Bridging Rotaxanes' Wheels—Cyclochiral Bonnanes

Rapid synthesis of hydrogen bond templated handcuff rotaxanes

The rapid synthesis of hydrogen bond templated handcuff rotaxanes is described. The isolated rotaxanes were characterized by NMR and IR spectroscopies and high resolution mass spectrometry. This report represents a rare demonstration of preparing (2)handcuff [2]rotaxanes by covalently linking separate axles threaded through the rings of a bis-macrocycle by use of the copper catalyzed azide-alkyne cycloaddition (CuAAC) reaction.

Rotaxanes with dynamic mechanical chirality: Systematic studies on synthesis, enantiomer separation, racemization, and chiral-prochiral interconversion

Dynamic mechanical chirality of [2]rotaxane consisting of a C s symmetric wheel and a C 2v symmetric axle is discussed via the synthesis, enantiomer separation, racemization, and chiral-prochiral interconversion. This [2]rotaxane is achiral and/or prochiral when its wheel locates at the center of the axle, but becomes chiral when the wheel moves from the center of the axle. These were proved by the experiments on the enantiomer separation and racemization. The racemization energy of the isolated single enantiomers was controlled by the bulkiness of the central substituents on the axle. Furthermore, the chiral-prochiral interconversion was achieved by relative positional control of the components. The present systematic studies will provide new insight into mechanically chiral interlocked compounds as well as the utility as dynamic chiral sources.

Mechanically interlocked molecular handcuffs

The field of mechanically interlocked molecules that employ a handcuff component are reviewed. The variety of rotaxane and catenane structures that use the handcuff motif to interlock different components are discussed and a new nomenclature, distilling diverse terminologies to a single approach, is proposed. By unifying the interpretation of this class of molecules we identify new opportunities for employing this structural unit for new architectures.

Mechanically interlocked molecules that employ a handcuff component provide a pathway to highly unusual structures, a new nomenclature is proposed which helps to identify opportunities for employing this structural unit for new architectures.

Precision synthesis of linear oligorotaxanes and polyrotaxanes achieving well-defined positions and numbers of cyclic components on the axle

Interest in macromolecules has increased because of their functional properties, which can be tuned using precise organic synthetic methods. For example, desired functions have been imparted by controlling the nanoscale structures of such macromolecules. In particular, compounds with interlocked structures, including rotaxanes, have attracted attention because of their unique supramolecular structures. In such supramolecular structures, the mobility and freedom of the macrocycles are restricted by an axle and dependent on those of other macrocycles, which imparts unique functions to these threaded structures. Recently, methods for the ultrafine engineering and synthesis, as well as functions, of "defined" rotaxane structures that are not statistically dispersed on the axle (i.e., control over the number and position of cyclic molecules) have been reported. Various synthetic strategies allow access to such well-defined linear oligo- and polyrotaxanes, including [1]rotaxanes and [n]rotaxanes (mostly n > 3). These state-of-the-art synthetic methods have resulted in unique functions of these oligo-and polyrotaxane materials. Herein, we review the effective synthetic protocols and functions of precisely constructed one-dimensional oligomers and polymers bearing defined threaded structures, and discuss the latest reports and trends.

Synthesis of a Mechanically Planar Chiral and Axially Chiral [2]Rotaxane

In this study, we synthesized a [2]rotaxane that was both mechanically planar chiral and axially chiral, comprising a symmetrical bis-crown ether featuring a biphenyl moiety (as the macrocyclic component) and a symmetrical bis-ammonium salt (as the dumbbell-shaped component).

A Chiral [3]Rotaxane Comprising Achiral Bis-macrocyclic and Dumbbell-Shaped Components

In this study, we synthesized a molecularly chiral [3]rotaxane comprising a calix-bis-crown ether (as the macrocyclic component) and two unsymmetrical dialkylammonium salts (as dumbbell-shaped components) without any chirality in any of the individual components. Chiral high-performance liquid chromatography was used to separate the enantiomers, which were characterized by circular dichroism spectroscopy. Density functional theory calculations gave an insight into the absolute configuration of each [3]rotaxane.

Synthesis of a Chiral [2]Rotaxane: Induction of a Helical Structure through Double Threading

A helically chiral [2]rotaxane featuring two ammonium ion recognition sites in the dumbbell-like component and a calix-bis-crown ether as the macrocyclic component was synthesized, but with no chirality in either individual component. The enantiomeric nature of the isomers, separated through chiral HPLC, was apparent in their CD spectra, which were mirror images for all wavelengths.

Chirality in rotaxanes and catenanes

Although chiral mechanically interlocked molecules (MIMs) have been synthesised and studied, enantiopure examples are relatively under-represented in the pantheon of reported catenanes and rotaxanes and the underlying chirality of the system is often even overlooked. This is changing with the advent of new applications of MIMs in catalysis, sensing and materials and the appearance of new methods to access unusual stereogenic units unique to the mechanical bond. Here we discuss the different stereogenic units that have been investigated in catenanes and rotaxanes, examples of their application, methods for assigning absolute stereochemistry and provide a perspective on future developments.

Synthesis of Mechanically Planar Chiral rac-[2]Rotaxanes by Partitioning of an Achiral [2]Rotaxane: Stereoinversion Induced by Shuttling

Mechanically planar chiral [2]rotaxanes were synthesized by the introduction of bulky pyrrole moieties into the axle component of an achiral [2]rotaxane. The enantiomers were separated by chiral HPLC. The shuttling of the ring component between the two compartments at high temperature induced the stereoinversion of the mechanically planar chiral [2]rotaxane. The rate of the stereoinversion was studied quantitatively, and the kinetic parameters were determined.

Asymmetric Catalysis with a Mechanically Point-Chiral Rotaxane

Mechanical point-chirality in a [2]rotaxane is utilized for asymmetric catalysis. Stable enantiomers of the rotaxane result from a bulky group in the middle of the thread preventing a benzylic amide macrocycle shuttling between different sides of a prochiral center, creating point chirality in the vicinity of a secondary amine group. The resulting mechanochirogenesis delivers enantioselective organocatalysis via both enamine (up to 71:29 er) and iminium (up to 68:32 er) activation modes.

An efficient approach to mechanically planar chiral rotaxanes

We describe the first method for production of mechanically planar chiral rotaxanes in excellent enantiopurity without the use of chiral separation techniques and, for the first time, unambiguously assign the absolute stereochemistry of the products. This proof-of-concept study, which employs a chiral pool sugar as the source of asymmetry and a high-yielding active template reaction for mechanical bond formation, finally opens the door to detailed investigation of these challenging targets.

The first case of an actinide polyrotaxane incorporating cucurbituril: a unique ‘dragon-like’ twist induced by a specific coordination pattern of uranium

A unique ‘dragon-like’ twist is induced by the specific coordination pattern of uranium in the first actinide polyrotaxane.

Sequence isomerism in [3]rotaxanes

We describe a strategy for assembling different macrocycles onto a nonsymmetrical rotaxane thread in a precise sequence. If the macrocycles are small and rigid enough so that they cannot pass each other then the sequence is maintained mechanically, affording stereoisomerism in a manner reminiscent of atropisomerism. The method is exemplified through the synthesis of a pair of [3]rotaxane diastereomers that are constitutionally identical other than for the sequence of the different macrocycles on the thread. The synthesis features the iterative binding of different palladium(II) pyridine-2,6-dicarboxamide complexes to a pyridine ligand on the thread followed by their macrocyclization by ring-closing olefin metathesis. Removal of the palladium(II) from the first rotaxane formed frees the pyridine site to coordinate to a second, different, palladium(II) pyridine-2,6-dicarboxamide unit which, following macrocyclization, provides a multiring rotaxane of predetermined macrocycle sequence.

Phenyl-(2-pyridyl)-(3-pyridyl)-(4-pyridyl)methane: synthesis, chiroptical properties, and theoretical calculation of its absolute configuration

The title compound, a prototypical chiral molecule based on a tetraarylmethane framework, has been synthesized in five steps from (2-pyridyl)-(3-pyridyl)ketone. X-ray crystallographic analysis revealed the tetraarylmethane framework of the molecule but did not determine the positions of the nitrogen atoms because the crystal is a racemic compound and the aryl groups are disordered in the crystal. The optical resolution of the title compound was achieved by chiral HPLC with a Chiralcel OD column. The CD spectra of the two fractions in acetonitrile exhibited opposite signs as expected for a pair of enantiomers. Their CD spectra are changed in 2 M HCl due to protonation. The calculated CD curve for the target molecule based on time-dependent density functional theory (TDDFT) reproduces the experimental result very well, thus suggesting that the first eluted fraction is the R isomer in terms of absolute configuration.

Polysaccharide derivatives as useful chiral stationary phases in high-performance liquid chromatography

The chromatographic separation of enantiomers using chiral stationary phases (CSPs) has significantly advanced. The esters and carbamates of polysaccharides coated on silica gel have been extensively studied and widely used as CSPs for high-performance liquid chromatography (HPLC). In order to overcome the strict solvent limitation on these coated CSPs, the preparation of a new generation of CSPs consisting of immobilized polysaccharide derivatives has become increasingly important. The universal solvent compatibility of the new CSPs provides flexibility in both analytical and preparative chromatographies.