Metallation. in situ

The commercial viability of the reaetion depends on the formation of a eatalytic cycle by reoxidizing the palladium metal in situ. This is achieved by the introduction of CUCI2 ... 

Another important catalytic reaction that has been most extensively studied is CO oxidation catalyzed by noble metals. In situ STM studies of CO oxidation have focused on measuring the kinetic parameters of this surface reaction. Similar to the above study of hydrogen oxidation, in situ STM studies of CO oxidation are often conducted as a titration experiment. Metal surfaces are precovered with oxygen atoms that are then removed by exposure to a constant CO pressure. In the titration experiment, the kinetics of surface reaction can be simplified and the reaction rate directly measured from STM images. 

A companion reagent to the ylide is the corresponding metalated sulfide 30 which arises by the lithiation of 29 with n-butyllithium 66). The latter forms by base closure of 28. Since closure of 28 b involves use of n-butyllithium, the cyclopropyl sulfide 29 simply becomes an intermediate which is metalated in situ to give 30 directly67). A non-metalation sequence involves 32 which undergoes reductive... 

The technology is commercially available and has been applied to leachates, wastewater processing, and contaminated groundwater. The technology has been modified to remove hexavalent chrominm and immobilize heavy metals in situ, but demonstration results have not been reported. 

LEADX is a chemical additive that was developed to treat material contaminated with lead or other heavy metals in situ. LEADX may be added directly to soil or incorporated into sandblasting materials or paint thinners. LEADX penetrates the contaminated material and chemically bonds with heavy-metal contaminant to form an insoluble, nonleachable compound. The vendor claims that once bonded, the lead cannot be absorbed by plants or animals and is chemically rendered immobile. According to the vendor, it has been used in the following applications for the treatment of lead ... 

Dehydrobromination of bromotrifluoropropene affords the more expensive trifluoropropyne [237], which was metallated in situ and trapped with an aldehyde in the TIT group s [238]synthesis of 2,6-dideoxy-6,6,6-trifluorosugars (Eq. 77). Allylic alcohols derived from adducts of this type have been transformed into trifluoromethyl lactones via [3,3] -Claisen rearrangements and subsequent iodolactonisation [239]. Relatively weak bases such as hydroxide anion can be used to perform the dehydrobromination and when the alkyne is generated in the presence of nucleophilic species, addition usually follows. Trifluoromethyl enol ethers were prepared (stereoselectively) in this way (Eq. 78) the key intermediate is presumably a transient vinyl carbanion which protonates before defluorination can occur [240]. Palladium(II)-catalysed alkenylation or aryla-tion then proceeds [241]. 

The palladium-catalyzed oxidation of ethylene to acetaldehyde (the Wacker process) was discovered by Smidt and co-workers514-518 in 1959. This process combines the stoichiometric reduction of Pd(II) with reoxidation of metal in situ by molecular oxygen in the presence of copper salts. The overall reaction constitutes a palladium-catalyzed oxidation of ethylene to acetaldehyde by molecular oxygen ... 

In one important series of compounds, the mono-halocyclopropane is generated in situ from a 1,3-dehydrohalogenation of a l,3-dihalopropan-2-one acetal using a metal amide with an excess of reagent, the cyclopropene produced in metallated in situ and can be further functionalized by trapping with electrophiles48 ... 

Badding J. V., Mao H. K., and Hemley R. J. (1991) High-pressure chemistry of hydrogen in metals in-situ study of iron-hydrite. Science 253, 421—424. 

Ruthenium hydrogenation catalyst. A 5% ruthenium-carbon (Norit) catalyst and a 5% Ru-AljO , catalyst are supplied by Engelhard Industries. Some investigators employ ruthenium dioxide, which is reduced to the metal in situ. 

The analysis of such inhibition curves has been based on the implied assumption that these metal chelate complexes (E. L. Smith and Hanson, 1949) are analogous to those observed in simple systems (Martell and Calvin, 1952), an assumption for which little experimental justification exists. It should be emphasized that a metalloenzyme inhibited by binding of a chelating agent to a metal in situ represents a mixed complex, consisting of two different ligands combined with one metal. Studies of the physical chemistry of simple complexes of this type are as yet few (Klotz and Loh-Ming, 1954 de Witt and Watters, 1954). The data on mixed complexes available to date are apparently not adequate to justify extrapolations to the interpretation of inhibition of metalloenzymes (Coryell, 1955, personal communication). 

Among the remediation technologies available for soils contaminated with heavy metals, in situ immobilization techniques are of particular interest because of their cost-effectiveness. The use of phosphorus (P) amendments has been identified as a potentially efficient in situ remediation technology. These amendments are available in various forms, and they are environmentally friendly and simple to use. Numerous studies have been conducted to demonstrate the mechanisms and effectiveness of heavy metal immobilization in soils using various P sources (Ma et al., 1993 Chen et al., 1997a Singh et al., 2001 Mavropoulos et al., 2002). 

Trace amounts of noble and semi-noble metals (e.g., Au, Ag, Hg, and Cu) are separated electrolytically from acid media on a small platinum or gold cathode [81]. In cementation methods (i.e., reduction to metal in situ by another metal) small amounts of semi-noble metals (Cu, Bi, Sb) are deposited on less noble metals such as tin, iron, or zinc. 

A -FluorohiXtrifluoromethanesulfonyl)imide is one of the most powerful electrophilic fluori-nating agents. It was successfully used to fluorinate diethyl cyanomethylphosphonate in the presence of w-BuLi (1 eq), and the resulting diethyl 1-fluoro-1-cyanomethylphosphonate can be isolated (51% yield) or metallated in situ and reacted with carbonyl compounds in a Homer-Wadsworth-Emmons reaction to give a-fluoroacrylonitriles in 30-58% overall yields (Scheme 3.21). 2 These results aie especially noteworthy because the fluorination of diethyl cyanomethylphosphonate with NFBS in the presence of LiHMDS (2 eq) produces the A -fluoro and not the expected C-fluoro phosphonate. 

It is often customary to date the emergence of histochemistry from the publication in 1936 of Lison s Histochimie animale. Rapid advances in the qualitative chemistry of organic compounds during the early decades of this century made the 1930 s ripe for the application of color tests and allied methods to tissue sections for the localization of known compounds or at least their functional groups. As early as 1850, however, histologists were attempting to identify specific substances, initially metals, in situ (Pearse, 1951). 

Metal Chlorination. With the exception of titanium and vanadium chlorides (TiCl4 and VCI4), the chlorides of the refractory metals are solids at room temperature and it is often expedient to chlorinate the metal in situ with chlorine or HCl as shown schematically in Fig. 15.3. A typical reaction is the formation of hafhium chloride as follows ... 

Binders Binders are fugitive materials which are required to increase viscosity and density of the vehicles developed for suspending metal powders during application of the ceramic. They must leave no deleterious residue following the firing operation and must hold the metal in situ until the particles become somewhat adherent. Typical binders are represented by acrylic polymers, commercial binders, nitrocellulose and pyroxyline resins. 

To avoid nucleophilic addition, a metalation/in situ trapping was used with a set of electrophiles. The simultaneous introduction of the electrophile and the compound 1241 prevent the dimer formation. Reaction of the lithioderivative 1245 with iodine or hexachloroethane as electrophiles gave 6-halopyrimidines in low yield whereas moderate or good yields observed for carbonyl compounds trimethylsilyl chloride and diphenyl sulphide (Scheme 275, Table 51) [760]. 

Oxidation and reduction reactions can be used to immobilize heavy metals in-situ. Oxidation of soluble Fe and Mn to their insoluble hydrous oxides, Fe203 xH20 and Mn02 xH20, respectively, can precipitate these metal ions and coprecipitate other heavy-metal ions. However, subsurface reducing conditions could later result in reformation of soluble reduced species. Reduction can be used in situ to convert soluble, toxic chromate to insoluble chromium(III) compounds. 

Soviet republics) each contain about 3 million tons or more, of lead (metal in situ) reserves. Taken together, the top ten deposits (see Table 3.2) account for 30 per cent of total world lead reserves. 

We have already met a number of these, particularly in the reduction of carbon-carbon multiple bonds (Section 11.5). The metals most commonly used are platinum, palladium, rhodium, and ruthenium. Adams catalyst, Pt02, is widely used for the reduction of alkenes and alkynes to alkanes. It gives more predictable results than finely divided or colloidal platinum and is easier to handle. The dioxide is reduced to the metal in situ, and the solid metal recovered (usually by filtration) after completion of the reaction may be pyrophoric and must be treated with care. Examples of its use are shown in Figure 23.19. 

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