Chlorinated cation radicals

Studies were made on a series of chlorinated dibenzo-p-dioxin cation radicals in trifluoromethanesulfonic acid (TFMS acid). TFMS acid was... 

Three isomeric tetrachlorodibenzo-p-dioxins were studied. All were insoluble in TFMS acid. To dissolve these compounds and form cation radicals, UV irradiation was necessary. The 1,2,3,4-tetrachloro compound was particularly sensitive to UV irradiation, and as a solid, even turned pink when exposed to ordinary fluorescent light. When subjected to constant UV irradiation, radical ions were induced rapidly. The change in the cation radical concentration was monitored by the ESR signal as illustrated in Figure 10. To determine whether the tetrachloro isomer had been converted to lower chlorinated derivatives after UV irradiation, the dissolved dioxin was then poured into ice water and recovered. The GLC retention time of the recovered dioxin was unchanged in addition, no new GLC peaks were observed. Moreover, the ESR spectrum see Figure 11) for the recovered material was not altered between widely... 

According to the results of Ben Taarit and co-workers (76) and Neikam (77) Ce(III) Y zeolites will not form anthracene cation radicals but upon oxidation to Ce(IV) the radicals are readily formed. This experiment suggests that one role of oxygen during calcination may be to oxidize certain cations. The surface may be oxidized by molecules other than oxygen since the chlorination of 7-alumina by carbon tetrachloride considerably increases the sites responsible for the acceptor character. These sites, which oxidize perylene into the paramagnetic radical ion, have been attributed to biocoordinated positive aluminum atoms (78). 

It is noteworthy that both thermal and photoinduced electron-transfer activation of the [ArH, IC1] complex leads to the ion-radical triad. Consequently, iodination versus chlorination represents the competition between ion-pair and radical-pair collapse. This is confirmed by reactivity studies of dimethoxybenzene cation radical with chloride and iodine (atom), respectively,225 i.e.,... 

As for chemical paths to cation-radical generation, the following oxidants deserve to be mentioned concentrated (98%) sulfuric acid (Carrington et al. 1959, Hyde and Brown 1962, Carter 1971), persufate (Minisci et al. 1983), iodosobenzene bis(trifluoroacetate) (Alberti et al. 1999), and chlorine dioxide (Handoo et al. 1985, Sokolov et al. 1999). The following metal ions were used in many applications Tl(III) (Elson and Kochi 1973, McKillop and Taylor 1973), Mn(III) (Andrulis... 

Methyl tricyclo[4.1.0.0 ]heptane-l-carboxylate gives a cation-radical in which the spin density is almost completely localized on C-1 while the positive charge is on C-7. The revealed structural feature of the intermediate cation-radical fairly explains the regioselectivity of N,N-dichlorobenzenesulfonamide addition to the molecular precursor of this cation-radical. In the reaction mentioned, the nucleophilic nitrogen atom of the reactant adds to electrophilic C-7, and the chlorine radical attacks C-1 whose spin population is maximal (Zverev and Vasin 1998, 2000). 

The mechanism of the reaction of thiophene with a variety of radicals as a function of pH has been studied using ESR (81JCS(P2)207). Attack by -OH at pH 6 proceeds by direct addition with a preference to add to the a-position the ratio of (226) to (227) is 4 1. At low pH the (3-adduct easily loses OH- to form the thiophene radical-cation, which may undergo rehydration. In the case of 2-methyIthiophene the radical-cation deprotonates to give the thenyl radical this is reminiscent of the electrochemical oxidation (Section 3.14.2.6). The radical-cations are also formed by direct electron abstraction from the thiophene substrates by chlorine anion-radicals. At pH >6, (226) starts disappearing with formation of ring-opened products (Scheme 61). 

It is quite evident that both anion and cation radicals cannot always leave the solvent cage and exist together in the bulk solution for a long time. One such rare example is the nucleophilic substitution of chlorine in 2,4-dinitrochlorobenzene (substrate) by the diethy-lamino group from triethylamine (reactant) (Scheme 4-16). 

The reaction starts with excitation of the quinone, followed by intersystem crossing and electron transfer from the thiophene to the triplet excited quinone. The ion radical pair collapses to a biradical which loses a chlorine and a hydrogen atom. Yields are high (65-78%) when R1 = halogen and R2 = H, fair (57%) when R1 = R2 = H and poor (2-17%) when R1 = H and R2 = halogen. The regioselectivity has been explained on the basis of calculated electron densities in the cation radicals of thiophenes. 

Enol silyl ethers can lead to a-chloro ketones on treatment with anhydrous copper(II) chloride in DMF or iron(lll) chloride in acetonitrile (equation 13, Table 1). The chlorination of (36 equation 14) proceeds through a cation radical intermediate formed by an electron-transfer process with metal halides. 

In addition, there have been several reports of reduced porphyrin or oxophlorin tt-cation radicals of Fe. Since it has been suggested that Fe isobacteriochlorin TT-cation radicals are viable species for at least two reaction states of the hemoprotein subunit of E. coli sulfite reductase and assimilatory nitrite reductases from several organisms, and at least one state of the heme d (chlorin) center of the dissimilatory nitrite reductases from several microorganisms, the investigation of the one-electron reduced states of these proposed intermediates has also been undertaken. 

E. S. Huyser (ed.), Methods in Free Radical Chemistry , Dekker, New York. The series includes chapters on (a) Free-Radical Chlorination via Nitrogen Cation Radicals, by N. C. Deno, 1971, vol. 3, p. 93 (b)... 

Cation-radicals from 2-arylfurans have been suggested to be involved in the oxidation of these compounds by ruthenium tetroxide. The suggestion is based on the observation of the capture by the heterocycle of the chlorine from chlorinated solvents and of a broad ESR signal which persists for several days. 

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