Showing papers by "J. Fraser Stoddart published in 2002"

Dynamic covalent chemistry.

Abstract: Dynamic covalent chemistry relates to chemical reactions carried out reversibly under conditions of equilibrium control. The reversible nature of the reactions introduces the prospects of "error checking" and "proof-reading" into synthetic processes where dynamic covalent chemistry operates. Since the formation of products occurs under thermodynamic control, product distributions depend only on the relative stabilities of the final products. In kinetically controlled reactions, however, it is the free energy differences between the transition states leading to the products that determines their relative proportions. Supramolecular chemistry has had a huge impact on synthesis at two levels: one is noncovalent synthesis, or strict self-assembly, and the other is supramolecular assistance to molecular synthesis, also referred to as self-assembly followed by covalent modification. Noncovalent synthesis has given us access to finite supermolecules and infinite supramolecular arrays. Supramolecular assistance to covalent synthesis has been exploited in the construction of more-complex systems, such as interlocked molecular compounds (for example, catenanes and rotaxanes) as well as container molecules (molecular capsules). The appealing prospect of also synthesizing these types of compounds with complex molecular architectures using reversible covalent bond forming chemistry has led to the development of dynamic covalent chemistry. Historically, dynamic covalent chemistry has played a central role in the development of conformational analysis by opening up the possibility to be able to equilibrate configurational isomers, sometimes with base (for example, esters) and sometimes with acid (for example, acetals). These stereochemical "balancing acts" revealed another major advantage that dynamic covalent chemistry offers the chemist, which is not so easily accessible in the kinetically controlled regime: the ability to re-adjust the product distribution of a reaction, even once the initial products have been formed, by changing the reaction's environment (for example, concentration, temperature, presence or absence of a template). This highly transparent, yet tremendously subtle, characteristic of dynamic covalent chemistry has led to key discoveries in polymer chemistry. In this review, some recent examples where dynamic covalent chemistry has been demonstrated are shown to emphasise the basic concepts of this area of science.

Two-dimensional molecular electronics circuits.

Abstract: Addressing an array of bistable [2]rotaxanes through a two-dimensional crossbar arrangement provides the device element of a current-driven molecular electronic circuit. The development of the [2]rotaxane switches through an iterative, evolutionary process is described. The arrangement reported here allows both memory and logic functions to use the same elements.

Interactions between Conjugated Polymers and Single-Walled Carbon Nanotubes

Abstract: The chemical interactions between single walled carbon nanotubes (SWNTs) and two structurally similar polymers, poly{(m-phenylenevinylene)-co-[(2,5-dioctyloxy-p-phenylene)vinylene]}, or PmPV, and poly{(2,6-pyridinylenevinylene)-co-[(2,5-dioctyloxy-p-phenylene)vinylene]}, or PPyPV, are investigated. The fundamental difference between these two polymers is that PPyPV is a base and is readily protonated via the addition of HCl. Both polymers promote chloroform solubilization of SWNTs. We find that the SWNT/PPyPV interaction lowers the pKa of PPyPV. Optoelectronic devices, fabricated from single polymer-wrapped SWNT structures, reveal a photogating effect on charge transport which can rectify or amplify current flow through the tubes. For PmPV wrapped tubes, the wavelength dependence of this effect correlates to the absorption spectrum of PmPV. For PPyPV, the wavelength dependence correlates with the absorption spectrum of protonated PPyPV, indicating that SWNTs assist in charge stabilization.

Chemical synthesis gets a fillip from molecular recognition and self-assembly processes

Abstract: By drawing on nature conceptually, the practice of chemical synthesis can now be broadened in scope to include the supramolecular synthesis of supermolecules and their postassembly covalent modification to form mechanically interlocked molecules, which the chemist can use to build functioning nanosystems after the style of those found in the living world.

An acid-base switchable [2]rotaxane.

Abstract: A chemically addressable, bistable [2]rotaxane, which incorporates a dumbbell-shaped component containing both secondary dialkylammonium and 1,2-bis(pyridinium)ethane recognition sites for its ring component, dibenzo[24]crown-8 (DB24C8), has been assembled. (1)H NMR spectroscopy has demonstrated that deprotonation (and reprotonation) of the secondary dialkylammonium (dialkylamine) recognition site induces the DB24C8 ring to move away from this site to the 1,2-bis(pyridinium)ethane one (and back again) in a discrete manner, particularly when the experiment is performed in CDCl(3) solution.

Photoinduced electron transfer in a triad that can be assembled/disassembled by two different external inputs. Toward molecular-level electrical extension cables.

Abstract: We have designed, synthesized, and investigated a self-assembling supramolecular system which mimics, at a molecular level, the function performed by a macroscopic electrical extension cable. The system is made up of three components, 12+, 2-H3+, and 3. Component 12+ consists of two moieties: a [Ru(bpy)3]2+ unit, which plays the role of an electron donor under light excitation, and a DB24C8 crown ether, which fulfills the function of a socket. The wire-type component 2-H3+ is also composed of two moieties, a secondary dialkylammonium-ion center and a bipyridinium unit, which thread into the DB24C8 crown-ether socket of 12+ and the 1/5DN38C10 crown-ether socket 3, respectively. The photochemical, photophysical, and electrochemical properties of the three separated components, of the 12+ ⊃ 2-H3+ and 2-H3+ ⊂ 3 dyads, and of the 12+ ⊃ 2-H3+ ⊂ 3 triad have been investigated in CH2Cl2 solution containing 2% MeCN. Reversible connection/disconnection of the two plug/socket systems can be controlled independently...

Ferrocene-containing carbohydrate dendrimers.

Abstract: Aliphatic amines, incorporating one or three (branched) acylated beta-D-glucopyranosyl residues, were coupled with the acid chloride of ferrocenecarboxylic acid and with the diacid chloride of 1,1'-ferrocenedicarboxylic acid to afford four dendrimer-type, carbohydrate-coated ferrocene derivatives in good yields (54-92%). Deprotection of the peracylated beta-D-glucopyranosyl residues was achieved quantitatively by using Zemplen conditions, affording four water-soluble ferrocene derivatives. When only one of the two cyclopentadienyl rings of the ferrocene unit is substituted, strong complexes are formed with beta-cyclodextrin in H2O, as demonstrated by liquid secondary ion mass spectrometry (LSIMS), 1H NMR spectroscopy, electrochemical measurements, and circular dichroism spectroscopy. Molecular dynamics calculations showed that the unsubstituted cyclopentadienyl ring is inserted through the cavity of the toroidal host in these complexes. The electrochemical behavior of the protected and deprotected ferrocene-containing dendrimers was investigated in acetonitrile and water, respectively. The diffusion coefficient decreases with increasing molecular weight of the compound. The potential for oxidation of the ferrocene core, the rate constant of heterogeneous electron transfer, and the rate constant for the energy-transfer reaction with the luminescent excited state of the [Ru(bpy)3]2+ complex (bpy = 2,2'-bipyridine) are strongly affected by the number (one or two) of substituents and by the number (one or three) of carbohydrate branches present in the substituents. These effects are assigned to shielding of the ferrocene core by the dendritic branches. Electrochemical evidence for the existence of different conformers for one of the dendrimers in aqueous solution was obtained.

A supramolecular approach for the formation of fullerene–phthalocyanine dyads

Abstract: A supramolecular Pc–C60 dyad with pseudorotaxane-like geometry has been assembled in solution by the threading of a dibenzylammonium unit attached to a fullerene through the crown ether of an unsymmetrically substituted phthalocyanine, which contains a DB24C8 moiety. Spectroscopic data as well as electrochemical studies suggest a conformation for the complex in which the two bulky subunits are far apart from each other, thus preventing electronic interactions between both electroactive subunits.

Large oligosaccharide-based glycodendrimers.

Abstract: Carbohydrate-based dendritic structures composed of 21 and 27 monosaccharide residues have been synthesized in a convergent manner from trisaccharide building blocks. The oligosaccharide AB2 monomers are based on a maltosyl beta(1-->6)galactose structure, which has been modified to include two methylamino groups at the primary positions of the glucosyl residues. Reductive alkylation of the secondary amino groups, with the innate formyl function of a second oligosaccharide monomer, allows for the chemoselective construction of dendritic wedges, while employing a minimal number of protecting groups. The first-generation dendron can be coupled either to another AB2 monomer, to give a second-generation dendron, or to a tris[2-(methylamino)ethyl]amine-based core moiety, to provide a carbohydrate-based dendrimer. Alternating alpha- and beta-glucosyl residues in the monomers and dendrons, simplifies 1H NMR spectra as a consequence of spreading out the anomeric proton signals. Monomers and dendrons were characterized by extensive one- and two-dimensional NMR spectroscopy in addition to FAB, electrospray, and MALDI-TOF mass spectrometry. Molecular dynamics simulations revealed similar conformations in the dendrons as in the isolated trisaccharide repeating units.

Post-assembly processing of [2]rotaxanes

Abstract: The concept of using [2]rotaxanes that carry one or more surrogate stoppers which can subsequently be converted chemically into other structural units, resulting in the formation of new interlocked molecular compounds, is introduced and exemplified. Starting from simple NH2(+)-centered/crown-ether-based [2]rotaxanes, containing either one or two benzylic triphenylphosphonium stoppers, the well-known Wittig reaction has been employed to make, 1) other [2]rotaxanes, 2) higher order rotaxanes, 3) branched rotaxanes, and 4) molecular shuttles--all isolated as pure compounds, following catalytic hydrogenations of their carbon-carbon double bonds, obtained when aromatic aldehydes react with the ylides produced when the benzylic triphenylphosphonium derivatives are treated with strong base. The two starting [2]rotaxanes were characterized fully in solution and also in the solid state by X-ray crystallography. The new interlocked molecular compounds that result from carrying out post-assembly Wittig reactions on two [2]rotaxanes were characterized by (dynamic) 1H NMR spectroscopy. In the case of a molecular shuttle in which the crown ether component is dibenzo[24]-crown-8 (DB24C8), shuttling is slow on the 1H NMR timescale, even at high temperatures. However, when DB24C8 is replaced by benzometaphenylene[25]-crown-8 as the ring component in the molecular shuttle, the frequency of the shuttling is observed to be around 100 Hz in [D4]methanol at 63 degrees C.

Probing polyvalency in artificial systems exhibiting molecular recognition.

Abstract: An approach to the study of polyvalencythe interaction of polyvalent receptors with polyvalent ligandsin unnatural systems is outlined. In this study, the complexation of dibenzylammonium cations by dibenzo[24]crown-8 or benzometaphenylene[25]crown-8 is utilized as the component receptor−ligand interaction. Two analogous multivalent receptorseach containing either seven dibenzo[24]crown-8 (DB24C8 CLUSTER) or seven benzometaphenylene[25]crown-8 (BMP25C8 CLUSTER) moieties appended to a modified β-cyclodextrin corewere prepared in moderate yields. For each of these multivalent receptors, complementary mono- and divalent ligands containing one or two dialkylammonium centers, respectively, were prepared in good yields. These ligands contained fluorine atom substituents to allow their interactions with crown ether compounds to be probed by ^(19)F NMR spectroscopy. The complexation of these monovalent ligands with the DB24C8 CLUSTER and the BMP25C8 CLUSTER was studied by determining the average binding constant (K_(AVE)) between the receptors and ligands. The abilities of the crown ether clusters to complex with these monovalent ligands was compared with those of the monovalent crown ethers dibenzo[24]crown-8 and benzometaphenylene[25]crown-8. In both instances, it was found that clustering seven crown ethers together into one molecule is detrimental to the abilities of the crown ether moieties to complex with monovalent dialkylammonium ligands. The complexation of the divalent ligands by the DB24C8 CLUSTER and the BMP25C8 CLUSTER was then studied again by determining K_(AVE) and their abilities to complex with these ligands was compared with those of their respective component interactions. By determining K_(AVE) for the polyvalent interaction, it was possible to calculate an association constant, K_(POLY), for the binding of the divalent ligands by the DB24C8 CLUSTER and the BMP25C8 CLUSTER compounds. In both instances K_(POLY) for the polyvalent interaction was found to be approximately 2 orders of magnitude higher than the association constants, K_A, for the component interaction.

An efficient approach towards the convergent synthesis of "fully-carbohydrate" mannodendrimers.

Abstract: Glycosylation of sugar trityl ethers with sugar 1,2-O-(1-cyano)ethylidene derivatives (the trityl-cyanoethylidene condensation) has been applied to the synthesis of highly branched (dendritic) mannooligosaccharides incorporating a Manα1→3(Manα1→6)Man structural motif. The convergent synthetic strategy used to assemble these oligosaccharides was based on the use of glycosyl acceptors and/or a glycosyl donor already bearing this structural motif. The former were represented by mono- and ditrityl ethers of ManαOMe, Manα1→3ManαOMe, and Manα1→3(Manα1→6)ManαX, where X=OMe or SEt. The pivotal glycosyl donor was the peracetylated 1,2-O-(1-cyano)ethylidene-3,6-di-O-(α-D-mannopyranosyl)-β-D-mannopyranose (1), prepared by orthogonal Helferich glycosylation of the known 1,2-O-(1-cyano)ethylidene-β-D-mannopyranose with tetra-O-acetyl-α-D-mannopyranosyl bromide followed by O-acetylation. Glycosylation of acetates of methyl 6-O-trityl-α-D-mannopyranoside and methyl 3,6-di-O-trityl-α-D-mannopyranoside with one equivalent of the donor 1 gave rise to the isomeric tetrasaccharide derivatives, Manα1→3(Manα1→6)Manα1→6ManαOMe and Manα1→3(Manα1→6)Manα1→3ManαOMe, respectively. The latter derivative was further mannosylated at the remaining 6-O-trityl acceptor site to give the protected pentasaccharide Manα1→3(Manα1→6)Manα1→3(Manα1→6)ManαOMe. The isomeric pentasaccharide, Manα1→3(Manα1→6)Manα1→6(Manα1→3)ManαOMe, was prepared by reaction of 1 with the 6-O-trityl derivative of (Manα1→3)ManαOMe. In a similar fashion, 6′- and 6″-O-trityl derivatives of the branched trisaccharide Manα1→3(Manα1→6)ManαOMe served as precursors for two isomeric mannohexaosides. The 3,6-di-O-trityl ether of ManαOMe and the 6′,6″-di-O-trityl ether of Manα1→3(Manα1→6)ManαX (X=OMe or SEt) were efficiently bis-glycosylated with the donor 1 to give the corresponding protected mannoheptaoside and mannononaoside. The yields of these glycosylations with the donor 1 ranged from 50 to 66 %. Final deprotection of all the oligosaccharides was straightforward and afforded the target products in high yields. Both the acylated and deprotected products were characterized, and the intersaccharide connectivities were elucidated by extensive one- and two-dimensional NMR spectroscopy. The described blockwise convergent approach allows assembly of a variety of 3,6-branched mannooligosaccharides.

Surrogate-stoppered [2]rotaxanes: A new route to larger interlocked architectures

Abstract: A novel synthetic strategy for exchanging stoppers on rotaxanes, without them losing their integrity as interlocked molecules, is presented. The surrogate-stoppered [2]rotaxane contains one inert stopper and a triphenylphosphonium group attached to a benzylic position as a second reactive stopper in the dumbbell-shaped component which contains an ammonium (NH2+) ion recognition site, encircled by a crown ether (24C8 or 25C8) component. The strategy for exchanging stoppers relies upon the ability of a benzylic triphenylphosphonium function to undergo a Wittig reaction with a bulky aromatic aldehyde to form a ‘stilbenoid’ [2]rotaxane as a mixture of cis and trans isomers, without the occurrence of any dethreading of the crown ether ring component. The CC double bonds can then be hydrogenated, using Adams' catalyst, to afford a new covalently modified [2]rotaxane with two inert stoppers. Utilizing this strategy, larger interlocked molecular structures including a two-stationed [2]rotaxane and a branched [4]rotaxane have been prepared. Furthermore, initial studies, aimed at using this methodology to gain access to poly[n]rotaxane architectures, are presented. Two ammonium ion/crown ether-based [2]rotaxane monomers–each incorporating (i) a dumbbell-shaped component, possessing the surrogate benzylic triphenylphosphonium stopper, and (ii) a crown ether ring component, bearing an aldehyde function on a fused benzenoid ring–undergo a sequence of Wittig reactions in which the surrogate benzylic triphenylphosphonium stopper is exchanged for a crown ether ring component either, (i) in the same rotaxane molecule, to give cyclic daisy-chains by an intramolecular, chain-terminating reaction or, (ii) in another rotaxane molecule, to give acyclic daisy chains by an intermolecular chain-propagating reaction. Copyright © 2003 John Wiley & Sons, Ltd.

Reversing a rotaxane recognition motif: threading oligoethylene glycol derivatives through a dicationic cyclophane.

Abstract: An already well-established recognition motif-namely one in which the NH2+ centers in the rod sections of the dumbbell components of rotaxanes are encircled by macrocyclic polyether components-has been turned simultaneously outside-in and inside-out, a fact that has been proved beyond any doubt by the stoppering of both ends of a [2]pseudorotaxane to give a stable [2]rotaxane. The [2]pseudorotaxane is formed in nitromethane when a benzylic dibromide, obtained after reacting an excess of 1,4-bis(bromomethyl)benzene with hexaethylene glycol, is added to an equimolar amount of a dicationic cyclophane in which two -CH2OCH2- chains link a pair of dibenzylammonium ions through the para positions on their phenyl rings. When the [2]pseudorotaxane is reacted in nitromethane with triphenylphosphine, a [2]rotaxane and the corresponding free dumbbell compound are isolated in 58 and 31% yields, respectively. The structure of the [2]rotaxane is established by using mass spectrometry (FABMS and ESMS) and NMR (1H and 13C) spectroscopy in nitromethane-d3. The [2]rotaxane exhibits quite dramatic changes in the 1H chemical shifts of the signals for its CH2N+ and CH2O protons compared with those in the free dumbbell compound. The 1H NMR spectrum of the [2]pseudorotaxane shows many similar features. Titration experiments with three of the six different CH2O probes give an average Ka value of 2900 +/- 750 M-1 in nitromethane-d3. The new recognition motif for the template-directed synthesis of rotaxanes can now be exploited at both the molecular and macromolecular levels of structure with numerous potential applications in sight.

Speed-Controlled Molecular Shuttles

Abstract: Five potential molecular shuttles based on degenerate [2]rotaxanes - wherein the ring component is a bis-1,4-phenylene[34]crown-10 and the dumbbell components are terminated by tetraarylmethane (slippage) stoppers and contain two identical bipyridinium recognition sites and a central 1,3-phenylene unit carrying a CH 2 OR (R = Me, Et, Ph, i-Pr, and t -Bu) substituent on its C-5 position - have been prepared by acylating the parent hydroxymethyl derivative with the appropriate reagents. Dynamic 1 H NMR spectroscopy, carried out on these five potential molecular shuttles in CD 3 CN solutions, has revealed average Gibbs energy barriers to shuttling of 11.5, 12.0, 14.6, and 14.8 kcal mol -1 for the [2]rotaxanes where R = Me, Et, i-Pr, and Ph, respectively. For the [2]rotaxane where R = t -Bu, the Gibbs energy barrier to shuttling must exceed 17.2 kcal mol -1 .

Glycodendrimers based on cellobiosyl-derived monomers

Abstract: Reductive amination of suitably functionalized trisaccharide monomers, based on cellobiosylgalacto residues, has made it possible to construct high-molecular-weight glycodendrons and glycodendrimers for the display of large bioactive oligosaccharides and proteins in a well-defined manner. Key words: cellobiose, glycodendrimers, reductive amination, trisaccharide monomers.

Molecular and supramolecular nanomachines

Abstract: Publisher Summary This chapter reviews molecular and supramolecular nanomachines A molecule or a supermolecule with the capacity to store information might be expected to behave like a molecular/supramolecular switch Such a switch can be envisaged as a molecule/supermolecule that can exist in two different states that can be interconverted reversibly by some external stimuli It has been demonstrated that the synthetic chemist possesses in his/her toolbox numerous simple building blocks that exhibit bistability and thus have molecular switching properties when brought under the influence of an external source These molecular switches are becoming an attractive alternative for the fabrication of devices capable of writing, reading, and storing information at the nanoscale levelwthe so-called bottom-up approach In addition, with the development of supramolecular science and self-assembling synthetic methodologies, relatively large, well-defined, and highly ordered superstructures can be constructed The order of these superstructures is controlled by weak and reversible noncovalent bonding interactions with small changes in the environment or in the stereoelectronic complementarity of the building blocks, producing dramatic changes in the shapes of the superstructures, sometimes destroying them The versatility of the approach is appreciated when it is realized that the molecular motions present in the supramolecules and molecules can be addressed by photochemical, electrochemical, and/or chemical means