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Photolysis of alkylthiols (RSH) in EtOH/EtOK in the presence of ButOOBut yields the disulfide radical anions RSSR˙– in conditions suitable for EPR detection in solution over a wide temperature range.

The photochemical reaction of the only isolable aromatic thioaldehyde ArCHS (1; Ar = 2,4,6-tri-tert-butylphenyl) in alkaline medium yields ArCH2CH2Ar (2), ArCH2SCH2Ar (3), 6,8-di-tert-butyl-3,4-dihydro-4,4-dimethyl-1H-2-benzothiine (4) and ArMe (5). A radical mechanism which accounts for the formation of these compounds is proposed, mainly on the basis of the direct observation (EPR spectroscopy) of a number of radicals such as ArCHS˙–(A), ArCHCH2Ar (B), and ArCH2˙(D). Results of scrambling of deuterium atoms confirmed the structure of these radicals through interpretation of the corresponding EPR spectra.

The relative conformer stability in a series of naphthoyl σ-radicals has been investigated by e.s.r. techniques over a wide temperature range. In the case of the 1-naphthoyl radical two rotamers, with a conformational ratio apparently different from that of the parent molecule, were detected. In contrast, in the 2-naphthoyl radical the relative amounts of the conformers are, most likely, dependent upon those in the precursor aldehyde. Structural assignments were achieved by comparing the experimental data with INDO h.f.s. constants. The steric effect on the ratio of the conformers caused by methyl substituents was also examined in order to ascertain the reliability of the INDO approximation.

Addition of trimethylsilyl (Me3Si·) and methylthiyl (MeS·) radical to Me3SiCH2–CHCH2 and MeSCH2–CHCH2 yields, respectively, the radicals Me3SiCH2ĊHCH2SiMe3(I) and MeSCH2ĊHCH2SMe (II) which were observed by e.s.r. spectroscopy. Restricted rotation about the ·CH–CH2 bonds was detected below –100 °C and the free energy of activation determined [3.4 for (I) and 2.85 kcal mol–1 for (II), respectively]. The features of the line shapes also showed that the conformation of (I) is symmetric whereas that of (II) is asymmetric. The geometry of these conformations was also assessed and the angle ϕ between the 2pz orbital of ·CH and the C–X bond was found to be 4–5°§ when X = SiMe3(I) and 10–11° when X = MeS (II). The dependence of the β-splitting on the conformation and on the electronic properties of silicon and sulphur was studied by INDO calculations and the hypothesis that sulphur-containing radicals must be distorted to account for their low aHβ splitting is challenged.

Radicals obtained by addition of Et3Si˙ to the double bond of methylenecycloalkanes have been detected by e.s.r. at low temperature. The three-, four-, five-, and six-membered rings have been investigated. In the last case different splitting constants were detected for axial and equatorial hydrogens at low temperatures whereas a single average value is observed at higher temperatures. In the intermediate range line broadening effects allowed ΔG‡ 5.2 kcal mol–1 for the ring reversal process to be developed. In the case of the five-membered ring experimental evidence for a non-planar conformation has been also obtained. On the other hand the four-membered ring seems to be planar, at least within the approximation involved in these determinations.

Rotational barriers of some para-substituted benzoyl radicals have been measured by e.s.r. techniques: the linear correlation between the e.s.r. ΔG‡ values and the corresponding n.m.r. ΔG‡ values allows us to estimate the free energy of activation of the unsubstituted benzoyl radical [ΔG‡ 2.8 ± 0.15 kcal mol–1(11.8 ± 0.6 kJ mol–1)].

The methylthiyl radical (MeS˙) generated by photolysis of dimethyl disulphide (MeSSMe) is shown to give three types of reactions with cyclic molecules containing carbon–carbon double bonds. E.s.r. spectra corresponding to radicals formed by addition of MeS˙, by hydrogen abstraction, and by a rearrangement (possibly involving 1,4-hydrogen shift between SMe and the radical centre) have been detected. In the case of cyclopentene, all three types of radicals were observed. In the case of cyclohexene, only radicals due to abstraction and rearrangement were detected. With methylenecycloalkanes there was no evidence for hydrogen abstraction but radicals from addition and rearrangement were identified.

The e.s.r. spectra of the hydronitroxide radicals ArCHX-NHȮ, obtained by reaction of a number of aromatic aldoximes ArCHNOH (Ar = phenyl, 3- and 4-pyridyl, 2- and 3-thienyl, 2-furyl) with radicals X˙(X = MeS, BunS, Et3Si, Ph3Ge), have been detected. Assignment of the α- and β-proton splittings has been achieved by means of deuterium labelling. The analysis of the hyperfine splittings supports the hypothesis that the radicals ArCH(SR)-NHȮ adopt a single preferred conformation whereas the radicals ArCH(SiEt3)-NHȮ and ArCH(GePh3)-NHȮ undergo a rotational pathway through a variety of torsional conformers.

Triethylsilyloxy nitroxide radicals obtained by photolysis from the three isomeric nitropyridines have been investigated by e.s.r. spectroscopy. The kinetic study showed a first-order decay of the radicals; the concentration was found to depend upon the square root of the light intensity and therefore a reaction mechanism is postulated involving the reversible formation of a diamagnetic dimer. Proof of this was obtained by rapidly raising the temperature during the course of radical decay.

The low-temperature 13C n.m.r. spectra (25.16 MHz) of thiophene-2- and -3-carbaldehyde [(1) and (2)] display line-broadening effects due to the exchange between their S,O-syn- and S,O-anti-conformers. In both cases the free energy of activation for rotation of the –CHO moiety was measured, the values being 10.15 and 8.5 kcal mol–1 for (1) and (2), respectively. In the case of thiophene-3-carbaldehyde (2) the signals of both conformers were detected, the ratios being 97:3 at –140 and 93:7 at –100 °C in favour of the S,O-anti-conformer. In thiophene-2-carbaldehyde (1) the minor conformer (S,O-anti) was present in too small an amount to be unambiguously detected by a conventional spectrometer, but spectra taken with a superconducting instrument (75.48 MHz) revealed ca. 1.5% of this conformer at –100 °C.

1 H and 13C n.m.r. measurements show that hydrazones containing the 2,2,6,6-tetra methylpiperidin-1-yl (TMP) ring (i.e. TMP–NCR1R2) have the–NC plane 90° twisted with respect to the dynamically averaged plane of the TMP ring itself. Restricted rotation occurs around the N–N bond, allowing the determination of the torsional barriers by means of line shape analysis of the variable temperature n.m.r. spectra. Steric and conjugative effects upon the barrier when R2 is an aromatic substituent (phenyl, naphthyl, pyridyl, thienyl, fury) are discussed.

Results are reported of an e.s.r. investigation of the reactions of the methyl radical, generated from dimethyl sulphoxide with the TiIII–H2O2 couple, with a variety of aliphatic compounds in aqueous solution. For example, with carboxylic acids and nitriles, the spectra of radicals formed by hydrogen-atom abstraction are detected: it is shown that the methyl radical is more selective than hydroxy, having a relatively greater propensity for abstracting hydrogen from the carbon atom adjacent to the functional group, and rate constants for these reactions, which are in the range ca. 102–104 l mol–1 s–1, have been determined. Reaction with nitromethane is more complex. The spectra observed are those of the nitroethane radical-anion or its conjugate acid, depending on the pH, and it is concluded that abstraction by the methyl radical to give ·CH2NO2 is followed by rapid one-electron reduction by TiIII to give the aci-compound to which methyl then adds. Nitroethane behaves analogously, and both EtNO2H and PriNO2H radicals are estimated to have pKa 4·4.

2′-(Phenylazo)biphenyl-2-yl radicals (3; Ar = Ph), produced by reduction of 2′-(phenylazo)biphenyl-2-diazonium tetrafluoroborate (6) with potassium iodide at –20 °C, undergo intramolecular 1,5-addition to the azo-group, affording N-(carbazol-9-yl)phenylaminyl radicals (4; Ar = Ph), from which 1,2-dicarbazol-9-yl-1,2-diphenylhydrazine (8) is formed by N,N-coupling. The hydrazine (8) is thermally unstable and easily fragments at room temperature into aminyl radicals (4; Ar = Ph), which appear to undergo hydrogen abstraction and C,N-coupling as the major reaction paths.

ion of an allylic hydrogen in methylene cyclo-propane, -butane, -pentane, and -hexane affords the corresponding allylic radicals. Their e.s.r. spectra can be detected in appropriate conditions below –100 °C. In the case of the six-membered ring radical the two pseudoaxial and the two pseudoequatorial hydrogens are diastereotopic, thus indicating that, as expected, a half-chair conformation is adopted. On the other hand the five-membered ring radical prefers an envelope conformation, whereas the four-membered ring radical (and obviously the three-membered) are planer.

A number of radicals generated by photolysis add to furan and thiophen giving radicals involved in homolytic aromatic substitution: the corresponding e.s.r. spectra were observed. Photolysis of (PhCOO)2 in furan or thiophen gives the spectra of both (A) and (B). On the other hand photolysis of (MeCOO)2 in furan gave the spectrum [graphic omitted] of (C) at –110° but that of (D) at higher temperature (–10°). The interpretation of the aH hyperfine splitting constants suggests that the structure of these radicals is not planar. They seem rather to prefer envelope-like conformations with the substituents either in pseudoaxial or in pseudoequatorial positions or in equilibrium between the two, depending on the substituent.

The e.s.r. spectra of the 9-phenylxanthenyl, 9-phenylthioxanthenyl, and 9-phenylselenoxanthenyl radicals have been interpreted; unambiguous assignments of the hyperfine splitting constants have been made by use of a number of selectively deuteriated derivatives. Although the theoretical models used to interpret the experimental data failed to explain all the details of the spin-density distribution, they allowed certain conclusions to be drawn with respect to the stereochemistry of the molecules.The results show that the phenyl groups do not rotate freely, but are locked at an angle of ca. 70–80° degrees to the heterocyclic plane.

E.s.r. spectra of some 5-substituted 2-thienyl nitroxides, generated by scavenging triethylsilyl radicals with substituted nitrothiophens, and nitro-anions have been recorded in order to determine the effective substituent delocalising power with respect to an unpaired electron in the thiophen series. The results are discussed in connection with chemical data. Hyperfine splitting constants calculated for the substituted nitro-anions by the INDO method, using an s,p basis set for the sulphur atom, are in good agreement with the experimental values.

Triethylsilyloxy nitroxide radicals of thiophen and thienothiophens have been produced from the corresponding nitro-derivatives. Their e.s.r. spectra showed the presence of rotational isomers at low temperature for the thiophen radicals and even at room temperature for the thienothiophen analogues. In the former case the kinetics of the interconversion could be followed by examining variations in the line shape with temperature. The activation parameters have been determined and the rotational barrier is larger for the 2-thienyl than for the 3-thienyl silyloxy nitroxide. For the purpose of comparison the parameters related to the internal rotation of phenyl triethylsilyloxy nitroxide have been obtained.

The homolytic reactivity of 2- and 3-methylthiophens towards hydrogen abstraction from the side-chain by t-butoxyl radicals has been investigated by determining the relative amount either of the coupling products of 2- and 3-thenyl radicals or of the nitroxides obtained by trapping with ButNO. The conditions for the latter experiment have been explored and are discussed. Both methods showed that 2-methylthiophen is 2·2 time more reactive than the 3-isomer.

Radical cations containing a thiophen ring substituted by 2,5-bisalkylthio-groups have been obtained and their e.s.r. spectra interpreted in terms of unequally populated rotational isomers. Assignment of their structure has been achived by examined the effect of steric hindrance caused by bulkyl substituents on the ratio of conformers. The hyperfine splitting constants have been assigned experimentally to the asymmetric conformers, the results being in agreement with the conclusions of theoretical methods. Theoretical calculations (CNDO/2) indicate that, for these derivatives, the most populated conformation of the radical cation corresponds to the most stable rotational isomer of the neutral molecule.

Alkali-metal reduction of some thienylethylene derivatives at –80° yields the corresponding radical anions whose e.s.r. spectra have been interpreted by the use of deuteriated derivatives. The results indicate that phenyl and thienyl groups are locked in a definite position, on the e.s.r. time scale; rotational isomers were detected in the case of the 1,2-di-(2-thienyl)ethylene. The experimental hyperfine splitting constants could be reproduced satisfactorily by molecular orbital calculations, assuming, in the p-model framework, twisting angles of 40° for the 3-thienyl, and 0° for the 2-thienyl and phenyl groups. An analogous theoretical approach, which takes into account the participation of the 3d orbitals of sulphur, was found to disagree with the experimental results.

The 13C n.m.r. spectra of the four symmetrical naphthyridines have been obtained and assignment of the signals performed by means of heteronuclear decoupling experiments and empirical additivity methods. The shifts were found to depend linearly upon the total charge density on the carbon atoms as calculated by self-consistent molecular orbital INDO and CNDO approximations.