Copper alkenylcopper

Mixed copper-zinc reagents also react with alkynes to give alkenylcopper species that can undergo subsequent electrophilic substitution. 

This method can be extended to the preparation of alkenylcopper compounds. Thus, treatment of the iodoalkenyl azide 10 with nBuLi at —100 °C (Scheme 2.3), followed by transmetalation with Znl2 in THF and then by a second transmetalation with CuCN-2LiCl, produces the copper species 11. This then effects a cis-selective carbocupration of ethyl propiolate to furnish the ( , ) diene 12 in 81% yield. 

One exception to this exclusion of late transition metals may be copper. CuH—MgXa complexes, prepared in situ from MgH2/CuX or from NaH/CuX/MgX2, react with terminal alkynes to give alke-nylcopper species. So far, these have only been used as sources for dialkenyl-coupling products, as in equation (54), but there is no obvious reason why other copper-based procedures should not be accessible, as was found for alkenylcopper obtained by transmetallation from Zr (Section 3.9.3.4.2). Al-kylcopper cannot be made by hydrometallation (nor by transmetallation), as CuH does not add to alkenes, except for special ones such as enones. ... 

Whereas the direct reaction of a-substituted alkenylzirconocene reagents with chlorophosphines does not lead to the related alkenylphosphines, two groups have now shown that if the reactions are carried out in the presence of copper(I) chloride the desired phosphines are obtained, initially as copper-complexes, from which they can be liberated by treatment with appropriate reagents. It is thought that alkenylcopper reagents are involved in these reactions, and, using this approach, new alkenylphosphines have been prepared, e.g., (41)," and the diphosphines (42)." ... 

Alkenylzirconium complexes can also undergo transmetalation from Zr to Cu to form alkcnylcoppcr(l) complexes, which are useful as synthetic intermediates. Since alkenylcopper(l) complexes are thermally unstable, dienes are oblainerl when the reaction is carried out at 0 20, with formation of a copper... 

Scheme 9.12 Preparation of functionalized alkenylcopper derivative via a halogen/copper-exchange.

Interestingly, Lipshutz and Wood have described a combination of hydrozirco-nation/transmetallation from zirconium to copper that is catalytic in copper salt. The zincate MesZnLi triggers the desired conversion of the alkenylcopper 68 to a more reductive cuprate at -78 C and yet does not compete with the cuprate in the Michael addition. MojZnli acts as a shuttle for the cuprate formation. In the example described in Scheme 9.19, only 10 mol% of cuprate is used, leading to the 1,4-addition product 69 in 87% yield [30]. 

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