Palladacycles carbopalladation

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. 

Arai et al. reported that asymmetric tandem cyclization of the dialkenyl alcohol 182 in the presence of Pd(II)— spiro bis(isoxazoline) catalyst gave the bicyclic heterocycle 183 in 89% yield with 82% ee (Scheme 61).132d The reaction proceeds through Wack-er-type oxypalladation, formation of the palladacycle 185 by carbopalladation of the resulting alkylpalla-dium intermediate 184, elimination of HX, and subsequent reductive elimination of Pd(0) to give the product 183. 

The full synthetic scope and utility of those reactions that involve carbopalladation including the Heck reaction became apparent only in the 1980s through extensive investigations by a number of workers, as detailed in Part IV. The scope of carbopalladation may conceptually be further expanded so as to include addition reactions of palladium-carbene complexes as well as palladacyclopropanes, palladacyclopropenes, and higher palladacycles. These reactions are also discussed in Part IV (i.e.. Sects. IV.9 and IV.IO). 

As mentioned earlier, oxidative complexation (pattern 3) and reductive decomplexa-tion (pattern 13) are nothing but alternate representations of 7r-complexation (pattern 2) and TT-decomplexation (pattern 12), respectively. It might be useful to note that the -TT-complex formalism and palladacyclopropane formalism correspond to the Walsh model and more commonly used bent o--bond representation of cyclopropanes, respectively. The palladacyclopropane formalism is useful in discussing the formation of larger palladacycles in terms of carbopalladation of palladacyclopropanes and paUadcyclo-propenes (Part IV). 

Allylic amines when treated with lithium tetrachloropaliadate and a stabilized carbanion undergo carbopalladation to form stable five-manbered palladacycles. The Pd atom may then be substituted by hydrogen, a functionalized alkene, or carboxylate, depending on the conditions employed (Scheme 5). The carbopalladation of allylic sulfides also results in the formation of five-membered chelates. ... 

Two mechanistic pathways for Pd-catalyzed enyne cycloisomerizations have been proposed. While the palladacyclization followed by fi-hydride elimination is likely to be operative for Pd(ll) catalysts, Pd(0) complexes in combination with catalytic amounts of carboxylic acids enter a hydropalladation-cyclic carbopalladation-fi-hydride elimination cycle. This latter mechanism is particularly suited for the development of sequentially catalyzed processes. 

Mono- or di-arylaied products may be formed by a-arylation of -dicarbonyl compounds, such as malonamide esters, with arynes generated by reaction of fluoride with ortho-silylaryltriflates. A similar method of benzyne generation has been used in the a-arylation of -ketoamides in a procedure which may be modified to yield asymmettic products. The reaction of 2-haloacetanilides with arynes in the presence of a palladium catalyst may produceiV-acylcarbazoles such as (26). Possible pathways are initial formation of a palladacycle with aryne followed by oxidative addition of the haloacetanilide, or direct insertion of the palladium into the carbon-halogen bond of the acetanilide followed by carbopalladation of the aryne. ... 

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