStereochemistry has played a key role in the development of synthetic chemistry for the simple reason that the function and properties of most molecules, from medicine to materials science, depend on their shape and thus the stereoisomer used. However, despite the potential for rotaxanes and catenanes to display unusual forms of stereochemistry being identified as early as 1961, this aspect of the mechanical bond remained underexplored and underexploited; until 2014 it was only possible to access chiral rotaxanes and catenanes whose stereoisomerism is solely attributable to the mechanical bond using chiral stationary phase high performance liquid chromatography, which limited their production on scale and thus inhibited the investigation of their properties and applications. Furthermore, the stereogenic units of such molecules and analogues were often poorly described, which made it hard to fully articulate both what had been achieved in the field and what problems were left to solve. Relatively recently, methods to access rotaxanes and catenanes that display mechanical stereochemistry selectively have been developed, making these intriguing structures available for study in a range of prototypical applications including catalysis, sensing, and as chiral luminophores.In this Account, we briefly discuss the history of mechanical stereochemistry, beginning in 1961 when the potential for mechanical stereoisomerism was first identified, before defining how mechanical stereochemistry arises from a structural point of view. Building on this, using simple stereochemical arguments, we confirm that the complete set of unique stereogenic units of two-component rotaxanes and catenanes have finally been identified and categorized unambiguously, with the last being identified only in 2024. After pausing to discuss some of the stereochemical curiosities that arise when molecules contain both covalent and mechanical stereogenic units, and the potential for stereoisomerism to arise due to co-conformational movement, we use our stereochemical framework to summarize our efforts to develop conceptually general approaches to [2]catenanes and [2]rotaxanes containing all of the possible mechanical stereogenic units. In particular, we highlight how the nature of a mechanical stereogenic unit affects the available strategies for their stereoselective synthesis. We finish by highlighting recent prototypical chemical applications of interlocked molecules that rely on their mechanical stereochemistry, before discussing future directions and challenges.Taken together, we propose that the transition of such molecules from being hard to make and poorly described, to being available in high stereopurity using clearly articulated methodological and stereochemical concepts suggests that the field is finally maturing. Thus, we are now coming to the end of the beginning of mechanical stereochemistry. The stage is now set for such molecules to play a functional role in a range of areas, indeed in any chemical or physical application where control over molecular shape is required.

Scaling-up VPT2: A feasible route to include anharmonic correction on large molecules

Vibrational analysis plays a crucial role in the investigation of molecular systems. Though methodologies like second-order vibrational perturbation theory (VPT2) have paved the way to more accurate simulations, the computational cost remains a difficult barrier to overcome when the molecular size increases. Building upon recent advances in the identification of resonances, we propose an approach making anharmonic simulations possible for large-size systems, typically unreachable by standard means. This relies on the fact that, often, only portions of the whole spectra are of actual interest. Therefore, the anharmonic corrections can be included selectively on subsets of normal modes directly related to the regions of interest. Starting from the VPT2 equations, we evaluate rigorously and systematically the impact of the truncated anharmonic treatment onto simulations. The limit and feasibility of the reduced-dimensionality approach are detailed, starting on a smaller model system. The methodology is then challenged on the IR absorption and vibrational circular dichroism spectra of an organometallic complex in three different spectral ranges.

Pitfalls in the Optimization of Conformer Populations to Maximize the Similarity between Predicted and Experimental Chiroptical Spectra

The conformational populations of pantolactone, epichlorohydrin, and N-acetyl-tryptophan methyl ester were investigated by using similarity analysis between their calculated and experimental chiroptical spectra. By performing the analysis on pantolactone using two different chiroptical methods, namely, vibrational circular dichroism and Raman optical activity, it was found that the optimal sets of conformers do not match between the two methods, indicating that the conformational populations obtained by optimizing the similarity between calculated and experimental spectra are unlikely to be more accurate than energy-based Boltzmann populations. Also, it was found for pantolactone, epichlorohydrin, and N-acetyl-tryptophan methyl ester that the similarity between calculated and experimental spectra would often not vary significantly if each of the populated conformers was discarded, one at a time. This observation indicates that more than one set of conformers can provide acceptable similarity between the predicted and experimental spectra. Therefore, the correct set of conformers cannot be accurately determined by similarity analysis.

Matrix effects in MI-VCD spectra of two chiral oxiranes and their potential microscopic origin

Combining vibrational circular dichroism (VCD) spectroscopy with the matrix isolation (MI) technique opens up interesting possibilities to study chiral molecules. MI involves the isolation of guest species in inert solid matrices at cryogenic temperatures. Hence, MI-VCD measures are solid-state VCD measurements, and as such, can suffer from mostly birefringance-related artefacts in the same way as common solid-state VCD measurements. In this contribution, we demonstrate that the sample preparation condition have tremendous impact on the quality and reliability of the recorded MI-VCD spectra. While MI-IR spectra are basically blind to these artefacts, the variation of deposition temperatures and host flow rates seem to control whether high quality MI-VCD spectra are obtained or if depolarization effects lead to completely obscured spectra. For two selected examples, styrene oxide (SO) and 1-phenyl propylene oxide (PPO), we discuss how the various experimental conditions may lead to the aforementioned effects and give a microscopic description of their origin.

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.

Modelling solute-solvent interactions in VCD spectra analysis with the micro-solvation approach

Vibrational circular dichroism (VCD) spectroscopy has become an important part of the (stereo-)chemists' toolbox as a reliable method for the determination of absolute configurations. Being the chiroptical version of infrared spectroscopy, it has also been recognized as being very sensitive to conformational changes and intermolecular interactions. This sensitivity originates from the fact that the VCD spectra of individual conformers are often more different than their IR spectra, so that changes in conformational distributions or band positions and intensities become more pronounced. What is an advantage for studies focussing on intermolecular interactions can, however, quickly turn into a major obstacle during AC determinations: solute-solvent interactions can have a strong influence on spectral signatures and they must be accurately treated when simulating VCD and IR spectra. In this perspective, we showcase selected examples which exhibit particularly pronounced solvent effects. It is demonstrated that it is typically sufficient to model solute-solvent interactions by placing single solvent molecules near hydrogen bonding sites of the solute and subsequently use the optimized structures for spectra simulations. This micro-solvation approach works reasonably well for medium-sized, not too conformationally flexible molecules. We thus also discuss its limitations and outline the next steps that method development needs to take in order to further improve the workflows for VCD spectra predictions.

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.

Synthesis and Vibrational Circular Dichroism Analysis of N-Heterocyclic Carbene Precursors Containing Remote Chirality Centers

VCD analysis of 16 diastereomeric pairs of NHC precursors containing two isolated chirality centers and different substitution patterns identified VCD transitions characteristic of the chirality center in the imidazolium ring or in the side chain, which, in contrast to ECD and OR, could be utilized to assign the two chirality centers separately by simple comparison, regardless of the type and position of achiral aromatic substituents. While the ECD and OR data showed great dependence on the position of an achiral substituent such as a methoxy group, characteristic experimental VCD transitions remained consistent and they could be used to determine the absolute configuration of all the regio- and stereoisomers and substituted analogues. VCD, ECD and OR approaches were evaluated, and several carbene precursors were found, for which only the VCD method could distinguish the four stereoisomers. With t-butyl, phenyl or 2-naphthyl substituents at the C-1′ chirality center, the ECD spectra of the C-1′ epimers were near-identical, and hence it was only the VCD approach that showed distinct differences suitable for the configurational assignment. The chiroptical characterization of our diastereomeric pairs of NHC precursors enables the future application of related derivatives having different substitution patterns in stereoselective transformations.

Achieving Adjustable Multifunction Based on Host-Guest Interaction-Manipulated Reversible Molecular Conformational Switching

Small molecules that are capable of toggling between multiple and definite conformational states under external stimuli have great potential for use in molecular switches or sensors. However, currently developed regulation approaches for these switchable molecules mostly involve covalent bond-breaking/reforming processes, thereby inevitably producing byproducts or causing fatigue accumulation. Herein, we report a simple but successful model whose molecular conformation can be precisely manipulated between stretched and folded forms by employing host-guest interactions with rigid macrocycles, thus avoiding possible side reactions and fatigue accumulation and possessing excellent reversibility. Moreover, the conformation states of this molecule can be visualized and identified by luminous readout, endowing it with real-time self-reporting features. Furthermore, this controllable and reversible conformational conversion is accompanied by various valuable functions, including controllable multicolor emission; ratiometric fluorescent thermosensing with high temperature resolution, excellent reversibility, lock/unlock switching, and especially linear detection range tunability; and in addition real-time intracellular temperature sensing and imaging, disclosing the intriguing microscopic "conformation-function" relationship based on a single molecule.

A chiral interlocking auxiliary strategy for the synthesis of mechanically planar chiral rotaxanes

Rotaxanes can display molecular chirality solely due to the mechanical bond between the axle and encircling macrocycle without the presence of covalent stereogenic units. However, the synthesis of such molecules remains challenging. We have discovered a combination of reaction partners that function as a chiral interlocking auxiliary to both orientate a macrocycle and, effectively, load it onto a new axle. Here we use these substrates to demonstrate the potential of a chiral interlocking auxiliary strategy for the synthesis of mechanically planar chiral rotaxanes by producing a range of examples in high enantiopurity (93–99% e.e.), including so-called ‘impossible’ rotaxanes whose axles lack any functional groups that would allow their direct synthesis by other means. Intriguingly, by varying the order of bond-forming steps, we can effectively choose which end of an axle the macrocycle is loaded onto, enabling the synthesis of both hands of a single target using the same reactions and building blocks.

Absolute configuration of a [1]rotaxane determined from vibrational and electronic circular dichroism spectra

The experimental vibrational circular dichroism (VCD) and electronic circular dichroism (ECD) spectra were measured for the enantiomers of [1]rotaxane 1. These experimental spectra have been analyzed using predicted VCD and ECD spectra for (S, R_mp ) or (S, S_mp ) diastereomers using density functional theory. This comparison allowed for a definitive assignment of the absolute configuration of 1.

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.

Spin-labelled mechanically interlocked molecules as models for the interpretation of biradical EPR spectra

Biradical spin probes can provide detailed information about the distances between molecules/regions of molecules because the through-space coupling of radical centres, characterised by J, is strongly distance dependent. However, if the system can adopt multiple configurations, as is common in supramolecular complexes, the shape of the EPR spectrum is influenced not only by J but also the rate of exchange between different states. In practice, it is often hard to separate these variables and as a result, the effect of the latter is sometimes overlooked. To demonstrate this challenge unequivocally we synthesised rotaxane biradicals containing nitronyl nitroxide units at the termini of their axles. The rotaxanes exchange between the available biradical conformations more slowly than the corresponding non-interlocked axles but, despite this, in some cases, the EPR spectra of the axle and rotaxane remain remarkably similar. Detailed analysis allowed us to demonstrate that the similar EPR spectral shapes result from different combinations of J and rates of conformational interconversion, a phenomenon suggested theoretically more than 50 years ago. This work reinforces the idea that thorough analysis must be performed when interpreting the spectra of biradicals employed as spin probes in solution.

Vibrational circular dichroism studies of exceptionally strong chirality inducers in liquid crystals

Chiral dopants are used in liquid crystal displays to introduce uniform helical alignment. VCD can provide unambiguous determination of the absolute configuration and structural details of such a dopant, while X-ray crystallography fails.