Metalametallation Bis-metallation

Metalametallations of alkenes and alkynes are useful methods for the construction of 1,2-dimetala-alkanes and 1,2-dimetala-l-alkenes, which react subsequently with suitable electrophiles to form substituted alkanes and alkenes. Metalametallation is carried out usually with bimetallic reagents of the type R Si-M R, or R Sn-M R in which M = B, Al, Mg, Cu, Zn, Si or Sn. Some metalametallations proceed without catalysts Cu, Ag and Pd compounds are good catalysts. The metalametallation with bimetallic compounds, such as Si-B, Si-Mg, Si-Al, Si-Zn, Si-Sn, Si-Si, Sn-Al or Sn—Sn bonds, catalysed by transition metal complexes, is explained by the oxidative addition of the bimetallic compounds to form 478, and insertion of alkene generates 479. Finally 1,2-dimetallic compounds 480 are formed by reductive elimination. Dimetallation of alkynes proceeds similarly to give 481. Dimetallation is syn addition. 

The cA-bis(boryl)alkenes 482 are obtained by bis-boration of terminal alkynes catalysed by a Pt complex [185]. Pd and Rh complexes are inactive. Then the (Z)-1,2-diphenylalkene 483 is prepared by Pd-catalysed Suzuki-Miyaura coupling of 482 with iodobenzene. 

Enones are also bis-silylated. Asymmetric 1,4-bis-silylation of the enone 505 catalyzed by Pd-BINAP afforded the silyl enol ether 507, and the keto alcohol 509 with 87% ee was obtained after hydrolysis and oxidation of 508 [195]. In this case, the unsymmetric disilane 506 should be used. 

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