Transmetallation alkenylmetals

Products of the carbometallation 442 and 443 contain very reactive metal carbon bonds, and can not be isolated in many cases. They undergo further reactions such as protonation, transmetallation and nucleophlic attack. The preparation of tri- or tetrasubstituted alkenes 444 and 445 from alkynes via the alkenylmetals 443 is particularly useful. Only catalysed carbometallations are treated here. 

B.iii.b. (Z)- Substituted Alkenylmetals. The stereodefined (Z)-/3-substituted aUcenyl-metals can generally be prepared by either metallation or metallation-transmetallation of the corresponding (Z)-alkenyl electrophiles. However, several other methods are notable.f i [5 ] P5] a stereoselective synthesis of (Z)-alkenylboranes via hydroboration... 

At present, (Z)-/3-substituted alkenylmetals containing other metals are less readily accessible than those mentioned above, even though the Zr-catalyzed carboalumination of ethyne has been shown to produce (Z)-/S-substituted alkenylalanesJ They are generally prepared via metallation-transmetallation of (Z)-/S-substimted hahdes. Despite this drawback, (Z)-/3-substituted alkenylzincs generated by this procedure have been shown to be superior reagents in the subsequent Pd-catalyzed cross-coupling reactiont (Table 11). 

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