1.4- Dioxins radical cations

Fluorescence was used to investigate the excited-state dynamics of the radical cation of thianthrene (TH ) <2001PCA6594>. The frequencies of the fundamental modes of the molecules of some dioxins in the ground electronic state were also determined by analysis of their fine-structure phosphorescence spectra <19990PS(86)239, 19970PS(83)92, 20000PS(89)42, 20000PS(88)339>. 

In this regard, clear chemical and spectroscopic evidence for the disproportionation of the intermediate radical cations, photochemically and/or thermally generated, were achieved on 2,3-diphenyl-5,6-dihydro-1,4-dioxin, and derivatives 22a - c [90, 111, 57-159]. In fact, the 2,4,4,6-tetrabromo-2,5-cyclohexadien-l-one (TBCHD)-sensitized photooxygenation of 22a affords the corresponding 1,2-ethanedioldibenzoate, 67, the cleavage product of the intermediate 1,2-dioxetane, 24a, together with minor amounts of 4a,8a-diphenyl-2,3,4a,6,7,8a-hexahydro-p-dioxino[2,3-b]-p-dioxin 68 [158] ... 

The same technique was employed to determine the kinetics of the inversion of the dioxine ring in the radical cations of 29-34 [100]. 

It has been shown voltammetrically that oxidation of 1,4-dioxin and of its 2,3,5,6-tetraphenyl, benzo, and dibenzo derivatives takes place at a rotating platinum electrode in acetonitrile in single-electron steps with the intermediate formation of radical cations.The simplest dioxin cation-radical for which an ESR spectrum has been obtained is 79, for which hyperfine splittings are indicated in gauss.The radical was prepared by treatment with sulfuric acid of the dimer which forms spontaneously on the storage of acetoin. 

Under conditions that are not strictly nonaqueous, the oxidized dimer may be trapped by water, as was observed during the oxidation of 1,1-diphenylethylene catalyzed by the radical cation of dibenzo-1,4-dioxin [91]. The dimer dication upon reaction with water undergoes a 1,2-phenyl shift, resulting finally in 1,2,4,4-tetraphenyl-3-buten-l-one [Eq. (42)], reminiscent of the 1,2-shifts observed during anodic oxidation of 1-phenyl- and 1,4-diphenylnaphthalene in acidic dime thy Iformamide (DMF) [92]. 

Voltammetric oxidation of 1,4-dioxins and 1,4-dithiins in acetonitrile containing perchlorate yields a radical cation in the first one-electron oxidation and a dication in the second one-electron oxidation. Both compounds are typical r-electron-rich heterocycles, whereas the dication contains six 7r-electrons, and the relatively high stability of the dication is connected with this aromaticity [296]. 

Among the reactions of 1,4-dioxins and 1,4-dithiins, oxidation of their tetraphenyl and dibenzo derivatives is important. This reaction leads to the formation of radical cations 7 and dications 8... 

Tetraphenyl-1,4-dioxin, with SbCl5, forms a blue-violet radical cation. On further oxidation, a green dication is obtained. However, tetraphenyl-1,4-dithiin with SbCl5 yields a violet dication as a hexachloroantimonate. The dications 8 can be regarded as Huckel aromatic 67c-systems, in contrast to the antiaromatic parent compounds 1-3. 

This color transformation has been observed in dibenzo-p-dioxin (Structure I) and in its bromo, chloro, nitro, methyl, and ethyl derivatives in addition, the observed electron spin resonance (ESR) signals indicated the presence of paramagnetic species (2, 3). This phenomenon has been attributed to the formation of cation radicals in acid solution. 

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

The 1-chloro- and 2-chlorodibenzo-p-dioxins, which readily dissolved in TFMS acid, formed cation radicals without UV irradiation or the addition of oxidizing agents. With the exception of broader resonance lines. Figure 3 shows that the five-line pattern observed with 1-chloro-dibenzo-p-dioxin is similar to that of the unsubstituted dibenzo-p-dioxin. Apparently, protons at the 2, 3, 7, and 8 positions became less equivalent... 

Four dichloro isomers, the 1,6-, 2,3-, 2,7-, and 2,8-dichlorodibenzo-p-dioxins, were studied. These compounds also dissolve in TFMS acid, forming cation radicals in the absence of oxidizing agents or UV irradiation. The 2,8-isomer (Figure 5) exhibited a three-line spectrum, in agreement with the two equivalent protons in the 3,6 positions. The 2,7-isomer should also exhibit a three-line spectrum, similar to the 2,8-... 

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... 

Isolation of salts of both the cation radicals and dications has been achieved for a number of systems. Thus, reaction of 2,3,4,5-tetraphenyl-l,4-dioxin with antimony pentachloride in chloroform gives the cation radical (63) as blue-violet crystals, and this can be oxidized by voltammetry to the green dication salt (64). Tetraphenyl-l,4-dithiin with antimony pentachloride in benzene affords the dication salt (65) directly as dark violet crystals (70ZC147). 6-Methyl-1,4-benzodithiin reacts with the same reagent to give a cation radical salt, which decomposes in air to a black resin (62JCS4963). 

The preparation of dibenzo[6,e ][ 1,4]dioxin cation radical (66) has been achieved by oxidation of the heterocycle in ethyl acetate-lithium perchlorate at a platinum anode, using a controlled potential of 1.2 volts vs. Ag-AgC104. The blue solid collected at the anode contained between 85-90% of (66) as the perchlorate (74JHC139). The purple cation radical perchlorate of phenoxathiin, (67), is obtained in high purity by oxidation of phenoxathiin in benzene with 70% perchloric acid-acetic acid (75JOC2756). Similar perchloric acid oxidation of thianthrene affords the dark reddish brown perchlorate of (68) (69JOC3368) and the heterocycle can also be oxidized on a preparative scale with antimony pentachloride (62JCS4963). 

It has been argued466 that the solution photolysis rates of polychlorinated dibenzo-/ -dioxins may be explained by the preferential photodissociation of chlorine atoms from a lateral vs a non-lateral position to yield the corresponding aryl radical and/or aryl cation-aryl carbene intermediate. 

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