Heterocycles radical ions

The generation and structure of heterocyclic radical ions has been discussed previously [2] and needs not be addressed at length in this context, because the main concern here is with reactivity. A few examples of the main structural differences are, however, reported in this section to facilitate subsequent discussion of chemical reactions. 

Electron attachment to dinucleoside phosphates leads to the formation of radical ions in which the electron has added to the more easily reduced heterocyclic base.35... 

Heterocycles are of great interest in organic chemistry due to their specific properties. Many of these cycles are widely present in natural and pharmaceutical compounds. Electrochemistry appears as a powerful tool for the preparation and the functionalization of various heterocycles because anodic oxidations and cathodic reductions allow the selective preparation of highly reactive intermediates (radicals, radical ions, cations, anions, and electrophilic and nucleophilic groups). In this way, the electrochemical technique can be used as a key step for the synthesis of complex molecules containing heterocycles. A review of the electrolysis of heterocyclic compounds is summarized in Ref. [1]. 

Each ion-radical reaction involves steps of electron transfer and further conversion of ion-radicals. Ion-radicals may either be consnmed within the solvent cage or pass into the solvent pool. If they pass into the solvent pool, the method of inhibitors will determine whether the ion-radicals are prodnced on the main pathway of the reaction, that is, whether these ion-radicals are necessary to obtain the hnal prodnct. Depending on its nature, the inhibitor may oxidize the anion-radical or reduce the cation-radical. Thns, quinones are such oxidizers whereas hydroquinones are reducers. Because both anion and cation-radicals are often formed at the first steps of many ion-radical reactions, qninohydrones— mixtures of quinones and hydroquinones—turn out to be very effective inhibitors. Linares and Nudehnan (2003) successfully used these inhibitors in studies on the mechanism of reactions between carbon monoxide and lithiated aromatic heterocycles. 

Thianthrene radical ion(H-) is generated upon treatment of the heterocycle with ozone an assumed initial adduct (27) is thought to dissociate (78JOC675) (Eq. 9). When haloazoxybenzenes are decomposed with sul-... 

A quite different mode of reaction was observed for the reactions between thianthrene radical ion(l+) and the heterocyclic bases pyridine (72JOC2691) and 2,3-diazabicyclo[2,2,2]oct-2-ene (88JA7880) thianthren-2-yl-N salts were obtained in each case. It was shown that 2 mol equivalents of the radical cation are required, the byproduct being thianthrene. 

This, after 1,3,6,8-tetraazapyrene, was only the second example of obtaining both a negative and a positive radical from the same heterocyclic compound. Attempts to generate radical-ions from other condensed thiophenes succeeded only with dithieno[2,3-d 2, 3 -radical-cation [HFSC 0.42 (2H), 2.36 (IH), and 2.98 (IH) Gauss]. 

The mass spectrometry of four-membered heterocycles has been considerably investigated by Scala and Colon. Holmes et have provided structures associated with metastabile ion peaks. The effect of ring size on the fragmentation process of cyclic sulfoxides was studied by Tamagaki and Oae. Thietane 1-oxide appears as a strong radical ion at mje 90. At 80 eV no appreciable deoxygenation takes place. Instead, the loss of OH and/or SOH is observed, giving the peak at mje a value of 73 for CaHjS" and the base peak at mje 41 for CaH ". ... 

The mass spectrum of compound 1 and of its 3-deutero derivative has been reported.33 Apart from a strong molecular ion, the principal feature is a peak at 91 m.u. due to a C6H5N radical ion. This ion or interconverting ions 139-141, which are obtained from many other compounds, heterocyclic and nonheterocyclic, have been studied by the technique of collision-induced decomposition (CID).210 A study of some triazolopyridine-7-methanols has shown that other pathways, notably loss of the CHN2 fragment, can become important.23... 

Heterocyclic radicals and radical ions, ESR spectra of solutions 88MI3. 

Radical-anion and radical-cation intermediates, for example, reaet with each other after PET in donor-acceptor systems. After proton reorganization they undergo cyclization to provide a direct synthetic route to macrocycles and A-heterocycles with a variety of ring sizes [230]. Cycloadditions via radical ion pairs [231] and the C -C bond formation between Ceo and A,0-ketene acetals [232] also fit this eategory. 

Because it is felt that particular attention should be given to the electron-transfer step, after a brief discussion of the structure of the radical ions of heteroaromatic molecules, the reactions are discussed in two sections, the first concerning the reactions in which the heterocycle is the donor, the latter reactions in which it is the acceptor. When both donor and acceptor are heterocycles, the reaction is presented in the most appropriate section. 

From a synthetic point of view, bond forming steps are the most important reactions of radical ions [202]. Several principle possibilities have been described in Section 8.1 and are summarized in Scheme 52. Many carbo- and heterocyclic ring systems can be constructed by (inter- and intramolecular) radical addition to alkenes, alkynes, or arenes. Coupling of carbonyl radical anions leads to pinacols either intra-or inter-molecular which can be further modified to give 1,2-diols, acyloins or alkenes. Radical combination reactions with alkyl radicals afford the opportunity to synthesize macrocyclic rings. These radical ion-radical pairs can be generated most efficiently by inter- or intramolecular photoinduced electron transfer. 

For the radical ions 75—82 derived from the nitrogen-containing heterocycles, which can be regarded as the heteroanalogues of benzenoid hydrocarbons, the available electronic spectral data have been obtained from direct measurements after y-irradiation or alkah-metal reduction " ) and inferred from the photoelectron spectra It may be assumed... 

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