Oxidative addition, zinc metal

Simmons—Smith Cyclopropanation. Simmons-Smith cyclopropana-tions have been known for many years as an efficient method for cyclopropanation via methylene transfer to olefinic substrates. Although a stoichiometric amount of reagent is mostly required, the superior reactivity and selectivity make the method inseparable from cyclopropanation. The classical Simmons-Smith cyclopropanation involves a zinc carbenoid reagent (IZnCH2l) generated by the oxidative addition of metallic zinc to diiodomethane by a copper metal as an activator (48). This somewhat cumbersome procedure was later replaced by other more easily accessible and reproducible methods. The benchmark replaces the... 

Antimicrobial agents are used where there is a need to inhibit bacterial and fungal growth. The additives can consist of copper, germanium, zinc and zinc compounds, metal oxides or sulfides, metal zeofltes, as well as silver and copper oxide-coated inorganic core particles (154—159) (see Industrial ANTIMICROBIAL AGENTS). 

Dehalogenation of monochlorotoluenes can be readily effected with hydrogen and noble metal catalysts (34). Conversion of -chlorotoluene to Ncyanotoluene is accompHshed by reaction with tetraethyl ammonium cyanide and zero-valent Group (VIII) metal complexes, such as those of nickel or palladium (35). The reaction proceeds by initial oxidative addition of the aryl haHde to the zerovalent metal complex, followed by attack of cyanide ion on the metal and reductive elimination of the aryl cyanide. Methylstyrene is prepared from -chlorotoluene by a vinylation reaction using ethylene as the reagent and a catalyst derived from zinc, a triarylphosphine, and a nickel salt (36). 

Accelerated sulphur systems also require the use of an activator comprising a metal oxide, usually zinc oxide, and a fatty acid, commonly stearic acid. For some purposes, for example where a high degree of transparency is required, the activator may be a fatty acid salt such as zinc stearate. Thus a basic curing system has four components sulphur vulcanising agent, accelerator (sometimes combinations of accelerators), metal oxide and fatty acid. In addition, in order to improve the resistance to scorching, a prevulcanisation inhibitor such as A -cyclohexylthiophthalimide may be incorporated without adverse effects on either cure rate or physical properties. 

Alkenes are reduced by addition of H2 in the presence of a catalyst such as platinum or palladium to yield alkanes, a process called catalytic hydrogenation. Alkenes are also oxidized by reaction with a peroxyacid to give epoxides, which can be converted into lTans-l,2-diols by acid-catalyzed epoxide hydrolysis. The corresponding cis-l,2-diols can be made directly from alkenes by hydroxylation with 0s04. Alkenes can also be cleaved to produce carbonyl compounds by reaction with ozone, followed by reduction with zinc metal. 

Certain oxides of divalent metals, those of ZnO, CuO, SnO, HgO, and PbO, form cements that are hydrolytically stable in addition MgO, CaO, BaO and SrO form cements that are softened when exposed to water. Compressive strengths of these materials range from 26 to 83 MPa, the strongest being the copper(II) and zinc polyacrylate cements (Table 5.1). Crisp, Prosser Wilson (1976) found that for divalent oxides the rate of reaction increased in the order... 

Metallic zinc is typically covered with a zinc oxide layer that must be removed before the metal can engage in an oxidative addition reaction with organic halides. This activation can be done in one of several ways. 

Whenever metallic zinc is to be used in oxidative addition processes, results are affected by the metal surface activity. Two strategies for the production of active zinc metal surfaces can be adopted (i) chemical or physical activation of commercial zinc powders, or (ii) in situ production of highly reactive metal powders by reduction of a zinc salt with a suitable reducing agent. 

Oxide of Zinc—ZnO—is obtained by distilling zinc from day retorts into chambers through which a current of air is maintained, the volatilized metal burns at the high temperature to which it is exposed, and the oxide is deposited in a scries of condensing chambers. The hydrated oxide may bo obtained by procipitetion from the solutions of the salts of zine by the addition of an alkali. An excess of the alkaline liquid redissolves the oxide. 

Enhancement of fluorescence due to the complexation of metal ions with fluoroionophores has been used as a well-precedented technique to analyze for the presence of metal ions [189-191], A number of studies have reported chelating fluorophores whose emission spectra change upon the addition of metal ions [192-198]. One remarkable result of this emission intensity enhancement is shown in Scheme 23, where the chelation of zinc chloride to 9,10-bis(((2-(dimethylamino)ethyl)methylamino)methyl)anthracene drastically enhances the observed fluorescence by a factor greater than 1000-fold [199], In the absence of Zn2+, the singlet excited state of anthracene moiety is strongly quenched by intramolecular photoinduced electron transfer from the amine to the anthracene moiety. The complex formation of Zn2+ with the amine moiety may result in the largely positive shift of the one-electron oxidation potential. Thus, intramolecular photoinduced electron transfer is strongly suppressed by the complexation of the amine moiety with Zn2+,... 

An older metallurgical process, which is still used extensively, involves a rather extended treatment of high-grade ore or concentrates obtained by a flotation process. In either case, the finely divided ore is roasted to convert sulfides and carbonates to oxides, which are then reduced by means of carbon at temperatures of 1200 to 1300°C. Since zinc boils at 907°C, the liberated metal distills from the earthenware retort and may be condensed in suitable receivers. If the temperature of the condenser is kept below the melting temperature of zinc (419.3°C), the metal is obtained in the form of zinc dust which, in addition to metallic impurities, contains approximately 5% zinc oxide. If, however, the zinc vapors are condensed at a temperature above... 

Early preparations of active zinc utilized the potassium or sodium metal reduction of anhydrous zinc salts in refluxing THF or DME (Protocol l).3,8 These highly divided zinc powders displayed high reactivity towards organic halides in oxidative addition. Alkyl iodides and bromides reacted with the zinc powders at room temperature. Even aryl bromides and iodides would react to form the corresponding arylzinc iodides or bromides at refluxing... 

---