Structure and reactivity of organic radical cations

An Electron Spin Resonance Investigation of the Radiolysis of Hexamethylbenzene and Hexamethyl(Dewar)Benzene

Exposure of hexamethyl(Dewar)benzene (HMD) to γ-rays in a solid CFC13 matrix at 77K generated the corresponding radical cation (HMD+•) as shown by the observation of hyperfine coupling (A = 9·5 G) to the protons of four equivalent methyl groups: thus the 2B2 state (I) of the ion is established. On annealing to 140 K, ring-opening occurred to give the hexamethylbenzene cation (HMB+•). When CF2ClCFCl2 was used as the matrix material, a completely different spectrum was produced, consisting of seven lines with a common spacing of 14 G. Results obtained using different batches of the freon, and from careful annealing and concentration studies, establish that the low-field line is not part of the remaining six-line spectrum which can be assigned to the allylic radical (III), formed by deprotonation of the parent cation. γ-Irradiation of HMD in 2- methyltetrahydro-furan (MeTHF) followed by annealing gave the ψs* Jahn-Teller distorted form of the (ring-opened) HMB-• radical anion [A(6H) = 9 G] and so the parent anion is unstable, as is the cation. This was confirmed by an identical spectrum being obtained after the irradiation of an authentic sample of HMB in MeTHF. γ-Irradiation of pure HMD gave rise to a spectrum that can be analysed in terms of (III) and the endo H-atom adduct of HMD (IV). Hexamethylbenzene (HMB) on γ-irradiation in solid CFC13 gave the parent radical cation (HMB+•) At 130 K, this has a 6·7 G splitting and correct binomial intensities for 18 equivalent protons; on cooling to 77 K, the cation distorts so that a ψA type SOMO is adopted with four strongly coupled (A = 11·5G) and two weakly coupled (A = −2·8G) methyl groups. γ-Irradiation of pure HMB gave only broad features which we ascribe to a mixture of pentamethylbenzyl (VIII) and hexamethylcyclohexadienyl (IX) radicals, in which proton hyperfine structure is unresolved due to a range of restricted conformations being adopted by various methyl groups because of steric hindrance between them.

Understanding Reactivity Patterns of the Dialkylaniline Radical Cation

N,N-Dimethylaniline and N,N-diethylaniline react with Cu2+ to form the corresponding amine radical cations. The radical cations were characterized by their absorption spectra. In the absence of any nucleophiles, the radical cations dimerize to give tetraalkylbenzidines, and this reaction can be monitored by absorption spectroscopy. In the presence of nucleophiles such as Cl[negative in circle], Br[negative in circle], or SCN[negative in circle], the radical cations undergo nucleophilic substitution to give para-substituted dialkylanilines in good yields.

Anodic voltammetric assay of lansoprazole and omeprazole on a carbon paste electrode

The electrochemical oxidations of lansoprazole and omeprazole have been studied at a carbon paste electrode by cyclic and differential-pulse voltammetry in Britton-Robinson buffer solutions (0.04 M; pH 6.0-10.0). The drug produced a single oxidation step. By differential-pulse voltammetry, a linear response was obtained in B-R buffer pH 6.0 in a concentration range from 2.0 x 10(-7) to 5.0 x 10(-5) M for lansoprazole or omeprazole. The detection limits were 1.0 x 10(-8) and 2.5 x 10(-8) M for lansoprazole and omeprazole, respectively. The method was successfully applied for the analysis of omeprazole and lansoprazole in capsules. The results were comparable to those obtained by spectrophotometry.

Omeprazole determination using HPLC with coulometric detection

A sensitive high performance liquid chromatography (HPLC) method for the determination of omeprazole and three related benzimidazoles is reported. Coulometric detection was carried out at +800 mV using a porous carbon electrode. The linear range is 0.01-10 microg/ml. The method has a high degree of precision; the relative standard deviation of omeprazole at a concentration of 1.06 microg/ml was 0.7% (n=4). The cyclic voltammogram of omeprazole is consistent with the hydrodynamic voltammogram exhibiting a single major irreversible oxidative wave with a peak potential at +1105 mV. The response factors for the four compounds are similar indicating that the oxidative process does not involve the sulfur moiety exclusively. The data are most consistent with oxidation primarily of the benzimidazole groups. The method was applied successfully to the determination of omeprazole in a paste formulation.

Preparation and Structures of Crystalline Aromatic Cation-Radical Salts. Triethyloxonium Hexachloroantimonate as a Novel (One-Electron) Oxidant

Triethyloxonium hexachloroantimonate [Et(3)O(+)SbCl(6)(-)] is a selective oxidant of aromatic donors (ArH), and it allows the facile preparation and isolation of crystalline paramagnetic salts [ArH(+)(*), SbCl(6)(-)] for the X-ray structure determination of various aromatic cation radicals. The mechanistic relationship between the Meerwein salt [Et(3)O(+)SbCl(6)(-)] and the pure Lewis acid oxidant SbCl(5) is based on a prior ethyl transfer from oxygen to chlorine within the ion pair.

The effect of meta- or para-cyano substitution on the reactivity of the radical cations of arylalkenes and alkanes. Radical ions in photochemistry, Part 34

The effect of electron-withdrawing substituents, meta- or para-cyano, on the reactivity of the radical cation of arylalkenes and alkanes has been determined. The radical cations were generated by single electron transfer (set) to an electron-accepting photosensitizer. Three reactions were studied: (i) the addition of nucleophile to the radical cation of arylalkenes, (ii) cleavage of the benzylic carbon–carbon bond of the radical cation of arylalkanes; and (iii) the deprotonation of the benzylic carbon–hydrogen bond of the radical cation of arylalkanes. The radical cations of 4-(1-phenylethenyl)benzonitrile (1b), 3-(1-phenylethenyl)benzonitrile (1c), 4-(2-methoxy-1-phenylethyl)benzonitrile (2b), 3-(2-methoxy-1-phenylethyl)benzonitrile (2c), cis- and trans-5-cyano-2-methoxy-1-phenylindane (6b-cis and -trans), and 6-cyano-3-phenylindene (7b) were generated, by single electron transfer to the lowest excited singlet state of 1,4-dicyanobenzene (3), in acetonitrile–methanol. The radical cations of 1b, 1c, and 7b react with methanol to yield the anti-Markovnikov adducts (2b, 2c, and 6b-cis and 6b-trans). The radical cations of 2b, 2c, and 6b-trans cleave at the benzylic carbon–carbon bond to give products derived from the radical and carbocation fragments. The radical cation of 6b-cis deprotonates from the benzylic position with subsequent formation of the diastereomer, 6b-trans. This behaviour can be explained/predicted on the basis of the proposed mechanisms for these reactions. Molecular orbital calculations (AM1) support the conclusions. Keywords: photosensitized, electron transfer, radical ions, radicals, molecular orbital calculations (AM1).