Copper lithium cuprates

The regiochcmistry for stoichiometric alkylation with butyl(cyano)copper magnesium bromide is the same as that for the copper cyanide catalyzed reaction. The regiochemistry with dibutyl-copper magnesium bromide is also very similar to that of the copper(I) bromide catalyzed reaction. Lithium cuprates do not exhibit y regioselectivity in this biased system. 

Thienyl(cyano)copper lithium S Cu(CN)Li xhe reagent is obtained by reaction of thiophene with BuLi in THF at - 78° and then with CuCN at - 40°. The reagent is fairly stable and can be stored in THF at - 20° for about 2 months. It is inert, but is readily converted by addition of RLi or RMgX into a higher-order mixed cuprate, which is as efficient as the freshly prepared cuprate."1... 

Scheme 2.9 Influence of copper salt and additives on the SN2 substitution of propargyl oxirane 27 with lithium cuprates. TBS = Si(tBu)Me2.

So far, only cuprates with a 1 1 copper/lithium ratio have been considered. Treatment of phenyllithium with various substoichiometric quantities of copper bromide in DMS as solvent afforded so-called higher order cuprates, of which two were characterizable by X-ray crystallography. These have the overall stoichiometries Cu2Li3Ph5(DMS)4 and Cu4Li5Ph9(DMS)4 [114, 115). The structure of the former compound in the solid state is shown in Fig. 1.26. 

The organocopper is shown here as Me-Cu because its precise structure is not known. But there are other organocopper reagents that also undergo conjugate addition and that are much better understood. The simplest result from the reaction of two equivalents of organolithium with one equivalent of a copper (I) salt such as CuBr in ether or THF solvent at low temperature. The lithium cuprates (I CuLi) that are formed are not stable and... 

Sakai and co-workers [38] studied the additions to conjugate esters protected with a cyclic diol as a chiral auxiliary. The ester is protected with enantiomerically pure ( R2R) rrfl/js-cyclohexane diol (see Table 4, entry 7). Addition of phenyl Grignard reagent, catalyzed by Cul, produced a moderate yield (50%) of a mixture of the two diastereomers (prochiral carbon R/5 = 78% 22%) with a d.e. of 55%. In contrast, the authors also reported that addition of the phenyl lithium cuprate gave 94 6 ratio with the major product having the S-configuration for the prochiral carbon. Because of the differences in the structures of the complexes, attack from the lithium cuprate comes from the Re-face, whereas attack from the copper-Grignard comes from the Si-face. 

The preparation of a,p-unsaturated ketones by direct acylation of vinylcopper reagents has proven more problematic, since lithium cuprates do add to the product enones. Better results are obtained with the less reactive monovinyl copper compounds in the presence of a palladium catalyst. Alkynic ketones have been prepared by a variation of the Stephens-Castro coupling. ... 

Tables of electronic wavefunctions have been compiled for the diatomic hydrides AH, where A denotes the elements Li through F, and Na through Cl.195 X-Ray diffraction patterns at high pressure show that LiH retains the low-pressure NaCl structure up to 12.0 GPa (120 kbar).196 The preparation of the first stable complex metal hydride of copper, lithium dihydro-cuprate(i), is reported from the reduction of LiCuMe2 by lithium aluminium hydride in ether at low temperatures. The solid compound, LiCuH2, is solvated by ether and stable under ambient conditions for several days.197...

An excellent illustration of both these types of vinyl anion equivalent appears in Corey and Wollenberg s synthesis40 of the antibiotic brefeldin A 163. This interesting molecule has two E-alkenes, one attached to a hydroxyl group as an allylic alcohol, and so accessible by direct addition of a vinyl lithium 165 to an aldehyde, and one in a 1,3-relationship with an alcohol and so accessible in theory by conjugate addition of a vinyl copper (or cuprate) 166 to the simple double electrophile 164. Each OH group must be protected. 

Lithium-cuprate complexes obtained by the reaction of Cu(I) iodide and the corresponding aryllithium were also successfully applied in the synthesis of unsymmetrical biaryls [23]. An alternative strategy involves the low temperature oxidative coupling of higher order mixed diarylcuprates [24]. The classical Ullmann reaction can be accelerated by using, instead of copper... 

The chemical reactivity displayed by the cuprate reagents is powerful nu-cleophilicity toward carbon, but with a strong preference for reaction at alkene or halide sites over carbonyl groups. The mechanism(s) of the reactions have not been established with certainty. A leading proposal " for the conjugate addition is that reaction is initiated by a one-electron transfer from the copper-lithium species. The... 

The low ionic character of the aluminium-silicon bond has been cleverly utilized to develop a very mild, general and effective synthesis of acyl silanes, successful for aliphatic, aromatic, heteroaromatic, a-aUcoxy, a-amino and even a-chiral and a-cyclopropyl acyl sUanes. Acyl chlorides are treated with lithium tetrakis(trimethylsilyl)aluminium or lithium methyl tris(trimethylsilyl) aluminium in the presence of copper(I) cyanide as catalyst to give the acyl silanes in excellent yields after work-up. Later improvements include the use of 2-pyridinethiolesters in place of acyl halides, allowing preparation of acyl silanes in just a few minutes in very high yields indeed (Scheme 9) °, and the use of bis(dimethylphenylsilyl) copper lithium and a dimethylphenylsilyl zinc cuprate species as nucleophiles. 

Intensive studies have been devoted to a mechanistic rationale of the conjugate addition of organocopper compounds and give a rather complex picture [141, 135, 136]. A simplified mechanistic model starts from dimeric lithium cuprates 131, wherein oxygen is assumed to coordinate to lithium and the carbon-carbon double bond to copper. The chelated cuprate 133, but also the nonchelated species 132 might function as intermediates for the formation of... 

---