Oxidation ferrous precursors

Synthesis of the 1,2-dihydrodiol of BcP by conventional methods was blocked by the failure of attempts to synthesize its potential synthetic precursors 1-keto-l,2,3,4-tetrahydro-BcP and 1,2-dihydro-BcP (66). However, BcP 1,2-dihydrodiol was obtained in low yield ( 1%) by oxidation of BcP with ascorbic acid-ferrous sulfate (66). 

The use of hydroxides as precursors is often very tricky, due to the presence of side reactions. Metal hydroxides such as ferrous hydroxide often will oxide to ferric oxide if oxygen is present, which would yield... 

Iron extraction values show that iron speciation varies significantly between layers in the cave (Fig. 6A). Values for amorphous, total, and ferrous iron range from 2.4 to 84 pmol/g. Extraction results indicate a significant amount of goethite in the lower layers of the sequence as determined by total minus ammonium-oxalate extractable iron (52 pmol/g in the bottom yellow layer) (Schwertmann and Taylor, 1977). The upper layers have total iron values represented almost entirely by ammonium-oxalate extractable iron (83 pmol/g in the red layer and 59 pmol/g in the top orange layer) suggestive of ferrihydrite (a necessary precursor to hematite formation). The ferrihydrite in the upper layers is indicative of formation by rapid oxidation of ferrous iron (Schwertmann, 1993). The black layer contains the only cave sediment with a significant amount (42 pmol/g) of extracted ferrous iron. 

The nature of the metal-ions in the active site also varies between species. Whereas the purple acid phosphatase isolated from red kidney beans (rkbPAP) contains Fe and Zn", the tartrate-resistant acid phosphatase isolated from rat osteoclasts (TRAcP) contains two iron atoms in different oxidation states, an stabilized Fe ion and a redox-active Fe ion. In this way, the ability of the ferrous ion to act as an electron donor confers to the enzyme an alternative function as generator of reactive oxygen species (ROS) [20, 21]. The enzyme may appear in an inactive purple form when the redox-active iron is oxidized to the ferric state, or it can be in an active pink form where the redox-active iron is reduced to the ferrous state [22]. In particular, the tartrate-resistant acid phosphatase isolated from osteoclasts is synthetized as a precursor which is activated by cysteine proteinases resulting in an active two subunit enzyme [23]. 

Numerous studies have attempted to elucidate the role of Mo in the passivity of stainless steel. It has been proposed from XPS studies that Mo forms a solid solution with CrOOH with the result tiiat Mo is inhibited from dissolving trans-passively [9]. Others have proposed that active sites are rapidly covered with molybdenum oxyhydroxide or molybdate salts, thereby inhibiting localized corrosion [10]. Yet another study proposed that molybdate is formed by oxidation of an Mo dissolution product [11]. The oxyanion is then precipitated preferentially at active sites, where repassivation follows. It has also been proposed that in an oxide lattice dominated by three-valent species of Cr and Fe, ferrous ions will be accompanied by point defects. These defects are conjectured to be canceled by the presence of four- and six-valent Mo species [1]. Hence, the more defect-free film will be less able to be penetrated by aggressive anions. A theoretical study proposed a solute vacancy interaction model in which Mo " is assumed to interact electrostatically with oppositely charged cation vacancies [ 12]. As a consequence, the cation vacancy flux is gradually reduced in the passive film from the solution side to the metal-film interface, thus hindering vacancy condensation at the metal-oxide interface, which the authors postulate acts as a precursor for localized film breakdown [12]. 

In summary, observation of reduction in situ revealed that the hydroxylated ferric surface of the precursor is converted into a largely ferrous surface, free of water after a long induction period. Only a fraction of the surface iron is reduced to an ill-defined metallic state. Neither a film of chemisorbed water nor other molecular contaminants block the surface and prevent the reduction from proceeding. This is in contrast to the kinetic studies described in Section 2.5, which deal with the high-pressure reduction of iron oxides where, under identical conditions, ferrous ions are more easily reduced than ferric ions. 

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