Heterocycles radical cations

A related question is proton transfer, where two aspects are relevant, viz. proton transfer from a heterocyclic radical cation after an ET step (e.g. after photoinduced ET from indole to anthracenes or pyrene) [38] and the possible role of ET in proton transfer from aromatic radical cations and heterocyclic nucleophiles, in particular to pyridine derivatives [37b, 39]. 

The authors proposed a mechanism that starts with the generation of a (trifluoroaceto>q )iodo benzene radical and an azide radical (Scheme 5.26). The azide radical abstracts a hydrogen atom from the cyclic alkane and the resulting hydrazoic acid reacts with a second FIFA molecule with release of trifluoroacetic acid that protonates the heterocycle and activates it for the addition of the allq l radical. The resulting heterocyclic radical cation is oxidized into the corresponding product by the (trifluoroaceto>y)iodo benzene radical. 

An interesting method for the substitution of a hydrogen atom in rr-electron deficient heterocycles was reported some years ago, in the possibility of homolytic aromatic displacement (74AHC(16)123). The nucleophilic character of radicals and the important role of polar factors in this type of substitution are the essentials for a successful reaction with six-membered nitrogen heterocycles in general. No paper has yet been published describing homolytic substitution reactions of pteridines with nucleophilic radicals such as alkyl, carbamoyl, a-oxyalkyl and a-A-alkyl radicals or with amino radical cations. 

Reactivity patterns of radical cations in transformations of heterocycles 97AG(E)2550. 

Quaternary ammonium salts are organically substituted nitrogen compounds in which the nitrogen atom is pentavalent. Four of the substituents are alkyl, aryl or heterocyclic radicals and the fifth is an anion (a cationic charge). This anion is mostly chloride. Therefore we call them benzalkonium chlorides. 

Cyclic voltametric analysis has been utilized to determine material properties of this class of heterocyclic compounds. All the DTPs 23 <2003JOC2921 > exhibited a well-defined irreversible oxidation presumably corresponding to the formation of the radical cation. When scanned to higher positive potentials, it resulted in two consecutive broad oxidations for most of the DTPs. The second oxidation is quite weak, followed by a more intense and well-defined third oxidation. Coupling of thiophene radical cation is usually rapid (r <10-5 s) <1995SM(75)95>. These additional broad waves most likely correspond to the oxidation of coupled products rather than further DTP oxidations. The electrochemical data of the DTP S 23 are given in the Table 10. 

The oxygen transfer to the -acetylenic carbon results in the very intense benzoyl cation, whereas the transfer to the a-carbon, via a series of fragments corresponding to the loss of OH , CO and CO2, respectively, leads to annelated heterocycles such as the radical cation of carbazole, as a result of elimination of CO81. The generation of the benzoyl cation was rationalized as shown in Scheme 1281. 

Another heterocyclization is presented by Panifilow et al. Cyclic acetals and ethers are obtained by electrochemical oxidation of the terpenoid alcohol linalool 57 in methanol containing alkaline and sodium methoxide as electrolyt [102]. Anodic oxidation of the C(6)-C 7) double bond of linalool leads to the radical cation 58. In addition to direct methoxylation of the radical cation an attack on the hydroxyl group takes place. After a second one-electron oxidation and following methoxylation the regioisomeric cyclic acetal and a subsequent 1,2-hydride shift, the cyclic acetal 60 and the cyclic ether 61 are finally formed in yields of 16 and 24%, respectively (Scheme 13). As shown by Utley and co-workers bicyclic lactones 65 and 66 can be synthesized by anodic oxidation... 

The photolysis of donor-acceptor systems shows a reaction pattern of unique synthetic value. Direct irradiation of the donor-acceptor pairs, such as arene-amine, leads by intramolecular electron transfer, to amine radical cations and arene radical anions. The generated radical cation and radical anion intermediates undergo cyclization reactions providing efficient synthetic routes to N-heterocycles with a variety of ring sizes. 

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. 

A great advantage of this method is the synthesis of bis(l,2,3-dithiazoles) from diaminodithiols (1997JA12136, 1999CM164, 2000JA7602). Reduction of radical cations formed with PhsSb led to neutral heterocycles 122 and 123 as air-stable crystalline solids (Scheme 61). 

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]. 

---