Transmetallation reactions organometallics

The general mechanism of coupling reactions of aryl-alkenyl halides with organometallic reagents and nucleophiles is shown in Fig. 9.4. It contains (a) oxidative addition of aryl-alkenyl halides to zero-valent transition metal catalysts such as Pd(0), (b) transmetallation of organometallic reagents to transition metal complexes, and (c) reductive elimination of coupled product with the regeneration of the zero-valent transition metal catalyst. 

The transmetallation reaction involves the transfer of the organic group from an organometallic species to a Pd(II) species and produces a trails Pd(II) species. Isomerization from the trans arrangement to a cis one is necessary prior to the reductive elimination step. Reductive elimination yields the coupled product and regenerates the transition metal catalyst. Because the reductive elimination is very fast, competing reactions leading to by-products are usually not observed. 

Leading examples of such performance are the transmetallation reactions of (Z)-vinylic tellnrides, easily generated by the hydrotelluration of alkynes, leading to (Z)-vinyl organometallic reagents, which are difficult to obtain by other currently available methods. 

In some cases, especially when alkynes are prepared by dehalogenations of haloalkenes, the intermediate organometallic compounds, depending on the nature of the metal, are sufficiently stable to be identified or even isolated at ambient temperatures. This property can be exploited in transmetalation reactions with suitable organometallic compounds, and thus new polylluorinated organometallic species may be prepared. 

Having in mind the fact that lithium is the most electropositive metal among those with wide synthetic applications (Li, Mg, Zn, Cd, Cu), the preparation of an organolithium compound formally represents the access to any other of such classes of organometallic compounds by a transmetallation reaction (Scheme 88).240... 

Transmetallation reactions of Grignard reagents with other organometallic compounds are rare. An example is shown here [70] other transmetallations have been briefly reported [71]. 

Of the two mechanistic pathways, i.e., via palladacyclization or via hydropalladation-cyclic carbopalladation, the latter seems to be more suitable for the development of sequentially catalyzed processes. Considering cycloisomerizations via the hydropalladation-cyclic carbopalladation route the catalytic reaction can terminate by /1-hydride elimination giving rise to the formation of dienes and derivatives thereof (Scheme 79). Alternatively, the alkyl-Pd species formed in the cyclic carbopalladation can be susceptible to subsequent transmetallation with organometallic substrates. Then, a reductive elimination could conclude this second Pd-mediated step releasing the Pd(0) species for a new catalytic cycle. 

Transmetallation reactions represent an important method for the synthesis of organoboranes. Organometallic... 

Substituted gem-dimetallic compounds, readily obtained via allylation of alkenyl organometallics, react with aldehydes in the presence of BF3-Et20 but do not react with ketones. When alkylidenemalonates are used instead of aldehydes, the Z-olefins see (E) (Z) Isomers) are obtained with a very high stereoselectivity (Scheme 35). A transmetallation reaction with copper cyanide significantly increases the reactivity of gm-dimetallic derivatives via formation of 1,1-zinca cyanocuprates. Indeed, when these compounds react with... 

For a brief discussion of the two types of transmetallation reactions, see E. Negishi, Organometallics in Organic Synthesis, Vol. 1, Wiley-Interscience, New York, 1980, Ch. 2. 

The organometallic reagent reacts in a transmetallation reaction with the alkenyl-palladium intermediate formed (TMS trimethyl silyl). 

Organolanthanide chemistry is dominated by the trivalent compounds. " Compounds in oxidation state (II) are restricted to derivatives of europium, samarium, and ytterbium, but they have considerable importance in both organic and organometallic syntheses because of their reducing properties. " Redox transmetalation reactions of organomercurials with lanthanide metals provide convenient syntheses of a number of diorganolanthanides, for example, R2M, R = CgFj or PhCC, M = Yb or Eu. " °... 

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