Carbocupration of acetylene

The carbocupration of acetylene takes place smoothly in a cis fashion, providing a reliable synthetic route to vinyl copper species (Eq. 10.8) [24]. Magnesium and zinc,... 

The carbocupration of acetylene has been studied systematically for five model species - MeCu, Me2Cu, Me2CuLi, Me2CuLi LiCl, and (Me2CuLi)2 [91] - all of which have been invoked once in a while in discussions of cuprate mechanisms. A few general conclusions have been made regarding the reactivities of these reagents with 71-acceptors ... 

Fig. 10.3. Snapshots of intermediates on the potential energy surface of carbocupration of acetylene.

Fig. 16.17. Mechanism of the carbocupration of acetylene (R = H) or terminal alkynes (R H) with a saturated Gilman cuprate. The unsaturated Gilman cuprate I is obtained via the cuprolithiation product E and the resulting carbolithiation product F in several steps—and stereoselectively. Iodolysis of I leads to the formation of the iodoalkenes J with complete retention of configuration. Note The last step but one in this figure does not only afford I, but again the initial Gilman cuprate A B, too. The latter reenters the reaction chain "at the top" so that in the end the entire saturated (and more reactive) initial cuprate is incorporated into the unsaturated (and less reactive) cuprate (I). - Caution The organometallic compounds depicted here contain two-electron, multi-center bonds. Other than in "normal" cases, i.e., those with two-electron, two-center bonds, the lines cannot be automatically equated with the number of electron pairs. This is why only three electron shift arrows can be used to illustrate the reaction process. The fourth red arrow—in boldface— is not an electron shift arrow, but only indicates the site where the lithium atom binds next.

Scheme 7-9 Carbocupration of acetylene with a funtionalized zinc reagent.

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