Showing papers by "J. Fraser Stoddart published in 1992"

Molecular LEGO. 1. Substrate-directed synthesis via stereoregular Diels-Alder oligomerizations

Abstract: The diastereoselective synthesis and structural characterization of a wide range of wholly-synthesis cyclic and acyclic polyacene derivatives has been achieved. These novel compounds are notable for (i) their rigidity, (ii) their highly-ordered structures, and (iii) the high stereochemical precision which governs their formation. A key feature in the success of the synthetic methodology described is the development of a repetitive Diels-Alder reaction sequence in which three distinct levels of diastereoselectivity are expressed during each cycloaddition involving bisdiene and bisdienophilic building blocks

Cyclobis(paraquat-p-phenylene) as a synthetic receptor for electron-rich aromatic compounds: electrochemical and spectroscopic studies of neurotransmitter binding

Abstract: The equilibrium binding constants of the charge-transfer complexes formed by the receptor cyclobis(paraquat-p-phenylene) with four neurotransmitters (dopamine, epinephrine, norepinephrine, and serotonin) and the related aromatic compounds indole and catechol were measured in aqueous media using a spectrophotometricmethod. The values obtained were in the range 1000-7500 M -1 . NMR studies confirmed that the surveyed π-donor guests are included inside the paraquat-lined cavity of the title receptor

A new design strategy for the self-assembly of molecular shuttles

Abstract: A strategy for the construction of a [2]rotaxane—comprised of a dumbbell-shaped component containing two π-electron-deficient 4,4′-bipyridinium units encircled by one π-electron-rich bisparaphenylene-34-crown-10 macrocycle—by a constitutionally determined self-assembly process is described.

The template-directed synthesis of porphyrin-stoppered[2]rotaxanes

Abstract: Two [2]rotaxanes, composed of (i) a polyether chain intercepted by (a) one centrally-located and (b) two symmetrically-located π-electron-rich hydroquinol rings and terminated by free-base and metallated (Zn) tetraaryl-porphyrin groups respectively and (ii) a tetracationic cyclophane constructed of two π-electron-deficient bipyridinium units linked by paraphenylenedimethyl residues, have been self-assembled by a clipping procedure.

The Mechanisms of Making Molecules to Order

Abstract: The mechanisms of formation of a [2]catenane and one of its molecular components have been investigated. While the synthesis of the tetracationic cyclophane, [BBIPYBIXYCY]4+ from bipyridine (BP) and 1,4-bis(bromomethyl)benzene (BBB), directed by the template, 1,5–bis[2(2–hydroxyethoxy)ethoxy]naphthalene(1/5BHEEN) becomes less efficient (23 to 5%) under ultra-high pressure reaction conditions (12 kbars), the self-assembly of {[2]-[BPP34C10]-[BBIPYBIXYCY] catenane}4+ from BP and BBB in the presence of BPP34C10 can be achieved with increased efficiency (18 to 42%) at 12 kbars. This difference in the trends of the yields can be ascribed to the enhanced templating action of BPP34C10 relative to that of 1/5BHEEN when two moles of BP and two moles of BBB are being employed to construct the tetracationic cyclophane. The self-assembly of the [2]catenane from BP, BBB, and BPP34C10 has been followed by 1H NMR spectroscopy in D7-DMF solution. On the basis of this spectroscopic evidence and supporting chemical data, the formation of {[2]-[BPP34C10]-[BBIPYBIXYCY] catenane}4+ from two moles of BP, two moles of BBB, and one mole of BPP34C10 is believed to proceed via the monoquatemary intermediate, [MBXYBIPY]+, which has not been isolated, and the dicationic species, [BBIPYXY]2+, which has been isolated and shown to be an intermediate in the self-assembly process leading to the [2]catenane, presumably via the [BXYBBIPYXY]3+ trication — the final intermediate which again has not been isolated.

Self‐assembling synthetic supramolecular polymers

Abstract: The solid state structure of a [2]catenane, composed of macrocyclic rings containing π-electron rich hydroquinol rings and π-electron deficient bipyridinium units has revealed an approach to the elaboration of supramolecular polymers with a double helical tertiary structure. Neutral molecules containing up to five hydroquinol rings, separated by trioxaundecane spacers and terminated by benzyl ether functions, have been shown by 1H NMR spectroscopy to form superstructures with possible double helical character as a result of π-π stacking and other non-covalent bonding interactions with polycations containing up to four bipyridinium units separated by p-xylyl spacers and terminated also by benzyl functions.

The fortuitous discovery of a synthetic cationic molecular receptor system for methanol

Abstract: A single crystal X-ray structural study of a diprotonated bis(1,7-diaza-12-crown-4), crystallised from a methanolic solution, reveals that a methanol molecule is bound to each protonated crown via both [O–H ⋯ N] and [N+-H ⋯ O] hydrogen bonds.

Conformational mobility in chemically-modified cyclodextrins

Abstract: The observation that per-2,6-0-methyl-3-0-benzoyl-α-cyclodextrin (1) displays some unusual conformational behaviour in solution has led to a detailed investigation by (dynamic) NMR spectroscopy of the equilibration process that occurs in solutions of per-2,3-0-benzoyl-α-cyclodextrin (3) and some related compounds (7—9) between conformational isomers with averaged C6 and C3 molecular symmetries in certain organic solvents such as benzene, dichloromethane, and chloroform The solvent dependence of the conformational equilibrium is also reflected in a spread of values for the specific optical rotations for 3 from +9° in 1,1,2,2-tetrachloroethane, where there is a degenerate equilibrium between species with C3 molecular symmetry, to + 92° in acetone where a species with averaged C6 symmetry is present