All-valence-electron scf-mo calculations on the electronic structure and reactivities of some halogenated benzene and azabenzene derivatives

All-valence-electron CNDO/2 SCF-MO calculations have been performed on some fluoro, chloro and methoxy derivatives of benzene, pyridine and the diazines. The computed charge distributions and the dipole moments are discussed and compared with available experimental data. The relative basicities of the derivatives of pyridine and the diazines have been investigated and an attempt made to correlate the results with known pK_a values. Localisation energies for the nucleophilic substitution of halogen by methoxide ion have also been calculated and the predicted relative reactivities compared with experimental observations, where these were available.     

2-alkyl-5-aryltetrazoles as hydrogen bond acceptors

The dissociation constants of the H-complexes formed by 2-alkyl-5-aryltetrazoles and p-fluorophenol in carbon tetrachloride (pK _HB 0.9–1.3) were determined by Fourier-transform IR spectroscopy. 2-Alkyl-5-aryltetrazoles were found to act as medium-strength hydrogen bond acceptors comparable with diazines. The thermodynamic parameters of the equilibrium formation of H-complex with 2-isopropyl-5-phenyltetrazole were determined. The electronic nature of substituents in the tetrazole ring only slightly affects the pK _HB values of tetrazoles.     

Selective formation of heteroaryl thioethers via a phosphonium ion coupling reaction

Heteroaryl thioethers, comprised of pyridines and diazines, are an important class of compounds with relevance to medicinal chemistry. Metal-catalyzed cross-couplings and S_NAr reactions are traditionally used to form C–S bonds in these systems but are limited by available halogenated precursors. An alternative approach is presented where pyridines and diazines are transformed into heterocyclic phosphonium salts and then C–S bonds are formed by adding thiolate nucleophiles. The process is 4-selective for pyridines, simple to execute and can be used to make derivatives of complex pharmaceuticals.     

Plasma polymerization. XI. A comparison of the plasma polymerization of the isomeric perfluorinated diazines

Plasma polymers synthesized by inductively coupled RF techniques have been investigated as a function of operating parameters for the isomeric perfluorinated diazines (pyrazine, pyrimidine, and pyridazine). A combination of ESCA and microanalytical studies shows that plasma polymers are produced by rearrangement mechanisms: the C/F and C/N stoichiometric ratios are similar to those of the starting monomers over a range of operating parameters. A comparison of rates of formation of plasma polymer films reveals distinctive differences between the isomeric diazines that suggest that equilibration of valence isomers occurs on a substantially slower time scale than for isomeric fluorinated benzenes^5–9 which, in general, polymerize at essentially the same rates. In contrast to the remarkably low critical surface tension of plasma polymers based on perfluorobenzenes (ca. 20 dynes cm^?2), the plasma polymer films from the perfluorinated diazines which are initially hydrophobic become hydrophilic in contact with water droplets. This is attributed to the hydrolytic instability of films associated with the labilizing influence of nitrogen on nucleophilic displacement of fluoride which is a feature of the chemistry of monomers.     

Phosphorus Compounds as Cyclization Promoted Reagents: Preparation of 1,4-α3,γ3-Diphospha-2,6-Diazines

Abstract

lmines are important classes of compounds in chemistry. In our continued investigation in the studies of the interactions betwcen group 4 (Zr, Ti) elements and main group elements we recently developped a new methodology for the preparation of various σ³, λ³ phospha-imines. Extension of this work allowed us to prepare unprecedented 8-π electron 6-membered phosphorus heterocycles; 1, 4 σ³, λ³ diphospha-2,6-diazines. The formation of these diazines involved for the first time the use of phosphorus derivatives as cyclization promoted reagents in zirconocene chemistry.     

On the amination of azaheterocycles. A new procedure for the introduction of an amino group

A new method of amination of diazines and triazines, using potassium amide, liquid ammonia and potassium permanganate, has been described.     

Theoretical Study on the Structures and Properties of Hydrogen Bonding Complexes between Diazines and Water

Density functional theory B3LYP method and second-order Moller-Plesset perturbation theory MP2 method were employed to obtain the optimized geometries of the ground state and interaction energy for diazines and water complexes. The results show that the ground state complexes have strong hydrogen bonding interaction with -20.99, -16.73 and -15.31 kJ/mol after basis set superposition error and zero-point vibration energy correction for pyridazine-water, pyrimidine-water and pyrazine-water, respectively, and large red-shift for the symmetric H-O stretching vibration frequencies due to the formation of N…H-O hydrogen bond in the diazine-water complexes. The NBO analysis indicates that intermolecular charge transfer are 0.0316, 0.0255 and 0.0265 e respectively. In addition, the first singlet （n,n＊） vertical excitation energy of the monomer and the hydrogen bonding complexes between diazines and water was investigated by time-dependent density functional theory.     

Quenching of triplet states of diazines by H-atom donors. Formation of azyl radicals

The laser flash photolysis of pyrazine in water and in organic solvents has been examined. The ³(n, π^*) state in water has absorption bands at 230, ≈ 260, ≈ 295, ≈ 640, 700 and 810 nm, and decays with k = 2.2 × 10⁵ sec^?1. It is quenched by oxygen with k_q = 3.2 × 10⁹ M^?1 sec^?1 and by various H-atom donors, e.g., k_q = 1.3 × 10⁸ M^?1 sec^?1 for isopropyl alcohol. On reaction with H-atom donors, the chemistry of ³(n, π^*) pyrazine produces the neutral pyrazyl-radical and the dihydro radical cation, whose characteristic absorption spectra have been identified. These results are discussed by comparison with ³(π, π^*) diazines and with ³(n, π^*) aromatic carbonyl compounds.     

The preparation of diazine derivatives containing the ferroin group

With the idea of providing new compounds capable of chelation with metal ions, certain derivatives of diazines, containing the ferroin (?N? C? C? N?) group have been prepared.     

13C-13C spin-spin coupling constants in six-membered ring azaaromatic compounds

The ¹³C-{¹H} NMR spectra of monomethyl substituted diazines, protonated at the picoline and 2-methylopyrimidine nitrogen atoms, have been analyzed, along with that of pyridine N-oxide and its 2-methyl derivatives protonated at the oxygen atom. Direct and vicinal ¹³C-¹³C spin-spin coupling constants (SSCC) have been measured. It was found that the ¹³C-¹³C SSCC in diazines follow additivity rules which are based on consideration of the number and mutual orientation or distribution of nitrogen atoms in the ring. It has also been demonstrated that increased direct ¹J_CC values involving methyl group carbon atoms in -positions to nitrogen atoms in the aromatic ring are due to effects associated with unshared electron pairs.Translated from Khimiya Geterotsiklieheskikh Soedinenii, No. 9, pp. 1243–1250, September, 1988.     

A spectroscopic study of the iodine complexes of donors—pyridines,phenanthrolines, bipyridines and diazines

Ultraviolet—visible spectral data of iodine complexes of n- and π-donors have been interpreted by considering that the repulsion energy responsible for the blue shift of the iodine band is also experienced by the donor partner which causes the blue shift of the original band of the donor. This reasoning explains the spectral data of iodine complexes of benzene, pyridine-N-oxide and stilbazoles. Ultraviolet—vis. spectra of the iodine complexes of pyridine, aminopyridines and diazines have been reinvestigated and discussed in the light of the above reasoning. The above reasoning is extended to the CT spectra of iodine complexes of twin-site donors such as 1,10-phenanthroline, its methyl and chloro derivatives, 1,7 and 4,7-phenanthrolines, 2,2′-bipyridine and 4,4′-bipyridine. Arguments are presented which indicate that the donors used in this study form only 1:1 complexes with iodine. The thermodynamic parameters were evaluated for iodine complexes of the above twin-site donors. The kinetics of transformation of outer CT complexes between the donors and iodine to inner complexes is presented and discussed. Two CT bands are observed for the iodine complexes of 1,10-phenanthroline and its substituents. These bands are explained by assuming a structure for the 1,10-phenanthroline complex in which iodine is in dynamic equilibrium between the two nitrogens.     

ZnO nanorods as an efficient catalyst for the synthesis of imidazo[1,2-a]azines and diazines

Abstract  

ZnO nanorods were synthesized using a novel and solvent-free procedure and their catalytic activity in the preparation of imidazo[1,2-a]azines and diazines was studied. The effects of the amount of catalyst and its reusability were investigated to optimize the reaction conditions. The ZnO nanorods exhibited higher catalytic activity than that of bulk ZnO.     

A directing effect of n–π* transitions on electronic charges

Changes of electronic charge distribution following the lowest singlet n–π* transitions of benzaldehyde, benzamide, benzoic acid, the phthalaldehydes, pyridine, and the diazines have been examined using the CNDO -S /CI method. A Singlet n–π* transition is found as a general rule to direct electronic charges to the para position of the atom in the ring that carries the lone pair or to which the substituent carrying the lone pair is attached.     

Potassium carbonate,a support for the green synthesis of azoles and diazines

A novel potassium carbonate mediated synthesis of 2‐amino‐1,3‐thia/oxa zoles, 2‐amino‐1,3,4‐thia/oxa diazines and 2‐hydrazino‐1,3,4‐thiadiazines has been described here. The use of K₂CO₃ allows for an aqueous workup thereby eliminating the organic solvent from both the reaction step and post reaction stage, without compromising the yield and reaction time.     

Reactivity of asymmetric benzo-condensed diazines with nitrilimine dipoles in the 1,3-dipolar cycloaddition reactions

The reactivity of asymmetric benzo-condensed diazines in the 1,3-dipolar cycloaddition reactions with nitrilimines was investigated. The results demonstrated that, at variance with the symmetric quinoxaline, a certain grade of diastereoselectivity emerged. Moreover in the case of the 5-methylquinoxaline and quinazoline a mono-cycloadduct was obtained.     

Conjugate addition of sodium methanesulfinate to vinyl pyridines and diazines for the synthesis of aliphatic sulfones

A convenient method to introduce a sulfone group to pyridines and diazines is described. This efficient method involves the conjugate addition of sodium methanesulfinate to vinyl heterocycles. This process tolerates a wide variety of functional groups and is performed in the presence of acetic or trifluoroacetic acid. A one-pot, two-step synthesis of sulfones starting from the corresponding heteroaryl halide is also described.     

Computational analysis of relative stabilities of polyazine N-oxides

There is considerable interest in polyazine N-oxides as potential frameworks for energetic compounds with relatively high enthalpies of formation and crystal densities. The N⁺→O^? linkages, if appropriately located, may diminish the destabilization associated with nitrogen catenation. We have computationally characterized 40 N-oxides of the isomeric diazines, triazines, and tetrazines in terms of their geometries, relative energies, and (for a representative selection) electrostatic potentials. The presence of N⁺→O^? linkages does partially counteract the destabilizing effects of nitrogen catenation, although the isomers with complete catenation remain the least stable. The stabilizing influence of N⁺→O^? groups, and the accompanying changes in bond lengths, can be understood in terms of resonance charge delocalization to the polyazine rings. The N(O)–N(O) bonds between nitrogens that both bear oxygens tend to be relatively weak. The electrostatic potentials above the polyazine rings become increasingly positive as there are more nitrogens and oxygens; eventually they are positive above all of the carbons and nitrogens and possibly even the oxygens, with negative regions only on the peripheries of the molecules. However, the nitrogens that bear oxygens always have more positive potentials than those that do not.     

Selective partial reduction of various heteroaromatic compounds with bridgehead nitrogen via Birch reduction protocol

[reaction: see text] For the first time various heteroaromatic compounds with bridgehead nitrogen, including indolizines, bispyrrolopyrimidines, pyrroloquinolines, pyrroloisoquinolines, and bispyrrolopyrazines, were selectively partially reduced under Birch reduction conditions. It was found that the double bond in the fused heterocycles which possesses the highest LUMO density can be selectively reduced under these conditions. Indolizine 6, containing an ester group at C-6, was reductively alkylated to give dihydroindolizines 8 and 9 possessing a quaternary carbon center in good yield. It was found that ambident substrate 12, under Birch reduction conditions, underwent smooth partial reduction to give 4,5-dihydroquinoline 14 as a sole product with no evidence of reduction of the side chain olefin. It was also shown that electron-rich pyrroloisoquinoline 15, which cannot be reduced via catalytic hydrogenation conditions, was efficiently transformed into its dihydrocounterpart 16 by using the Birch reduction protocol. Finally, it was shown that various fused diazines were smoothly and stereoselectively reduced under Birch reduction conditions to give trans-4,5-disubstituted dihydropyrimidines 30 and 32 in virtually quantitative yields.     

Analysis of azines and diazines by HPLC on bonded alkyl stationary phases. Pyridine and quinoline in environmental samples

Summary Several heterocyclic compounds have been investigated by HPLC on RP-8 and RP-18 with methanol-water mixtures as mobile phase. The chromatographic systems developed showed good selectivity for determination of azines and diazines. Pyridine and quinoline have been determined in environmental samples and in food. Samples for analysis were preconcentrated by solid phase extraction on-column with octadecyl silane packing.