Energy transfer switching in a bistable molecular machine

[reaction: see text] A novel [2]rotaxane, containing pyrene and perylene bisimide as both stoppers and photoactive units, has been prepared. The shuttling of the protonated macrocycle switched the energy transfer (EN) from a pyrene moiety to a perylene moiety, which resulted in changes of fluorescence of the perylene moiety.     

Diversity Oriented Preparation of Pillar[5]arene-Containing [2]Rotaxanes by a Stopper Exchange Strategy

Rotaxane building blocks bearing 3,5-bis(trifluoromethyl) benzenesulfonate (BTBS) stoppers have been efficiently prepared from a pillar[5]arene derivative, 3,5-bis(trifluoromethyl) benzenesulfonyl chloride (BTBSCl) and different diols, namely 1,10-decanediol and 1,12-dodecanediol. The BTBS moieties of these compounds are good leaving groups and stopper exchange reactions could be achieved by treatment with different nucleophiles thus affording rotaxanes with ester, thioether or ether stoppers.     

Active Esters as Pseudostoppers for Slippage Synthesis of [2]Pseudorotaxane Building Blocks: A Straightforward Route to Multi‐Interlocked Molecular Machines

The efficient synthesis and very easy isolation of dibenzo[24]crown‐8‐based [2]pseudorotaxane building blocks that contain an active ester motif at the extremity of the encircled molecular axle and an ammonium moiety as a template for the dibenzo[24]crown‐8 is reported. The active ester acts both as a semistopper for the [2]pseudorotaxane species and as an extensible extremity. Among the various investigated active ester moieties, those that allow for the slippage process are given particular focus because this strategy produces fewer side products. Extension of the selected N‐hydroxysuccinimide ester based pseudorotaxane building block by using either a mono‐ or a diamino compound, both containing a triazolium moiety, is also described. These provide a pH‐dependent two‐station [2]rotaxane molecular machine and a palindromic [3]rotaxane molecular machine, respectively. Molecular machinery on both interlocked compounds through variation of pH was studied and characterized by means of NMR spectroscopy.     

A Fullerene‐Substituted Pillar[5]arene for the Construction of a Photoactive Rotaxane

Transformation of a methylene group of the pillar[5]arene scaffold into a ketone has been achieved by treatment with N‐bromosuccinimide followed by hydrolysis of the bromide intermediate and oxidation of the resulting secondary benzylic alcohol with BaMnO₄. Condensation of the resulting macrocycle including a ketone function with p‐toluenesulfonyl hydrazide followed by reaction of the corresponding tosylhydrazone with C₆₀ under modified Bamford–Stevens conditions gave a fulleropillar[5]arene derivative. This building block has been used to prepare a rotaxane. The resulting molecule combining the fullerene‐functionalized macrocycle with an axle bearing a porphyrin stopper is a photoactive molecular device in which the porphyrin emission is efficiently quenched by the fullerene moiety.     

Size Complementarity of Macrocyclic Cavities and Stoppers in Amide-Rotaxanes

New [2]rotaxanes were prepared by the threading and the slipping procedure, the latter having the advantage of not needing templating interactions. As a consequence, the first [2]rotaxane consisting of a tetraamide macrocycle and a pure hydrocarbon thread was synthesized (see 12a in Scheme 2). Sterically matching wheels and axles being the basic requirement of a successful slipping approach to rotaxanes, mono- and bishomologous wheels 5b , c with larger diameters than the parent 5a were synthesized and mechanically connected to amide axles 10a – c which were stoppered with blocking groups of different spatial demand (Scheme 1). The deslipping kinetics of the resulting rotaxanes 8a – c and 9a , b were measured and compared; it emerges that even slight increases in the wheel size require larger stoppers to stabilize the mechanical bond. Moreover, when the deslipping rate of 8a (amide wheel and amide axle) was determined in either DMF or THF, a strong dependence on the solvent polarity, which is caused by a differing extent of intramolecular H-bonds between the wheel and the axle, was observed. As expected, no such dependence was detected for rotaxane 12a (amide wheel and hydrocarbon axle) whose components cannot interact via H-bonds. The comparison of the sterically matching pairs of macrocycles and blocking groups, found by a systematic fitting based on the results of slipping and deslipping experiments, with other rotaxane types bearing similar stoppers allows conclusions concerning the relative cavity size of wheels of various structure.     

Selective photoinduced energy transfer from a thiophene rotaxane to acceptor

An energy transfer process was investigated using cyclodextrin-oligothiophene rotaxanes (2T-[2]rotaxane). The excited energy of 2T-[2]rotaxane is transferred to the sexithiophene derivative which is included in the cavity of β-CD stoppers of 2T-[2]rotaxane.     

High-yield diastereoselective synthesis of planar chiral [2]- and [3]rotaxanes constructed from per-ethylated pillar[5]arene and pyridinium derivatives

Planar chiral [2]- and [3]rotaxanes constructed from pillar[5]arenes as wheels and pyridinium derivatives as axles were obtained in high yield using click reactions. The process of rotaxane formation was diastereoselective; the obtained [2]rotaxane was a racemic mixture consisting of (pS, pS, pS, pS, pS) and (pR, pR, pR, pR, pR) forms of the per-ethylated pillar[5]arene (C2) wheel, and other possible types of the [2]rotaxane did not form. Isolation of the enantiopure [2]rotaxanes with one axle through (pS, pS, pS, pS, pS)-C2 or (pR, pR, pR, pR, pR)-C2 wheels was accomplished. Furthermore, pillar[5]arene-based [3]rotaxane was successfully synthesized by attachment of two pseudo [2]rotaxanes onto a bifunctional linker. [3]Rotaxane formed in a 1:2:1 mixture with one axle threaded through two (pS, pS, pS, pS, pS)-C2, one (pS, pS, pS, pS, pS)-C2 and one (pR, pR, pR, pR, pR)-C2 (meso form), or two (pR, pR, pR, pR, pR)-C2 wheels. The [3]rotaxane enantiomers and the meso form were successfully isolated using appropriate chiral HPLC column chromatography. The procedure developed in this study is the starting point for the creation of pillar[5]arene-based interlocked molecules.     

Synthesis,characterisation and photoswitchability of a new [2]rotaxane of α-cyclodextrin with a diazobenzene containing π-conjugated molecular dumbbell

In this work, a new [2]rotaxane consisted of a diazobenzene containing π-conjugated linear compartment, including the 4,4′-bipyridyl moiety and α-cyclodextrin (α-CD) as the macrocyclic compartment, was synthesised with yields of nearly 57% and fully characterised. α-CD easily assembled with the linear compartment and suitable bulky ends (stoppers) in water to give a new [2]rotaxane. The characterisation of this supramolecular compound was accomplished using several spectroscopic techniques such as ¹H NMR, ¹³C NMR and 2D NMR spectroscopy, powder X-ray diffraction, fourier transform infrared spectroscopy, induced circular dichroism and UV–vis spectrophotometry, as well as scanning electron microscopy and Energy Dispersive X-ray. Furthermore, the reversible E–Z photoisomerisation of both [2]rotaxane and its molecular dumbbell was investigated by irradiation with UV light.     

Complexes of a [2]rotaxane ligand with terminal terpyridine groups

Permanently interlocked [2]rotaxane ligands can be created by capping a pyridine terminated [2]pseudorotaxane with terpyridine containing stoppers. The robust nature of the resulting [2]rotaxane ligand allows coordination to inert metals such as Ru(II) not possible under standard self-assembly conditions.     

Dynamic covalent chemistry of a boronylammonium ion and a crown ether: formation of a C3-symmetric [4]rotaxane

This Letter describes an efficient method for constructing a C₃-symmetric [4]rotaxane through hydrogen bond-guided self-assembly and boroxine formation. The reactions proceed under mild conditions in solution, with entropically driven forces promoting the formation of the [4]rotaxane.     

Synthesis and characterization of a metallocycle-based molecular shuttle

Kinetically stable metallocycle-based molecular shuttles of [2]rotaxanes 4a and 4b, along with [3]rotaxanes 5a and 5b, have been prepared using the rhenium(I)-bridged metallocycle 2 and the dumbbell components containing two stations, 3a and 3b. The rotaxanes were self-assembled by hydrogen bonding interactions upon heating a Cl(2)CHCHCl(2) solution containing their components at 70 degrees C. Each rotaxane was isolated in pure form by silica gel chromatography under ordinary laboratory conditions and fully characterized by elemental analysis and various spectroscopic methods. The (1)H NMR signals for the amide NH and the methylene -(CH(2))(4)- of the station were considerably changed when occupied by the metallocycle. In [2]rotaxane 4b, which has a larger naphthyl spacer, the occupied and unoccupied stations gave widely separated signals in the (1)H NMR spectroscopy at room temperature, but averaged signals of two stations were observed in [2]rotaxane 4a, which has a smaller phenyl spacer. This is attributed to the shuttling of the metallocycle between two stations. The coalescence temperature experiment gave a shuttling rate of approximately 670 s(-)(1) at 19 degrees C in CDCl(3), corresponding to an activation free energy (DeltaG()) of 13.3 kcal/mol. With respect to the relative position of the chloride in the rhenium(I) center, two diastereomers are possible in the [2]rotaxane and three diastereomers are possible in the [3]rotaxane. In fact, the rotaxanes exist as diastereomeric mixtures in nearly equal amounts of all possible diastereomers on the basis of the amide NH signals of the station in the (1)H NMR spectroscopy.     

Redox-controllable amphiphilic [2]rotaxanes

With the fabrication of molecular electronic devices (MEDs) and the construction of nanoelectromechanical systems (NEMSs) as incentives, two constitutionally isomeric, redox-controllable [2]rotaxanes have been synthesized and characterized in solution. Therein, they both behave as near-perfect molecular switches, that is, to all intents and purposes, these two rotaxanes can be switched precisely by applying appropriate redox stimuli between two distinct chemomechanical states. Their dumbbell-shaped components are composed of polyether chains interrupted along their lengths by i) two pi-electron rich recognition sites-a tetrathiafulvalene (TTF) unit and a 1,5-dioxynaphthalene (DNP) moiety-with ii) a rigid terphenylene spacer placed between the two recognition sites, and then terminated by iii) a hydrophobic tetraarylmethane stopper at one end and a hydrophilic dendritic stopper at the other end of the dumbbells, thus conferring amphiphilicity upon these molecules. A template-directed protocol produces a means to introduce the tetracationic cyclophane, cyclobis(paraquat-p-phenylene) (CBPQT(4+)), which contains two pi-electron accepting bipyridinium units, mechanically interlocked around the dumbbell-shaped components. Both the TTF unit and the DNP moiety are potential stations for CBPQT(4+), since they can establish charge-transfer and hydrogen bonding interactions with the bipyridinium units of the cyclophane, thereby introducing bistability into the [2]rotaxanes. In both constitutional isomers, (1)H NMR and absorption spectroscopies, together with electrochemical investigations, reveal that the CBPQT(4+) ring is predominantly located on the TTF unit, leading to the existence of a single translational isomer (co-conformation) in both cases. In addition, a model [2]rotaxane, incorporating hydrophobic tetraarylmethane stoppers at both ends of its dumbbell-shaped component, has also been synthesized as a point of reference. Molecular synthetic approaches were used to construct convergently the dumbbell-shaped compounds by assembling progressively smaller building blocks in the shape of the rigid spacer, the TTF unit and the DNP moiety, and the hydrophobic and hydrophilic stoppers. The two amphiphilic bistable [2]rotaxanes are constitutional isomers in the sense that, in one constitution, the TTF unit is adjacent to the hydrophobic stopper, whereas in the other, it is next to the hydrophilic stopper. All three bistable [2]rotaxanes have been isolated as green solids. Electrospray and fast atom bombardment mass spectra support the gross structural assignments given to all three of these mechanically interlocked compounds. Their photophysical and electrochemical properties have been investigated in acetonitrile. The results obtained from these investigations confirm that, in all three [2]rotaxanes, i) the CBPQT(4+) cyclophane encircles the TTF unit, ii) the CBPQT(4+) cyclophane shuttles between the TTF and DNP stations upon electrochemical or chemical oxidation/reduction of the TTF unit, and iii) folded conformations are present in which the CBPQT(4+) cyclophane, while encircling the TTF unit, interacts through its pi-accepting bipyridinium exteriors with other pi-donating components of the dumbbells, especially those located within the stoppers.     

Solvent-free synthesis of heterocyclic compounds using microwaves

Microwave-enhanced solvent-free synthetic approach is described that features simplicity, manipulative ease of the operation and conservation of solvents as the main advantages. This eco-friendly approach, which has found application in facile organic functional group transformations, is applied to rapid assembly of heterocyclic compounds. A variety of solid state reactions are described that occur rapidly at ambient pressure under solventless conditions and provide ready access to intermediates such as enamines and α-tosyloxyketones which can be transformed in situ to biologically significant heterocyclic compounds such as isoflav-3-enes, flavones, quinolones, 2-aroylbenzo[b]furans and thiazoles in one-pot operation. Multicomponent reactions under these solvent-free conditions can be adapted for high speed parallel synthesis and are exemplified by assembly of dihydropyrimidine-2(1H)-ones (Biginelli reaction) and imidazo[1,2-a]annulated pyridines, pyrazines and pyrimidines (Ugi reaction) which may have potential in building a library of such compounds.     

A self-threaded "molecular 8"

A new type of [1]rotaxanes containing two aliphatic bridges between axle and wheel is obtained in 39% yield in a one-step synthesis starting from a [2]rotaxane which contained one sulfonamide group each in both the wheel and the axle. Temperature controlled chemoselective substitution reactions first at these sulfonamide nitrogens and then subsequently at the various other carboxamide nitrogens in the wheel and axle give rise to the formation of an isomeric mixture of three double-bridged [1]rotaxanes which could be separated by HPLC. Structure determination of the main product 3a was possible by NMR experiments supported by molecular modeling calculations. Using different reaction conditions, a double-substituted but not yet bridged [2]rotaxane 4 could be isolated as an intermediate giving further evidence for the assigned structure of 3a and the way of its formation. The shape of this double-bridged [1]rotaxane 3a reminds of a self-intertwining chiral "molecular 8", in which any possible racemization due to deslipping is hindered by the two stoppers originating from the former rotaxane axle. Hence, to the best of our knowledge this is the first example of a molecule in which both concepts, cycloenantiomerism and helical chirality, are realised in one structure. Enantiomer separation of the main product was possible by further HPLC using chiral stationary phases. The Cotton effects of the circular dichrograms are different to those of the already synthesized [1]rotaxanes bearing just one aliphatic bridge between axle and wheel.     

En route to a molecular sheaf: active metal template synthesis of a [3]rotaxane with two axles threaded through one ring

We report that a 2,2':6',2″-terpyridylmacrocycle-Ni complex can efficiently mediate the threading of two alkyl chains with bulky end groups in an active metal template sp(3)-carbon-to-sp(3)-carbon homocoupling reaction, resulting in a rare example of a doubly threaded [3]rotaxane in up to 51% yield. The unusual architecture is confirmed by X-ray crystallography (the first time that a one-ring-two-thread [3]rotaxane has been characterized in the solid state) and is found to be stable with respect to dethreading despite the large ring size of the macrocycle. Through such active template reactions, in principle, a macrocycle should be able to assemble as many axles in its cavity as the size of the ring and the stoppers will allow. A general method for threading multiple axles through a macrocycle adds significantly to the tools available for the synthesis of different types of rotaxane architectures.     

Cyclodextrin-Based [c2]Daisy Chain Rotaxane Insulating Two Diarylacetylene Cores

A [c2]daisy chain rotaxane with two diarylacetylene cores was efficiently synthesized in 53 % yield by capping a C₂-symmetric pseudo[2]rotaxane composed of two diarylacetylene-substituted permethylated α-cyclodextrins (PM α-CDs) with aniline stoppers. The maximum absorption wavelength of the [c2]daisy chain rotaxane remained almost unchanged in various solvents, unlike that of the stoppered monomer, indicating that the two independent diarylacetylene cores were insulated from the external environment by the PM α-CDs. Furthermore, the [c2]daisy chain rotaxane exhibited fluorescence emission derived from both diarylacetylene monomers and the excimer, which implies that the [c2]daisy chain structure can undergo contraction and extension. This is the first demonstration of a system in which excimer formation between two π-conjugated molecules within an isolated space can be controlled by the unique motion of a [c2]daisy chain rotaxane.     

Insights into the Difference Between Rotaxane and Pseudorotaxane

Rotaxane and pseudorotaxane are two types of mechanically interlocked molecular architectures, and there is a clear topological difference and boundary between them. In this work, a “suggested [2]rotaxane 1 ⊂α‐CD” was constructed based on axle molecule 1 bearing two terminal ferrocene groups and a wheel component α‐cyclodextrin (α‐CD), but the result obtained indicated that the ferrocene group cannot prevent α‐CD dethreading under UV irradiation. That is, 1 ⊂α‐CD is just a pseudo[2]rotaxane. Furthermore, the two ferrocene groups in 1 ⊂α‐CD were encapsulated by two cucurbit[7]uril (CB[7]) units to obtain a heteropseudo[4]rotaxane 1 ⊂α‐CD⋅2CB[7]. This heteropseudo[4]rotaxane displayed high stability towards harsh temperatures and the isomerization of azobenzene in 1 , so it can be regarded as a [2]rotaxane. In this [2]rotaxane, the stoppers are not the bulky groups covalently bonded to the axle, but the cyclic CB[7] units connected through noncovalent interactions.     

Langmuir and Langmuir–Blodgett Films from Amphiphilic Pillar[5]arene‐Containing [2]Rotaxanes

Amphiphilic pillar[5]arene‐containing [2]rotaxanes have been prepared and fully characterized. In the particular case of the [2]rotaxane incorporating a 1,4‐diethoxypillar[5]arene subunit, the structure of the compound was confirmed by X‐ray crystal structure analysis. Owing to a good hydrophilic/hydrophobic balance, stable Langmuir films have been obtained for these rotaxanes and the size of the peripheral alkyl chains on the pillar[5]arene subunit has a dramatic influence on the reversibility during compression–decompression cycles. Indeed, when these are small enough, molecular reorganization of the rotaxane by gliding motions are capable of preventing strong π–π interactions between neighboring macrocycles in the thin film.     

Rotaxanes with fluorocarbon blocking groups

A novel [2]-rotaxane has been prepared in which perfluoropropyl stoppers have been utilized as electronic blocking groups. To further investigate this capability, a linear compound that was precapped with perfluoropropyl groups was also prepared. This compound did not demonstrate rotaxane formation when mixed with dibenzo-24-crown-8, providing further evidence that the perfluoropropyl groups were effective electronic stoppers for the macrocycle. [structure: see text]     

Controlling stereoselectivity with the aid of a reagent-directing group: hydroformylation, cuprate addition, and domino reaction sequences

The specific introduction of an appropriately designed reagent-directing group into an organic substrate allows the more efficient use of substrate direction to allow high levels of acyclic stereocontrol in both rhodium-catalyzed hydroformylation and cuprate addition to enoates. This provides access to major building blocks of the polyketide class of natural products. Incorporation of these directed reactions into sequential transformations holds promise for new particularly efficient synthetic methods.