Copper Compounds Lithium dimethylcuprate

The 2 1 species are known as cuprates and are the most common synthetic reagents. Disubstituted Cu(I) species have the 3c 10 electronic configuration and would be expected to have linear geometry. The Cu is a center of high electron density and nucleophilicity, and in solution, lithium dimethylcuprate exists as a dimer [LiCu(CH3)2]2.3 The compound is often represented as four methyl groups attached to a tetrahedral cluster of lithium and copper atoms. However, in the presence of Lil, the compound seems to be a monomer of composition (CH3)2CuLi.4... 

In 1952 Gilman and co-workers reported that the yellow, ether-insoluble methylcopper dissolved to give a clear colorless solution on addition of a further equivalent of methyllithium 119). The new copper compound, of composition LiCu(CH3)2, is an ate complex now usually known as lithium dimethylcuprate. Mixed cuprates can be prepared from a preformed organocopper compound, RCu, and a lithium reagent 147, 259, 298, 301). 

Compounds with acidic hydrogen atoms react rapidly with cuprates. Phenylacetylene has been mentioned as one example (223). Another is diethyl phenylmalonate (144), which on addition to lithium dimethylcuprate gave a rapid evolution of methane and the formation of a methyl-copper-like precipitate which did not redissolve. Subsequent to the addition of benzoyl chloride and the customary work-up, only acetophenone and the phenylmalonate were isolated. The reaction may be summarized by Eq. (23). The failure to isolate the acylated product may be ascribed to the formation of the enolate, (II). 

However, good yields of ketones can be obtained for an equivalent quantity of acid halide per group attached to copper. Suitable conditions exist for the coupling of dialkylcuprates with acid chlorides which contain other functional groups (233), such as carbonyl, carboxy, and cyano. The order of reactivity of these groups towards organocopper compounds is CHO > CO > COgR > CN. Benzoyl fluoride reacts very slowly with lithium dimethylcuprate. 

On reaction of lithium dimethylcuprate with a vinylic halide which also contained a tetrahydropyranyl ether-protected allylic alcohol function (152), the product of coupling, (XVI), was obtained in poor yield (30%) and was accompanied by a 60% total yield of three other compounds [Eq. (103)]. The most significant of these represent products arising from the splitting off of the THP group, perhaps because of its allylic nature, by attack of the dimethylcuprate species and by an intermediate copper(III) compound on (XVI) or its precursor. 

The difficulties of direct oxidative insertion with metals other than Mg or Li mean that o-complexes are often made from organo-lithium or Grignard reagents by metal exchange. This reaction amounts to a nucleophilic substitution at the metal without a change of oxidation state so the metal is used in whatever oxidation state is finally needed. Attack of methyl lithium on a Cu(I) halide gives methyl copper 50, a o-complex of Me- and Cu(I). If an excess of MeLi is present an ate complex is formed, lithium dimethylcuprate 51. This is formally a compound of a copper anion 51a, just as BF4 is a borate. The term ate complex refers to such formally anionic complex in which the metal has one extra anionic ligand. Its true structure is dimeric 51b and it exists as an equilibrium with 52 in solution.20... 

Until recently, the magnesium, lithium, and, to a lesser extent, cadmium compounds were the only organometallic systems of much use in organic synthesis. This situation has changed drastically in the last few years. Compounds of copper have proven to be especially valuable. One of the first examples of these compounds is a reagent prepared from methyllithium and cuprous iodide this reagent is referred to as lithium dimethylcuprate. The reagent adds selectively at the /3-carbon... 

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