Active zinc in organic synthesis

In 1973, the direct potassium metal reduction of zinc salts was reported.3 This active zinc powder reacted with alkyl and aryl bromides to form the alkyl- and arylzinc bromides under mild conditions.4 The reduction of anhydrous zinc salts by alkali metals can be facilitated through the use of electron carriers. Lithium and sodium naphthalenide reduce zinc salts to give highly reactive metal powders under milder and safer conditions. Graphite5 and liquid ammonia6 have also been employed as electron carriers in producing zinc powders. A highly dispersed reactive zinc powder was formed from the sodium metal reduction of zinc salts on titanium dioxide.7 

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An organozinc compound that occupies a special niche in organic synthesis is iodo-methylzinc iodide (ICH2ZnI). It is prepared by the reaction of zinc-copper couple [Zn(Cu), zinc that has had its surface activated with a little copper] with diiodomethane in diethyl ether. 

The presence of halides in the coordination sphere with aldehydes is important as zinc halides are typically used in organic synthesis as organic carbonyl activators. Large excesses of aldehydes and anhydrous zinc halides were necessary. Both monomeric and polymeric structures with halide bridges were observed. Tetrahedral geometries were observed for mixed ligand, aldehyde, and halide complexes.353... 

Remarkable advances in the magnesium and zinc Lewis acid catalysts have been reviewed and considered in terms of their catalytic activity, stereoselectivity, and the assumed intermediates. Their high potential in organic synthesis has been clarified, especially in asymmetric synthesis. Increasingly sophisticated reactions with greater selectivity and catalytic performance, including overall efficiency, are to be expected. 

Enantioselective vanadium and niobium catalysts provide chemists with new and powerful tools for the efficient preparation of optically active molecules. Over the past few decades, the use of vanadium and niobium catalysts has been extended to a variety of different and complementaiy asymmetric reactions. These reactions include cyanide additions, oxidative coupling of 2-naphthols, Friedel-Crafts-type reactions, pinacol couplings, Diels-Alder reactions, Mannich-type reactions, desymmetrisation of epoxides and aziridines, hydroaminations, hydroaminoalkylations, sulfoxida-tions, epoxidations, and oxidation of a-hydroxy carbo) lates Thus, their major applications are in Lewis acid-based chemistiy and redox chemistry. In particular, vanadium is attractive as a metal catalyst in organic synthesis because of its natural abundance as well as its relatively low toxicity and moisture sensitivity compared with other metals. The fact that vanadium is present in nature in equal abundance to zinc (albeit in a more widely distributed form and more difficult to access) is not widely appreciated. Inspired by the activation of substrates in nature [e.g. bromoperoxidase. 

Indeed, many metals other than magnesium have been activated and used in the synthesis of many organic compounds [55]. Most notably, copper, zinc, calcium, indium, cadmium, nickel, aluminum, and barium have been activated and used in the synthesis of many complex molecules [56]. 

It has been reported that Neurospora crassa, grown on zinc-deficient media, was devoid of ADH activity, which was not restored by the addition of Zn++. The structured association of zinc with the ADH apoenzyme was not considered as a possible explanation for the resultant functional derangements (Nason, Kaplan, and Colowick, 1951), which were attributed to an indirect influence on the synthesis of the apoenzjune (McElroy and Nason, 1954). In the absence of zinc, this organism is apparently unable to form the functional metalloenzyme molecule. It would seem that the ADH of Neurospora, like the ADH of yeast, is a zinc metalloenzyme. 

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