Epoxidation of polybutadiene by a topologically linked catalyst

Silk-inspired low-molecular-weight organogelator

A minimal sequence from natural silks, the tetrapeptide GAGA, has been used as inspiration for the design of a new organogelator. Gels were obtained in several organic solvents, and their microscopic aspects were studied by transmission and cryo-scanning electron microscopies (TEM and cryo-SEM). FT-IR spectroscopy, circular dichroism, and wide-angle X-ray diffraction were used to study the self-assembly features of this molecule, and evidence of an antiparallel beta-sheet organization was obtained. Remarkably, the precise secondary structure uniqueness found in Nature was successfully transferred into a small synthetic analogue that self-assembles driven only by noncovalent interactions.

Efficient Synthesis of a Chiral [4]Pseudocatenane and Its Derivatives: A Novel Ship's Wheel-like Interlocked Structure

A novel chiral[4]pseudocatenane 5H(3)[PF(6)](3) was synthesized efficiently by treatment of a solution of chiral triptycene-based tri(crown ether) 1 and three equivalents of a bis[p-(but-3-enyloxy)benzyl]ammonium salt in CH(2)Cl(2) with a Grubbs II catalyst, followed by hydrogenation. It was found that the ammonium groups in 5H(3)[PF(6)](3) could be deprotonated by 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in acetonitrile or dimethyl sulfoxide (DMSO). Consequently, N-acylation of the ammonium groups was easily performed in the presence of DBU, which resulted in a new class of neutral highly ordered interlocked molecules in good yields. In particular, the incorporation of stopper units, for example, diethyl phosphoramidate, lead to the isolation of the interlocked molecule 10 with an interesting ship's wheel-like structure, which was structurally studied with the help of detailed NMR experiments. Compared with 1, it was further found that the Cotton effect of (R)-1,1'-binaphthyl chromophore at 241 nm was greatly reduced in 5H(3)[PF(6)](3) and its derivatives. Moreover, a new positive Cotton effect at 248 nm appeared in the interlocked molecules; this observation could be attributed to the chirality transfer from the binaphthyl units to the macrocycles lying in the cavities of 1.

Ligand-Template Directed Assembly: An Efficient Approach for the Supramolecular Encapsulation of Transition-Metal Catalysts

Supramolecular encapsulation of small guest molecules inside well-defined cavities of molecular capsules has witnessed broad attention because of the unusual behaviour of these systems. The molecular capsules generally consist of rigid complementary building blocks that are held together by multiple, complementary non-covalent interactions. Interestingly, it has been shown that chemical transformations can take place inside these capsules and in some examples the reaction is accelerated, while in other cases otherwise instable intermediates could be isolated in the capsulated form. Many reactions of interest require a transition-metal (TM) catalyst, and the creation of new capsules in which such catalysts are implemented within the structure is thus required for the development of resourceful type of catalyst systems for these processes. In this concept article we will discuss new strategies to arrive at such systems, with a focus on a ligand-templated approach. In this approach, multifunctional ligands are used as templates for the encapsulation process by supramolecular building blocks and concomitantly for the formation of TM complexes that are active in catalytic processes. The obtained encapsulated transition-metal catalysts show unusual reactivity and selectivity behaviour that will be discussed in detail.

Substituent Effects Control the Self-Association of Molecular Clips in the Crystalline State

We report the X-ray crystal structure of 11 molecular clips and analyze the influence of substituents (e.g., OMe, Me, and NO2) and their location on the observed crystal packing. Molecular clips 3a and 3b form tapelike structures in the crystal due to pi-pi interactions between the aromatic walls. Compounds 3d, 3eC, and 3fC form dimers driven by critical C-H...O interactions and then form tapes driven by pi-pi interactions in the crystal. These two building motifs, pi-pi and C-H...O interactions, can be used to rationalize the enantio- and diastereoselectivity observed in the X-ray crystal structures of the remaining five molecular clips. For example, the C-H...O interactions are found to dictate the formation of homochiral dimers in the structures of (+/-)-3eT and (+/-)-3fT and to control the diastereoselective formation of 6a2-6c2 dimeric motifs with internal p-dimethoxy-o-xylylene walls. Overall, the results suggest that substituent effects that induce even weak intermolecular interactions (e.g., C-H...O) can be used to reliably control crystal packing within glycoluril-based systems.

A Comparison of Shuttling Mechanisms in Two Constitutionally Isomeric Bistable Rotaxane-Based Sunlight-Powered Nanomotors

To find out how best to optimize shuttling of the macrocycle in a particular class of photochemically driven molecular abacus, which has the molecular structure of BR-I6+ in its Mark I prototype (Ashton et al., Chem. Eur. J. 2000, 6, 3558), we have synthesized and characterized a Mark II version of this kind of two-station rotaxane comprised of six molecular modules, namely (a) a bisparaphenylene[34]crown-10 electron donor macrocycle M and its dumbbell-shaped component which contains (b) a Ru(ii)-polypyridine photoactive unit P2+ as one of its stoppers, (c) a p-terphenyl-type ring system as a rigid spacer S, (d) 4,4′-bipyridinium (A12+) and (e) 3,3′-dimethyl-4,4′-bipyridinium (A22+) electron acceptor units that can play the role of stations for the macrocycle M, and (f) a tetraarylmethane group T as the second stopper. This Mark II version is identical with BR-I6+ in the Mark I series that works as a sunlight-powered nanomotor (Balzani et al., Proc. Natl. Acad. Sci. USA 2006, 103, 1178), except for the swapping of the two stations A12+ and A22+ along the dumbbell-shaped component, i.e. the Mark I and II bistable rotaxanes are constitutionally isomeric. We have found the closer the juxtaposition of the electron transfer photosensitizer P2+ to the better (A12+) of the two electron acceptors, namely the situation in BR-II6+ compared with that in BR-I6+ results in an increase in the rate — and hence the efficiency — of the photoinduced electron-transfer step. The rate of the back electron transfer, however, also increases. As a consequence, BR-II6+ performs better than BR-I6+ in the fuel-assisted system, but much worse when it is powered by visible light (e.g. sunlight) alone. By contrast, when shuttling is electrochemically driven, the only difference between the two bistable rotaxanes in the Mark I and Mark II series is that the macrocycle M moves in opposite directions.

Chiral sensing for amino acid derivative based on a [2]rotaxane composed of an asymmetric rotor and an asymmetric axle

A racemic [2]rotaxane, composed of an asymmetric rotor and an asymmetric axle, formed a diastereomer with an amino acid derivative, and showed an optical response for the chiral recognition.

Artificial nanomachines based on interlocked molecular species: recent advances

The bottom-up construction and operation of nanoscale machines and motors, that is, supramolecular systems wherein the molecular components can be set in motion in a controlled manner for ultimately accomplishing a function, is a topic of great interest in nanoscience and a fascinating challenge of nanotechnology. The field of artificial molecular machines and motors is growing at an astonishing rate and is attracting a great deal of interest. Research in the last decade has shown that species made of interlocked molecular components like rotaxanes, catenanes and related systems are most attractive candidates. In recent times, the evolution of the structural and functional design of such systems has led to the construction and operation of complex molecular machines that, in some cases, are able to do specific tasks. This tutorial review is intended to discuss the design principles for nanomachines based on interlocked molecules, and to provide a timely overview on representative prototype systems.

Artificial Molecular Motors Powered by Light

The bottom-up construction and operation of machines and motors of molecular size is a topic of great interest in nanoscience, and a fascinating challenge of nanotechnology. The problem of the energy supply to make molecular motors work is of the greatest importance. Research in the last ten years has demonstrated that light energy can indeed be used to power artificial nanomotors by exploiting photochemical processes in appropriately designed systems. More recently, it has become clear that under many aspects light is the best choice to power molecular motors; for example, systems that show autonomous operation and do not generate waste products can be obtained. This review is intended to discuss the design principles at the basis of light-driven artificial nanomotors, and provide an up-to-date overview on the prototype systems that have been developed.

Light-Driven Molecular Motors: Stepwise Thermal Helix Inversion during Unidirectional Rotation of Sterically Overcrowded Biphenanthrylidenes

To investigate the unidirectional rotation of chiral overcrowded biphenanthrylidenes in more detail, the size of the substituent next to the double bond responsible for the unidirectionality of rotation was varied. The thermal and photochemical isomerization of three sterically overcrowded alkenes is described. The behavior of the biphenanthrylidenes with methyl and ethyl substituents is rather similar, and these compounds undergo a unidirectional 360 degrees rotation around the central double bond in a four-step sequence involving two photochemical cis-trans isomerizations and two thermal helix inversions. The only difference between these two true molecular motors was a small entropic effect, which causes the ethyl substituted molecular motor to rotate slightly faster. The behavior of the i-propyl substituted compound differs significantly from that of the other two. Although not all different isomers of the i-propyl substituted molecular motor were detected spectroscopically, experimental data led to the conclusion that this compound can also be considered as a molecular motor and is capable of performing a 360 degrees unidirectional rotation. (1)H NMR and X-ray analysis show a meso-like form as an intermediate in the unidirectional rotation, which proves that the thermal helix inversion is a stepwise process.

Dynamic covalent and supramolecular direction of the synthesis and reassembly of copper(I) complexes

The technique of subcomponent self-assembly has been applied to the preparation of a set of copper(I) complexes from diamines and aldehydes in aqueous solution. These complexes may be synthesized alongside one another in solution despite the chemical non-orthogonality of their respective starting materials; thermodynamic equilibration eliminates all mixed products. The reactivity of these complexes has been studied, revealing that in certain cases, the substitution of both ligands and ligand subcomponents could be independently carried out. In one particular case, a complex was shown to be inert to ligand substitution but readily underwent ligand subcomponent substitution, creating the possibility of a previously undocumented kind of cascade reaction: Once ligand subcomponent substitution had occurred, ligand exchange could then happen, allowing both reactions to be triggered by a single chemical event.

Chemical peristalsis

Molecules that emulate in part the remarkable capabilities of protein motors were recently chemically synthesized. A promising approach is based on physically interlocked macromolecular complexes such as rotaxanes and catenanes. Using the latter, Leigh et al. [Leigh, D. A., Wong, J. K. Y., Dehez, F. & Zerbetto, F. (2003) Nature 424, 174-179] constructed a molecular rotor in which two small rings are induced by pulses of light to move unidirectionally around a third, larger ring. The mechanism is similar to that by which a peristaltic pump operates. Unlike macroscopic peristalsis, however, in which a traveling wave forces material through a series of one-way valves, the chemical peristaltic mechanism does not directly cause the small rings to move but only alters the energetics, with the motion itself arising by thermal activation over energy barriers. Engines operating by this mechanism are "Brownian" motors. Here we describe a minimal two-state mechanism for a catenane-based molecular motor. Although fluctuations caused by equilibrium processes cannot drive directed motion, nonequilibrium fluctuations, whether generated externally or by a far-from-equilibrium chemical reaction, can drive rotation even against an external torque. We discuss a possible architecture for input and output of information and energy between the motor and its environment and give a simple expression for the maximum thermodynamic efficiency. The proposed Brownian motor mechanism is consistent with the high efficiency observed by Yasuda et al. [Yasuda, Y., Noji, H., Kinoshita, K. & Yoshida, M. (1998) Cell 93, 1117-1124] for the F(1)-ATP synthase operating as an ATP-powered molecular rotor.

Dynamic combinatorial olefin metathesis: templated synthesis of porphyrin boxes

A porphyrin macrocyclic square is efficiently prepared by a dynamic combinatorial approach to olefin metathesis and shown by scanning tunneling microscopy (STM) to self-assemble into highly ordered arrays on a graphite surface.

Catalytic molecular motors: fuelling autonomous movement by a surface bound synthetic manganese catalase

A molecular approach to the powering of multi-component nano-devices capable of autonomous translational and rotational motion through the conversion of chemical to kinetic energy is reported.

A dynamic supramolecular system exhibiting substrate selectivity in the catalytic epoxidation of olefins

A dynamic supramolecular system involving hydrogen bonding between a Mn(III) salen catalyst and a Zn(II) porphyrin receptor exhibits selectivity for pyridine appended cis-beta-substituted styrene derivatives over phenyl appended derivatives in a catalytic epoxidation reaction.

Selection Rules for Helicate Ligand Component Self-Assembly: Steric, pH, Charge, and Solvent Effects

The reaction between 1,10-phenanthroline-2,9-dicarboxaldehyde, copper(I), and certain primary amines was found to give quantitatively a dicopper double-helicate product (two of which were crystallographically characterized) by imine self-assembly around Cu(I) templates. The parameters of this reaction were investigated, and important roles were found to be played by (i) the steric bulk of the amine, (ii) the charge of the amine, (iii) the solvent used, and (iv) the pH of the solution. Water was found to allow the broadest range of structures to form, and ligand-component exchange reactions (involving the substitution of an aromatic for an aliphatic amine) were demonstrated to proceed readily in this solvent.

A High-Throughput Screen for Porphyrin Metal Chelatases: Application to the Directed Evolution of Ferrochelatases for Metalloporphyrin Biosynthesis

Porphyrins are of particular interest in a variety of applications ranging from biocatalysis and chemical synthesis to biosensor and electronic technologies as well as cancer treatment. Recently, we have developed a versatile system for the high-level production of porphyrins in engineered E. coli cells with the aim of diversifying substitution patterns and accessing porphyrin systems not readily available through chemical synthesis. However, this approach failed to produce significant amounts of the metalloporphyrin in vivo from overproduced protoporphyrin due to insufficient metal insertion. Therefore, we systematically assessed the activity of the B. subtilis ferrochelatase in vivo and in vitro. A true high-throughput-screening approach based on catalytic in vivo ferrochelatase activity was developed by using fluorescence-activated cell sorting (FACS). This assay was used to screen a library of 2.4 x 10(6) ferrochelatase mutants expressed in protoporphyrin-overproducing recombinant E. coli cells. Several selected protein variants were purified, and their improved catalytic activity was confirmed in vitro. In addition to ferrochelatase activity, metal transport into E. coli was identified as another limitation for in vivo heme overproduction. Overexpression of the metal transporter zupT as part of the assembled pathway increased the overall metalloporphyrin production twofold. This report represents the most exhaustive in vitro evolution study of a ferrochelatase and demonstrates the effectiveness of our novel high-throughput-screening system for directed evolution of ferrochelatases based on their catalytic activity.

Asymmetric Benzoin Condensation Catalyzed by Chiral Rotaxanes Tethering a Thiazolium Salt Moiety via the Cooperation of the Component: Can Rotaxane Be an Effective Reaction Field?

Although some reactions on rotaxanes have been reported, the characteristic features of the rotaxanes providing unique reaction fields have hardly been studied, especially as catalyst. In our continuous studies on interlocked molecules such as rotaxanes and catenanes, we have noticed the importance of such interlocked structures with high freedom in functionalized materials such as molecular catalyst. For catalytic asymmetric benzoin condensations, two optically active rotaxanes possessing thiazolium salt moieties were prepared using the binaphthyl group as the chiral auxiliary. The benzoin condensations of aromatic aldehydes catalyzed by the chiral rotaxanes as catalysts gave optically active benzoins with ca. 30% ee in moderate to high chemical yields depending upon the structure of rotaxane and the reaction conditions employed. From the results, two intrarotaxane chirality transfers are confirmed: (i) through-space chirality transfer from wheel to axle and (ii) through-bond chirality transfer controlled with an achiral wheel. Because these asymmetric reaction fields are specific to the rotaxane structure, the importance and possibility of the "rotaxane field" as a particular reaction field is demonstrated in this work.

Increased Speed of Rotation for the Smallest Light-Driven Molecular Motor

In this paper we present the smallest artificial light-driven molecular motor consisting of only 28 carbon and 24 hydrogen atoms. The concept of controlling directionality of rotary movement at the molecular level by introduction of a stereogenic center next to the central olefinic bond of a sterically overcrowded alkene does not only hold for molecular motors with six-membered rings, but is also applicable to achieve the unidirectional movement for molecular motors having five-membered rings. Although X-ray analyses show that the five-membered rings in the cis- and trans-isomer of the new molecular motor are nearly flat, the energy differences between the (pseudo-)diaxial and (pseudo-)diequatorial conformations of the methyl substituents in both isomers are still large enough to direct the rotation of one-half of the molecule with respect to the other half in a clockwise fashion. The full rotary cycle comprises four consecutive steps: two photochemical isomerizations each followed by a thermal helix inversion. Both photochemical cis-trans isomerizations proceed with a preference for the unstable diequatorial isomers over the stable diaxial isomers. The thermal barriers for helix inversion of this motor molecule have decreased dramatically compared to its six-membered ring analogue, the half-life of the fastest step being only 18 s at room temperature.