Structure-kinetics relations holding in the amination of six-membered nitrogen-containing heterocyclic compounds

Relative rates of the amination of 3-X- and 4-X-substituted pyridines (X = H, 3-Me, 4-Me, 3-F₃C, 3-CN, 4-CN, 3-Cl, 3-Br, 4-MeO, 4-Me₂N), pyrazine, quinoline, isoquinoline, 2,2′- and 4,4′-bipyridines, and 1,10-phenanthroline with O-mesitylenesulfonylhydroxylamine were estimated by NMR spectroscopy. Correlations were found between logarithms of the relative amination rate constants and substituent constants σ or σ_I and σ_R for X-substituted pyridines. A wide series of nitrogen-containing heterocyclic compounds turned out to fit correlations between logarithms of the relative amination rate constants, on the one hand, and relative stabilities of N-aminoazinium cations, energies of activation of the amination process, and gas-phase proton affinities, calculated by the DFT/PBE/3z and DFT/B3LYP/L2 methods, on the other.     

Transferable potentials for phase equilibria. 9. Explicit hydrogen description of benzene and five-membered and six-membered heterocyclic aromatic compounds

The explicit hydrogen version of the transferable potentials for phase equilibria (TraPPE-EH) force field is extended to benzene, pyridine, pyrimidine, pyrazine, pyridazine, thiophene, furan, pyrrole, thiazole, oxazole, isoxazole, imidazole, and pyrazole. While the Lennard-Jones parameters for carbon, hydrogen (two types), nitrogen (two types), oxygen, and sulfur are transferable for all 13 compounds, the partial charges are specific for each compound. The benzene dimer energies for sandwich, T-shape, and parallel-displaced configurations obtained for the TraPPE-EH force field compare favorably with high-level electronic structure calculations. Gibbs ensemble Monte Carlo simulations were carried out to compute the single-component vapor-liquid equilibria for benzene, pyridine, three diazenes, and eight five-membered heterocycles. The agreement with experimental data is excellent with the liquid densities and vapor pressures reproduced within 1 and 5%, respectively. The critical temperatures and normal boiling points are predicted with mean deviations of 0.8 and 1.6%, respectively.     

Gas chromatographic indicator of the ability of five- and six-membered heterocyclic nitrogen-containing compounds for self-association in pure liquids

The retention indices of five- and six-membered nitrogen-containing heterocyclic compounds (pyrrole, pyrazole, imidazole, 1,2,4-triazole, pyridine, pyridazine, pyrimidine, pyrazine, 1,3,5-triazine, isoxazole, and oxazole) were determined on a capillary column with the OV-101 nonpolar stationary phase. The difference between the experimentally determined boiling point of the sorbate and that calculated from GC data (δT _b.p.) was proposed as a gas chromatographic indicator sensitive to the ability of substances to form self-associates in pure liquids. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 730–733, April, 1999.     

Spectroscopic properties of lanthanoid benzene carboxylates in the solid state: Part 2. Polar substituted benzoates

Europium and terbium complexes of ortho, meta and para substituted benzoate ligands including nitrobenzoate (NBA), aminobenzoate (ABA), hydroxybenzoate (OHBA) and methoxybenzoate (MeOBA) have been synthesised by metathesis reactions, carried out in aqueous media. The complexes were characterised by elemental, compositional and structural investigations, including microanalysis, EDTA titrations, differential thermal analysis, infra red spectroscopy, X-ray powder diffraction and single crystal structural analyses. Besides this, strong emphasis was on the determination of the optoelectronic properties of the compounds in the solid state. In this regard, reflectance, excitation and emission spectra were recorded. From these, the emission and excitation efficiencies were determined. The relative intensities as well as the splitting patterns of the ⁵D₀ → ⁷F_J transitions in the europium emission spectra are discussed.     

The tertiary amino effect in heterocyclic synthesis : mechanistic and computational study of the formation of six-membered rings

The mechanism of the ring closure of [2-(1-pyrrolidinyl)phenylmethylene]-propanedinitrile (2a) to l,2,3,3a,4,5-hexahydropyrrolo[1,2-a]quinoline-4,4-dicarbonitrile (3a) has been studied by kinetic measurements using ¹H-NMR spectroscopy. It could be shown that the rate-determining step consists of an intramolecular 1,5 hydrogen transfer, which is accompanied by charge separation within the- molecule. The calculated (AM1) and experimental (X-ray) molecular structure of 2a are in fairly good agreement. In the ground state geometry a 1,5 hydrogen transfer will most likely take place suprafacially. Subsequent rotation of the former vinyl group and C-C-bond formation, leading to a six-membered ring, also take place in a stereochemically defined way.     

Orthogonal contacts between benzene rings: A special type of specific intermolecular interactions

The crystal structures of benzene and some of its derivatives are analyzed in detail. The structures are characterized by orthogonal contacts between the benzene cycles (OBzC). Although the orthogonal contacts conflict with the close packig law, they are an important structure-forming factor, being thus a specific type of intermolecular interactions. The OBzC system, which is inherent in the orthorhombic crystals of benzene, remains invariable in 2-aminophenol and 2-amino-4-chlorophenol crystals, notwithstanding other specific interactions (H-bonding and halogen...halogen contacts). OBzC are also observed in para- and ortho-cresol crystals. A classification of OBzC is given, and an especially effective projection method (rectangular projection) is suggested. M. V. Lomonosov Moscow State University, Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 5, pp. 775–789, September–October, 1995. Translated by L. Smolina     

N-Methyl inversion and structure of six-membered heterocyclic rings: rotational spectrum of 1-methyl-4-piperidone

The conformational and structural properties of the six-membered heterocyclic ring of 1-methyl-4-piperidone have been observed in a jet-cooled supersonic expansion using Fourier transform microwave spectroscopy (FT-MW). The rotational spectrum evidenced two different conformations originated by nitrogen inversion, with the N-methyl group in either equatorial (most stable) or axial position. Additional observation of the rotational spectra for all possible carbon, nitrogen, and oxygen monosubstituted species (4 × (13)C, (15)N, (18)O) in natural abundance allowed us to determine substitution (r(s)) and effective structures (r(0)) for the equatorial conformer. Additional ab initio and DFT calculations provided comparative rotational parameters, structural data, conformational energies, and the axial-equatorial interconversion barrier. The structural data were compared with the related azabicycle of tropinone, revealing the molecular changes and structural relaxation associated with the presence of the two-carbon bridge in the latter molecule.     

Aggregation of star-shaped tris(tetrathiafulvalenylethynyl) benzene in solution and in the solid state

Neutral hexabutylthio-1,3,5-tris(tetrathiafulvalenylethynyl)benzene aggregates in CD₃CN-CDCl₃ and CDCl₃, whereas stronger stacking of the cationic counterparts in solution has been observed. In the solid state, X-ray analysis of hexamethylthio-1,3,5-tris(tetrathiafulvalenylethynyl)-benzene shows a columnar structure, reflecting the threefold face-to-face stacking interaction of the TTF units.     

A preference for edgewise interactions between aromatic rings and carboxylate anions: the biological relevance of anion-quadrupole interactions

Noncovalent interactions are quite important in biological structure-function relationships. To study the pairwise interaction of aromatic amino acids (phenylalanine, tyrosine, tryptophan) with anionic amino acids (aspartic and glutamic acids), small molecule mimics (benzene, phenol or indole interacting with formate) were used at the MP2 level of theory. The overall energy associated with an anion-quadrupole interaction is substantial (-9.5 kcal/mol for a benzene-formate planar dimer at van der Waals contact distance), indicating the electropositive ring edge of an aromatic group can interact with an anion. Deconvolution of the long-range coplanar interaction energy into fractional contributions from charge-quadrupole interactions, higher-order electrostatic interactions, and polarization terms was achieved. The charge-quadrupole term contributes between 30 to 45% of the total MP2 benzene-formate interaction; most of the rest of the interaction arises from polarization contributions. Additional studies of the Protein Data Bank (PDB Select) show that nearly planar aromatic-anionic amino acid pairs occur more often than expected from a random angular distribution, while axial aromatic-anionic pairs occur less often than expected; this demonstrates the biological relevance of the anion-quadrupole interaction. While water may mitigate the strength of these interactions, they may be numerous in a typical protein structure, so their cumulative effect could be substantial.     

1,3-diindolylureas and 1,3-diindolylthioureas: anion complexation studies in solution and the solid state

1,3-Diindolylureas and thioureas have been synthesised and their anion complexation properties in solution studied. Whilst diindolylthioureas showed only moderate affinities and selectivities, diindolylureas show remarkably high affinity for dihydrogen phosphate in solution for an acyclic, neutral receptor in water/[D(6)]DMSO mixtures. These easy-to-make compounds adopt relatively planar conformations in the solid-state and are able to donate four hydrogen bonds and yet not fill the coordination sphere of carbonate or phosphate, allowing two or three receptors to bind to each anion in the solid-state.     

Spectral and photophysical studies on cruciform oligothiophenes in solution and the solid state

The photophysical and spectroscopic properties of a new class of oligothiophene derivatives, designated as cruciform oligomers, have been investigated in solution (room and low temperature) and in the solid state (as thin films in Zeonex matrixes). The study comprises absorption, emission, and triplet-triplet absorption spectra, together with quantitative measurements of quantum yields (fluorescence, intersystem crossing, internal conversion, and singlet oxygen formation) and lifetimes. The overall data allow the determination of the rate constants for all decay processes. From these, several conclusions are drawn. First, in solution, the main deactivation channels for the compounds are the radiationless processes: S(1) --> S(0) internal conversion and S(1) --> T(1) intersystem crossing. Second, in general, in the solid state, the fluorescence quantum yields decrease relative to solution. A comparison is made with the analogous linear alpha-oligothiophenes, revealing a lower fluorescence quantum efficiency and, in contrast to the normal oligothiophenes, that internal conversion is an important channel for the deactivation of the singlet excited state. Replacement of thiophene by 1,4-phenylene units in the longer-sized cruciform oligomer increases the fluorescence efficiency. The highly efficient generation of singlet oxygen through energy transfer from the triplet state (S(Delta) approximately 1) provides support for the measured intersystem crossing quantum yields and suggests that reaction with this may be an important pathway to consider for degradation of devices produced with these compounds.     

In situ spectroelectrochemical studies on ladder-type oligomers in solution and the solid state

A series of thermally stable fluoranthenopyracylene oligomers with extended pi conjugation were studied by in situ ESR-UV/Vis/NIR spectroelectrochemistry with respect to their application in devices such as organic light-emitting diodes and field-effect transistors. The oligomers are both soluble in o-dichlorobenzene and form thin films by evaporation in the temperature range of 300-500 degrees C in vacuum. Their electrochemical behavior was studied in reduction (n doping) and oxidation (p doping) under standard voltammetric and thin-layer conditions. The HOMO and LUMO energies and the band gaps of all compounds under study were estimated from both electrochemical and UV/Vis/NIR spectroscopic data. The fluorene-type oligomers A(2)-A(6) and B(2) bearing flexible alkyl chains exhibit both reversible multistep reductions and oxidations. The spectroelectrochemistry indicates substantial differences in delocalization of the positive and negative charges in these ladder-type oligomers. The formation of doubly charged sigma dimers was observed for the first time for both the radical anion and radical cation of the same molecule (B(1)). The redox behavior of the oligomers was studied in the solid state and in solution.     

Photochemical studies on acyclic alkyl aromatic ketones in the solid state: asymmetric induction and increased chemoselectivity

The acyclic alkyl aromatic ketones, 2-isobutyl-4-methyl-1-arylpentan-1-one derivatives were designed and synthesized for photochemical investigations. Irradiation of such compounds in acetonitrile solution led to the Yang cyclization and Norrish type II cleavage photoproducts with a ratio of 1:1, whereas the reaction conducted in the solid state using the ionic chiral auxiliary method led to only the Yang cyclization product with as high as 99% ee. Furthermore, the conformational transitions were first observed in the reaction.     

Bonding studies in group IV substituted anilines : IV. Excited state interactions in the trimethyl derivatives

The solution ultraviolet spectra for N,N-dimethylaniline, p-t-butyl-, and p-trimethylsilyl-, p-trimethylgermyl-, and m-trimethylsilyl-N,N-dimethylaniline in acetonitrile and pentane are reported. The Group IV substituents perturb the excited states to a much larger extent than the ground states. Both the symmetric and antisymmetric π^* levels are affected by π interaction with the trimethylsilyl and trimethylgermyl substituents. The magnitude of the effects are proportional to the π density at the point of substitution. Simple Hückel calculations correlate well with experimental transition energies. The model appears to exaggerate π interactions between silicon or germanium and the π^* molecular orbitals.     

ESR studies of poly(aniline-co-m-aminophenol) in the solid state and nonaqueous solution

The copolymers, poly(aniline-co-m-aminophenol)s, used for the ESR studies were synthesized chemically in the solutions consisting of different concentration ratios of m-aminophenol to aniline. On the basis of the ESR measurements, the unpaired spin (polaron) densities of the copolymers were calculated to be 1.14 x 10(19) spins per gram for copolymer-A with the conductivity of 7.02 x 10(-6) S cm-1 and 2.03 x 10(20) spins per gram for copolymer-C with the conductivity of 2.34 S cm-1. The ESR measurements of the copolymers in the solid states show that the peak-to-peak line width DeltaHpp decreases with a decreasing concentration ratio of m-aminophenol to aniline, but the g-value hardly changes. A conversion of Curie spins to Pauli spins for the copolymers is observed as the temperature changes in going from low temperature to high temperature between 136 and 356 K. The ESR studies of the copolymers in a nonaqueous solution first reveal that there are two free radicals in the copolymer, and the unpaired spins in the copolymers arise from nitrogen nuclei.     

Additivity of 13C-1H and 1H-1H spin-spin coupling constants in six-membered aromatic nitrogen-containing heterocycles

We have analyzed proton-coupled ¹³C NMR and PMR spectra of pyridazine, pyrimidine, pyrazine, 1,3,5-triazine, and 1,2,4-triazine for one-molar solutions of the compounds in DMSO-D₆. Comparison of the values obtained for the ¹³C-¹H and ¹H-¹H spin-spin coupling constants with the values calculated on the basis of the spin-spin coupling constants in pyridine and benzene has shown that it is possible to predict the constants using linear additive equations. Substantial nonlinear deviations (5–6 Hz) are observed for ¹J_CH when the carbon atom is located between two adjacent nitrogen atoms.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 986–990, July, 1985.     

Understanding of assembly phenomena by aromatic-aromatic interactions: benzene dimer and the substituted systems

Interactions involving aromatic rings are important in molecular/biomolecular assembly and engineering. As a consequence, there have been a number of investigations on dimers involving benzene or other substituted pi systems. In this Feature Article, we examine the relevance of the magnitudes of their attractive and repulsive interaction energy components in governing the geometries of several pi-pi systems. The geometries and the associated binding energies were evaluated at the complete basis set (CBS) limit of coupled cluster theory with singles, doubles, and perturbative triples excitations [CCSD(T)] using a least biased scheme for the given data set. The results for the benzene dimer indicate that the floppy T-shaped structure (center-to-center distance: 4.96 A, with an axial benzene off-centered above the facial benzene) is isoenergetic in zero-point-energy (ZPE) corrected binding energy (D0) to the displaced-stacked structure (vertical interplanar distance: 3.54 A). However, the T-shaped structure is likely to be slightly more stable (D0 approximately equal to 2.4-2.5 kcal/mol) if quadruple excitations are included in the coupled cluster calculations. The presence of substituents on the aromatic ring, irrespective of their electron withdrawing or donating nature, leads to an increase in the binding energy, and the displaced-stacked conformations are more stabilized than the T-shaped conformers. This explains the wide prevalence of displaced stacked structures in organic crystals. Despite that the dispersion energy is dominating, the substituent as well as the conformational effects are correlated to the electrostatic interaction. This electrostatic origin implies that the substituent effect would be reduced in polar solution, but important in apolar media, in particular, for assembling processes.     

Solvent effects on valence tautomerism: A comparison between the interconversion in solution and solid state

A detailed comparative study of the valence tautomeric interconversion of complexes 1–3 in three different environments is reported. The three environments are solid state (both crystalline and amorphous materials), solution and embedded into a polymeric matrix. The VT behavior of the three complexes strongly differs from one to the other, though no systematization can be established. In solution, different solvatational parameters seem to affect the equilibrium. However, such solvate effect cannot be translated either into the solid state or polymeric matrix, where the equilibrium is controlled by the vibrational relaxation of the network.