Cadmium radical anions

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. 

The very reactive Rieke cadmium metal was allowed to react under vacuum with benzoquinone in THF. This resulted in the formation of several different paramagnetic species, [Cd(C6H402 )(THF)3]+,C6H402 or [Cd(C6H402 )(THF)3]+ + C6H402 [215]. The tetrahedral coordination sphere for each species consists of three THF molecules and an anion radical that is asymmetrically or symmetrically coordinated to the metal dication. The original apparatus for the generation of the benzoquinone anion radical via electron transfer from cadmium metal was reported... 

Zinc and cadmium alkyls have not been successful as stereospecific catalysts in the absence of co-catalysts, presumably because they do not complex strongly enough with the monomer and the metal-carbon bonds are too covalent. Cadmium alkyls were first reported by Furukawa and coworkers (260) to induce vinyl polymerization, but it was shown later (267, 262) that oxygen was a co-catalyst and the reactions were free radical in nature. Similar free radical results were obtained with zinc alkyls (261—263) and vinyl monomers. However, with more basic and more easily polarized monomers, such as olefin oxides and aldehydes, the zinc catalysts operate by a coordinated anionic mechanism (250). 

Trialkyl boron was first claimed as a new anionic initiator for the polymerization of vinyl compounds (264), although it was rather improbable in view of the low ionic character of the boron-carbon bond. The error was quickly corrected when it was shown that free radicals were involved (265, 266) and that oxygen, peroxides, silver salts and copper salts were co-catalysts (262, 267). Aluminum alkyls can also initiate radical polymerizations in the presence of oxygen (267,262) but, as in the case of zinc, cadmium or boron alkyls, the products were not stereoregular. Thus, complexing between catalyst and monomer probably does not occur. 

Another way of carrying out electron-transfer mediated oxidation reactions is to use semiconductors as catalysts (Mozzanega et al., 1977). Titanium dioxide will, photocatalyse the oxidation of substituted toluenes to benz-aldehydes by electron transfer from toluene into the photogenerated hole. The electron in the conduction band will reduce oxygen giving the superoxide anion. Reaction of the superoxide anion with the hydrocarbon radical cation produces the aldehyde. A similar mechanism has been used to explain the observation that dealkylation of Rhodamine B (which contains N-ethyl groups) occurs when the dye is irradiated in the presence of cadmium sulphide (Watanabe et al., 1977). 

In a reaction similar to that shown above for aliphatic esters [Eq. (13)], it has been reported that aromatic esters may also be reduced to the corresponding diketone provided the reduction is carried out using a cadmium cathode and magnesium anode [60]. Earlier work demonstrated that phenyl benzoate anion radical cleaves to form phenolate in good yield and presumably benzoyl radical, which leads to dibenzoyl [16]. Nitro-substituted phenyl benzoates are similarly cleaved on reduction to give phenolate [62,63]. 

Exposure to cadmium may produce oxidative stress, which may result directly in toxicity or may occur secondary to cadmium toxicity. Results from studies using cultured cells have demonstrated cadmium-induced formation of superoxide anion radicals (Amoruso et al. 1982) and implicated superoxide anions in Cd-induced DNA single-strand scissions (Ochi et al. 1983). Cadmium inhibited superoxide dismutase in vivo, resulting in elevated superoxide levels (Shukla et al. 1987). Cadmium has been shown to increase peroxidation of lipids in isolated rat hepatocytes (Stacey et al. 1980) and in other target tissues in vivo and in vitro (Gabor et al. 1978 Wahba and Waalkes 1990) thus, increased levels of lipid peroxides following exposure to cadmium could constitute a source of active oxygen species. 

Amoruso MA, Witz G, Goldstein BD (1982) Enhancement of rat and human phagocyte superoxide anion radical production by cadmium in vitro. Toxicol Lett 10 133-138... 

Especially transition metal icMis play an important role in the induction of oxidative DNA damage. While neither superoxide anions nor hydrogen peroxide are able to react with DNA directly, in the presence of transition metals like iron, copper, cobalt, or nickel they are converted into highly reactive hydroxyl radicals by Fenton-type reactions. In contrast, cadmium ions are not able to participate in redox reactions under physiological conditions, yet, oxidative stress and the interference with cellular redox regulation may be of high relevance in cadmium-induced carcinogenicity. Increased levels of ROS due to cadmium exposure have been observed both in vitro and in vivo [31]. Different cadmium compounds have been shown to induce DNA strand breaks and oxidative DNA base modifications in... 

While recent oligosilanyl zinc chemistry was studied by several research groups not much has been done with respect to cadmium. In contrast to this a large variety of oligosilanyl mercury compounds were studied almost exclusively by Apeloig and coworkers, who utilized the unique properties of Si-Hg compounds for the generation of silyl radicals or anions. 

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