Dimers cuprates

Conjugate additions lo a,/ -unsalutaled kelones and eslets ate die most Impotlanl ctiptale reactions. Kinetic studies by Ktauss and Sniidi on MezCuIi and a variety of ketones teveaied die following kinetic cliatacterislics lEq. 10.5), fitsl otdet bodi in cuprate dimer and in die etione [60]. 

Conjugate additions to a, j5-unsaturated ketones and esters are the most important cuprate reactions. Kinetic studies by Krauss and Smith on Me2CuLi and a variety of ketones revealed the following kinetic characteristics (Eq. 10.5), first order both in cuprate dimer and in the enone [60]. 

This rate expression is consistent with the reaction scheme shown in Eq. 10.6, formulated on the basis of the Krauss-Smith paper. Thus, the initially formed cuprate dimer/enone complex with lithium/carbonyl and copper/olefin coordinations [71, 72] transforms into the product via an intermediate or intermediates. A lithium/ carbonyl complex also forms, but this is a dead-end intermediate. Though detailed... 

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... 

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... 

House pioneered synthetic and mechanistic studies of cuprate reactions in the 1970s. His papers proposed a mechanism (Scheme 10.4) that assumes a singleelectron transfer (SET) from the dimer, producing a Cu" intermediate [56, 57). The SET/Cu theorem had a strong following for many years. However, most of the experimental facts listed below, once considered to support the SET process, are now no longer accepted as evidence of SET. Only the Cu" hypothesis has survived the test of time. 

Alkylation reactions reveal a mechanistic aspect of the cuprate reactions different from that of addition reactions. Theoretical analyses of reactions of alkyl halides (Mel and MeBr) [123, 124] and epoxides (ethylene oxide and cyclohexene oxide) [124] with lithium cuprate clusters (Me2CuLi dimer or Me2CuLi-LiCl, Scheme 10.11) resolved long-standing questions on the mechanism of the alkylation reaction. Density functional calculations showed that the rate-determining step of the... 

Experimental evidence indicates that cuprates in solution exist largely as dimer (RiCuLi) (25), which is found as the reactive species in conjugate addition. The kinetic results were consistent with the participation of the dimer (R2CuLi)2/enone complex 26 (Figure 24) in the C—C bond-forming process of the conjugate addition. Relatively unreactive a, -unsaturated ketones, esters and nitriles were also found to form complexes represented by 26 in Figure 24. 

Since NMR studies suggest a Ca—Cf, double bond in 26 to be significantly weak, a better representation of the cuprate/enone complex is as a cupriocyclopropene 27, which is represented in the reaction shown in Figure 25. However, the indispensability of the dimeric cluster in the crucial C—C bond-forming step is not clearly understood. 

SCHEME 7. Structures of lithium cuprates and the Schlenck dimer formed by Li reduction of... 

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