Shedding lead oxides

Studies on elementary reactions of acetylenes with metal complexes are now beginning to shed some light on the nature of activation caused by complexation. This activation is not a simple process. Many low-valent d8-d10 metal complexes and also some early transition metal compounds with higher oxidation state (d -d2 complexes) are capable of activating acetylenes. As already described, in the former complexes, interaction (c ) would lead to activation of an r)2-acctylcne ligand to an -acetylene having some radical as well as some anionic character ... 

The predominance of nuclear methoxylation over competing alkyl side-chain oxidation processes for these systems was largely attributed to an intramolecular attack by the appended side-chain alkoxy radical on the anodically generated aromatic radical cation in the usual EEQCp mechanism [99]. A more recent mechanistic study of anodic oxidation of chiral alkoxynaphthalene derivative (LXXXIX) sheds some additional light on this process [Eq. (46)] [100]. The isolation of a 2 1 mixture of enantiomeric methoxylated products indicated that intramolecular attack by the appended alkoxy radical on the aromatic radical cation (path B) was disfavored at —7S°C (a nearly 1 1 mixture of enantiomers was obtained at 25 C), since the intermediate cyclic ketal resulting from such a process would be a meso form that would lead to a racemic mixture of methoxylated products. Note that the chirality of the appended hydroxyether side-chain disappears upon cycliza-tion to the acetal. 

Because of their electrophilic and reactive nature, arene oxides also may undergo spontaneous reactions with nucleophilic functionalities present on biomacromolecules. Such reactions lead to modified protein. DNA. and RNA structures and often cau.se dramatic alterations in how these macromoleculcs function. Much of the cytotoxicity and irreversible lesions caused by arcnc oxides are presumed to result from their covalent binding to cellular components. Several wcll-establi.shed examples of reactive arene oxides that cause serious toxicity ate presented below. 

Although several hypotheses have been proposed, the mechanisms of electrode degradation involved in shedding and inactivation processes are still not clear. The method of material preparation plays a substantial role here. For example, positive material prepared by oxidation of needle like crystals of tetrabasic lead sulfate HPbO PbS04) maintains the latter s morphology and the electrode s superior performance during cycling of stationary cells." ... 

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