Active‐Metal Template Synthesis of a Halogen‐Bonding Rotaxane for Anion Recognition

The synthesis of an all‐halogen‐bonding rotaxane for anion recognition is achieved by using active‐metal templation. A flexible bis‐iodotriazole‐containing macrocycle is exploited for the metal‐directed rotaxane synthesis. Endotopic binding of a Cu^I template facilitates an active‐metal CuAAC iodotriazole axle formation reaction that captures the interlocked rotaxane product. Following copper‐template removal, exotopic coordination of a more sterically demanding rhenium(I) complex induces an inversion in the conformation of the macrocycle component, directing the iodotriazole halogen‐bond donors into the rotaxane’s interlocked binding cavity to facilitate anion recognition.     

N‐Heterocyclic Carbene Copper(I) Rotaxanes Mediate Sequential Click Ligations with All Reagents Premixed

We have prepared NHC‐Cu^I complexes with a rotaxane structure and used them as sterically sensitive catalysts for one‐pot sequential copper‐catalyzed azide/alkyne cycloadditions in solutions containing all of the coupling partners premixed in unprotected form. Most notably, a photolabile and sterically encumbered complex first catalyzed the coupling of a less bulky azide/alkyne pair; after removing the protective macrocyclic component from the rotaxane structure, through irradiation with light, the exposed dumbbell‐shaped NHC‐Cu^I complex catalyzed the second click reaction of a bulkier azide/alkyne pair. Using this approach, we obtained predominantly, from a single sealed pot, a bis‐triazole product (84 %) from a mixture of two sterically distinct azides and a diyne.     

An Interwoven Metal‐Organic Framework Combining Mechanically Interlocked Linkers and Interpenetrated Networks

New dibenzo[24]crown‐8 ether derivatives were prepared that contain appendages with thioether donors that can coordinate to a metal ion. These macrocycles were then combined with 1,2‐bis(pyridinium) ethane axles to create two types of [2]rotaxane ligands; those with the four thioether donors on the crown ether and those with six donor groups, four from the crown ether and two more attached to the stoppering groups of the dumbbell. The crown ethers and both types of [2]rotaxane ligands were allowed to react with Ag^I ions to form metal‐organic rotaxane framework (MORF) style coordination polymers. The interlocked hexadentate ligand forms the first example of a new type of lattice containing interwoven frameworks resulting from both interpenetration of frameworks due to the presence of an interlocked ligand and more classical interpenetration of independent frameworks.     

A Simple and Highly Effective Ligand System for the Copper(I)‐Mediated Assembly of Rotaxanes

A [2]rotaxane was produced through the assembly of a picolinaldehyde, an amine, and a bipyridine macrocycle around a Cu^I template by imine bond formation in close‐to‐quantitative yield. An analogous [3]rotaxane is obtained in excellent yield by replacing the amine with a diamine, thus showing the suitability of the system for the construction of higher order interlocked structures. The rotaxanes are formed within a few minutes simply through mixing the components in solution at room temperature and they can be isolated through removal of the solvent or precipitation.     

Fast Pirouetting Motion in a Pyridine Bisamine‐Containing Copper‐Complexed Rotaxane

The present work reports the introduction of pyridine bisamine terdentate ligands in the structure of a pirouetting copper rotaxane. Rotaxane 2 [PF₆] constitutes the first example of the incorporation of imine‐based dynamic covalent chemistry in the synthesis of switchable copper‐complexed interlocked systems. In this rotaxane, the substitution of the classical terpyridine terdentate unit by a pyridine bisamine moiety has led to a significant stabilization of the pentacoordinated site. That fact has been evidenced by EPR spectroscopy and cyclic voltammetry. Regarding the tetracoordinated site, the congestion around the coordination sphere has been reduced to accelerate the typically slow reorganization of the Cu^II. Ethynyl‐3,8‐substitution on the axis phenanthroline along with the 2,9‐diphenyl‐1,10‐phenanthroline (dpp) present in the macrocycle afforded a very stable coordination environment for Cu^I, which is at the same time labile upon oxidation. In summary, the incorporation of a pyridine bisamine unit as a terdentate ligand and the optimization of the bidentate ligand of the axle not only has led to a simplification of the synthetic procedures, but it has also given rise to a bistable systems with an enhanced energetic separation between states and an acceleration of the reorganization processes. Thus far, rotaxane 2 [PF₆] presents the fastest switching cycle reported to date in copper‐interlocked dynamic systems.     

Olefin Cyclopropanation by Radical Carbene Transfer Reactions Promoted by Cobalt(II)/Porphyrinates: Active‐Metal‐Template Synthesis of [2]Rotaxanes

A Co^II/porphyrinate‐based macrocycle in the presence of a 3,5‐diphenylpyridine axial ligand functions as an endotopic ligand to direct the assembly of [2]rotaxanes from diazo and styrene half‐threads, by radical‐carbene‐transfer reactions, in excellent 95 % yield. The method reported herein applies the active‐metal‐template strategy to include radical‐type activation of ligands by the metal‐template ion during the organometallic process which ultimately yields the mechanical bond. A careful quantitative analysis of the product distribution afforded from the rotaxane self‐assembly reaction shows that the Co^II/porphyrinate subunit is still active after formation of the mechanical bond and, upon coordination of an additional diazo half‐thread derivative, promotes a novel intercomponent C?H insertion reaction to yield a new rotaxane‐like species. This unexpected intercomponent C?H insertion illustrates the distinct reactivity brought to the Co^II/porphyrinate catalyst by the mechanical bond.     

Efficient Intramolecular Energy Transfer between Two Fluorophores in a Bis‐Branched [3]Rotaxane

A bis‐branched [3]rotaxane, with two [2]rotaxane arms separated by an oligo(para‐phenylenevinylene) (OPV) fluorophore, was designed and investigated. Each [2]rotaxane arm employed a difluoroboradiaza‐s‐indacene (BODIPY) dye‐functionalized dibenzo[24]crown‐8 macrocycle interlocked onto a dibenzylammonium in the rod part. The chemical structure of the [3]rotaxane was confirmed and characterized by ¹H and ¹³C NMR spectroscopy and high‐resolution ESI mass spectrometry. The photophysical properties of [3]rotaxane and its reference systems were investigated through UV/Vis absorption, fluorescence, and time‐resolved fluorescence spectroscopy. An efficient energy‐transfer process in [3]rotaxane occurred from the OPV donor to the BODIPY acceptor because of the large overlap between the absorption spectrum of the BODIPY moiety and the emission spectrum of the OPV fluorophore; this shows the important potential of this system for designing functional molecular systems.     

Acid‐Base Switchable [2]‐ and [3]Rotaxane Molecular Shuttles with Benzimidazolium and Bis(pyridinium) Recognition Sites

For the purpose of developing higher level mechanically interlocked molecules (MIMs), such as molecular switches and machines, a new rotaxane system was designed in which both the 1,2‐bis(pyridinium)ethane and benzimidazolium recognition templating motifs were combined. These two very different recognition sites were successfully incorporated into [2]rotaxane and [3]rotaxane molecular shuttles which were fully characterized by ¹H NMR, 2D EXSY, single‐crystal X‐ray diffraction and VT NMR analysis. By utilizing benzimidazolium as both a recognition site and stoppering group it was possible to create not only an acid/base switchable [2]rotaxane molecular shuttle (energy barrier 20.9 kcal?mol^?1) but also a [3]rotaxane molecular shuttle that displays unique dynamic behavior involving the simultaneous motion of two macrocyclic wheels on a single dumbbell. This study provides new insights into the design of switchable molecular shuttles. Due to the unique properties of benzimidazoles, such as fluorescence and metal coordination, this new type of molecular shuttle may find further applications in developing functional molecular machines and materials.     

Synthesis,Solid‐State NMR Characterization,and Application for Hydrogenation Reactions of a Novel Wilkinson’s‐Type Immobilized Catalyst

Silica nanoparticles (SiNPs) were chosen as a solid support material for the immobilization of a new Wilkinson’s‐type catalyst. In a first step, polymer molecules (poly(triphenylphosphine)ethylene (PTPPE); 4‐diphenylphosphine styrene as monomer) were grafted onto the silica nanoparticles by surface‐initiated photoinferter‐mediated polymerization (SI‐PIMP). The catalyst was then created by binding rhodium (Rh) to the polymer side chains, with RhCl₃ ? x H₂O as a precursor. The triphenylphosphine units and rhodium as Rh^I provide an environment to form Wilkinson’s catalyst‐like structures. Employing multinuclear (³¹P, ²⁹Si, and ¹³C) solid‐state NMR spectroscopy (SSNMR), the structure of the catalyst bound to the polymer and the intermediates of the grafting reaction have been characterized. Finally, first applications of this catalyst in hydrogenation reactions employing para‐enriched hydrogen gas (PHIP experiments) and an assessment of its leaching properties are presented.     

Iodide Recognition and Sensing in Water by a Halogen‐Bonding Ruthenium(II)‐Based Rotaxane

The synthesis and anion‐recognition properties of the first halogen‐bonding rotaxane host to sense anions in water is described. The rotaxane features a halogen‐bonding axle component, which is stoppered with water‐solubilizing permethylated β‐cyclodextrin motifs, and a luminescent tris(bipyridine)ruthenium(II)‐based macrocycle component. ¹H NMR anion‐binding titrations in D₂O reveal the halogen‐bonding rotaxane to bind iodide with high affinity and with selectively over the smaller halide anions and sulfate. The binding affinity trend was explained through molecular dynamics simulations and free‐energy calculations. Photo‐physical investigations demonstrate the ability of the interlocked halogen‐bonding host to sense iodide in water, through enhancement of the macrocycle component’s Ru^II metal–ligand charge transfer (MLCT) emission.     

A Dual‐Response [2]Rotaxane Based on a 1,2,3‐Triazole Ring as a Novel Recognition Station

Two novel multilevel switchable [2]rotaxanes containing an ammonium and a triazole station have been constructed by a Cu^I‐catalyzed azide–alkyne cycloaddition reaction. The macrocycle of [2]rotaxane containing a C6‐chain bridge between the two hydrogen bonding stations exhibits high selectivity for the ammonium cation in the protonated form. Interestingly, the macrocycle is able to interact with the two recognition stations when the bridge between them is shortened. Upon deprotonation of both [2]rotaxanes, the macrocycle moves towards the triazole recognition site due to the hydrogen‐bond interaction between the triazole nitrogen atoms and the amide groups in the macrocycle. Upon addition of chloride anion, the conformation of [2]rotaxane is changed because of the cooperative recognition of the chloride anion by a favorable hydrogen‐bond donor from both the macrocycle isophthalamide and thread triazole CH proton.     

Gold(I)‐Catalyzed Divergence in the Preparation of Bicyclic Enol Esters: From Exclusively [3C+2C]‐Cycloaddition Reactions to Exclusive Formation of Vinylcyclopropanes

With the use of benzonitrile‐stabilized Au^I catalyst [Au(IPr)(NCPh)]SbF₆ ( Ic ; IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene), a spectrum of reactivity is observed for propargyl ester 4 a with cyclic vinyl ethers, ranging from exclusively [3C+2C] cycloaddition reactions to exclusively cyclopropanation depending only on the structure of the substrate. Some initially formed cyclopropanation products rearrange into the corresponding formally [3C+2C] cycloaddition products after treatment with fresh Au^I complex at 80 °C. Vinylcyclopropanes formed from dihydrofuran and dihydropyran resisted such rearrangement, even in the presence of fresh Au^I catalyst at elevated temperature. This study addresses an important mechanistic question concerning whether the five‐membered‐ring products were produced by a direct [3C+2C] cycloaddition reaction or by a sequential cyclopropanation/ring‐expansion reaction. A dual pathway is proposed for the Au^I‐catalyzed reactions between propargyl esters and cyclic vinyl ethers. The different behavior among vinyl cyclic ethers is attributed to the difference in the polarization of the π bond. Highly polarized bonds appear to undergo the cycloaddition reaction whereas less polar π‐bonds produce cyclopropanes.     

Macrocyclic Dinuclear Palladium Complex as a Novel Doubly Threaded [3]Rotaxane Scaffold and Its Application as a Rotaxane Cross‐Linker

A dinuclear Pd^II complex possessing a cyclic ligand was developed as a novel doubly threaded [3]rotaxane scaffold and applied as a rotaxane cross‐linker reagent. The dinuclear complex (PdMC)₂ was prepared by one‐step macrocyclization followed by the double palladation reaction. ¹H NMR analysis and UV/Vis measurements revealed the formation of a doubly threaded pseudo[3]rotaxane by the complexation of (PdMC)₂ with 2 equivalents of 2,6‐disubstituted pyridine 3 through double metal coordination. The treatment of (PdMC)₂ with 2 equivalents of 4‐vinylpyridine (VP) afforded a doubly threaded [3]rotaxane cross‐linker (PdMC‐VP)₂ . Radical co‐polymerization of VP and t‐butylstyrene in the presence of (PdMC‐VP)₂ afforded a stable rotaxane cross‐linked polymer (RCP). An elastic RCP was also prepared by using n‐butyl acrylate as a monomer. The obtained RCPs exhibited higher swelling ability and higher mechanical toughness compared with the corresponding covalent cross‐linked polymers.     

Energy Transfer and Concentration‐Dependent Conformational Modulation: A Porphyrin‐Containing [3]Rotaxane

A zinc porphyrin‐containing [3]rotaxane A was synthesized through a copper(I)‐catalyzed azide–alkyne cycloaddition (CuAAC) reaction. Energy donors and acceptor porphyrin were introduced to dibenzo[24]crown‐8 (DB24C8) and dibenzyl ammonium (DBA) units of [3]rotaxane A to understand the intramolecular energy transfer process. Investigations of the photophysical properties of [3]rotaxane A demonstrated that the intramolecular efficient energy transfer readily occurred from the donors on the wheels to the porphyrin center on the axis. The fluorescence of energy donors in the region of 400 to 450 nm was efficiently absorbed by the porphyrin acceptor under irradiation at 345 nm, and finally a red light emission at about 600 nm was achieved. Further investigation indicated that the conformation of [3]rotaxane A was self‐modulated by changing its concentration in CH₂Cl₂. The triazole groups on the wheel coordinated or uncoordinated to Zn²⁺ through intramolecular self‐coordination with the change in the concentration of [3]rotaxane A in CH₂Cl₂. Therefore, this conformational change was reversible in a non‐coordinating solvent such as CH₂Cl₂ but inhibited in a coordinating solvent such as THF. Such interesting behaviors were rarely observed in porphyrin derivatives. This self‐modulation feature opens up the possibility of controlling molecular conformation by varying concentration.     

Enantioselective Rhodium‐Catalyzed Cycloisomerization of (E)‐1,6‐Enynes

An enantioselective rhodium(I)‐catalyzed cycloisomerization reaction of challenging (E)‐1,6‐enynes is reported. This novel process enables (E)‐1,6‐enynes with a wide range of functionalities, including nitrogen‐, oxygen‐, and carbon‐tethered (E)‐1,6‐enynes, to undergo cycloisomerization with excellent enantioselectivity, in a high‐yielding and operationally simple manner. Moreover, this Rh^I‐diphosphane catalytic system also exhibited superior reactivity and enantioselectivity for (Z)‐1,6‐enynes. A rationale for the striking reactivity difference between (E)‐ and (Z)‐1,6‐enynes using Rh^I‐BINAP and Rh^I‐TangPhos is outlined using DFT studies to provide the necessary insight for the design of new catalyst systems and the application to synthesis.     

Unprecedented Trinuclear AgI Complex with 2,4,6‐Tris(2‐pyrimidyl)‐1,3,5‐triazine as an Efficient Catalyst for the Aziridination of Olefins

An unprecedented trinuclear heteroleptic Ag^I complex was isolated using a stable multidentate 2,4,6‐tris(2‐pyrimidyl)‐1,3,5‐triazine (TPymT) ligand. The obtained compound is an efficient catalyst for the direct aziridination of terminal olefins.     

Highly Efficient CSeCF3 Coupling of Aryl Iodides Enabled by an Air‐Stable Dinuclear PdI Catalyst

Building on our recent disclosure of catalysis at dinuclear Pd^I sites, we herein report the application of this concept to the realization of the first catalytic method to convert aryl iodides into the corresponding ArSeCF₃ compounds. Highly efficient C? SeCF₃ coupling of a range of aryl iodides was achieved, enabled by an air‐, moisture‐, and thermally stable dinuclear Pd^I catalyst. The novel SeCF₃‐bridged dinuclear Pd^I complex 3 was isolated, studied for its catalytic competence and shown to be recoverable. Experimental and computational data are presented in support of dinuclear Pd^I catalysis.     

One‐pot synthesis of polyrotaxane by clipping and cyclopolymerization of α,ω‐diethynyl isophthalamide with pyridiniumdicarboxamide chloride

The one‐pot synthesis of a main chain‐type polyrotaxane composed of axle molecules threaded through the macrocyclic units on the polymer main chain was achieved via the combination of cyclopolymerization and clipping procedures. The cyclopolymerization of an α,ω‐diethynyl monomer bearing an isophthalamide moiety ( 1 ), which clips onto an axle component bearing a pyridiniumdicarboxamide moiety ( 2·Cl ) through a chloride anion was carried out in chloroform with the monomer concentration of 0.06 mol L^?1 at 40 °C using [Rh(nbd)Cl]₂/Et₃N as a catalyst to afford a gel‐free polymer. The resulting polymer was assigned to the main chain‐type polyrotaxane with a poly(phenylacetylene) backbone (poly‐ 3·Cl ) based on size exclusion chromatography and ¹H NMR measurements. The diffusion‐order two‐dimensional NMR and circular dichroism spectra provided definitive proof of the rotaxaned architecture in the polymer. The mole fraction of the rotaxane unit in the total cyclic repeating unit was determined to be 26.3%. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Copper‐Mediated Controlled/“Living” Radical Polymerization in Polar Solvents: Insights into Some Relevant Mechanistic Aspects

The field of transition‐metal‐mediated controlled/“living” radical polymerization (CLRP) has become the subject of intense discussion regarding the mechanism of this widely‐used and versatile process. Most mechanistic analyses (atom transfer radical polymerization (ATRP) vs. single‐electron transfer living radical polymerization (SET‐LRP)) have been based on model experiments, which cannot correctly mimic the true reaction conditions. We present, for the first time, a determination of the [Cu^IBr]/[L] (L=nitrogen‐based chelating ligand) ratio and the extent of Cu^IBr/L disproportionation during CLRP of methyl acrylate (MA) in dimethylsulfoxide (DMSO) with Cu⁰ wire as a transition‐metal catalyst source. The results suggest that Cu⁰ acts as a supplemental activator and reducing agent of Cu^IIBr₂/L to Cu^IBr/L. More importantly, the Cu^IBr/L species seem to be responsible for the activation of SET‐LRP.