Magnesium zincates

Lithium or magnesium zincates are readily prepared by transmetalation reactions from the corresponding magnesium or lithium reagents and are well suited to halogenzinc exchange reactions, as shown in Scheme 7.15 (Uchiyama et al. 1996) (Scheme 7.21). 

Zincate reagents can add to imines with or without Lewis acid catalysis. Alkylimines require BF3 but imines of pyridine-2-carboxaldehyde react directly. If the imines are derived from chiral amines, diastereoselectivity is observed. Both a-phenylethyl amine and ethyl valinate have been tried. Higher enantioselectivity was observed with mixed magnesium reagents.175... 

In contrast to organomagnesium see Beryllium Magnesium Organometallic Chemistry) componnds, dialkylzinc derivatives do not disproportionate in coordinated zincate anions and organozinc cations under addition of cryptand... 

In a typical reaction (Scheme 69), 2- or 3-bromo-6-methoxypyridine is lithiated in THF with rerf-butyllithium in THF at - 78°C. Low temperature and a bulky lithiating reagent are used to limit (or avoid) the tendency for adduct formation between the 7r-deficient heterocycles and the reagent. The lithiated species is reacted in the cold with zinc chloride to form the zincated derivative (299). 3-Bromo-2,6-dimethylpyridine, a benzenoid bromide, is zincated by heating in THF with magnesium, and the metal-metal exchange is effected by zinc chloride (300). The pyridylzinc chlorides can be coupled with 3-bromo- or iodo-2-quinolines using Pd-... 

Scheme 9.161. The Reformatsky reaction. A representation of the conversion of an a-haloester to the corresponding zincate, which, less reactive than the corresponding magnesium or lithium reagents, does not react with the ester functional group. The reaction with a ketone (cyclohexanone) of the typical ethyl bromo-zincethanoate (ethyl bromozincacetate, BrZnCH2C02CH2CH3) is shown.

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