Zinc reagents characteristics

A general discussion of the superior thermal stability characteristics of fluorovinylzinc reagents has been presented vide supra). Specifically, the fluorinated zinc reagents exhibit exceptional thermal stability. A sample of a [(Z)-pentafluoroprop-l-enyl]zinc halide in triglyme shows no loss of activity after three days at room temperature and only 10% of the zinc reagent decomposes after 36 days at room temperature. Upon heating at 65 C over three days, only 5 % of the zinc reagent decomposes, and a 25 % loss of activity is observed at the same tern-... 

Nickel- or palladium-catalyzed coupling reactions of alkyl Grignard or zinc reagents can be applied to heteroaromatic halides and sulfides. The characteristic features are, therefore, based on those described in the preceding sections and hence details are not repeated in this section. Aspects are summarized by the types of heteroaromatic compounds NiCh(DPPP) is used as catalyst, unless stated otherwise hereafter. 

Unfortunately, catalytic asymmetric additions of organic alanes to imines are almost unknown, and admittedly, zinc reagents are better applicable for this purpose. But the characteristics of aluminum organyls reveal their potential the high Lewis acidity should enable activation of the rather imreactive CNl-double bonds and the low Br0nsted basicity should preclude deprotonation and formatiOTi of the respective azaenolates. Because chiral a-tertiary and a-secondaiy amines are ubiquitous structural motifs in natural products and synthetic bioactive compounds, exploration of alane additions is highly relevant. 

Cyclic Peroxides. CycHc diperoxides (4) and triperoxides (5) are soHds and the low molecular weight compounds are shock-sensitive and explosive (151). The melting points of some characteristic compounds of this type are given in Table 5. They can be reduced to carbonyl compounds and alcohols with zinc and alkaH, zinc and acetic acid, aluminum amalgam, Grignard reagents, and warm acidified iodides (44,122). They are more difficult to analyze by titration with acidified iodides than the acycHc peroxides and have been sucessfuUy analyzed by gas chromatography (112). 

Jiang Hao, Hu Yuehua, Xu Jing, 2000. Electrochemical characteristics of couple electrode of galena-pyrite in different solutions. Trans. Nonferrous Met. Soc. China, 10 87 - 88 Johnson, N. W. and Munro, P. D., 1988. h-pH measurements for problem solving in a zinc reverse flotation process. Austral Inst. Min. Metall., 239(3) 53 - 58 Karkovsky, I. A., 1957. Physicochemical properties of some flotation reagents and their salts with ions of heavy iron-ferrous metals. Proc. 2nd Int. Congr. Surface Activity, London, 4 225 - 237... 

Not only a zinc-copper couple, but also a zinc-lead couple forms a gem-dimetal species from diiodomethane, according to the 1975 Nysted patent5. He also insisted that treatment of dibromethane with a zinc-lead couple in THF at 80 °C forms a characteristic gem-dizinc species 1 (equation 3). However, there was no further evidence concerning the structure except H NMR data, which was not enough for the complete structural determination. The obtained compound was definitely a gem-dizinc species, but the written structure 1 was not fully characterized. The white solid 1 is obtained as a dispersion in THF, and does not dissolve in DMF and DMI (l,3-dimethyl-2-imidazolidinone). This THF dispersion is commercially available from Aldrich Co. as Nysted reagent. Nysted also showed that this dizinc compound is effective for the methylenation of a-hydroxy ketone moiety in steroid derivatives5. 

Acidify the test solution with acetic acid, and add a few drops of the reagent. The organic phase turns red in the presence of zinc. Cu2+, Hg2+, Hgf+, Ag+, Au3+, and Pd2+ ions interfere. Upon adding sodium hydroxide solution to the mixture and shaking, the aqueous phase turns to red also. This reaction is characteristic for zinc only. (In alkaline solutions the sensitivity of the test is less.)... 

The most satisfactory precipitation for sodium ions is that with uranyl magnesium or zinc acetate (Section III.37, reactions 1 and 3). The flame test, in which an intense persistent yellow colouration is produced, is characteristic. Traces of sodium may be introduced from the reagents during the analysis, and hence it is important to look for a strong persistent yellow colouration a feeble yellow colouration may be ignored. 

In order to reduce an orange dye which is formed when the diazonium salt is added to the bisulfite solution, a small amount of zinc and acetic acid is added. To prepare the base, the hydrochloride is treated with excess 30 per cent sodium hydroxide solution, and the oily base is extracted with ether. The ethereal extract is dried and, after removal of ether, is distilled under reduced pressure. Substituted phenylhydrazines, such as p-nitrophcnylhydrazine and p-bromophenylhydrazine, are prepared in the same manner. Arylhydrazines are strong bases forming characteristic salts, which are important reagents in the identification of aldehydes, ketones, and sugars (see preparation of phenylhydrazones and osazones). 

With these examples the list of the many-sided reactions of the halogen alkyls is not exhausted they are also used for the preparation of the- metallic alkyls, e.g., zinc alkyls for the preparation of the phosphines, and for many other compounds. Finally, attention is called to the characteristic difference between the organic and inorganic halides. While, e.g., potassium chloride, bromide, or iodide in solution act instantly with a silver nitrate solution to form a quantitative precipitate of silver chloride, bromide, or iodide respectively, silver nitrate in a water solution does not act on most organic halides, so that this reagent does not serve in the usual way to show the presence of a halogen. 

Many organic ions are stable only in organic solvents, in particular aprotic media. Thus the oxidation of Grignard reagents and of the alkyls and aryls of alkali metals can be conducted only in ethers. The field has been reviewed by several authors. " The products are characteristic of radical intermediates, hydrocarbons being formed by disproportionation and dimerisation reactions and metal alkyls by reaction of the radicals with the anode material when anodes of lead, cadmium, zinc or aluminium are employed. 

Detection limits down to the pg per kg level have been attained for the most favorable instrumental analyses (e.g., carbon and nitrogen in molybdenum and tungsten) and for radiochemical analyses (e.g., cadmium and thallium in zinc) at least if no nuclear interferences occur. This is below the concentration levels at which these impurities influence the material characteristics and below the detection limit attainable by more common methods of analysis. A precision (reproducibility) of a few percent is possible at the mg per kg concentration level in the most favorable cases. However, at higher concentration levels the precision will not improve significantly. Many systematic errors can be checked experimentally (e.g., interferences, yield of a radiochemical separation) others can be avoided experimentally (e.g., surface contamination). Systematic errors due to reagent blanks do not arise. 

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