Organocopper complexes preparation

A less convenient route for the preparation of cyclopentadienylcopper-ligand complexes involves the use of Cu(I) oxide. Purification of the organocopper complexes is difficult because of the decomposition products formed by subsequent hydrolysis by H2O produced in the metallation reaction, e.g. ° ... 

An ethereal soln. of 2-cyclohexenone added to Li-di-n-buty Icupr ate complex prepared from tri-n-butylphosphine-CuLcomplex and n-butyllithium in ether-hexane at -78° for 0.5 hr., stirred 0.5 hr. at the same temp., acetyl chloride in ether and hexamethylphosphoramide added rapidly at 0°, and stirred 4 hrs. at room temp. 2-acetyl-3- -butylcyclohexanone. Y 92%. F. e. s. T. Tanaka et al., Tetrah. Let. 1975, 1535 reactions with organocopper compds., review, s. J. P. Marino, Ann. Rep. Med. Chem. 10, 327 (1975). 

In addition to the ate complexes prepared from 2 equiv of RLi, the organocopper(I) conpounds RCu, prepared from 1 equiv of RLi and CuL have found synthetic application in the presence of additives, which enhance the reactivity of these otherwise relatively unreactive compounds. Both Lewis bases and Lewis acids " have been used for this purpose, and their utility has been extended to the organocuprates as well. Examples of the former are phosphines, such as tri-butylphosphine, and sulfides, such as dibutyl sulfide and dimethyl sulfide examples of the... 

When the CuBr-SMe2 complex is used in preparing the organocopper reagent, and Me2S is a cosolvent in the addition, higher yields may be achieved. Under such conditions even the otherwise unreactive methylcopper adds to terminal alkynes to generate 2-methyl-susbtituted alkenylcopper derivatives.520... 

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). 

The nature of organocopper reagents appears to be dependent on the method of preparation and the stoichiometry. Specific examples are methylcopper (76, 310), phenylcopper 73), and pentafluorophenyl-copper 34, 37, 147). The best method of preparing pentafluorophenyl-copper of composition CeFsCu appears to be via the addition of copper(I) bromide to pentafluorophenylmagnesium bromide 34, 37), since the lithium reagent and copper iodide gives an ate complex 147). An ate complex was also obtained from pentafluorophenyllithium and silver chloride in equimolar proportions 265). As shown in Table III, many of the isolated copper compounds gave somewhat incorrect or irreproducible analyses, and others contain metal halide and solvent molecules. 

RCu CN) Bu)JLi. The complex /i-BuiNCu(CN)i (1) is prepared from CuCN and m-Bu4NCN (1 equiv.). This new class of cuprate reagents was introduced to effect coupling of the vinyllithium 2 with the iodide 3, which proceeds in low yield with a variety of organocopper reagents. However, the cuprate prepared from 2 and I equiv. of 1 reacts with 3 to form the desired coupling product in 69% yield. 

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]. 

Complexing the unstable organocopper compounds with Tr-bonding ligands fails to confer much additional stability, but several complexes are sufficiently stable to be isolated. It seems best to prepare them from an existing copper(I) complex, e.g. (167),... 

For the preparation of the organocopper compounds, RCu, Cu(I) halides (preferably the bromide) are employed, although Cu(I)CN Cu(I)SCN Cu(I)03SCF3 (soluble in benzene ) or the ether-soluble phosphine complexes of these salts [e.g., Cu(I)I[P(Bu-n)3]-tetramer] can be used also. 

The cuprate complexes have better thermal stability and solubility than the parent organocopper compounds. They can be prepared directly in high yields from the 2 1 reaction of RLi with CuBr [Eqs. (c) -1- (d)] or quantitatively via the 1 1 reaction of... 

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