Benzene Radical Ions

Kimura Y, Alfano JC, Walhout PK, Barbara PF (1994) Ultrafast transient absorption spectroscopy of the solvated electron in water. J Phys Chem 98 3450-3458 Li X, Sevilla MD, Sanche L (2003) Density functional theory studies of electron interaction with DNA can zero eV electrons induce strand breaks J Am Chem Soc 125 13668-13699 Lind J, Shen X, Eriksen TE, Merenyi G, Eberson L (1991) One-electron reduction of N-bromosuccin-imide. Rapid expulsion of a bromine atom. J Am Chem Soc 113 4629-4633 Marasas RA, lyoda T, Miller JR (2003) Benzene radical ion in equilibrium with solvated electrons. J Phys Chem A 107 2033-2038... 

The position of the benzene radical ions among the other benzenoid systems is somewhat exceptional. The presence of the degenerate frontier molecular orbitals makes it difficult to perform standard open-shell calculations because the LHP method is inherently incapable of accommodating systems of this type and the Roothaan procedure diverges here The simple Cl treatments based on the HMO computational... 

XI. Benzene Radical Ions A. Benzene Radical Cation ... 

Xu and Li (1989) investigated H — CIDNP spectra of fifteen substituted benzene-diazonium ions during reduction with NaBH4. The spectra are consistent with a mechanism in which the first step is the addition of a hydride ion to the diazonium ion. The diazene formed (Ar - N2 - H) is assumed to dimerize and disproportionate into a radical pair [Ar-N-NH2 N = N — Ar] which loses one equivalent of N2 yielding [Ar—N —NH2 Ar] and recombines to give the diarylhydrazine. A proportion of the aryl radicals escape and form the hydro-de-diazoniation product. 

Detection of an Intermediate. In many cases, an intermediate cannot be isolated but can be detected by IR, NMR, or other spectra. The detection by Raman spectra of NOj was regarded as strong evidence that this is an intermediate in the nitration of benzene (see 11-2). Free radical and triplet intermediates can often be detected by ESR and by CIDNP (see Chapter 5). Free radicals [as well as radical ions and EDA complexes] can also be detected by a method that does not rely on spectra. In this method, a doublebond compound is added to the reaction mixture, and its fate traced. One possible result is cis-trans conversion. For example, cis-stilbene is isomerized to the trans isomer in the presence of RS- radicals, by this mechanism ... 

Allyl (27, 60, 119-125) and benzyl (26, 27, 60, 121, 125-133) radicals have been studied intensively. Other theoretical studies have concerned pentadienyl (60,124), triphenylmethyl-type radicals (27), odd polyenes and odd a,w-diphenylpolyenes (60), radicals of the benzyl and phenalenyl types (60), cyclohexadienyl and a-hydronaphthyl (134), radical ions of nonalternant hydrocarbons (11, 135), radical anions derived from nitroso- and nitrobenzene, benzonitrile, and four polycyanobenzenes (10), anilino and phenoxyl radicals (130), tetramethyl-p-phenylenediamine radical cation (56), tetracyanoquinodi-methane radical anion (62), perfluoro-2,l,3-benzoselenadiazole radical anion (136), 0-protonated neutral aromatic ketyl radicals (137), benzene cation (138), benzene anion (139-141), paracyclophane radical anion (141), sulfur-containing conjugated radicals (142), nitrogen-containing violenes (143), and p-semi-quinones (17, 144, 145). Some representative results are presented in Figure 12. 

Another type of interaction is the association of radical ions with the parent compounds. Recently (118), a theoretical study was reported on the interaction of butadiene ions with butadiene. Assuming a sandwich structure for the complex, the potential curve based on an extended Hiickel calculation for two approaching butadienes (B + B) revealed only repulsion, as expected, while the curves for B + and B + B" interactions exhibit shallow minima (.068 and. 048 eV) at an interplanar distance of about 3.4 A. From CNDO/2 calculations, adopting the parameter set of Wiberg (161), the dimer cation radical, BJ, appears to be. 132 eV more stable than the separate B and B species, whereas the separate B and B species are favored by. 116eV over the dimer anion radical, BJ. This finding is consistent with experimental results formation of the dimer cation radical was proved in a convincing manner (162) while the attempts to detect the dimer anion radical have been unsuccessful. With other hydrocarbons, the reported formation of benzene dimer anion radical (163) represents an exceptional case, while the dimeric cation radical was observed... 

Both benzene dimer ion radicals were a subject of a recent theoretical study (166). 

New synthetic transformations are highly dependent on the dynamics of the contact ion pair, as well as reactivity of the individual radical ions. For example, the electron-transfer paradigm is most efficient with those organic donors yielding highly unstable cation radicals that undergo rapid unimolecular reactions. Thus, the hexamethyl(Dewar)benzene cation radical that is generated either via CT activation of the [D, A] complex with tropylium cation,74... 

It is now well established that when a surface presents electron donor or electron acceptor sites, it is possible to ionize molecules of relatively high electron affinity (> 2 eV) or low ionization potential values, resulting in paramagnetic radical ions. For instance anthracene and perylene are easily positively ionized on alumina (7 ) (IP = 7.2 and 6.8 eV respectively). The adsorption at room temperature of benzenic solution of perylene, anthracene and napthalene on H-ZSM-5 and H-ZSM-11 samples heated up to 800°C prior to adsorption did not give rise to the formation of the corresponding radical cation. For samples outgassed at high... 

Many other ion-molecule reactions involving highly unsaturated hydrocarbon ions and neutral olefins or the equivalent strained cycloalkanes have been studied by mass spectrometry98. For example, we may mention here the addition of ionized cyclopropane and cyclobutane to benzene radical cations giving the respective n-alkylbenzene ions but also isomeric cyclodiene ions such as ionized 8,9-dihydroindane and 9,10-dihydrotetralin, respectively. Extensive studies have been performed on the dimerization product of charged and neutral styrene4. 

M. Iwamoto, K. Matsukami, and S. Kagawa, Catalytic oxidation by oxide radical ions. 1. One-step hydroxylation of benzene to phenol over group 5 or 6 oxides supported on silica gel, J. Phys. Chem. 87,903 (1983). 

A review considering the generation and characterization of radical ions, their reactions, formation of species with three-electron bonds, and radical cations of strained systems has been published." The redox and acidity properties of a number of substituted benzene radical cations were smdied by pulse radiolysis. ... 

Optically active benzene(poly)carboxamides and benzene(poly)carboxy-lates were used by Inoue and co-workers as sensitizers for the geometrical photoisomerization of (Z)-cyclooctene and (Z,Z)-cyclooctadienes in various solvents at different temperatures. Under energy-transfer conditions, enantiomeric excesses up to 64% ee in unpolar solvents like pentane were reported. The use of polar solvents diminished the product ee s due to the intervention of a free or solvent-separated radical ion pair generated through the electron transfer from the substrate to the excited chiral sensitizer (Scheme 58) [105-109]. 

The benzyne radical ion can also be accessed by dissociation of benzene ion. The recent dissociative REMPI measurements by Neusser and co-workers for the reaction. 

Hiberty PC (1990) The Distortive Tendencies of Delocalized tt Electronic Systems. Benzene, Cyclobutadiene and Related Heteroannulenes. 153 27-40 Hintz S, Heidbreder A, Mattay J (1996) Radical Ion Cyclizations. 177 77-124 Hirao T (1996) Selective Transformations of Small Ring Compounds in Redox Reactions. 178 99-148... 

Of the radical ions derived from aromatic hydrocarbons, we mention the ions of benzene and tetracene. For benzene, both the positive and negative ions have been characterized by ESR spectroscopy. The radical anion shows seven evenly spaced lines ( H = 0.341 mT), ° suggesting that the spin density is distributed evenly over the six carbons (or that there is a fast equilibrium between structures corresponding to the two degenerate antibonding SOMOs Fig. 6.7). Introducing a single D is sufficient to disturb the equilibrium ( h = 0.3983 mT, 4H ah = 0.3454 mT,... 

Hexakis(benzylthio)benzene acts as a rather efficient electron donor. The ESR spectrum obtained with in situ electrolysis indicates a pattern corresponding to the twelve equivalent protons in the benzyl positions. However, cyclovoltammet-ric measurements reveal that the radical cation exists in a complicated equilibrium with the dication and the parent neutral precursor53. Radical ions generated from C60 have been the subject of several publications and discussions within the last years. Several authors have postulated that C6o + can be observed in solid C60. However it could be demonstrated that this signal has to be ascribed to C6a peroxide or its decomposition products54. 

Mass spectra of hydroxy- and alkoxy-coumarins have been very intensively studied. The decomposition sequence of 3-hydroxycoumarin is initiated by carbon monoxide loss from the molecular ion giving a 2-hydroxybenzofuran ion. Subsequent fragmentation occurs by two major pathways, involving a further loss of CO and expulsion of a formyl radical. The former leads to the base peak, and thence by another loss of CO to give the abundant benzene radical cation at m/e 78. The other main pathway gives a benzoyl cation which leads to the phenonium ion at m/e 77 (77IJC(B)816). 

Initially, the products of these reactions suggested radical ions were involved138. In particular, when hexamethyl Dewar-benzene was epoxidized with MCPBA, the nature of the products depended on whether or not the iron(III) porphyrin hemin was added to the reaction mixture1381 . Furthermore, when Z-stilbene was epoxidized with dioxygen, catalyzed by (tetraphenylporphorinato)iron(III) chloride, -stilbene appeared in the reaction mixture139. 

Electron spin resonance (ESR) spectroscopy is of application to organic species containing unpaired electrons radicals, radical ions and triplet states, and is much more sensitive than NMR it is an extremely powerful tool in the field of radical chemistry (see Chapter 10). Highly unstable radicals can be generated in situ or, if necessary, trapped into solid matrices at very low temperatures. Examples of the application of this techniques include study of the formation of radical cations of methoxylated benzenes by reaction with different strong oxidants in aqueous solution [45], and the study of the photodissociation of N-trityl-anilines [46],... 

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