Preparation and Structure of Organocopper Reagents

The synthetic application of organocopper compounds received a major impetus from the study of the catalytic effect of copper salts on reactions of Grignard reagents with y.,(i-unsaturated ketones.1 Whereas Grignard reagents normally add to conjugated enones to give the 1,2-addition product, the presence of catalytic amounts of Cu(I) results in 

Many subsequent studies have led to the characterization of several types of organocopper compounds that result from reaction of organolithium reagents with copper salts.2 

The 2 1 species are known as cuprates and are the most important as synthetic reagents. 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 

Discrete diarylcuprate anions have been observed in crystals in which the lithium cation is complexed by crown ethers.5 Both tetrahedral Ph4Cu4 and [Ph2Cu] units have been observed in complex cuprates containing (CH3)2S as a ligand. [Ph3Cu]2- units have also been observed as parts of larger aggregates.6 

Cuprates with two different copper substituents have been developed. These compounds can have important advantages in cases where one of the substituents is derived from a valuable synthetic intermediate. Table 8.1 presents some of these mixed cuprate reagents. 

There has been a great deal of study concerning the effect of solvents and other reaction conditions on the stability and reactivity of organocuprate species.8 These studies have found, for example, that (CH3)2S-CuBr, a readily prepared and purified complex of CuBr, is an especially reliable source of Cu(I) for cuprate preparation.9 Copper(I) cyanide and iodide are also generally effective and, in some cases, preferable.10 

An important type of mixed cuprate is prepared from a 2 1 ratio of an alkyllithium and CuCN.11 Called higher-order cyanocuprates, their composition is R2CuCNLi2 in THF solution, but it is thought that most of the molecules are probably present as dimers. The cyanide does not seem to be bound directly to the copper, but rather to the lithium cations.12 The dimers most likely adopt an eight-membered ring motif.13 

Mixed ligand reagent Reactivity and properties Reference 

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Diastereoselective addition in Et20/HMPT (1 1) leads to the (5)-aldehyde with an enantiomeric excess of 40%, whereas in -hexane the (2 )-aldehyde is formed with 80% ( ) enantiomeric excess. The (2 )-configured aldehyde is also obtained in benzene and in dichloromethane, but with lower ee values of 50% and 25%, respectively. Inverse results were obtained with a chiral oxazolidine prepared from ( )-cinnamaldehyde and (+)-ephedrine. Here, the (5)-aldehyde with ee = 79% is formed in -hexane, and the (7 )-aldehyde in Et20/HMPT (1 1) with ee = 50% [703], This result may be due to different structures of the organocopper reagent, and hence of the diastereomorphic activated complexes, in nonpolar solvents ( -hexane, benzene, dichloromethane) and in EPD solvents (Et20/HMPT) [703]. 

To prepare the parent bisallene 118, 116 is first converted into its Grignard reagent (known from spectroscopic studies to possess the allenic structure), from which, presumably, by the addition of cuprous chloride the organocopper intermediate 117 is generated. Addition of further 116 subsequently provides a mixture of 118 and propargylallene (l,2-hexadien-5-yne) (29) (see below) in a 2 3 isomer ratio [44],... 

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