Lithium naphthalenide, highly reactive preparation

At this time the applicability of organozinc chemistry had been limited to those zinc reagents that could be prepared by insertion of zinc powder into the corresponding alkyl iodide.20 Further developments, notable the use of highly reactive zinc (Rieke zinc), obtained by the reduction of zinc halides with lithium naphthalenide, allowed the preparation of zinc reagents from otherwise unreactive organic substrates, such as aryl iodides and aryl bromides (Scheme 1.7).21... 

Highly reactive calcium can be readily prepared by the reduction of calcium halides in tetrahydrofuran solution with preformed lithium biphenylide under an argon atmosphere at room temperature.5 This colored calcium species seems to be reasonably soluble in THF. However, the reactive calcium complex prepared from preformed lithium naphthalenide was insoluble in THF solution and precipitated out of solution to give a highly reactive black solid. The exact nature of this black calcium complex has not been determined. Acid hydrolysis of the black material releases naphthalene as well as THF. Accordingly, the most likely structure of the black material is a Ca-naphthalene-THF complex similar in nature to the soluble magnesium-anthracene complex recently reported.6... 

A highly reactive zero-valent copper species has been prepared by direct reduction of lithium 2-thienylcyanocuprate in THF with lithium naphthalenide <89SC1833>. 

A far superior method of preparing the highly reactive zinc is to use lithium metal as the reducing agent along with an electron carrier such as naphthalene (Method 2). This approach is considerably safer as there is no rapid burst of heat. The dark green lithium naphthalenide also serves as an indicator, signaling when the reduction is over. 

Secondary alkyl bromides are sufficiently reactive under these conditions, but primary alkyl bromides are usually inert and much better results are obtained by using highly activated Rieke-zinc. Thus, the reduction of zinc chloride with lithium naphthalenide, freshly prepared from finely cut lithium and naphthalene in THF, produces within 1.5 h highly reactive zinc (Rieke-zinc). This form of zinc readily reacts with secondary and tertiary alkyl bromides. Adamantyl bromide (217) was converted into the corresponding organozinc reagent 218. Its reaction with cyclohexenone in the presence of BF3 OEt2 and TMSCl afforded the 1,4-addition product 219 in 54% yield (Scheme 2-87). 

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