Dependence of the reactivity of five-membered aromatic heterocycles on their structure. 4. Proton affinity of N-aminoazoles

Proton affinity of the amino groups of 10 N-aminoazoles is calculated using the STO-3G basis set. The nature of the heterocyclic effect on proton affinity of the amino groups depends on its conformation which in turn is determined by the number of nitrogen atoms in the -position. The accuracy of the proton affinity calculation can be raised considerably by taking into account the interaction of heteroatoms in the ring.For Communication 3, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1221–1223, September, 1989.     

Dependence of the reactivity of five-membered aromatic heterocycles on their structure. 2. Effect of aza substitution on the proton affinity of aminofurans

The proton affinities of a series of aminofurans and their aza derivatives were calculated by a nonempirical method in the STO-3G basis set. A correlation was established between the proton affinities and the position of the heteroatoms in the ring.For Communication 1, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 175–178, February, 1987.     

Dependence of the reactivity of five-membered heterocycles on their structure. 3. Proton affinity of azoles and oxazoles

The proton affinity (PA) of a number of azoles and oxazoles was calculated by MNDO and STO-3G ab initio methods. In spite of the fact that these methods predict poorly the absolute values of the PA, a good correlation between the PA values calculated by STO-3G and the experimental values of PA as well as pK_a values exists. Correlation of the experimental values with MNDO results are much worse.For Communication 2, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 508–511, April, 1989.     

Dependence of the reactivity of five-membered aromatic heterocycles on their structure. 1. Effect of the number,type, and position of nitrogen atoms on the proton affinity of aminoazoles

The proton affinities (PA) of 16 aminoazoles were calculated by the MNDO and ab initio methods. It was assumed that the effect of the heteroatom on the PA is determined by its type and position and does not depend on the presence of other heteroatoms in the ring. Quantitative characteristics reflecting the effect of heteroatoms at various positions on the PA were obtained by the method of least squares.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 54–58, January, 1987.     

Phosphorylation of five-membered aromatic heterocycles with phosphorus tribromide

Phosphorus trichloride in a basic medium is a convenient reagent for the direct C-phosphorylation of electron-rich aromatic heterocycles. The compounds obtained can be used for the synthesis of various types of heteroaryl and diheteroaryl derivatives of tri- and pentavalent phosphorus. © 1995 John Wiley & Sons, Inc.     

NMR spectra, GIAO and charge density calculations of five-membered aromatic heterocycles

The B3LYP/6-31+G(d) molecular geometry optimized structures of 17 five-membered heterocycles were employed together with the gauge including atomic orbitals (GIAO) density functional theory (DFT) method at the B3LYP/6-31+G(d,p), B3LYP/6-311++G(d,p) and B3LYP/6-311+G(2d,p) levels of theory for the calculation of proton and carbon chemicals shifts and coupling constants. The method of geometry optimization for pyrrole (1), N-methylpyrrole (2) and thiophene (7) using the larger 6-311++G(d,p) basis sets at the B3LYP/6-31+G(d,p), B3LYP/6-311++G(d,p), B3LYP/6-31+G(2d,p) and B3LYP/cc-pVTZ levels of theory gave little difference between calculated and experimental values of coupling constants. In general, the (1)H and 13C chemical shifts for all compounds are in good agreement with theoretical calculations using the smaller 6-31 basis set. The values of nJHH(n=3, 4, 5) and rmnJ(CH)(n=1, 2, 3, 4) were predicted well using the larger 6-31+G(d,p) and 6-311++G(d,p) basis sets and at the B3LYP/6-31+G(d,p), B3LYP/6-311++G(d,p), B3LYP/6-31+G(2d,2p) levels of theory. The computed atomic charges [Mülliken; Natural Bond Orbital Analysis (NBO); Merz-Kollman (MK); CHELP and CHELPG] for the B3LYP/6-311++G(d,p) geometry optimized structures of 1-17 were used to explore correlations with the experimental proton and carbon chemical shifts.     

Rearrangements of cyclopropenes into five-membered aromatic heterocycles: mechanistic aspect

This review covers rearrangements of cyclopropenes into aromatic five-membered heterocyclic compounds, namely furans and pyrroles; accompanied by small ring cleavage proceeding in the presence of catalytic amounts of transition metals, Lewis acids, or under UV irradiation.     

Electronic structures,physical properties,and reactivities of five-membered aromatic heterocycles (review)

The possibilities of modern quantum-chemical methods for the investigation of the electronic structures and reactivities of five-membered heteroaromatic compounds are examined.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 3–22, January, 1977.The authors thank Professor Ya. L. Gol'dfarb for the introduction he wrote for this review and for a number of valuable comments, as well as Doctor of Chemical Sciences L. I. Belen'kii for his useful discussion.     

Synthesis and reactivity of acyclic and macrocyclic uracils bridged with five-membered heterocycles

Replacement of terminal atoms of Br in 1,3-bis(bromopentyl)-5(6)-substituted uracils with 2-mercapto-5-methyl-1,3,4-thiadiazole, 2,5-dimercapto-1,3,4-thiadiazole, 2-mercaptoimidazole, and 2-mercaptobenzimidazoles resulted in a series of acyclic compounds and isomeric heterocyclophanes. Structures of macrocyclic regioisomers were unambiguously determined by NMR data. One of the regioisomers exhibits a hypochromic effect with respect to model compounds. The acyclic uracils obtained bridged with five-membered heterocycles are alkylated with methyliodide and methyl tosylate, and oxidated with m-CPBA, H₂O₂, and I₂.     

Quantum-chemical investigation of the 1,2-proton shift in protonated five-membered aromatic heterocycles

Calculations of the energetics of the 1,2-proton shift in protonated five-membered aromatic heterocycles — pyrrole, furan, and thiophene — have been carried out by the SCF MO LCAO method in the MINDO/3 approximation and nonempirically on the OST-3GF (OST-3GF*) basis. The general features of this process, and also the influence of solvation and of taking into account the vacant d-AOs of the sulfur atom in the protonated form of thiophene on the results of the calculation are considered. The results obtained have been used for a discussion of the activity and selectivity of the heterocycles considered in aromatic electrophilic substitution reactions.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1186–1190, September, 1984.     

Synthesis,insecticidal activity and stability study of novel nitromethylene neonicotinoids with five-membered aromatic heterocycles

Twenty-four novel neonicotinoid analogues with nitromethylene and five-membered aromatic heterocycles were designed and synthesized. All target molecular structures have been confirmed by analytical and spectral data. Some compounds exhibited notable insecticidal activities against aphid (Aphis medicaginis) and brown planthopper (Nilaparvata lugens). The aqueous stability test confirmed that the stabilities of those compounds were superior to the leading compound, and the photostability was even better than that of imidacloprid.     

Quantum chemical study of the structure and thermochemistry of the five-membered nitrogen-containing heterocycles and their anions and radicals

The nitrogen-containing heterocycles are of interest as high-energy-density materials for use as propellants and explosives, while the pyrolysis of these compounds is also important in understanding the evolution of unwanted NO and NO2 (NOx) from organic fuels such as coal and biomass. We have used ab initio and density functional methods to study the molecular structures and thermochemical properties of the five-membered nitrogen-containing heterocycles and their anions and radicals corresponding to respective heterolytic and homolytic loss of a hydrogen atom from either a nitrogen or carbon site. Many of these thermochemical properties have not previously been measured, especially for the heterocycles containing three and four nitrogen atoms. Using the theoretical methods CBS-APNO, G3, and G3B3, we calculate enthalpies of formation of 26.5, 42.4, 31.9, 63.7, 46.8, 81.0, and 79.0 kcal mol-1 for pyrrole, pyrazole, imidazole, 1,2,3-triazole, 1,2,4-triazole, 1H-tetrazole, and 2H-tetrazole. A correlation is developed between the number of nitrogen atoms in a heterocycle and its enthalpy, and we extrapolate this relationship to predict the enthalpy of formation of pentazole. N-H BDEs in the heterocycles typically increase with the number of nitrogen atoms in the molecule, while C-H BDEs are similar in all of the studied heterocycles, at around 120 kcal mol-1. In all cases the N-H BDEs are weaker than the C-H BDEs, suggesting abstraction of the N-H hydrogen atom is more likely. Deprotonation enthalpies and free energies reveal that the N-H protons become more acidic with increasing number of nitrogen atoms in the heterocycle. C-H protons are less acidic than N-H protons by ca. 49 kcal mol-1, or ca. 35 kcal mol-1 when adjacent to the NH group. Trends in N-H and C-H acidities can be qualitatively explained by electrostatic effects and electron affinities. From its use as a reference species in our calculations, we identify that the experimental enthalpy of pyrimidine (1,3-diazine) may be in error by ca. 1-3 kcal mol-1, and we recommend an enthalpy of formation of 44.8 +/- 1.0 kcal mol-1.     

Opening of the ring of five-membered aromatic heterocycles under the influence of bases (review)

The review is devoted to a discussion of the various mechanisms of dehydration of five-membered aromatic heterocycles under the influence of bases; this reaction can be used in many cases for the synthesis of hard-to-obtain polyunsaturated acyclic compounds.Part of the material in this review was presented in a lecture given by S. Gronowitz at Moscow State University on May 31, 1976.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 435–451, April, 1978.     

Electrophilic behavior of the pi delocalized azepinium ion: Friedel-Crafts reactions with benzenes and five-membered aromatic heterocycles

[reaction: see text] Although the reactivity of tropylium ion with aromatic substrates is low, the reaction of azepinium ion with aromatic substrates such as benzene, phenol, furan, and thiophene resulted in the formation of 2-aryl-2H-azepine as a major product. An exceptional result in the formation of ring-contracted product was observed in the reaction with pyrrole.     

Single-molecule vibrations, conformational changes, and electronic conductivity of five-membered heterocycles

Using an ultrahigh vacuum scanning tunneling microscope (STM), we have explored the interactions of isolated five-membered heterocycles, pyrrole, thiophene, pyrrolidine, and tetrahydrothiophene, with the Cu(001) surface at 9 K. Pyrrolidine was also studied on the Ag(001) surface. Important distinctions in bonding, vibrational spectra, and vibrationally mediated negative differential resistance were observed with the aid of single-molecule inelastic electron tunneling spectroscopy (STM-IETS).