Sulfur Dioxide Anion-Radical

Dissociation of the dithionite ion can proceed both in aqueous and in nonaqueous media. There is a special equation to determine the average values of equilibrium constants of the forma- 

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Mottley C, Harman LS, Mason RP (1985) Microsomal reduction of bisulfite (aqueous sulfur dioxide)—sulfur dioxide anion free radical formation by cytochrome P-450. Biochem Pharmacol 34 3005-3008... 

Hydroxyl radicals, generated from hydrogen peroxide and titanium trichloride, add to the sulfur atom of 2-methylthiirane 1-oxide leading to the formation of propene and the radical anion of sulfur dioxide (Scheme 102) (75JCS(P2)308). 

Another way to prepare fluorinated sulfides is the photochemical alkylation of sulfides or disulfides by perfluoroalkyl iodides [69, 70, 71] (equations 62-64). Reaction of trifluoromethyl bromide with alkyl or aryl disulfides in the presence of a sulfur dioxide radical anion precursor, such as sodium hydroxymethanesulfi-nate, affords trifluoromethyl sulfides [72] (equation 65). 

Fluorinated sulflnates are prepared from sodium dithionite and liquid per-fluoroalkyl halides [74] (equation 67). For the transformation of the gaseous and poorly reactive trifluoromethyl bromide, it is necessary to use moderate pressure [75] (equation 68) These reactions are interpreted by a SET between the intermediate sulfur dioxide radical anion and the halide The sodium trifluorometh-anesulfinate thus obtained is an intermediate for a chemical synthesis of triflic acid. 

Perfluoroalkylatwn ofpyridines by perfluoroalkyl bromides or iodides occurs m the presence of sulfur dioxide radical anion precursors, such as sodium hydroxy-methanesulfinate [755, 757, 158] (equation 135)... 

Between sulfur dioxide radical anions, dithionite, and sulfoxylate/sulfite there exists a pH-dependent equilibrium465 (equation 86). Therefore, dithionite has been used as a source of sulfoxylate in order to prepare sulfinate and hence sulfones. Alkylation with triethyl oxonium fluoroborate leads to ethyl ethanesulfinate, alkyl iodides lead to symmetrical sulfones466 (equation 87). 

Table 3 presents the hyperfme splitting constants of some sulfur-containing aromatic radical anions. The series studied included the monoxides and dioxides of dibenzothio-phene 1, thioxanthene 2, thioxanthone 3, dibenzo[b,/] thiepin 4 and dithienothiophene dioxide 5. 

The electrophilic character of sulfur dioxide does not only enable addition to reactive nucleophiles, but also to electrons forming sulfur dioxide radical anions which possess the requirements of a captodative" stabilization (equation 83). This electron transfer occurs electrochemically or chemically under Leuckart-Wallach conditions (formic acid/tertiary amine - , by reduction of sulfur dioxide with l-benzyl-1,4-dihydronicotinamide or with Rongalite The radical anion behaves as an efficient nucleophile and affords the generation of sulfones with alkyl halides " and Michael-acceptor olefins (equations 84 and 85). 

Microsomes are capable of oxidizing not only organic substrates but also inorganic ones. An interesting example is the metabolism of bisulfite (aqueous sulfur dioxide) in microsomes. Although mitochondrial sulfite oxidase is responsible for the in vivo oxidation of bisulfite by a two-electron mechanism, cytochrome P-450 is also able to reduce bisulfite to the sulfur dioxide radical anion [56] ... 

Analogous consideration of aryldiazonium salts and sulfur dioxide reaction with a-nitro olefins in the presence of cupric chloride gave rise to a conclusion that the process also includes the electron-transfer step and develops according to anion-radical mechanism (Bilaya et al. 2004). The reaction eventually leads to the formation of p-arylsulfonyl a-nitroethanes. 

In the upper atmosphere such oxidation of sulfur dioxide to its trioxide forming sulfuric acid or sulfate anion may occur at ambient temperature at a much slower rate in the presence of various free radicals. 

In the first step, lattice sulfide is oxidized to the S radical (Eq. 22), which then reduces oxygen according to Eq. 23. The resulting sulfur dioxide radical anion is... 

The azide radical is linear, as is its anion. Ozone, sulfur dioxide, and N02 are bent in the neutral form and in the anion. The neutrals of C02, CS2, COS, and N20 are linear and the anions bent, as predicted by simple molecular orbital theory. The electron affinities of these species are uncertain [90-102], In Figure 9.17 the ECD data for these compounds are shown. The diversity in the temperature dependence of the isoelectronic molecules is remarkable. The ECD response of C02 is unexplained. The temperature dependence for CS2 is typical of nondissociative electron attachment to two states. The low-temperature dependence of COS is unusual, but can be attributed to attachment to two states with a low A. Dissociative electron attachment and molecular ion formation are observed in the ECD data for N20. Consequently, only an upper limit to the electron affinity may be obtained [90-93],... 

It is also possible that electron exchange between sulfur dioxide radical anions create sulfur dioxide and sulfur dioxide dianions (sulfoxylates), as shown in Equation 13.4. These species are also known for their reducing properties. 

Dence [11] presented various reaction mechanisms present during hydrosulfite bleaching for ort/jo-benzoquinoid structure by a sulfur dioxide radical anion (top of Fignre 13.4), and for the reduction of coniferaldehyde type structures (bottom of Figure 13.4). 

This condensation with sulfur dioxide is rather peculiar. To the difference with carbonyl electrophiles, sulfur dioxide is more easily reduced than trifluoromethyl bromide. As already pointed out, initial consumption of zinc by this anhydride was obvious, producing the sulfoxylate radical anion which is known to be in equilibrium with the dithionite anion (Fig. 15). Incidentally, this salt mixed with sodium bicarbonate in aqueous acetonitrile was used for the transformation of liquid perhalogenoalkyl halides into their corresponding sulfmates (ref. 24). We have been able to transform the gaseous and poorly reactive trifluoromethyl bromide into sodium trifluoromethanesulfinate. However, the reaction conditions (Fig. 16) (ref. 25) were modified because no transformation occurred in the medium employed for the sulfinato-dehalogenation of the liquid halides. 

In this experiment, a decimolar quantity of zinc and sulfur dioxide was used. In order to explain such a catalytic effect, we have considered that the sulfur dioxide which is formed in the medium (Fig. 15) can be reduced back to its radical anion by an intermediate cyclohexadienyl radical. This step could induce a chain formation of the trifluoromethyl radical (Fig. 20) (refs. 26, 27). 

Experiments were performed with various sulfoxylate radical anion precursors sodium dithionite, sodium hydroxymethanesulfinate or a mixture of sulfur dioxide with a reductant, such as zinc or sodium formate (refs. 29, 30).In contradistinction with the trifluoromethylation of aromatic compounds (Figs. 19,20), a stoiechiometric amount of the sulfoxylate radical anion precursor was necessary. In the disulfide case, there is no intermediate able to reduce back the sulfur dioxide which is formed in the medium (Fig. 22). 

Another characteristic of 02 is its ability to act as a moderate one-electron reducing agent. For example, combination of 02 with 3,5-di-r-butylquinone (DTBQ) in DMF yields the semiquinone anion radical DTBSQ as the major product. The relevant redox potentials in DMF are 02/02 , E° = —0.60 V versus NHE, and DTBQ/DTBSQ , ° = —0.25 V versus NHE, which indicate that the equilibrium constant K for the reaction of O2 with DTBQ has a value of 0.8 x 10 (equation 148). Electrochemical studies of sulfur dioxide (S02/S02 , ° = —0.58 V vs. NHE) and of molecular oxygen in DMF indicate that the equilibrium constant K) for the reaction of SO2 with 02 has a value of 1.1 (equation 149). 

PBOCST is readily synthesized from the polymerization of r-hutoxycarhonyl oxystyrene via radical or cationic polymerization in liquid sulfur dioxide or alternatively by reacting poly(hydroxystyrene) with di-tert-h xty dicarbonate in the presence of a base PBOCST polymers with narrow dispersity have been prepared by living anionic polymerization of 5-tert-butyl(dimethyl)silyloxystyr-ene), followed by desilylation with HCl to form PHOST and protection with di-tert-butyl carbonate PBOCST is very transparent around the 250-nm region of the spectrum (absorbance <0.1/ p,m), thus making it an ideal candidate for DUV 248-nm lithography. 

In the first step, lattice sulfide is oxidized to the S radical (Eq. 22), which then reduces oxygen according to Equation 23. The resulting sulfur dioxide radical anion is finally oxidized to the sulfate ion (Eqs. 24, 25). Note the surprising feature that oxygen is reduced via a preceding primary oxidation step (Eq. 22). The intermediate S radical has been detected at ZnS powder by ESR [93,94] and at colloidal zinc and cadmium sulfide by pulse radiolysis with UV-VIS detection (absorption maximum around 500 nm) [26, 95]. However, the sulfide radical anion can also be formed via the primary reduction S- -e S when elemental sulfur is present as impurity. 

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