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Toshikazu Takata - One of the best experts on this subject based on the ideXlab platform.
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induction of single handed helicity of polyacetylenes using mechanically chiral Rotaxanes as chiral sources
Angewandte Chemie, 2017Co-Authors: Fumitaka Ishiwari, Yasuhito Koyama, Kazuko Nakazono, Toshikazu TakataAbstract:Effective induction of preferred-handed helicity of polyacetylenes by the pendant mechanically chiral Rotaxanes is discussed. Polyacetylenes possessing optically active mechanically chiral Rotaxanes in the side chains were synthesized by the polymerization of the corresponding enantiopure [2]Rotaxane-type ethynyl monomers prepared via the threading and end-capping protocol followed by the chiral HPLC separation. The CD Cotton effects revealed that the polyacetylenes took preferred-handed helical conformations depending on the Rotaxane's chirality. The preferred-handed helix was not disturbed by the additional chiral substituent on the Rotaxane side chain. These results for the first time demonstrate the significance and utility of mechanically chiral Rotaxanes for the effective construction of asymmetric fields.
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diastereoselective synthesis of optically active Rotaxane amine n oxides via through space chirality transfer
Tetrahedron Letters, 2016Co-Authors: Kazuko Nakazono, Toshikazu TakataAbstract:Abstract Selective synthesis of optically active Rotaxane amine N -oxides was achieved with high diastereoselectivity via the effective through-space chirality transfer. Oxidation of tert -amine moiety on axle component of Rotaxane with an optically active wheel component having ( R )-binaphthyl group was carried out. The oxidation of several Rotaxanes with dimethyldioxirane was conducted in dichloromethane at −78 °C to give the corresponding amine N -oxides with high diastereoselectivity up to 95%, indicating the conversion via the effective through-space chirality transfer. Higher diastereoselectivity was observed with the Rotaxane possessing the rigid skeleton and the N -benzyl substituent. The optimized structures suggested the stereochemistry of the nitrogen center of ( R )-configuration.
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neutralization of a sec ammonium group unusually stabilized by the Rotaxane effect synthesis structure and dynamic nature of a free sec amine crown ether type Rotaxane
Chemistry: A European Journal, 2010Co-Authors: Kazuko Nakazono, Toshikazu TakataAbstract:A fifteen-year riddle has been settled: neutralization, the most popular chemical event, of a crown ether/sec-ammonium salt-type Rotaxane has been achieved and a completely nonionic crown ether/sec-amine-type Rotaxane isolated. A [2]Rotaxane was prepared as a typical substrate from a mixture of dibenzo[24]crown-8 ether (DB24C8) and sec-ammonium hexafluorophosphate (PF6) with a terminal hydroxy group through end-capping with 3,5-dimethylbenzoic anhydride in the presence of tributylphosphane as a catalyst in 90 % yield. A couple of approaches to the neutralization of the ammonium Rotaxane were investigated to isolate the free sec-amine-type Rotaxane by decreasing the degree of thermodynamic and kinetic stabilities. One approach was the counteranion-exchange method in which the soft counterion PF6− was replaced with the fluoride anion by mixing with tetrabutylammonium fluoride, thus decreasing the cationic character of the ammonium moiety. Subsequent simple washing with a base allowed us to isolate the free sec-amine-type Rotaxane in a quantitative yield. The other approach was a synthesis based on a protection/deprotection protocol. The acylation of the sec-ammonium moiety with 2,2,2-trichloroethyl chloroformate gave an N-carbamated Rotaxane that could be deprotected by treating with zinc in acetic acid to afford the corresponding free sec-amine-type Rotaxane in a quantitative yield. The structure of the free sec-amine-type Rotaxane was fully confirmed by spectral and analytical data. The generality of the counteranion-exchange method was also confirmed through the neutralization of a bisammonium-type [3]Rotaxane. The mechanism was studied from the proposed potential-energy diagram of the Rotaxanes with special emphasis on the role of the PF6− counterion.
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sequential o and n acylation protocol for high yield preparation and modification of Rotaxanes synthesis functionalization structure and intercomponent interaction of Rotaxanes
Journal of Organic Chemistry, 2006Co-Authors: Yuya Tachibana, Nobuhiro Kihara, Hiroaki Kawasaki, Toshikazu TakataAbstract:A pseudoRotaxane consisting of a 24-membered crown ether and secondary ammonium salt with the hydroxy group at the terminus was quantitatively acylated by bulky acid anhydride in the presence of tributylphosphane as catalyst to afford the corresponding Rotaxane in high yield. Large-scale synthesis without chromatographic separation was easily achieved. The ammonium group in the resulting Rotaxane was quantitatively acylated with excess electrophile in the presence of excess trialkylamine. Various N-functionalized Rotaxanes were prepared by this sequential double-acylation protocol. 1H NMR spectra and X-ray crystallographic analyses of the Rotaxanes showed that the crown ether component was captured on the ammonium group in ammonium-type Rotaxane by strong hydrogen-bonding intercomponent interaction. The conformation around the ammonium group was fixed by the hydrogen-bonding interaction. Meanwhile, the conformation of the amide-type Rotaxane was determined by the weak CH/π interaction between the methylen...
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synthesis and photoinduced electron transfer processes of Rotaxanes bearing 60 fullerene and zinc porphyrin effects of interlocked structure and length of axle with porphyrins
Journal of Physical Chemistry B, 2005Co-Authors: Atula S D Sandanayaka, Yoshio Furusho, Nobuhiro Kihara, Nobuhiro Watanabe, Keiichiro Ikeshita, Yasuyuki Araki, Osamu Ito, Toshikazu TakataAbstract:Three Rotaxanes, with axles with two zinc porphyrins (ZnPs) at both ends penetrating into a necklace pending a C60 moiety, were synthesized with varying interlocked structures and axle lengths. The intra-Rotaxane photoinduced electron transfer processes between the spatially positioned C60 and ZnP in Rotaxanes were investigated. Charge-separated (CS) states (ZnP•+, C60•-)Rotaxane are formed via the excited singlet state of ZnP (1ZnP*) to the C60 moiety in solvents such as benzonitrile, THF, and toluene. The rate constants and quantum yields of charge separation via 1ZnP* decrease with axle length, but they are insensitive to solvent polarity. When the axle becomes long, charge separation takes place via the excited triplet state of ZnP (3ZnP*). The lifetime of the CS state increases with axle length from 180 to 650 ns at room temperature. The small activation energies of charge recombination were evaluated by temperature dependence of electron-transfer rate constants, probably reflecting through-space ele...
Paul D. Beer - One of the best experts on this subject based on the ideXlab platform.
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enantioselective anion recognition by chiral halogen bonding 2 Rotaxanes
Journal of the American Chemical Society, 2017Co-Authors: Jason Y C Lim, Igor Marques, Vitor Felix, Paul D. BeerAbstract:The application of chiral interlocked host molecules for discrimination of guest enantiomers has been largely overlooked, which is surprising given their unique three-dimensional binding cavities capable of guest encapsulation. Herein, we combined the stringent linear geometric interaction constraints of halogen bonding (XB), the noncovalent interaction between an electrophilic halogen atom and a Lewis base, with highly preorganized and conformationally restricted chiral cavities of [2]Rotaxanes to achieve enantioselective anion recognition. Representing the first detailed investigation of the use of chiral XB Rotaxanes for this purpose, extensive 1H NMR binding studies and molecular dynamics (MD) simulation experiments revealed that the chiral Rotaxane cavity significantly enhances enantiodiscrimination compared to the non-interlocked free axle and macrocycle components. Furthermore, by examining the enantioselectivities of a family of structurally similar XB [2]Rotaxanes containing different combination...
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Enantioselective Anion Recognition by Chiral Halogen-Bonding [2]Rotaxanes
2017Co-Authors: Jason Y. C. Lim, Igor Marques, Vítor Félix, Paul D. BeerAbstract:The application of chiral interlocked host molecules for discrimination of guest enantiomers has been largely overlooked, which is surprising given their unique three-dimensional binding cavities capable of guest encapsulation. Herein, we combined the stringent linear geometric interaction constraints of halogen bonding (XB), the noncovalent interaction between an electrophilic halogen atom and a Lewis base, with highly preorganized and conformationally restricted chiral cavities of [2]Rotaxanes to achieve enantioselective anion recognition. Representing the first detailed investigation of the use of chiral XB Rotaxanes for this purpose, extensive 1H NMR binding studies and molecular dynamics (MD) simulation experiments revealed that the chiral Rotaxane cavity significantly enhances enantiodiscrimination compared to the non-interlocked free axle and macrocycle components. Furthermore, by examining the enantioselectivities of a family of structurally similar XB [2]Rotaxanes containing different combinations of chiral and achiral macrocycle and axle components, the dominant influence of the chiral macrocycle in our Rotaxane design for determining the effectiveness of chiral discrimination is demonstrated. MD simulations reveal the crucial geometric roles played by the XB interactions in orientating the bound enantiomeric anion guests for chiral selectivity, as well as the critical importance of the anions’ hydration shells in governing binding affinity and enantiodiscrimination
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halotriazolium axle functionalised 2 Rotaxanes for anion recognition investigating the effects of halogen bond donor and preorganisation
Chemistry: A European Journal, 2014Co-Authors: James M Mercurio, Richard C Knighton, James Cookson, Paul D. BeerAbstract:The anion-templated synthesis of three novel halogen-bonding 5-halo-1,2,3-triazolium axle containing [2]Rotaxanes is described, and the effects of altering the nature of the halogen-bond donor atom together with the degree of inter-component preorganisation on the anion-recognition properties of the interlocked host investigated. The ability of the bromotriazolium motif to direct the halide-anion-templated assembly of interpenetrated [2]pseudoRotaxanes was studied initially; bromide was found to be the most effective template. As a consequence, bromide anion templation was used to synthesise the first bromotriazolium axle containing [2]Rotaxane, the anion-binding properties of which, determined by (1) H NMR spectroscopic titration experiments, revealed enhanced bromide and iodide recognition relative to a hydrogen-bonding protic triazolium Rotaxane analogue. Two halogen-bonding [2]Rotaxanes with bromo- and iodotriazolium motifs integrated into shortened axles designed to increase inter-component preorganisation were also synthesised. Anion (1) H NMR spectroscopic titration experiments demonstrated that these Rotaxanes were able to bind halide anions even more strongly, with the iodotriazolium axle integrated Rotaxane capable of recognising halides in aqueous solvent media. Importantly, these observations suggest that a halogen-bonding interlocked host binding domain, in combination with increased inter-component preorganisation, are requisite design features for a potent anion receptor.
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Rotaxane and catenane host structures for sensing charged guest species
Accounts of Chemical Research, 2014Co-Authors: Matthew J Langton, Paul D. BeerAbstract:ConspectusThe promise of mechanically interlocked architectures, such as Rotaxanes and catenanes, as prototypical molecular switches and shuttles for nanotechnological applications, has stimulated an ever increasing interest in their synthesis and function. The elaborate host cavities of interlocked structures, however, can also offer a novel approach toward molecular recognition: this Account describes the use of Rotaxane and catenane host systems for binding charged guest species, and for providing sensing capability through an integrated optical or electrochemical reporter group. Particular attention is drawn to the exploitation of the unusual dynamic properties of interlocked molecules, such as guest-induced shuttling or conformational switching, as a sophisticated means of achieving a selective and functional sensor response. We initially survey interlocked host systems capable of sensing cationic guests, before focusing on our accomplishments in synthesizing Rotaxanes and catenanes designed for the ...
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lanthanide appended Rotaxanes respond to changing chloride concentration
Chemical Science, 2013Co-Authors: Clemence Allain, Paul D. Beer, Stephen Faulkner, Michael W Jones, Alan M Kenwright, Nathan L Kilah, Richard C Knighton, Thomas Just Sorensen, Manuel TropianoAbstract:Lanthanide appended Rotaxanes have been prepared by the CuAAC ‘click’ reaction between an azide appended Rotaxane and lanthanide complexes of propargyl DO3A. The resulting complexes are luminescent, and exhibit chloride responsive luminescence behavior consistent with the existence of two independent halide binding pockets, one in the Rotaxane cavity and one on the ninth (axial) coordination site of the lanthanide. Strong halide binding to europium gives rise to changes in the relative intensity of the hypersensitive ΔJ = 2 transition compared to the rest of the europium emission spectrum, combined with quenching of the overall intensity of emission as a consequence of non-radiative quenching by the bound halide. The weaker interaction with the Rotaxane pocket mediates a subsequent recovery of intensity of the europium centered luminescence despite the considerable separation between the lanthanide and the Rotaxane binding pocket.
David A Leigh - One of the best experts on this subject based on the ideXlab platform.
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spontaneous assembly of Rotaxanes from a primary amine crown ether and electrophile
Journal of the American Chemical Society, 2018Co-Authors: Stephen D P Fielden, David A Leigh, Charlie T Mcternan, Borja Perezsaavedra, Inigo J VitoricayrezabalAbstract:We report the synthesis of crown ether-ammonium, amide and amine [2]Rotaxanes via transition state stabilization of axle-forming reactions. In contrast to the two-step “clipping” and “capping” strategies generally used for Rotaxane synthesis, here the components assemble into the interlocked molecule in a single, reagent-less, step under kinetic control. The crown ether accelerates the reaction of the axle-forming components through the cavity to give the threaded product in a form of metal-free active template synthesis. Rotaxane formation can proceed through the stabilization of different transition states featuring 5-coordinate (e.g., SN2) or 4-coordinate (e.g., acylation, Michael addition) carbon. Examples prepared using the approach include crown-ether-peptide Rotaxanes and switchable molecular shuttles.
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Spontaneous Assembly of Rotaxanes from a Primary Amine, Crown Ether and Electrophile
2018Co-Authors: Stephen D P Fielden, David A Leigh, Charlie T Mcternan, Borja Pérez-saavedra, Iñigo J. Vitorica-yrezabalAbstract:We report the synthesis of crown ether-ammonium, amide and amine [2]Rotaxanes via transition state stabilization of axle-forming reactions. In contrast to the two-step “clipping” and “capping” strategies generally used for Rotaxane synthesis, here the components assemble into the interlocked molecule in a single, reagent-less, step under kinetic control. The crown ether accelerates the reaction of the axle-forming components through the cavity to give the threaded product in a form of metal-free active template synthesis. Rotaxane formation can proceed through the stabilization of different transition states featuring 5-coordinate (e.g., SN2) or 4-coordinate (e.g., acylation, Michael addition) carbon. Examples prepared using the approach include crown-ether-peptide Rotaxanes and switchable molecular shuttles
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Goldberg Active Template Synthesis of a [2]Rotaxane Ligand for Asymmetric Transition-Metal Catalysis
2015Co-Authors: Steven Hoekman, David A Leigh, Matthew O. Kitching, Marcus Papmeyer, Diederik RokeAbstract:We report on the active template synthesis of a [2]Rotaxane through a Goldberg copper-catalyzed C–N bond forming reaction. A C2-symmetric cyclohexyldiamine macrocycle directs the assembly of the Rotaxane, which can subsequently serve as a ligand for enantioselective nickel-catalyzed conjugate addition reactions. Rotaxanes are a previously unexplored ligand architecture for asymmetric catalysis. We find that the Rotaxane gives improved enantioselectivity compared to a noninterlocked ligand, at the expense of longer reaction times
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nitrone 2 Rotaxanes simultaneous chemical protection and electrochemical activation of a functional group
Journal of the American Chemical Society, 2010Co-Authors: Daniel M Dsouza, David A Leigh, Simon J Teat, Loic Mottier, Francesco Paolucci, Songwei ZhangAbstract:We report on the use of the hydrogen-bond-accepting properties of neutral nitrone moieties to prepare benzylic amide macrocycle-containing [2]Rotaxanes in yields as high as 70%. X-ray crystallography showed the presence of up to four intercomponent hydrogen bonds between the amide groups of the macrocycle and the two nitrone groups of the thread. Dynamic 1H NMR studies of the rates of macrocycle pirouetting in nonpolar solutions indicated that the amide−nitrone hydrogen bonds are particularly strong (∼1.3 and ∼0.2 kcal mol−1 stronger than similar amide−ester and amide−amide interactions, respectively). In addition to polarizing the N−O bond through hydrogen bonding, the Rotaxane structure affects the chemistry of the nitrone groups in two significant ways: first, the intercomponent hydrogen bonding activates the nitrone groups to electrochemical reduction, a one-electron-reduction of the Rotaxane being stabilized by a remarkable 400 mV (8.1 kcal mol−1) with respect to the same process in the thread; secon...
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catalytic active metal template synthesis of 2 Rotaxanes 3 Rotaxanes and molecular shuttles and some observations on the mechanism of the cu i catalyzed azide alkyne 1 3 cycloaddition
Journal of the American Chemical Society, 2007Co-Authors: Vincent Aucagne, Stephen M. Goldup, David A Leigh, Jose Berna, James D Crowley, Kevin D Hanni, Paul J Lusby, Vicki E Ronaldson, Alexandra M Z Slawin, And Aurelien ViterisiAbstract:A synthetic approach to Rotaxane architectures is described in which metal atoms catalyze covalent bond formation while simultaneously acting as the template for the assembly of the mechanically interlocked structure. This “active-metal” template strategy is exemplified using the Huisgen−Meldal−Fokin Cu(I)-catalyzed 1,3-cycloaddition of azides with terminal alkynes (the CuAAC “click” reaction). Coordination of Cu(I) to an endotopic pyridine-containing macrocycle allows the alkyne and azide to bind to metal atoms in such a way that the metal-mediated bond-forming reaction takes place through the cavity of the macrocycleor macrocyclesforming a Rotaxane. A variety of mono- and bidentate macrocyclic ligands are demonstrated to form [2]Rotaxanes in this way, and by adding pyridine, the metal can turn over during the reaction, giving a catalytic active-metal template assembly process. Both the stoichiometric and catalytic versions of the reaction were also used to synthesize more complex two-station molecular s...
Stephen M. Goldup - One of the best experts on this subject based on the ideXlab platform.
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synthesis of a mechanically planar chiral Rotaxane ligand for enantioselective catalysis
Chem, 2020Co-Authors: Andrew W Heard, Stephen M. GoldupAbstract:Rotaxanes are interlocked molecules in which a molecular ring is trapped on a dumbbell-shaped axle because of its inability to escape over the bulky end groups, resulting in a so-called mechanical bond. Interlocked molecules have mainly been studied as components of molecular machines, but the crowded, flexible environment created by threading one molecule through another has also been explored in catalysis and sensing. However, so far, the applications of one of the most intriguing properties of interlocked molecules, their ability to display stereogenic units that do not rely on the stereochemistry of their covalent subunits, termed "mechanical chirality," have yet to be properly explored, and prototypical demonstration of the applications of mechanically chiral Rotaxanes remain scarce. Here, we describe a mechanically planar chiral Rotaxane-based Au complex that mediates a cyclopropanation reaction with stereoselectivities that are comparable with the best conventional covalent catalyst reported for this reaction.
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Stereoselective Synthesis of Mechanically Planar Chiral Rotaxanes.
Angewandte Chemie, 2018Co-Authors: Michael A. Jinks, Alberto De Juan, Mathieu Denis, Catherine J. Fletcher, Marzia Galli, Ellen M. G. Jamieson, Florian Modicom, Zhihui Zhang, Stephen M. GoldupAbstract:: Chiral interlocked molecules in which the mechanical bond provides the sole stereogenic unit are typically produced with no control over the mechanical stereochemistry. Here we report a stereoselective approach to mechanically planar chiral Rotaxanes in up to 98:2 d.r. using a readily available α-amino acid-derived azide. Symmetrization of the covalent stereocenter yields a Rotaxane in which the mechanical bond provides the only stereogenic element.
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Rotaxane-Based Transition Metal Complexes: Effect of the Mechanical Bond on Structure and Electronic Properties
2018Co-Authors: Martina Cirulli, Stephen M. Goldup, Zhihui Zhang, Amanpreet Kaur, James E. M. Lewis, Jonathan A. Kitchen, Maxie M. RoesslerAbstract:Early work by Sauvage revealed that mechanical bonding alters the stability and redox properties of their original catenane metal complexes. However, despite the importance of controlling metal ion properties for a range of applications, these effects have received relatively little attention since. Here we present a series of tri-, tetra-, and pentadentate Rotaxane-based ligands and a detailed study of their metal binding behavior and, where possible, compare their redox and electronic properties with their noninterlocked counterparts. The Rotaxane ligands form complexes with most of the metal ions investigated, and X-ray diffraction revealed that in some cases the mechanical bond enforces unusual coordination numbers and distorted arrangements as a result of the exclusion of exogenous ligands driven by the sterically crowded binding sites. In contrast, only the noninterlocked equivalent of the pentadentate Rotaxane CuII complex could be formed selectively, and this exhibited compromised redox stability compared to its interlocked counterpart. Frozen-solution EPR data demonstrate the formation of an interesting biomimetic state for the tetradentate CuII Rotaxane, as well as the formation of stable NiI species and the unusual coexistence of high- and low-spin CoII in the pentadentate framework. Our results demonstrate that readily available mechanically chelating Rotaxanes give rise to complexes the noninterlocked equivalent of which are inaccessible, and that the mechanical bond augments the redox behavior of the bound metal ion in a manner analogous to the carefully tuned amino acid framework in metalloproteins
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Macrocycle size matters: "small" functionalized Rotaxanes in excellent yield using the CuAAC active template approach.
Angewandte Chemie (International ed. in English), 2011Co-Authors: Hicham Lahlali, Kajally Jobe, Michael Watkinson, Stephen M. GoldupAbstract:By shrinking the macrocycle in the CuAAC active template reaction not only is it demonstrated to be possible to use smaller macrocycles, but, surprisingly, that smaller macrocycles lead to higher yields of Rotaxane product. The synthesis of “small” functionalized [2]Rotaxanes showcases this as a method for the production of materials with potential applications in molecular electronics, drug delivery, sensing, and enantioselective catalysis.
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catalytic active metal template synthesis of 2 Rotaxanes 3 Rotaxanes and molecular shuttles and some observations on the mechanism of the cu i catalyzed azide alkyne 1 3 cycloaddition
Journal of the American Chemical Society, 2007Co-Authors: Vincent Aucagne, Stephen M. Goldup, David A Leigh, Jose Berna, James D Crowley, Kevin D Hanni, Paul J Lusby, Vicki E Ronaldson, Alexandra M Z Slawin, And Aurelien ViterisiAbstract:A synthetic approach to Rotaxane architectures is described in which metal atoms catalyze covalent bond formation while simultaneously acting as the template for the assembly of the mechanically interlocked structure. This “active-metal” template strategy is exemplified using the Huisgen−Meldal−Fokin Cu(I)-catalyzed 1,3-cycloaddition of azides with terminal alkynes (the CuAAC “click” reaction). Coordination of Cu(I) to an endotopic pyridine-containing macrocycle allows the alkyne and azide to bind to metal atoms in such a way that the metal-mediated bond-forming reaction takes place through the cavity of the macrocycleor macrocyclesforming a Rotaxane. A variety of mono- and bidentate macrocyclic ligands are demonstrated to form [2]Rotaxanes in this way, and by adding pyridine, the metal can turn over during the reaction, giving a catalytic active-metal template assembly process. Both the stoichiometric and catalytic versions of the reaction were also used to synthesize more complex two-station molecular s...
Nobuhiro Kihara - One of the best experts on this subject based on the ideXlab platform.
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sequential o and n acylation protocol for high yield preparation and modification of Rotaxanes synthesis functionalization structure and intercomponent interaction of Rotaxanes
Journal of Organic Chemistry, 2006Co-Authors: Yuya Tachibana, Nobuhiro Kihara, Hiroaki Kawasaki, Toshikazu TakataAbstract:A pseudoRotaxane consisting of a 24-membered crown ether and secondary ammonium salt with the hydroxy group at the terminus was quantitatively acylated by bulky acid anhydride in the presence of tributylphosphane as catalyst to afford the corresponding Rotaxane in high yield. Large-scale synthesis without chromatographic separation was easily achieved. The ammonium group in the resulting Rotaxane was quantitatively acylated with excess electrophile in the presence of excess trialkylamine. Various N-functionalized Rotaxanes were prepared by this sequential double-acylation protocol. 1H NMR spectra and X-ray crystallographic analyses of the Rotaxanes showed that the crown ether component was captured on the ammonium group in ammonium-type Rotaxane by strong hydrogen-bonding intercomponent interaction. The conformation around the ammonium group was fixed by the hydrogen-bonding interaction. Meanwhile, the conformation of the amide-type Rotaxane was determined by the weak CH/π interaction between the methylen...
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synthesis and photoinduced electron transfer processes of Rotaxanes bearing 60 fullerene and zinc porphyrin effects of interlocked structure and length of axle with porphyrins
Journal of Physical Chemistry B, 2005Co-Authors: Atula S D Sandanayaka, Yoshio Furusho, Nobuhiro Kihara, Nobuhiro Watanabe, Keiichiro Ikeshita, Yasuyuki Araki, Osamu Ito, Toshikazu TakataAbstract:Three Rotaxanes, with axles with two zinc porphyrins (ZnPs) at both ends penetrating into a necklace pending a C60 moiety, were synthesized with varying interlocked structures and axle lengths. The intra-Rotaxane photoinduced electron transfer processes between the spatially positioned C60 and ZnP in Rotaxanes were investigated. Charge-separated (CS) states (ZnP•+, C60•-)Rotaxane are formed via the excited singlet state of ZnP (1ZnP*) to the C60 moiety in solvents such as benzonitrile, THF, and toluene. The rate constants and quantum yields of charge separation via 1ZnP* decrease with axle length, but they are insensitive to solvent polarity. When the axle becomes long, charge separation takes place via the excited triplet state of ZnP (3ZnP*). The lifetime of the CS state increases with axle length from 180 to 650 ns at room temperature. The small activation energies of charge recombination were evaluated by temperature dependence of electron-transfer rate constants, probably reflecting through-space ele...
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Is the tert-butyl group bulky enough to end-cap a pseudoRotaxane with a 24-crown-8-ether wheel?
Organic Letters, 2004Co-Authors: Yuya Tachibana, Yoshio Furusho, Nobuhiro Kihara, Toshikazu TakataAbstract:Although Rotaxane chemists have long believed that the tert-butyl group is bulkier than the cavity of dibenzo-24-crown-8-ether (DB24C8), it is essentially smaller than the cavity of DB24C8. The tert-butyl (or 4-tert-butylphenyl) group can actually function as an end-cap of DB24C8-based Rotaxanes when the intercomponent interaction is effectively operative. When such attractive interaction is removed, deslippage occurs. [structure: see text]
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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
Journal of the American Chemical Society, 2004Co-Authors: Yuya Tachibana, Nobuhiro Kihara, Toshikazu TakataAbstract: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 controll...
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dynamic covalent approach to 2 and 3 Rotaxanes by utilizing a reversible thiol disulfide interchange reaction
Chemistry: A European Journal, 2003Co-Authors: Yoshio Furusho, Nobuhiro Kihara, Tomoya Oku, Toshihide Hasegawa, Akiyoshi Tsuboi, Toshikazu TakataAbstract:A dynamic covalent approach to disulfide-containing [2]- and [3]Rotaxanes is described. Symmetrical dumbbell-shaped compounds with two secondary ammonium centers and a central located disulfide bond were synthesized as components of Rotaxanes. The Rotaxanes were synthesized from the dumbbell-shaped compounds and dibenzo-[24]crown-8 (DB24C8) with catalysis by benzenethiol. The yields of isolated Rotaxanes reached about 90 % under optimized conditions. A kinetic study on the reaction forming [2]Rotaxane 2 a and [3]Rotaxane 3 a suggested a plausible reaction mechanism comprising several steps, including 1) initiation, 2) [2]Rotaxane formation, and 3) [3]Rotaxane formation. The whole reaction was found to be reversible in the presence of thiols, and thermodynamic control over product distribution was thus possible by varying the temperature, solvent, initial ratio of substrates, and concentration. The steric bulk of the end-capping groups had almost no influence on Rotaxane yields, but the structure of the thiol was crucial for reaction rates. Amines and phosphines were also effective as catalysts. The structural characterization of the Rotaxanes included an Xray crystallographic study on [3]Rotaxane 3 a.
