Macrocyclic Ethers Research Articles

Metal complexes of edta-derived macrocyclic ether bis(lactones). Hydrolysis of the macrocycles and the metal catalysis effect

Four edta-derived macrocycles containing an ether moiety of increasing length, (CH 2CH 2O) n CH 2CH 2 with n=0–3, H 2L 1H 2L 4 were prepared. The protonation constants of the compounds and their stability constants with the alkaline-earth metal ions, and Cu 2+, Zn 2+, Cd 2+ and Pb 2+ were determined by potentiometric techniques at 298 K and ionic strength 0.10 mol dm −3 in tetramethyl ammonium nitrate. The overall basicity of those compounds is very low when compared with ethylenediaminotetracetic acid, H 4edta or H 4L 7, as expected, due to the electron-withdrawing ability of the ester functions incorporated on the macrocyclic backbone in close vicinity of the nitrogen sites. The 1H NMR spectra, obtained at three distinguishable pD values of H 2L 2 in D 2O, have suggested that the first protonation occurs at the nitrogen atoms, and the second one in a carboxylate group. The values of stability constants are the expected ones taking into account the basicity of the ligands. Those compounds are not stable in water at pH values higher than 7 (even at low pH on prolonged storage in the case of H 2L 1) but form the open-chain monoester by hydrolysis of one ester group. The presence of metal ions enhances hydrolysis of all compounds, where the hydrolysis rate seem to correlate roughly with the stability constants of the ML species and the size of the metal ion. 1H and 13C NMR spectroscopy of the Zn 2+ and Pb 2+ complexes of H 2L 1H 2L 3 showed that complexation occurs mainly in the edta-derived part of the molecule, but also proved that for the complex [Pb(L 2)] the ether oxygen of H 2L 2 is directly co-ordinated to the metal. EPR spectroscopy of the copper(II) complexes showed two types of signals corresponding to Δ M s=1 and Δ M s=2 transitions, which reveal the presence of dinuclear complexes.

Atropselective macrocyclization of diaryl ether ring systems: application to the synthesis of vancomycin model systems.

Vancomycin is the last line of defense available in the clinic for treating multidrug-resistant bacterial infections. Vancomycin contains two 16-membered diaryl ether macrocycles, each of which contains a stereogenic axis across the diaryl ether linkage. Since an effective total synthesis of vancomycin requires that these stereogenic axes be formed in a stereoselective manner, we have developed an atropselective variation of the triazene mediated diaryl ether forming reaction. This variation introduced an energetic penalty into the transition state of the undesired atropisomer. This reaction is used to synthesize the C-O-D diaryl ether macrocycle found in vancomycin with high diastereoselectivity (de > 90%), providing the naturally occurring atropisomeric configuration.

Metal complexation of thiacrown ether macrocycles by electrospray ionization mass spectrometry.

In the present study, electrospray ionization mass spectrometry is used to evaluate the metal-binding selectivities of an array of novel caged macrocycles for mercury(II), lead(II), cadmium(II), and zinc(II) ions. In homogeneous methanol/chloroform solutions as well as extractions of metals from aqueous solution by macrocycles in chloroform, it is found that the type of heteroatom (S, O, N), cavity size, and presence of other substituents influence the metal selectivities. Several of the macrocycles in this study bind mercury ion very selectively and efficiently in the presence of many other metal ions and have an avidity toward mercury that was tunable by the size and combination of heteroatoms in the macrocycle ring and the number of cage groups attached. The extraction mechanism was further investigated by determining the variation in extraction selectivity as a function of the counterions of the mercury salts.

Early Transition Metal Complexes of Triphosphorus Macrocycles

The reactions of 1,5,9-triethyl-1,5,9-triphosphacyclododecane, [12]aneP3Et3, with first-row transition metal halides MCl3 [M = Ti (1), V (2), and Cr (3)] or their THF adducts are reported. The oxidation of 2 gives the complex ([12]aneP3Et3)V(O)Cl2 (4). This is the first example of a phosphane ligand that is trans to a vanadyl moiety. Also reported are VIII (5) and CrIII (6) complexes of the pendant ether macrocycles [12]aneP3(C2H4OEt)3 and 12[ane]P3(C3H6OMe)3 respectively. The complexes 1−6 have been characterised crystallographically and represent a rare class of phosphane complexes of these metals. The paramagnetic d1 compounds are characterised by EPR and ENDOR spectroscopy, simulation of which gives valuable structural information of these species in frozen solution. With [12]aneP3Et3, NbIII and NbIV chlorides give air-sensitive materials which could not be fully characterised although by comparison with compound 4, EPR and ENDOR spectroscopy indicate the principal oxidation product to be ([12]aneP3Et3)Nb(O)Cl2 (7). (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Protons as the triggers to regulate hydrogen-bonding receptors.

[reaction: see text] The protonation of alkylamines in two novel receptors results in intramolecular host-guest associations between the resulting ammonium ions and crown ether macrocycles. These interactions result in conformational changes of the receptors and prevent them from acting as hydrogen bond complements for uracil and carboxylate guest species.

Complexation of silver(I) with different substituted diaza-18-crown-6 ethers in methanol

A large number of derivatives of the diaza-18-crown-6 ligand have been synthesized and characterized. These derivatives have different alkyl or aryl side arms at the nitrogen donor atoms of the macrocyclic diazacrown ether. In some side chains additional donor atoms are available. These diazacrown ethers are used as ligands for the complexation of AgI in MeOH. The complexation reactions have been studied using potentiometric and calorimetric methods. The thermodynamic data show the absence of a lariat effect. The increase of the values of the reaction enthalpies are due only to changes in solvation of the different ligands.

The Synthesis of Dibenzo[3n]crown-n by Novel Methods and their Cation Binding Studied by Fluorescence Spectroscopy. Part IV

The dibenzo[3n]crown-n were synthesised from1,2-bis(o-hydroxyphenoxy)ethane obtained from 1,2-bis(o-formylphenoxy)ethane via Bayer-Willigeroxidations with H2O2/CH3COOH in good yields. The cyclic condensation of 1,2-bis(o-hydroxyphenoxy)ethanewith dichlorides, and ditosylates of polyethylene glycols in DMF/Me2CO3 gave the macrocyclesdibenzo[15]crown-5, dibenzo[18]crown-6, dibenzo[21]crown-7 anddibenzo[24]crown-8. The structures were identified using IR, mass, 1H and 13C NMR spectroscopy. Therecognition of the molecules for the cations, Li+, Na+, K+, Rb+ and Zn2+were conducted quantitatively with steady state fluorescencespectroscopy. The 1:1 association constants in acetonitrileshowed a good relation of the appropriate size of the macrocyclic ether towards the fitting cationradii. Namely, dibenzo[15]crown-5 was the best for Li+ binding and more than 100 times better thanNa+ and K+. Dibenzo[21]crown-7 was excellent for Rb+ binding while K+ is 100 timesless preferred. The largest crown ether studied, dibenzo[24]crown-8, exhibited the order of binding power,Rb+ > K+ > Na+. Zn2+ displayed, however, a marked binding with only dibenzo[18]crown-6.p>

Synthesis and ring-expanding oligomerization of an extremely strained macrocyclic aromatic ether-sulfone.

[reaction: see text] An extremely strained, 5-ring, macrocyclic aromatic ether-sulfone, [-1,4-C(6)H(4)SO(2)-1,4-C(6)H(4)O-1,3-C(6)H(4)O-1,4-C(6)H(4)SO(2)-1,4-C(6)H(4)-], synthesized by nickel-catalyzed intramolecular coupling of a bis-chlorophenylene-terminated precursor, undergoes fluoride-promoted ring expansion in dilute solution, giving a series of higher macrocyclic oligomers including cyclic dimer, trimer, and tetramer. The latter contains 20 aromatic residues in a 92-atom ring and is one of the largest synthetic organic macrocycles to be crystallographically characterized. The synthesis, structure, and ring expansion of a related macrocyclic ether-ketone are also reported.

The use of 18-crown-6 as an ionizable phase label for the expedited synthesis of small molecules

A silica-supported propylbenzenesulfonic acid ion-exchange column was treated with a methanol solution of potassium carbonate to give the corresponding potassium sulfonate salt. This column was shown to effectively sequester 18-crown-6 and its derivatives presumably due to chelation of the macrocyclic ether with the ionically bound potassium cation. This phenomenon was also observed to a lesser extent with ammonium ion but not with other cations such as lithium and cesium. This crown ether technique was then used to facilitate the purification of an amidation reaction.

Allosteric Binding of K+ to Crown Ether Macrocycles Appended to a Lanthanum Double Decker System

Abstract A benzocrown-appended lanthanum(III) porphyrin double decker has been synthesized which is capable of binding simple K+ in a cooperative, allosteric fashion to form a 1 : 4 complex. Analysis of this system by means of the Hill equation gave an overall binding constant (Kb) of 1.0 × 1014 (mol dm−3)−4 and a Hill coefficient of 4.0. This same system, however, binds Na+ in a conventional, linear fashion. These results indicate that the novel allosteric binding occurs as the result of 1 : 2 sandwich complexes between K+ and appended crown ether moieties of the two joined porphyrins. The suitability of lanthanum(III) over cerium(IV) for preparing this highly crowded double decker porphyrin is also noted.

EFFECTIVENESS OF SOME CROWN COMPOUNDS ON INHIBITION OF POLYPHENOLOXIDASE IN MODEL SYSTEMS AND IN APPLE

Enzymatic browning is (in most cases) an undesirable reaction which usually impairs the sensory properties and chemical changes in raw fruits and vegetables after mechanical operations (such as peeling, coring or slicing). A great emphasis is put on research to develop new methods to prevent enzymatic browning especially in fresh-cut (minimally processed) fruits and vegetables. The inhibition effect of crown compounds, macrocyclic ethers, benzo-18-crown-6 with sorbic acid and benzo-18-crown-6 with potassium sorbate, on polyphenoloxidase (PPO) activity was studied. The effectiveness of these compounds was evaluated by using 3,4-dihydroxy phenylalanine (L-DOPA), and chlorogenic acid (3-o-caffeoyl-D-quinic acid), the most widespread natural PPO substrates in fruits and vegetables, as well as browning inhibition substances on the cut surface of apples. Results showed that crown compounds used in this study were effective, both as inhibitors of the oxidation of phenolic compounds (PPO substrates) in model solutions and as inhibitors of enzyme discolorations of real systems (fresh-cut apples). In the earlier published papers (V UKOVI C 'et al., 1999) the synthesis of crown compound used in this study was presented.

Macrocyclic Polyether Phase Transfer Catalysts for Free Radical Polymerization of Acrolein

AbstractMacrocyclic polyethers, e.g., crown ethers and cryptands, were prepared and employed as phase transfer catalysts for free radical polymerization of acrolein, a vinyl monomer, with persulfates (S2O82–) as initiators. The catalytic abilities of various macrocyclic polyethers as catalysts for the free radical polymerization of acrolein were found to be in the order: benzo‐15‐crown‐5 > dibenzo‐18‐crown‐6 > 12‐crown‐4 > 15‐crown‐5 > 18‐crown‐6 > cryptand‐22 with sodium persulfate (Na2S2O8) as initiator. Sodium persulfate proved to be a better initiator than ammonium persulfate or potassium persulfate with benzo‐15‐crown‐5 as a catalyst. Effects of solvents and temperature on the catalytic polymerization were also investigated. The polymerization rates in various solvents were in the order: dioxane > benzene > acetonitrile > acetone > dichloromethane > hexane > water. Comparison between bulk polymerization and solution polymerization was also made. Higher polymerization rate was observed at higher temperature. The molecular weights of polyacrolein and the conversion of monomer in reaction period were determined with gel permeation chromatography and ultra‐violet spectrophotometry, respectively. Concentration effects of crown ether and initiator were also investigated and discussed.

Synthesis of Chiral Crownophanes Having Two Phenolic Groups via Tandem Claisen Rearrangement and Their Chiral Recognition

Asymmetric crownophanes having a chiral binaphthyl unit and two phenolic hydroxyl groups were thermally synthesized from the corresponding macrocyclic ethers via tandem Claisen rearrangement. Circular dichroism (CD) spectroscopic studies and HPLC experiments confirmed that little racemization of these crownophanes occurred during the thermal rearrangement. The association constants for the interaction of the chiral crownophanes with the enantiomers of phenylethylamine, phenylglycinol, and phenylalaninol were determined by a 1H NMR titration method in CD2Cl2. As a result, the 27 membered crownophane has some chiral recognition for phenylglycinol.

Serial synthesis of oxa[3.n]cyclophanes and homooxacalix[n]arenes via reductive coupling of arenedialdehydes, and their X-ray structures†

A wide range of oxa[3.n]cyclophanes and homooxacalix[n]arenes is prepared from the corresponding arenedialdehyde via reductive homocoupling reaction in a one-pot fashion. Heterocoupling reaction of arenedialdehyde with bis(trimethylsilyloxymethyl)benzene provides a series of macrocyclic ethers including a new type of oxacalixarene, which consists of the moieties of oxa[3.n]cyclophane and homooxacalix[n]arene; m- or p-phenylene, and 5-substituted 2-methoxy-m-phenylene are tethered by CH2OCH2 linkages. A series of macrocycles are separated with gel permeation chromatography (GPC) and identified with NMR (1H, 13C) and mass (MALDI-TOF) spectra. Their solid-state conformations are elucidated by X-ray crystallographic analyses.

CHEMOSELECTIVE OXIDATION OF 1,4-DIHYDROPYRIDINES WITH [NO+.CROWN.H(NO3)2 −

An ionic complex has been obtained from N2O4 in the presence of the macrocyclic ether 18-crown-6. This crystalline compound was used as an effective oxidizing agent for the oxidation of dihydropyridines to their corresponding pyridine derivatives at room temperature with excellent yields.

Cation Recognition with Fluorophore Crown Ethers

The fluoroionophore behavior of various macrocycles with oxygen and nitrogen donors was reviewed in the present paper, and the cationic recognition of alkali and alkaline-earth metals involving different photophysical effects depending on the fluoroionophore structures was summarized. Spectrofluorometry is a very sensitive technique measuring both emission and excitation intensities of a fluorescent molecule that is influenced by the environment. The cationic recognition of fluoroionophore crown ethers which possess at least two molecular components with sites that interact with photons as well as ions is studied. Cations mostly induce the changes in triplet energy relative to the excited singlet state, S1 → T1, and the ground state, T1 → S0. In the presence of metal cations, the increased phosphorescence lifetime of luminescent macrocycles, in general, gave complexation-enhanced quenching fluorescence spectra that reduce the fluorescence lifetime. However, if the phosphorescence lifetime is reduced, the fluorescence life is increased and the complexation-enhanced fluorescence spectra would be observed. Optical responses originating from the different photophysical mechanisms of photoinduced charge transfer, electronic energy transfer, monomer/excimer equilibrium, and internal charge transfer were represented. The recent studies on the cation recognition of fluorophore macrocyclic ethers are exemplified and discussed in the present paper.

Cation complexing of crown ethers using fluorescence spectroscopy, part II

The complex formations of benzo[12]crown-4, benzo[15]crown-5 and benzo[18]crown-6 with perchlorate salts of Mg 2+, Li + and Na + were investigated using the steady state fluorescence emission spectroscopy in acetonitrile. The complexation enhanced quenched fluorescence spectra, (CEQFS) exhibited the ion complexation role of the macrocyclic ethers and equilibrium constant, K e of 1:1 stoichiometry were estimated. The K e were found in the order of Mg +>Na +>Li + for benzo[15]crown-5 whilst Na +>Mg +>Li + order was found with benzo[12]crown-4 at 298 K.

Recent Trends in the Search for New Extractants

Since the introduction of solvent extraction as a separation method for metals, such solvating and acidic extractants have predominantly been used which are mono- or bifunctional compounds. They are able to form a five- or six-membered chelate ring, binding metal ions typically through one or two oxygen atoms, or through oxygen and nitrogen atoms. They can exhibit very useful selectivity, which is dependent on the localization of binding sites in various functional groups (acidic or neutral), chains or heterocyclic rings. However, a new approach in the search of new extractants is needed, if unusual selectivities are to be attained, e.g. within groups of chemically similar elements. Development of radius sensitive compounds represents one of the recent trends. The selectivity of macrocyclic ethers, mixed-heteroatom macrocyclic compounds and calixarenes is discussed in this review, with some emphasis laid on potential separation within groups of alkali and alkaline earth metals. Potential separation of lanthanides and actinides is also discussed, but less attention is paid to transition metals which can be separated by conventional extractants. Variables influencing the sensitivity are molecular configurations of the extractants nature of anions participating in the formation of extracted complexes, presence of mineral acids in the aqueous phase, nature of diluent and temperature. To separate elements with very similar ionic radii, extractants with less common, “soft” donor atoms can be applied. The separation of transplutonides (III) from lanthanides (III) is taken as an example, and the separation potential of acidic sulfur donor as well as neutral nitrogen donor extractants is discussed.

Influence of cation size on ionic glass formation and ionic conductivity

The salts Na[CF 3SO 2N(CH 2) 3OCH 3] and K[CF 3SO 2N(CH 2) 3OCH 3] form amorphous complexes with cryptand and crown ether macrocycles, which exhibit substantial ionic conductivity. The formation of an amorphous phase is thought to be favored by a mismatch between the cavity size of the macrocycle and the diameter of the cation. Complexes were examined by thermal, spectroscopic and conductivity methods to determine the role of the cation and the size mismatch with complexing macrocycles on ionic glass formation.

ChemInform Abstract: The Conformational Analysis of 14‐Membered Macrocyclic Ethers.

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