Pyrometallurgy zinc

Eisch and Piotrowski reported the preparation of gem-dizinc compounds from diiodomethane and zinc powder in the titanocene chloride mediated methylenation of ketones in 1983 (equation 5)7. In this case, it was not mentioned that they had used pyrometallurgy zinc. The Tebbe-type reagent 2 was shown as an intermediary species. Before addition of titanocene chloride, the amount of methane was measured after hydrolysis of the reaction mixture to determine the formation of gem-dizinc species7. 

Alkylidenation of esters by treatment with a mixture of Zn (containing 0.04-0.07% of lead), RCHBr2, TiCLt and TMEDA works well (Scheme 17). The reaction requires a large excess of reagent. It should be noted that zinc powder in this procedure is also a pyrometallurgy zinc. When pure zinc without lead is used, a catalytic amount of PbCl2... 

In these reports, the zinc powder that had been used by Nozaki and coworkers in Kyoto University was pyrometallurgy zinc. It contains 0.04—0.07% lead (Pb) originally. This lead played an important role for the acceleration of further reduction of Simmons-Smith reagent into gem-dizinc species, according to Takai and coworkers report in 1994 [11], This effect was consonant with Nysted s result [9]. 

First, when 1,1-dihaloalkanes are reduced with low-valent titanium derived from TiCU and zinc, it is necessary to add a catalytic amount of PbCU (or Pb) for reproducibility (Equations (5) and (6)). Two kinds of zinc powder are available for laboratory use electrolytic zinc derived by hydrometallurgy and distilled zinc derived by pyrometallurgy. 

The reactivity of zinc metal has been recently found to depend on the method of production of zinc. The two kinds of zinc powder available are electrolytic zinc derived from hydrometallurgy and distilled zinc derived from pyrometallurgy. They are produced by different smelting methods, and thus, their purities, especially their lead contents, are quite different. A systematic study of the Simmons-Smith reaction of cyclooctene with diiodomethane and zinc derived from various sources has revealed that a trace amount of lead found in zinc derived from the pyrometallurgic smelting method decreases the reactivity of zinc substantially. The addition of a catalytic amount of chlorotrimethylsilane to a zinc suspension has been found to suppress the effect of lead, and to accelerate the reaction of diiodomethane and iodoalkenes with zinc. 

Sulfur dioxide is made on a vast industrial scale by burning sulfur, by burning H2S, or by roasting sulfide ores as part of the pyrometallurgy of zinc, molybdenum, and other metals. The burning of high-sulfur coals and fuel oil serves as the major source of SO2 produced as an environmental pollutant. 

When common oxides, such as for instance of lead, are reduced, they form molten metal. Zinc oxide does not exactly follow the other oxides, because zinc has such a high vapor pressure at the temperature needed that it disappears as a vapor, from which the metal has to be condensed. In view of these facts, it is not strange that the pyrometallurgy of zinc was delayed and so also was knowledge about zinc in its metallic form. 

The most common minerals of high metric tonnage (iron, aluminum, copper, titanium, nickel, chromium, magnesium, zinc, etc.) are found in nature as oxides and sulfides and as a combination of both. Ores are sometimes a mixture of rich metal composition and poorer compositions called gangue. The gangue can be acidic or alkaline, and determines the type of flux used for pyrometallurgy. Since ores come in all levels of complexity, various methods of processing have been developed over the years. 

Zn in these equations denotes pyrometallurgy grade zinc, containing 0.04-0.07% lead. When pure, lead-free zinc is used, a catalytic amount of PbCl2 should be added. 

On the other hand, it is not necessary to prepare such unstable gem-dihaloalkanes in advance. Esters can be alkylidenated by treatment with Zn-CH3CHBr2-TiCl4-TMEDA (Scheme 5.18) [15]. This reaction requires a large excess of the reagent. It should be noted that the zinc powder used in these procedures was also pyrometallurgy grade zinc, containing about 0.04-0.07% of lead. 

Esters were also alkylidenated by treatment with Zn (pyrometallurgy grade zinc, containing lead)-Me3SiCHBr2-TiCl4-TMEDA (Scheme 5.24) [33]. This reagent was found to convert thioesters into the vinyl sulfides. 

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