Star-shaped supramolecular ionic liquid crystals based on pyridinium salts

Series of novel star-shaped liquid crystals containing [1,1?-biphenyl]-4,4?-diyl diisonicotinate mesogens with various counterions (Br^?, B-SO₃^?,C-SO₃^?,H₂PO₄^?, BF₄^?) have been synthesized and characterized, which display a nematic phase. The molecular structures of the liquid crystals, thereof consisting of rod-like mesogens linked together by different long alkyl spacers to a small disc-like core of central benzene-1,3,5-triyl triisonicotinate, have been fully characterized by ¹H NMR. Their thermal and supramolecular organization behaviours have been studied by combining polarized optical microscopy, differential scanning calorimetry and small-angle X-ray scattering. These investigations showed that the mesophase temperature range increased with increasing alkyl chain length and the clearing point decreased with increase of anion size. These star-shaped liquid crystals have a long and ordered molecular structure, the electron delocalization of the π-π stacking conjugation effect and the electrostatic attraction of ionic make them have good ferroelectric properties and ionic conductivity properties. The interest in preparing ionic liquid crystal with a nematic phase lies in the technological applications as it is well known that the nematic phase has the highest fluidity of all liquid crystalline phases and hence the possibility to align it by applying an external electric/magnetic field, commonly used in electro-optical devices.     

Matrix elements of one- and two-electron operators

A graphical representation of matrix elements of spin-free one- and two-electron operators is used for deriving a simple algorithm for the evaluation of their values. The method covers all the cases which may occur when wave functions are taken as mutually orthogonal antisymmetrized products of spinorbitals (which are assumed to form an orthonormal set) and are eigenfunctions of L ² and L _z operators. The resulting formulas are suitable as well for computer programming as for hand calculations.

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Zusammenfassung Mit Hilfe einer graphischen Darstellung wird ein einfacher Algorithmus zur Bestimmung der Werte der Matrix-Elemente von spinfreien Ein- und Zweielektronenoperatoren abgeleitet. Durch diese Methode werden alle Fälle erfaßt, die auftreten, wenn die Wellenfunktionen wechselweise orthogonale, antisymmetrisierte Produkte von Spinorbitalen (von denen angenommen wird, daß sie einen orthogonalen Satz bilden) darstellen und Eigenfunktion der L ² und L _z Operatoren sind. Die erhaltenen Formeln eignen sich für Rechnungen mit und ohne Verwendung eines Computers.

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Sponsored by the Mathematical Institute, Polish Academy of Sciences.     

Symmetry properties of one- and two-electron molecular integrals

The maximum numbers of distinct one- and two-electron integrals that arise in calculating the electronic energy of a molecule are discussed. It is shown that these may be calculated easily using the character table of the symmetry group of the set of basis functions used to express the wave function. Complications arising from complex group representations and from a conflict of symmetry between the basis set and the nuclear configuration are considered and illustrated by examples.     

The reaction of N-nitrosamines with pyridinium salts

N-Nitrosamines react readily with pyridinium salts containing an activated methylene group to give hydrazones. This reaction may serve as the basis for the routine determination of N-nitrosamines.     

Catalytic hydrogenation of pyridinium salts

Catalytic hydrogenation of polysubstituted pyridinium salts leads to piperidines and their condensed analogs The spatial properties and conformational properties of the saturated azaheterocycles have been determined by¹³C NMR spectorscopy. It was shown that hydrogenation of the pyridinium salts occurs stereoselectively to form cis isomers in most cases.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 68–72, January, 1994.     

Unexpected reactivity of pyridinium salts toward alkynyl Fischer complexes to produce oxo‐heterocycles

The unprecedented reaction of ketone‐containing aromatic pyridinium salts 3a ‐ e and alkynyl Fischer complexes 1a ‐ f proceeds via a mild domino process to provide 4,6‐disubstituted pyran‐2‐ones 5a ‐ k and 2,3,5‐trisubstituted furans 6a ‐ h (45‐97%). According of the results of isotopic labeling experiments, a mechanism involving an initial Michael addition appears to be the key step, obtaining a mesomeric structure responsible for the formation of both products.     

Recent developments in the theory of one- and two-electron molecules

Recent theoretical work dealing with one- and two-electron molecules is reviewed. Results obtained using both the variational method and the perturbation theory are discussed for the ground states as well as for excited states of the molecules. Emphasis is placed on those results which are sufficiently accurate to yield definite conclusions with regard to either the system or the method.     

Microwave-promoted synthesis of chiral pyridinium salts

The synthesis of several chiral pyridinium salts via Zincke’s reaction can be easily accomplished by domestic microwave oven irradiation. Yield enhancements, reduction of reaction time, and less racemization were observed under microwave heating when compared to conventional heating in similar conditions.     

Synthesis of indoles from pyridinium salts

The detailed analysis of the products of the interaction of nitropyridinium salts with ketones and alkylamines resulted in new data testifying in favor of the previously proposed scheme for the formation of indoles from pyridinium salts.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 488–496, April, 1987.     

Imination of pyridinium and quinolinium salts

A new method for imination of N-alkyl pyridinium- and quinolinium salts is described. It involves a low temperature oxidation of a solution of appropriate substrates in liquid ammonia with potassium permanganate.     

Polarographic reduction of benzazolylsubstituted pyridinium salts

N-Methyl- and N-benzyl-substituted 2-, 3-, and 4-benzazolypyridinium (BAP) salts are reduced on a dropping mercury electrode (DME) to give a free radical in the first step and the corresponding dihydropyridine derivative in the second step. The free radicals of 2- and 4-BAP salts are distinguished by high stability as compared with other radicals with a pyridine structure owing to delocalization of the unpaired electron over the conjugated system and can be recorded by ESR spectroscopy. A peculiarity of the BAP salts is their facile reduction on the electrode; this is particularly true of the 2- and 4-benzoxazolyl- and benzothiazolyl-substituted salts.     

Charge-transfer interaction of some pyridinium salts with hen egg-white lysozyme

The charge-transfer interaction of pyridinium salts with lysozyme and L-tryptophan has been investigated by visible absorption, circular dichroism and fluorescence spectroscopy. Four quaternized and substituted pyridinium salts were used as electron acceptors. L-tryptophan was used as a model compound of electron donor for the purpose of the comparison with lysozyme. The binding constants obtained from absorption and circular dichroism spectra were 2.0–13 M^–1 and the sequence of the magnitude was found as EPCl(1-ethylpyridinium chloride) < MPCl(1-methylpyridinium chloride) < MNCl(1-methylnicotinamide chloride) < MNMCl(1-methyl-3-carbomethoxypyridinium chloride), which could be explained by the electron affinity of the pyridinium salt. The fluorescence spectra of lysozyme and L-tryptophan in the absence and presence of pyridinium salts were interpreted by Stern-Volmer plot. The static constants thus determined were in agreement with the binding constants obtained by other methods.     

Reactions of hydrated electrons with pyridinium salts in aqueous solutions

Rate coefficients for the reactions of the hydrated electron (e(aq)(-)) with pyridinium salts in aqueous solutions have been determined using pulse radiolysis techniques. The rate coefficients for pyridine, 1-hydropyridinium chloride, and 1-hydropyridinium nitrate were observed to be 1.4 x 10(10), 4.5 x 10(10), and 5.3 x 10(10) M(-1) s(-1), respectively. The e(aq)(-) was found to primarily attack the pyridine ring, the proton coordinated to the nitrogen atom, and the nitrate counterion, but not the chloride. Results for the corresponding dimer structures of 4,4'-dipyridyl, 1,1'-dihydro-4,4'-bipyridinium dichloride, and 1,1'-dihydro-4,4'-bipyridinium dinitrate had similar trends for e(aq)(-) attack sites. The rate coefficients for pyridinium salts were lower when the pyridinium nitrogen atom is coordinated to a methyl group rather than to a proton. This reduction is probably due to the increase in electron density of the pyridine ring due to the electron-donating methyl group. Pyridinium salts are not major contributors to the production of molecular hydrogen in the radiolysis of aqueous solutions and actually decrease molecular hydrogen yield due to scavenging reactions of the e(aq)(-). The yield of molecular hydrogen decreases from 0.45 to approximately 0.2 molecule/(100 eV) over the scavenging capacity range for the e(aq)(-) of 10(5)-10(9) s(-1). Absorption spectra of the transient species produced by the reactions of pyridinium salts with OH radical and H atom formed in water radiolysis were observed, and rate coefficients for these reactions were determined.     

Synthesis of indoles from pyridinium salts.

The reaction of 1,2,5,6-tetramethyl-3-nitropyridinium perchlorate with acetone and piperidine on a solid carrier has given the novel, relatively stable 3-nitropyridine anhydro-base, 1,2,5-trimethyl-3-nitro-6-methylene-1,6-dihydropyridine, which bears alkyl groups only. This anhydro-base has been shown not to be an intermediate in the synthesis of indoles from 3-nitropyridinium salts.See [1] for Communication 7.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1214–1216, September, 1990.     

Reactions of isoquinoline derivatives with pyridinium salts yielding 4-naphthylpyridines

Regiospecific introduction of the 2-naphthyl residue into position 4 of the pyridine ring occurs in the reactions of isoquinolinium salts with 4-methylpyridinium salts through the intermolecular transformation of the isoquinoline bicyclic system involving the methyl group of the pyridinium salt. The reaction occurs under the action of methylammonium sulfite in an aqueous medium on heating. This method provides ring transformation not only for isoquinolinium salts but even for unsubstituted isoquinoline.     

N-Heterocyclic carbene-catalyzed deaminative cross-coupling of aldehydes with Katritzky pyridinium salts

By employing an N-heterocyclic carbene (NHC) catalyst, we developed a versatile catalytic system that enables deaminative cross-coupling reactions of aldehydes with redox-active pyridinium salts. Katritzky pyridinium salts behave as single-electron oxidants capable of generating alkyl radicals enabled by the redox properties of the enolate form of Breslow intermediates. The resultant alkyl radical undergoes efficient recombination with the NHC-bound aldehyde-derived carbonyl carbon radical for the formation of a C–C bond. The mild and transition metal-free reaction conditions tolerate a broad range of functional groups, and its utility has been further demonstrated by the modification of a series of peptide feedstocks and application to the three-component dicarbofunctionalization of olefins.

By employing an N-heterocyclic carbene (NHC) catalyst, we developed a versatile catalytic system that enables deaminative cross-coupling reactions of aldehydes with redox-active pyridinium salts.     

One- versus two-electron reaction pathways in the electrocatalytic reduction of benzyl bromide at silver cathodes

The electrocatalytic reduction of benzyl bromide at a silver cathode has been investigated in acetonitrile in the absence and presence of acids, using cyclic voltammetry (CV) and controlled-potential electrolysis (CPE). CV gives rise to two reduction waves, which represent the dissociative 1e⁻ reduction of PhCH₂Br to and Br⁻ followed by a further reduction of the benzyl radical to at more negative potentials. The charge stoichiometry (1e⁻ vs 2e⁻/molecule) and product distribution depend on the applied potential and reaction medium. In the absence of added acids, the reduction of PhCH₂Br at potentials of the first wave is a 1e⁻ process mainly yielding bibenzyl, whereas toluene becomes the principal product at potentials beyond the second wave. The addition of acids strongly modifies the dependence of selectivity on the applied potential. The presence of a strong acid changes the mechanism of the process, which now becomes a 2e⁻ reduction to toluene, even at potentials corresponding to the first reduction wave.