Dibenzodioxin radical cation

The thin layer cyclovoltammogram of 35a showed two independent oxidation processes (i) an irreversible wave at Epa = 1034 mV (vs. Fc/Fc+) (fcpi = -232 mV (vs. Fc/ Fc+)) and (ii) a reversible wave (E1//2 (het-ox), which corresponds to the formation of the radical cation of the dibenzodioxin subunit (Figure 28). The irreversible wave represents a two-step process involving a one-electron oxidation of the DHA subunit followed by a chemical step (EC-type mechanism) leading to a significant change in the molecular structure. Since polyenic radical cations have a preference for dimerization, 34 it is reasonable to speculate on the formation of the dimeric dication species as shown in structure 41. The chemical reversibility of this EC-type process was confirmed by multisweep thin layer experiments. 

To date the mercurated arene radical cation is known for biphenylene [87], ace-naphthene, pyracene, hexahydropyrene, triptycene, p-terphenyl, tetramethylnaph-thopyran, anthracene, dibenzodioxin [89], and 4-tert-butylanisole [88]. In certain cases multiple mercuration is observed, for example in case of diphenylene [87] and dibenzodioxin [89]. Mercuration causes a decrease in -value and always occurs at the site where the local coefficient of the Huckel HOMO of the hydrocarbon is greatest, and there is a constant ratio of about 20.6 between the hyperfine couplings by the Hg [l, abundance 16.84 %) which has been introduced, and by the proton which has been displaced [89]. EPR spectroscopic evidence is reported for 8, 9, 10, 11, 12, 13, 14, 15 and 16 as new examples of recently recognized alternative mechanism of arene mercuration in which collapse of ArH +Hg(TFA)2 radical ion pair leads to arylmercury trifluoroacetate ArHg(TFA) + [90]. 

Step. As an example the role of an ET step in the interaction between dibenzodioxin and thianthrene radical cation with aromatic donors such as anisole and anthracene has been evaluated through kinetic studies [37]. 

The reaction in Scheme 5.11 gives the snlfoninm salt (anion CIO4 ) in a 90% yield (ronte a). One-electron reduction of the thianthrene cation-radical by anisole is the side reaction (ronte b). Route b leads to products with a 10% total yield. Addition of the dibenzodioxine cation-radical accelerates the reaction 200 times. The cation-radicals of thianthrene and dibenzodioxine are stable. Having been prepared separately, they are introdnced into the reaction as perchlorate salts. 

To increase the stationary concentration of complex (HetH - - - ArH) +, a stronger oxidizer, as compared to the cation-radical of thianthrene, should be introduced into the reaction. This is the cation-radical of dibenzodioxine. It increases the rate of the reaction by 2 orders. 

For oxidations, the cation radicals of aromatic compounds like 9,10-diphenyl-antracene, thiantrene, phenoxathiine, or dibenzodioxine are likely candidates. Their reactivity towards nucleophiles, however, limits their application to media of low nucleophilicity. Sometimes the stability of such cation radicals can be enhanced through blocking the reactive positions by substituents. For example, para-substituted triarylamines deliver cation radicals with often excellent stability even in methanol. The stability is further increased by incorporation of urzAu-substituents. Other mediators which have been applied in indirect electrosyntheses are those which are able to abstract hydrogen atoms or hydride atoms. 

Dibenzodioxin cation radical perchlorate has been made by anodic oxidation (Cauquis and Maurey-Mey, 1972). This salt undergoes further (unknown) reactions on the anode and therefore only small amounts can be deposited at a time (Shine and Shade, 1974). Anodic oxidation in trifluoroacetic acid can give trifluoroacetatcs or (if another anion is added after electrolysis) other salts (Hammerich et al, 1972). 

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