Nucleophilic addition to a p-benzyne derived from an enediyne: a new mechanism for halide incorporation into biomolecules

Biosynthesis of haloaromatics ordinarily occurs by electrophilic attack of an activated halogen species on an electron-rich aromatic ring. We now present the discovery of a new reaction whereby a nucleophilic halide anion can be attached even to an aromatic ring without activating substituents. We show that the enediyne cyclodeca-1,5-diyn-3-ene, in the presence of lithium halide and a weak acid, is converted to 1-halotetrahydronaphthalene. The kinetics are consistent with rate-limiting cyclization to a p-benzyne biradical that rapidly adds halide and is then protonated. This reaction has interesting mechanistic features and important implications for incorporation of halide into biomolecules.     

Pd-mBDPP-catalyzed regioselective internal arylation of electron-rich olefins by aryl halides

meso-2,4-Bis(diphenylphosphino)pentane (mBDPP) has proved to be an effective regiocontrolling ligand for palladium-catalyzed internal arylation by aryl bromides of electron-rich olefins in a common solvent DMSO with no need for any halide scavengers. The arylation of the benchmark electron-rich olefin butyl vinyl ether took place smoothly to afford exclusively alpha-arylated product with high isolated yields. The better performance of mBDPP, compared with that of the commonly used DPPP [1,3-bis(diphenylphosphino)propane], highlights the important but subtle effect of ligand on the regioselectivity of the Heck arylation reactions.     

HALOGENATION OF AROMATIC METHYL KETONES USING OXONE® AND SODIUM HALIDE

Aromatic methyl ketones undergo α-halogenation on treatment with sodium halide in the presence of oxone® in aqueous methanol. However, electron-rich hetero aromatic methyl ketones afford ring bromination products preferentially.     

Brønsted acid-promoted cyclizations of 1-siloxy-1,5-diynes

We have developed the first HNTf2-promoted 5-endo-dig cyclizations of 1-siloxy-1,5-diynes, which proceed with concomitant formation of C-Hal bonds as a result of halide abstraction from a halocarbon by the intermediate alkenyl cation. This process is enabled by a chemoselective activation of the more electron-rich siloxy alkyne moiety of the diyne cyclization precursor and represents an efficient and highly diastereoselective method for assembly of a range of beta-halo enones.     

New strapped porphyrins as hosts for fullerenes: synthesis and complexation study

New strapped porphyrin-based hosts with different π-conjugated moieties and linkers have been prepared and their ability to bind with fullerenes was studied in dilute solution. We found that the ability of these hosts to bind with fullerenes strongly depends on their chemical nature and more precisely on the substitution pattern of the porphyrin deck. As expected, the more electron-rich hosts containing either an exTTF or a porphyrin unit as the strap bind fullerenes more efficiently with association constants of up to 3.9 × 10(5) M(-1). The results clearly demonstrate the potential of such hosts as a supramolecular scaffold for surface immobilization of pristine fullerenes.     

Theoretical analysis of the structure and intramolecular dynamics of imidazolium halide and N-methylimidazolium halide ion pairs

A plant for the theoretical analysis of the structure and intramolecular dynamics of ion pairs of organic salts based on the combined use of the results of quantum-chemical calculations and molecular mechanics has been proposed. The potential-energy surfaces of the interionic interactions in imidazolium halide and N-methylimidazolium halide ion pairs have been investigated, and the equilibrium geometric parameters and frequencies of the interionic vibrations have been determined. It has been established that these ion pairs should be classified as stereochemically nonrigid molecules.Deceased.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 5, pp. 522–529, September–October, 1985.     

Hydrogen-bond-directed catalysis: faster, regioselective and cleaner Heck arylation of electron-rich olefins in alcohols

A general method for the regioselective Heck reaction of electron-rich olefins is presented. Fast, highly regioselective Pd-catalysed alpha-arylation of electron-rich olefins, vinyl ethers (1 a-d), hydroxyl vinyl ethers (1 e, f), enamides (1 g, h) and a substituted vinyl ether (1i) has been accomplished with a diverse range of aryl bromides (2 a-r), for the first time, in cheap, green and easily available alcohols with no need for any halide scavengers or salt additives. The reaction proceeds with high efficiency, leading exclusively to the alpha-products, in 2-propanol and particularly in ethylene glycol. In the latter, the arylation can be catalysed at a palladium loading of 0.1 mol% and finish in as short a time as 0.5 h. The remarkable performance of the alcohol solvents in promoting alpha regiocontrol is attributed to their hydrogen-bond-donating capability, which is believed to facilitate the dissociation of halide anions from PdII, and hence, the generation of a key ionic PdII-olefin intermediate responsible for the alpha product. This belief is further strengthened by the study of a benchmark arylation reaction in 21 different solvents. The study revealed that exclusive alpha-regioselective arylation takes place in almost all of the protic solvents, and there is a rough correlation between the alpha-arylation rates and the solvent parameter E(T)N. The method is simpler, cleaner and more general than those established thus far.     

Quantum chemical studies of the chemisorption of water and of unhydrated and hydrated halide ions on mercury

Local structures on electrode interfaces can be explored by quantum chemical investigation of medium-sized systems consisting of a cluster of substrate (metal) atoms, one or several solvent molecules, and/or at least one ion to be adsorbed at the interface. For the study of water adsorption and halide ion adsorption (unhydrated as well as hydrated) on a mercury surface, we have used the standard CNDO method together with geometrical optimization of the atom positions.In this paper, the following topics have been treated: (a) adsorption of a single water molecule in different positions on a close-packed plane cluster of seven mercury atoms; (b) adsorption of unhydrated halide ions (Cl^?, Br^?, I^?) in the “on-top” or hollow position on the mercury surface; (c) adsorption of monohydrated halides on the mercury surface. Further studies including solvation by six water molecules are discussed.The calculations provide information about minimum-energy geometries, energetic data, and local charges. Furthermore, they allow some conclusions about water mobility and reorientation on a close-packed metal surface, water orientation under the combined influence of an adsorbed ion and the metal surface, and trends of charge distribution in the halide series to be drawn. Calculations are critically discussed in the light of experimental and other quantum chemical data.     

Wurster''s crownophanes: an alternate topology for para-phenylenediamine-based macrocycles

Six redox-active cyclophane/crown hybrid molecules (crownophanes) were prepared via cyclization reactions involving N,N′-dimethyl-p-phenylenediamine and tosylated oligoethylene glycols of varying length. These new host molecules differ from other phenylenediamine-containing crown ethers in that the electron-rich π face is designed to be part of the ligating group. Their electrochemical properties were determined by cyclic voltammetry with a correlation found between macrocyclic architecture and ease of oxidation. The affinity of the smaller crownophanes for cations was studied by cyclic voltammetry with the result that these hosts show no electrochemical response to alkali metal cations, but, dependent on macrocycle size, modest selectivity for alkaline earth metal cations. This stands in contrast to previously reported phenylenediamine-containing crown ethers in which the redox centers are linked to guest ions through a macrocyclic amino group.     

Palladium-catalyzed regiocontrolled internal heteroarylation of electron-rich olefins with heteroaryl halides

A highly efficient palladium-catalyzed Heck coupling reaction of heteroaryl halides with electron-rich vinyl ether and hydroxyalkyl vinyl ethers is described. It was found that the choice of solvent, ligand, and reaction temperature had a fundamental influence on the regioselectivity and reactivity of the reaction, and the combination of Pd(OAc)₂ and DPPF in ethylene glycol led to the most effective catalytic system. Under these conditions, a variety of heteroaryl halides reacted very quickly with electron-rich olefins to afford exclusively the branched products in good to excellent yields without employing triflates, halide scavengers, or ionic liquids.     

A simple and facile iodination method of didechlorotiacumicin B and aromatic compounds

Tiacumicin B(also known as fidaxomicin or difimicin) is a marketed 18-membered macrolide antibiotic for the treatment of Clostridium difficile infections. Tiacumicin B is structurally characterized with two chlorine atoms substituted on the aromatic ring, which is installed by the halogenase Tia M. During the study on halide compatibility of Tia M, a facile and nonenzymatic iodination method of didechlorotiacumicin B was discovered, requiring Na I, acid and air for iodination at room temperature.This method was also amenable to other 9 aromatic substrates with electron-rich sites for iodination.     

Concurrent cyclopropanation by carbenes and carbanions

The generation of phenylhalocarbenes in the presence of varying quantities of halide ions and electron-poor alkenes affords concurrent cyclopropanation of the alkenes by an equilibrating mixture of phenylhalocarbenes and phenylhalomethide carbanions, which permits the smooth modulation of selectivity between electron-poor alkenes and electron-rich alkenes, a feature of potential synthetic utility.     

Palladium-catalyzed alpha-arylation of oxindoles

A catalyst generated from Pd(dba)2 and the bulky electron-rich phosphine ligand 2-(dicyclohexylphosphino)-2',4', 6'-tri-i-propyl-1-1'-biphenyl is effective for the alpha-arylation of oxindoles. Generation of the potassium-enolates of a range of oxindoles allows coupling with aryl chlorides, bromides, and triflates. Significant variation of the substitution pattern on both the oxindole and aryl halide is possible.     

Mass spectrometry of polymethine dyes

The mass spectra of some unsubstituted (in the heteroresidues and polymethine chain) thiacarbo- and polycarbocyanines were investigated. It was ascertained that the first act of disintegration of the dye molecules is splitting out of an alkyl halide ion to give an anhydro base ion. Some regularities between the structure of the dyes and the fragmentation of the anhydro ions obtained from them are exposed. The peculiarities of the dissociative ionization of the dye molecules as the polymethine chain is lengthened are shown.     

Highly regioselective internal heck arylation of hydroxyalkyl vinyl ethers by aryl halides in water

Highly regioselective and fast Pd(0)-catalyzed internal alpha-arylation of ethylene glycol vinyl ether with aryl halides was shown to be possible in water without the need for any halide scavengers or ionic liquid additives. This presents, to our knowledge, the first case of water being utilized in the selective arylation of electron-rich olefins. Resulting alpha-products were hydrolyzed and isolated as corresponding acetophenones in good to excellent yields when using aryl bromides and with good to moderate yields in the case of aryl iodides. Microwave irradiation was shown to be beneficial in activation of aryl chlorides toward the internal Heck arylation. The scope of the protocol was further increased to include different hydroxyalkyl vinyl ethers, these all giving selectively only branched alpha-products. Finally, the active role of the hydroxy group in directing the regioselectivity toward internal arylation of electron-rich olefins, even in nonpolar toluene, was revealed.     

A general method for copper-catalyzed arylation of arene C-H bonds

A general method for copper-catalyzed arylation of sp (2) C-H bonds with p K a's below 35 has been developed. The method employs aryl halide as the coupling partner, lithium alkoxide or K 3PO 4 base, and DMF, DMPU, or mixed DMF/xylenes solvent. A variety of electron-rich and electron-poor heterocycles such as azoles, caffeine, thiophenes, benzofuran, pyridine oxides, pyridazine, and pyrimidine can be arylated. Furthermore, electron-poor arenes possessing at least two electron-withdrawing groups on a benzene ring can also be arylated. Two arylcopper-phenanthroline complex intermediates were independently synthesized.     

Thermodynamic quantities of surface formation of aqueous electrolyte solutions. VI. Comparison with typical nonelectrolytes, sucrose and glucose

To demonstrate an important distinction between the electrolytes and nonelectrolytes, surface tension of aqueous solutions of typical nonelectrolytes, sucrose and glucose, was measured as a function of temperature and concentration. The presence of sucrose or glucose molecules in the surface region affects the surface tension in the same way as the presence of an ion does. There is, however, a difference in the temperature coefficient of the surface tension between typical nonelectrolyte solutions, sucrose and glucose, and alkali halide solutions. The entropy of surface formation of sucrose and glucose solutions is the same as that of pure water, while that of alkali halide solutions decreases with concentration. The relation between this entropy change and the formation of electric double layers was discussed.     

IR absorption spectra and association of acridinium salts with hydrogen halide molecules

It was observed that the reaction of acridine with hydrogen halides under heterogeneous conditions proceeds in two steps: An acridinium salt is formed in the first step, while a complex of the salt with hydrogen halide molecules is formed in the second step. The formation of a complex leads to substantial changes in the region of the stretching vibrations of NH⁺ bonds, and this makes it possible to propose the direct participation of these bonds in complexing. The band of the stretching vibrations of NH⁺ bonds of the acridinium ion has a complex structure that changes markedly as a function of the sort of halogen ion and replacement of hydrogen by deuterium and in the case of complexing with the hydrogen halide. It is proposed that the structure of this band is due to Fermi resonance.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 99–101, January, 1981.     

Selective Encapsulation of Chloride Ions within Novel Cage Host Complexes in the Presence of Equimolar Amounts of Chloride and Bromide Ions

Four macrotricyclic cage hosts which feature four positive binding sites oriented toward the center of the intramolecular cavity are presented as promising candidates for anion receptors and they have been expected to play a important role in the selective encapsulation of the halide ion Cl⁻ or Br⁻. The complementarity between a macrotricyclic quaternary ammonium ion and Cl⁻ was achieved by fine-tuning of the four ammonium nitrogen atoms and the endocyclic methylene groups. The cage hosts [R₄N₄(C₅H₁₀)₄(C₆H₁₂)₂]⁴⁺ (abbreviated as [556]) showed perfect encapsulation of all chloride ions in acetonitrile at 0<r=([Cl⁻]_o/[[556]]_o)≤1 within the sensitivity of the ¹H NMR spectra in combination with a rather slow chemical exchange of the Cl⁻ ion in an encapsulation/decapsulation equilibrium with [556]. Further, the selective encapsulation of all the chloride ions into [556] cage occurs unambiguously at r=1 in the presence of equimolar amounts of Br⁻. The structural complementarity of the newly designed [556] host prevails over the Hofmeister-series restraints determined by differences in Gibbs free energy of halide anion solvation.     

Luminescence and structural comparisons of strong-acid sensor molecules. 2

Anthraquinone-containing cyclic polyether hosts form 1:1 complexes with hydronium ion, producing large enhancements in luminescence via inversion of npi* and pipi* exited states. We have characterized the binding of hydronium ion within these host molecules and have synthesized a large variety of analogous hosts that contain different structural and electronic features that allow better understanding of what controls binding and luminescence capacity in this class of fluorescent sensor molecules. X-ray crystallography of an anthraquinone host that contains terminal amine functional groups rather than terminal ether groups is investigated, and complete proton transfer to carbonyl groups is observed in concentrated sulfuric acid media that also produces a previously unobserved luminescence.