Giorgio Martelli

Conformational isomers and rotational barriers in the ketyl radicals of thiophen and thienothiophens

The conformational isomers of the ketyl radicals (I)–(III) generated from di-2-thienyl ketone, bis-2-thieno[3,2-b] thienyl ketone, and bis-2-thieno[2,3-b]thienyl ketone, have been detected by e.s.r. spectroscopy. The two preferred conformations are cis–trans and trans–trans. Study of the line shape variations with temperature of the e.s.r. spectra allowed the determination of the activation parameters for the rotation of the aromatic rings. The activation energies follow the same order as the electron-delocalizing power of the nuclei, the values being 31·30, 37·95, 35·73, and 26·57 kJ mol–1 for (I), (II), (III) and for benzophenone ketyl, respectively.

Rotational isomerism in the radical anions of bi- and ter-aryls containing the thiophen ring

The conformational behaviour of the radical anions of several bi- and ter-aryls has been investigated by e.s.r. spectroscopy. Comparison with their diamagnetic precursors shows that the addition of the unpaired electron to the π-system greatly increases the conformational rigidity of these compounds. This result is accounted for by INDO MO calculations, and can be rationalized in terms of the occupancy by the unpaired electron of the lowest vacant orbital of neutral bi- and ter-aryls which has a strong bonding character in the interannular regions.

Radical anions of substituted thienothiophens

The e.s.r. spectra of the radical anions of carbonyl-, nitro-, and cyano-derivatives of thieno[3,2-b]thiophen and thieno[2,3-b]thiophen have been investigated in order to compare their properties with those of the corresponding substituted thiophens. From an examination of the hyperfine splitting constants it is concluded that in the case of thieno[3,2-b]thiophen the unpaired electron is delocalized over the whole molecule, while in thieno[2,3-b]thiophen the condensed thieno-group is scarcely involved. Nitrogen coupling constants for (II) and (III), as well as the proton splittings in the mixed terms of ketones (I) indicate that thieno[3,2-b]thiophen has a delocalization power greater than the thieno[2,3-b]thiophen which in its turn is slightly more efficient than thiophen.

Radical anions and nitroxides from alkylthio-, alkylsulphinyl-, and alkylsulphonyl-nitrobenzenes

The e.s.r. spectra of the radical anions and nitroxides from nitrobenzenes substituted with SR, SOR, and SO2R groups are reported. The effect of the substituent on the spin density distribution as revealed by the variations of the nitrogen and ring protons splitting constants, is discussed. While the alkylsulphinyl and alkylsulphonyl groups behave as electron attractors, their electron-withdrawing ability being little dependent on the nature of the alkyl substituent, the alkylthio-group has an electron-acceptor character strongly dependent on the dimensions and therefore on the conformation of the aliphatic substituent. As a possible explanation it is suggested that σ-π conjugation with electron release from the π orbital containing the unpaired electron to the low lying σ* antibonding orbital of the C–S bond is responsible for this behaviour.

Structural effects on the reactivity of carbon radicals in homolytic aromatic substitutions. Part III. Reaction of the 1-adamantyl radical with benzene derivatives

Homolytic substitution of monosubstituted benzenes by 1-adamantyl radical has been carried out and the reactivity and the polar character of the radical have been determined by examining the effects of substituents on isomer distributions and on relative reactivities. The results indicated that the 1-adamantyl radical has nucleophilic properties which are more pronounced than that of other more strained bridgehead radicals; it is suggested that this behaviour is attributable to the different role played by polar effects during the addition of bridgehead radicals to aromatic substrates. The syntheses of several monosubstituted 1-aryladamantanes are also described.

The reactivity of thieno[3,2-b]thiophen and thieno[2,3-b]thiophen with phenyl radicals

The isomeric thieno[3,2-b]thiophen (I) and thieno[2,3-b]thiophen (II) have been phenylated with phenyl radicals derived from the thermal decomposition of N-nitrosoacetanilide and from the reaction of aniline and pentyl nitrite. The radical substitution is favoured in the 2-position in the case of compound (I), but in structure (II) the 2- and 3-positions have comparable reactivities. The values of the relative reactivities show that for homolytic phenylation the order of decreasing reactivity is thieno[3,2-b]thiophen > thiophen > thieno[2,3-b]thiophen. The experimental results may be rationalised on the assumption that the contribution of the condensed thieno-group to the stabilisation of the intermediates is greater in (I) than in (II).The syntheses of 2- and 3-phenylthieno[3,2-b]thiophens and of 2- and 3-phenylthieno[2,3-b]thiophens are described.

Homolytic aromatic substitution of heterocyclic compounds. Part V. The phenylation of benzo[b]thiophen and benzo[b]furan

The homolytic phenylation of benzo[b]thiophen and benzo[b]furan has been carried out, with the thermal decomposition of N-nitrosoacetanilide as the source of phenyl radicals. The isomer ratios and the relative reactivities have been determined. Homolytic substitution in benzo[b]furan takes place almost exclusively in the heterocyclic ring, in contrast with benzo[b]thiophen, in which all the available positions show a comparable reactivity towards substitution by phenyl radicals.The syntheses of a number of phenylbenzo[b]furans and phenylbenzo[b]thiophens are reported.