Synthesis of thiophene-capped [2]rotaxanes

Two types of thiophene-capped [2]rotaxanes, i.e., bithienyl (2T)- and bis(3,4-ethylenedioxythiophene)-yl (BEDOT)-capped [2]rotaxanes, were synthesized. The electron-deficient cyclophane of cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺) was used as a macrocycle. Association constants for inclusion complexation of 2T- and BEDOT-derivatives with CBPQT⁴⁺ were obtained by ¹H NMR titration. Due to the donor-acceptor charge transfer absorption band, 2T- and BEDOT-capped [2]rotaxanes have red and green colors, respectively. On the basis of electrochemical analysis, we confirmed that only BEDOT-capped [2]rotaxane is a promising candidate for [3]rotaxane synthesis through oxidation coupling of the thiophene unit.     

Synthesis of a Five‐Membered Molecular Necklace: A 2+2 Approach

Like with a string of pearls , four molecular “beads” are threaded on a molecular rectangle to form a molecular necklace. This rectangular species is synthesized from two L‐shaped, preorganized pseudorotaxanes with two molecular beads each (cucurbituril, schematically symbolized by the barrels), held together by Cu²⁺ ions [Eq. (1)].     

Orientational Isomers of Cyclodextrin Semirotaxanes and Rotaxanes With Organic and Transition Metal Complex Stoppers

The novel asymmetric dicationic ligands [Quin(CH₂)₁₀tbp]²⁺, [Quin(CH₂)₁₀bpy]²⁺, and [Lut(CH₂)₁₀bpy]²⁺ (Quin⁺ = quinuclidinium, Lut = 3,5-lutidinium, tbp⁺ = 4-tert-butylpyridinium, and bpy⁺ = 4,4′-bipyidinium) form [2]semirotaxanes with α- and β-cyclodextrins in aqueous solution, with the cyclodextrin passage possible only over the 4-tert-pyridinium or bipyridinium end groups to yield two orientational isomers. The kinetics of the formation and dissociation of the kinetically and thermodynamically preferred orientational isomers of the [Quin(CH₂)₁₀tbp]²⁺[2]semirotaxane with α-CD have been investigated by ¹H NMR spectroscopy. Complexation of the free nitrogen on the 4,4′-bipyridinium end groups of the [R(CH₂)₁₀bpy]²⁺ ligands by the aquapentacyanoferrate(II) ion results in the formation of the corresponding [2]rotaxanes.     

Synthesis and characterization of a persistent paramagnetic rotaxane based on α-cyclodextrin and α,ω-alkyl disulfides

A new synthetic strategy for the preparation of persistent paramagnetic cyclodextrin-based rotaxanes is described. The method consists in the formation of inclusion complexes between α-cyclodextrin (α-CD) and α,ω-dithiols containing an octamethylene chain covalently trapped by bulky stoppers composed of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) radical fragments. Interaction of α-CD (the bead) and 1,8-octanedithiol (the thread) occurs in aqueous alkaline media and encapsulation is obtained by nucleophilic substitution at both termini of the linear component with a bulky paramagnetic iodide [2,2,6,6-tetramethyl-4-(2-iodoacetamide)piperidine-N-oxyl]. Structure determination of the new [2]rotaxane by ¹H NMR is reported and the spectroscopic data are discussed.     

Self-Organization of a Heteroditopic Molecule to Linear Polymolecular Arrays in Solution

Like the proverbial monkey chain one heteroditopic self-complementary molecule, comprising a crown ether unit and a paraquat unit, catches a second such molecule in solution and thus by self-organization forms novel linear oligo- and polymolecular arrays (shown schematically; the crown ether unit is denoted by the ellipse, and the paraquat unit by the rectangle).     

Self-Assembly of Novel Polyrotaxanes: Main-Chain Pseudopolyrotaxanes with Poly(ester crown ether) Backbones

Not threading rings on chains, but chains through rings results in a new class of main-chain polyrotaxanes. The cations of bipyridinium salts thread through the crown ether units of a poly(ester crown ether) chain (shown below). The threading efficiency m/n of the polyrotaxane is controlled by stoichiometry and temperature.     

Reactivity of Glutaconamides Within [2]Rotaxanes: Mechanical Bond Controlled Chemoselective Synthesis of Highly Reactive α-Ketoamides and their Light-Triggered Cyclization

Glutaconamide-based [2]rotaxanes are efficiently oxidized to the respective interlocked α-ketoamides, whereas their non-interlocked threads afford hydroxycyclohexene tetraamides under similar reaction conditions. These results showcase the mechanically interlocking of highly reactive substrates as a powerful tool for controlling their chemical behavior. Inside the macrocycle and under irradiation with light, the α-ketoamide threads convert, in a divergent manner, into the corresponding interlocked hydroxy-β-lactams or oxazolidinones by two modes of Norrish/Yang type-II intramolecular cyclizations, processes that are efficiently chemocontrolled by the mechanical bond.     

Novel [2]rotaxanes based on the recognition of pillar[5]arenes to an alkane functionalized with triazole moieties

Host-guest complexation between an alkane functionalized with triazole moieties and three pillar[5]arenes was studied. Three pillar[5]arene-based [2]rotaxanes were constructed based on this new recognition motif. The sequence of the yields of these [2]rotaxanes was consistent with the order of association constants between the three corresponding pillar[5]arenes and the alkane. ¹H NMR, electrospray ionization mass spectrometry, and NOESY NMR were employed to characterize these [2]rotaxanes.