Triplet states of furan,thiophene, and pyrrole

Low-lying triplet electronic states have been detected in furan, thiophene, and pyrrole by the method of variable-angle, electron-impact spectroscopy. Singlet → triplet transitions occur with maximum intensity at 3.99 eV and 5.22 eV in furan, 3.75 eV and 4.62 eV in thiophene, and 4.21 eV in pyrrole. A weak transition at 5.22 eV in pyrrole is assigned as the lowest observed singlet → singlet excitation in that molecule.     

The direction of the dipole moments of furan,thiophen, and pyrrole: A controversial question

The report that the dipole moment in the five-membered heterocyclic rings, furan, thiophene, and pyrrole has the positive pole on the heteroatom is contradicted with arguments based on reactivity data, theoretical calculations and moment values of substituted derivatives. In actuality, the dipole moment in furan and thiophene is directed from the ring (positive pole) to the heteroatom (negative pole).     

Basicity and structure of α, β -unsaturated heterocyclic ketones

The protolytic equilibrium constants of chalcone and its furan and thiophene analogs have been measured spectrophotometrically in sulfuric acid — glacial acetic acid solution. It is shown that the furan ketones are more basic than the thiophene ones, basicity depending on the position of the carbonyl group in the conjugated chain. In the systems studied, 2-furyl exhibits an appreciably greater positive dynamical conjugation effect than 2-thienyl, but as compared with the latter it gives rise to less steric hindrance to solvation of carbonium ions.For Part II see [2].     

Cytotoxicity of cobalt complexes of furan oximes in murine and human tissue‐cultured cell lines

The copper complexes of 2‐furaldehyde and furan oximes have previously demonstrated potent cytotoxicity, L1210 DNA synthesis inhibition, DNA topoisomerase II inhibition and DNA fragmentation. Currently a series of cobalt metal complexes of 2‐furaldehyde oximes were compared with copper complexes of furan oximes to determine whether the type of metal is important to the cytotoxicity and mode of action of the complexes. The cobalt complexes of furan oximes, like the copper complexes, were shown to be cytotoxic to suspended tumor cell lines, e.g. leukemias, lymphomas, acute monocytic leukemia and HeLa‐S³ uterine carcinoma. The cobalt complexes did not demonstrate dramatic cytotoxicity against the growth of tumors derived from solid human tumor lines. The cobalt complexes preferentially inhibited L1210 DNA synthesis, followed by inhibition of RNA and protein synthesis from 25 to 100 µM over 60 min. These agents, like the copper complexes of 2‐furaldehyde and furan oximes, were inhibitors of DNA polymerase α activity and de novo purine synthesis with marginal inhibition of ribonucleoside reductase and dihydrofolate reductase activities with DNA fragmentation. Unlike the copper complexes, the cobalt complexes did not inhibit L1210 DNA topoisomerase II activity but did reduce thymidylate synthetase activity. Thus, varying the type of metal within the complexes of 2‐furaldehyde and furan oximes produces differences in both cytotoxicity and mode of action. Copyright © 1999 John Wiley & Sons, Ltd.     

Activity of catalysts in thiophene synthesis from furan and hydrogen sulfide

Thiophene synthesis from furan and H₂S over acid catalysts is reported. Proton donor catalysts are low-active, nonselective, and prone to deactivation. Alumina-supported catalysts having Lewis acid sites, such as alumina-supported catalysts, are more efficient. With these catalysts, the thiophene formation rate per Lewis acid site increases with increasing site strength. It is assumed that the reaction proceeds via the formation of a surface intermediate consisting of an acid site bonded to an α carbon atom of the furan ring and an H₂S molecule nondissociatively adsorbed on a basic site. At atmospheric pressure, T = 250–450°C, initial furan concentrations of 1–20 vol %, and H₂S/furan = 0.4–20 (mol/mol), the thiophene formation reaction is first-order with respect to both reactants and its rate constant increases with increasing temperature. The thiophene formation rate depends on the H₂S/furan molar ratio. Under optimal conditions, the thiophene yield is 95–98 mol % and the thiophene formation rate is high.     

Theoretical fine spectroscopy with symmetry-adapted-cluster configuration-interaction method: outer- and inner-valence ionization spectra of furan, pyrrole, and thiophene

Theoretical fine spectroscopy has been performed for the valence ionization spectra of furan, pyrrole, and thiophene with the symmetry-adapted-cluster configuration-interaction general-R method. The present method described that the pi(1) state interacts with the pi(3) (-2)pi*, pi(2) (-2)pi*, and pi(2) (-1)pi(3) (-1)pi* shake-up states providing the split peaks and the outer-valence satellites, both of which are in agreement with the experiments. The intensity distributions were analyzed in detail for the inner-valence region. In particular, for furan, theoretical intensities were successfully compared with the intensity measured by the electron momentum spectroscopy. The interactions of the 3b(2) and 5a(1) states with the shake-up states were remarkable for furan and pyrrole, while the 4b(2) state of thiophene had relatively large intensity.     

2-Mono- and 2,5-bis[bis[(4-dimethylamino)phenyl]hydroxymethyl]-substituted furans,thiophenes and N-methylpyrroles: New color-formers for carbonless imaging

2-Mono- and 2,5-bis[bis[(4-dimethylamino)phenyl]hydroxymethyl]substituted furan, thiophene and N-methylpyrrole were synthesized by mono- and dilithiation of furan, thiophene and N-methylpyrrole, respectively, and subsequent reaction with Michler's ketone. The title compounds develop black or dark-blue colors in acidic media.     

Triplet π→π* transitions in thiophene,furan and pyrrole by low-energy electron-impact spectroscopy

Low-energy (0–3 V above threshold) electron-impact excitation spectra and transmission spectra are presented for thiophene, furan and pyrrole. For each molecule two low-lying triplet states are reported. An additional new transition has been observed in thiophene at 6.5 eV. Assignments are suggested for some of the observed direct and resonance excitation processes.     

Angular distributions in the photoelectron spectroscopy of furan,thiophene, and pyrrole

The photoelectron angular distributions of furan, thiophene, and pyrrole are reported. The trends in the value of the asymmetry parameter β and in the shapes and positions of bands in the spectra are used to assign the peaks that correspond to ionization from the π orbitals in these molecules.     

Conformational preferences and electronic effects in selenophene and tellurophene carbonyl derivatives investigated by lanthanide induced shifts

Computer simulation of the lanthanide induced shifts has been applied to study of the conformational preferences in the 2-formyl and 2-acetyl derivatives of furan, thiophene, selenophene and tellurophene. The results assign a nearly equipopulated mixture of s-cis and s-trans conformers to the furan, and a preponderance of the s-trans form to the thiophene, selenophene and tellurophene derivatives. This difference is interpreted as due to the interaction between the heteroatom and carbonyl oxygen lone pairs. The 2-N,N-dimethylcarboxyamide derivatives of furan, thiophene and selenophene are found to exist mainly in a quasi-planar s-cis form. The barriers to the rotation about the amide bond in these amides have been measured and related to the electronegativity of the heteroatom.     

Alkylation of furan,thiophene, and pyrrole with furfuryl alcohol in the presence of the strongly acidic Amberlyst 15 sulfo cation-exchange resin

The corresponding 2-furylhetarylmethanes were obtained by the reaction of furan, thiophene, or pyrrole with furfuryl alcohol in the presence of the strongly acidic Amberlyst 15 cation-exchange resin in the H⁺ form. The alkylation of furan and thiophene takes place regiospecifically in the 2 position, whereas 2-furyl-2-pyrrolyl- and 2-furyl-3-pyrrolylmethane in a ratio of 6.21 are formed in the case of pyrrole.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 746–749, June, 1989.     

Ab Initio studies on the rotational equilibria of 2-substituted furan and thiophene carbonyl derivatives

Summary The relative stabilities of syn- and anti-isomers of 2-substituted furan and thiophene carbonyl derivatives are investigated by theab initio MO method. The energy differences between the rotamers are 1–3 kcal mol^–1 but the barriers to rotation are ca. 10 kcal mol^–1 so that free rotational mode is predicted to be a rather difficult process. Application of the self-consistent reaction field (SCRF) method to account for the solvent effect indicates that the isomer with a higher dipole moment (syn) is favored in solution. An electron withdrawing 2-substituent favors syn-isomers for furan carbonyls in contrast to thiophene carbonyls for which anti-isomers are favored. These trends are ascribable to a decrease in electrostatic repulsive and attractive interactions, respectively, in the syn forms of furan and thiophene carbonyls. Contribution of non-bonded repulsive interaction in the anti-isomer is important for the relative stability of the syn-isomer of furan carbonyl derivative. Solvent effects due to higher dielectric continuum are small on the absolute values of energy differences but can reverse the order of stability of the two isomers due to a greater stability acquired by an isomer (syn) with higher dipole moment in solution. The major factor determining stability, or instability, of syn-isomer is a repulsive electrostatic interaction between the two oxygen atoms for furan carbonyls and an attractive electrostatic interaction between the sulfur and oxygen atoms for thiophene carbonyls.     

Thiophene, furans, and related aromatic compounds from Eupatorium heterophyllum

A thiophene, two furans, a dihydro furan, and two acetylenic benzene derivatives were isolated from the roots of Eupatorium heterophyllum collected in China. Structures were established on the basis of spectroscopic analyses as well as chemical transformations.     

Reaction between Furan‐ or Thiophene‐2‐carbonyl Chloride,Isocyanides, and Dialkyl Acetylenedicarboxy­lates: Multicomponent Synthesis of 2,2′‐Bifurans and 2‐(Thiophen‐2‐yl)furans

An efficient multi‐component synthesis of highly functionalized 2,2′‐bifurans and 2‐(thiophen‐2‐yl)furans is described. A mixture of furan‐ or thiophene‐2‐carbonyl chloride, an isocyanide, and a dialkyl acetylenedicarboxylate undergoes a smooth addition reaction in dry CH₂Cl₂ at ambient temperature to produce 2‐amino‐5‐(4‐chlorofuran‐2‐yl)furan‐3,4‐dicarboxylates and 2‐amino‐5‐(4‐chlorothiophen‐2‐yl)furan‐3,4‐dicarboxylates. A single‐crystal X‐ray‐analysis of a derivative conclusively confirms the structure of these 2,2′‐bifurans and 2‐(thiophen‐2‐yl)furans. A novel electrophilic aromatic substitution reaction can justify the formation of the Cl‐substituted furan or thiophene rings.     

Synthesis of methyl-substituted benzofuro-, benzothieno-, and benzoselenopheno[2,3-b] pyridines

Three-ring benzo[b]furan, benzo[b]selenophene, and benzo [b]thiophene systems condensed with a pyridine ring and having a methyl group in the γ position of the pyridine ring were synthesized.     

The vibrational structures of furan, pyrrole, and thiophene cations studied by zero kinetic energy photoelectron spectroscopy

The vibrational structures of the electronic ground states ((approximately)X (2)A(2)) of furan, pyrrole, and thiophene cations have been studied by zero kinetic energy (ZEKE) photoelectron spectroscopic method. In addition to the strong excitations of the symmetric a(1) vibrational modes, other three symmetric vibrational modes (a(2), b(1), and b(2)) have been observed unambiguously. These results which cannot be explained by the Franck-Condon principle illustrate that the vibronic coupling and the Coriolis coupling may play important roles in understanding the vibrational structures of the five-membered heterocycle cations. The vibrationally resolved ZEKE spectra are assigned with the assistance of the density function theory calculations, and the fundamental frequencies for many vibrational modes have been determined for the first time. The first adiabatic ionization energies for furan, pyrrole, and thiophene were determined as 8.8863, 8.2099, and 8.8742 eV, respectively, with uncertainties of 0.0002 eV.     

A new facile approach to the synthesis of 3-methylthio-substituted furans, pyrroles, thiophenes, and related derivatives

2-(methylthio)-1,4-diaryl-2-butene-1,4-dione (3) are prepared from readily available aryl methyl ketones in the presence of copper(II) oxide, iodine, and dimethyl sulfoxide. The success of the cross-coupling reaction of 4-chloroacetophenone with 2-acetylthiophene confirms a proposed self-sorting tandem reaction mechanism. Both Z- and E-isomers of compound 3 are readily converted into the corresponding 3-methylthio 2,5-diaryl furan 7 in good yield through a domino process involving the reduction of the double bond followed by the Paal-Knorr furan synthesis. Meanwhile, 4-bromo-3-methylthio 2,5-diaryl furan 10 is obtained either by the treatment of furan 7 with molecular bromine or by the treatment of diketone 3 with 30% hydrogen bromide in acetic acid solution in one pot. Removal of the methylthio group is accomplished by the treatment of 7 with Raney Ni in ethanol, which affords the diaryl-substituted furan 11 in excellent isolated yield. Selective reduction of the double bond of compound 3 leads to the formation of the saturated 1,4-diketone 13, which is easily converted to the corresponding 3-methylthio-2,5-diaryl-substituted pyrrole 14 and thiophene 15 via the Paal-Knorr cyclization reaction.     

NMR studies on 4‐thio‐5‐furan‐modified and 4‐thio‐5‐thiophene‐modified nucleosides

Systematic NMR characterization of 4‐thio‐5‐furan‐pyrimidine nucleosides or 4‐thio‐5‐thiophene‐pyrimidine nucleosides (ribonucleosides and 2′‐deoxynucleosides) was performed. All proton and carbon signals of 4‐thio‐5‐thiophene‐ribouridine and related analogues were unambiguously assigned. The orientations of the base (4‐thiouridine or its deoxy analogue) relative to the ring (furan or thiophene) are explored by a NMR approach and further supported by X‐ray crystallographic studies. The procedures presented here would be applicable to other modified nucleosides and nucleotides. Copyright © 2016 John Wiley & Sons, Ltd.     

Synthesis and characterization of benzo[c]thiophene analogs incorporating benzo[b]thiophene/1-hexylindole/benzo[b]furan

Synthesis of 1,3-disubstituted benzo[c]thiophene analogs incorporating heterocycles such as benzo[b]thiophene/1-hexylindole/benzo[b]furan and thiophene units is described. Optical and electrochemical studies of the benzo[c]thiophene analogs are also reported.     

Cyclopentadienes Annelated with Five-membered Heterocycles: Methods of Synthesis, Heteroorganic Derivatives, and Synthetic Precursors. (Review)

The review is devoted to cyclopentadienes annelated with five-membered heterocycles (furan, pyrrole, thiophene, and their benzo analogs) (hetarenocyclopentadienes), their heteroorganic derivatives, and the synthetic precursors (hetarenocyclopentanones).