Organocuprates magnesium

Similarly to the alkyl derivatives, the most common route for arylcopper compounds is the reaction of a copper halide and aryllithium compounds (Equation (4)). Organocuprates with aryl groups are obtained by using an appropriate excess of the lithium reagent. Magnesium aryls have also been employed in transmetallation reactions with Cu(l) salts to yield both arylcopper compounds and arylcuprates (Equations (5) and (6)). 

AUcyl transfer from the magnesium hahde to the chiral Cu complexes generates the Cu complex A, as deduced from NMR experiments. Very likely, this complex functions in a similar manner as previously postulated for organocuprate additions . 

The initial interest in vinyl derivatives was due to their importance in the production of polymers. At present, however, vinyl halides are also widely used in organic syntheses as both precursors for the generation of nucleophilic reagents like vinyllithium or -magnesium derivatives, and as formal equivalents of the vinyl cations in couplings with various organocuprates (Scheme 2.51). 

The stereochemical outcome of the addition was explained in terms of steric interactions, as illustrated in Scheme 10.64. The carbonyl groups of both the crotonyl and pivaloyl esters are co-ordinated by the magnesium organocuprate or the magnesium halides formed during the reaction. The crotonyl ester 191 probably prefers an -rran -conformation. As the /-face of the double bond is shielded by the bulky pivaloyl group, the cuprate reagent approaches from the less... 

Improved organocuprate reagents, obtained from CuCN or CuSCN and organolithium (magnesium) compounds, used in addition, substitution, selective ligand transfer, epoxide opening. 

In the example in Scheme 1.170, the TJ-alkene is formed from the -alkenyl halide. Likewise, the Z-alkene is formed selectively from the Z-alkenyl halide. Alkenyl tri-llates are also good substrates for alkylation with organocuprate reagents. Treatment of the alkenyl triflate 170 with methyl magnesium bromide and copper(I) iodide as catalyst, gave the alkylated product 171, used in a synthesis of paeonilactone A (1.171). 27... 

---