Lithium naphthalenide, highly reactive

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... 

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... 

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... 

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. 

It should be pointed out that the homocoupling of alkyl halides could be completely eliminated when the CuI/PPhs complex vras used. The cycloalkylation of 5-bromo-l,2-epoxypentane with highly reactive copper, which was generated from lithium naphthalenide and CuI/PPhs, gave a 37 1 mixture of cyclopentanol and cyclobutylmethanol in excellent yield (87%) with no evidence of any homocoupling reaction. 

A new, highly reactive copper species made from the reduction of copper(I) complexes with two equivalents of preformed lithium naphthalenide leads to a reagent that behaves chemically as a formal copper anion solution [20]. Lithium naphthalenide provides a suitable reducing agent for a variety of metals and offers unique advantages over other techniques used in metal activation, such... 

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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