One-electron oxidants

The. more tightly held an electron is. the more difficult it is to remove, hence the higher the electrode potential necessary to remove it. Make the reasonable hypothesis that the electron removed in a one-electron oxidation comes from the highest occupied orbital. HOMO. Using SHMO. determine the HOMO for ben7 ene, biphenyl, and naphthalene. 

Subsequent studies (63,64) suggested that the nature of the chemical activation process was a one-electron oxidation of the fluorescer by (27) followed by decomposition of the dioxetanedione radical anion to a carbon dioxide radical anion. Back electron transfer to the radical cation of the fluorescer produced the excited state which emitted the luminescence characteristic of the fluorescent state of the emitter. The chemical activation mechanism was patterned after the CIEEL mechanism proposed for dioxetanones and dioxetanes discussed earher (65). Additional support for the CIEEL mechanism, was furnished by demonstration (66) that a linear correlation existed between the singlet excitation energy of the fluorescer and the chemiluminescence intensity which had been shown earher with dimethyl dioxetanone (67). 

Reactions with Parting of Radicals. The one-electron oxidation of cationic dyes yields a corresponding radical dication. The stabihty of the radicals depends on the molecular stmcture and concentration of the radical particles. They are susceptible to radical—radical dimerization at unsubstituted, even-membered methine carbon atoms (77) (Fig. 6). 

Fig. 6. One-electron oxidation and dimerization where (21a) is a dye, (21b) a radical cation, and (21c) a dimer.

Polymerization Initiator. Some unsaturated monomers can be polymerized through the aid of free radicals generated, as transient intermediates, in the course of a redox reaction. The electron-transfer step during the redox process causes the scission of an intermediate to produce an active free radical. The ceric ion, Ce" ", is a strong one-electron oxidizing agent that can readily initiate the redox polymerization of, for example, vinyl monomers in aqueous media at near ambient temperatures (40). The reaction scheme is... 

Radical cations can be derived from aromatic hydrocarbons or alkenes by one-electron oxidation. Antimony trichloride and pentachloride are among the chemical oxidants that have been used. Photodissociation or y-radiation can generate radical cations from aromatic hydrocarbons. Most radical cations derived from hydrocarbons have limited stability, but EPR spectral parameters have permitted structural characterization. The radical cations can be generated electrochemically, and some oxidation potentials are included in Table 12.1. The potentials correlate with the HOMO levels of the hydrocarbons. The higher the HOMO, the more easily oxidized is the hydrocarbon. 

Wurstert salts. Generated by one-electron oxidation of the diamine. Indefinitely stable. 

One-electron oxidation of carboxylate ions generates acyloxy radicals, which undergo decarboxylation. Such electron-transfer reactions can be effected by strong one-electron oxidants, such as Mn(HI), Ag(II), Ce(IV), and Pb(IV) These metal ions are also capable of oxidizing the radical intermediate, so the products are those expected from carbocations. The oxidative decarboxylation by Pb(IV) in the presence of halide salts leads to alkyl halides. For example, oxidation of pentanoic acid with lead tetraacetate in the presence of lithium chloride gives 1-chlorobutane in 71% yield ... 

An intriguing class of persistent radicals are those formed by the one-electron oxidation of the hexagonal prismatic clusters Li2[E(N Bu)3] 2 (3.21, E = S, Se). The air oxidation of 3.21 produces deep blue (E = S) or green (E = Se) solutions in toluene. The EPR spectra of these solutions consist of a septet (1 3 6 7 6 3 1) of decets (Eig. 3.5). This pattern results from interaction of the unpaired electron with three equivalent 7=1 nuclei, i.e., and three equivalent I = 3/2 nuclei, i.e., Ei. It has been proposed that the one-electron oxidation of 3.21 is accompanied by the removal of an Ei" cation from the cluster to give the neutral radical 3.22 in which the dianion [S(N Bu)3] and the radical monoanion [S(N Bu)3] are bridged by three Ei" cations. 

The oxidation of the macrocycle in phthalocyanines does not generally lead to stable molecules. One-electron oxidations of the macrocyclc(s) in bis(phthalocyanines) leading to species which can be isolated have been carried out chemically or electrochemically,... 

Tetrabutylammonium [bismuth(III) bis(phthalocyanine)] undergoes a one-electron oxidation (dichloromethane, U = IV, platinum electrode, several days) to give bismuth bis(phthalocyanine).167... 

Scheme 10.16 Redox cycle of flavins. The cycle is depicted with a two-electron reduction of flavin by NAD(P)H and two one-electron oxidations.

K Fe(CN)6 oxidation Compound F is stoichiometrically inactivated by oxidation with K.3Fe(CN)6 (Shimomura and Johnson, 1967) thus, it is possible to estimate the molecular extinction coefficient (e) of the 388-390 nm absorption peak by titrating F with K.3Fe(CN)6- The e value obtained by the titration in 50% ethanol was 15,400 (assuming the reaction to be one-electron oxidation) or 30,800 (assuming two-electron oxidation). Two other methods of lesser precision were used to determine the true s value 1) the dry weight of the ethyl acetate extract of an acidified solution of F gave an e value of 14,100 2) the comparison of NMR signal intensities gave a value of 11,400 2,000 in water (H. Nakamura, Y. Oba, and A. Murai, 1995, personal... 

One-electron oxidation to [Rh2(OCOMe)4(H20)2]+ leads to an ion (violet to orange, depending on solvent) with a shorter Rh-Rh bond (2.317 A) than that in the neutral molecule (2.385 A), suggesting the electron has been removed from an orbital with anti-bonding character. 

The dinuclear rhodium(II) acetate is described in section 2.8.2 the dinuclear structure is retained on one-electron oxidation, but when ozone is used as the oxidant, a compound with a trinuclear Rh30 core is formed, analogous to those formed by Fe, Cr, Mn and Ru. (It can also be made directly from RhCl3.)... 

A final class of multifunctional initiators is based on the use a (muUi)functional polymer and a low molecular weight redox agent. Radicals on the polymer chain arc generated from the polymer bound functionality by a redox reaction. Ideally, no free initiating species are formed. The best known of this class are the polyol-redox and related systems. Polymers containing hydroxy or glycol and related functionality are subject to one electron oxidation by species such as ceric ions or periodate (Scheme 7.23).266,267 Substrates such as cellulose,... 

Transition metal catalysts arc characterized by their redox ehemistry (catalysts can be considered as one electron oxidants/reductants). They may also be categorized by their halogen affinity. While in the initial reports on ATRP (and in most subsequent work) copper266,267 or ruthenium complexes267 were used, a wide range of transition metal complexes have been used as catalysts in ATRP. 

Ce4+ is a versatile one-electron oxidizing agent (E° = - 1.71 eV in HC10466 capable of oxidizing sulfoxides. Rao and coworkers66 have described the oxidation of dimethyl sulfoxide to dimethyl sulfone by Ce4+ cation in perchloric acid and proposed a SET mechanism. In the first step DMSO rapidly replaces a molecule of water in the coordination sphere of the metal (Ce v has a coordination number of 8). An intramolecular electron transfer leads to the production of a cation which is subsequently converted into sulfone by reaction with water. The formation of radicals was confirmed by polymerization of acrylonitrile added to the medium. We have written a plausible mechanism for the process (Scheme 8), but there is no compelling experimental data concerning the inner versus outer sphere character of the reaction between HzO and the radical cation of DMSO. 

The stable (f-Bu2MeSi)3Si radical reacts with lithium in hexane at room temperature affording the silyllithium 7, with the central anionic silicon atom almost planar, whereas the one-electron oxidation with tiiphenylmethyl ion produces the corresponding silylium ion 8 (Scheme... 

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