Alkyl derivatives 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. 

Besides these alkylations of soft nucleophiles with cr-alkenylpalladium intermediates, 7T-allylpalladium complexes, formed by carbopalladation of an allene with an arylpalla-dium halide intermediate, are able to cyclize by attacking a dialkyl malonate moiety (Scheme 20)P° Instead of allenes, which give methyleneindane derivatives, cyclic and acyclic 1,4-dienes have been employed to yield tetrahydronaphthalene derivatives in good yieldsl ... 

The carbopalladation is a central reaction in organopalladium chemistry and is extensively presented in Part IV. hi most reactions, a discrete organopalladium intermediate adds to a donble or triple bond. In this section, the reaction of an alkyl iodide with di-ethylzinc in the presence of a palladium(O) catalyst is presented. Such reaction conditions generate an alkyl radical that readily adds intramolecularly to a double bond, leading to an organozinc derivative (Scheme 1). The combination of a radical cyclization with the formation of an organometallic product allows new synthetic applications that will be discnssed. Closely related Ni-catalyzed cyclizations will also be briefly presented. 

The complex catalytic cycle commences with oxidative palladium insertion on the aryl iodide bond. The following carbopalladation with norbornene creates a bulky intermediate that is not prone to p-hydride elimination and allows C-H activation of the ortho position of the aryl moiety. Subsequent oxidative addition of the alkyl iodide allows cross-coupling and elimination of the norbornene by retro-carbopalladation. The remaining intermediate 146 then undergoes aMizoroki-Heck reaction with the acrylate, which completes the catalytic process and furnishes the desired cinnamate 148 derivative. 

The aryl (or alkyl) palladium species needed for a Heck reaction can be generated fi om boronic acids and their derivatives if the overall reaction is conducted under oxidative conditions. In contrast to haloarenes, transmetalation is leaving the metal oxidation state unchanged. Therefore, an additional oxidant is needed to gain back Pd(ll) after a completed catalytic circle. So-called oxidative Heck reactions have been reported for the first time by Cho and Uemura and Karami et al The group of Park has shown that enamides are suitable substrates for this carbopalladation mode (Scheme 5-40). ... 

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