Pd-alkene complex

The Wacker Reaction and Related Oxidations. An important industrial process based on Pd-alkene complexes is the Wacker reaction, a catalytic method for conversion of ethene to acetaldehyde. The first step is addition of water to the Pd(n)-activated alkene. The addition intermediate undergoes the characteristic elimination of Pd(0) and H+ to generate the enol of acetaldehyde. 

The key steps are attack of H20 on an electrophilic Pd-alkene complex, then (3-hydride elimination to give the enol. 

The results outlined above highlight numerous similarities between dioxygen-and BQ-mediated oxidation of palladium(O). The key steps in the respective pathways appear virtually identical (Scheme 10). Recent studies of the reactivity of dioxygen and electron-deficient alkenes with Pd°-alkene complexes reinforce these similarities. 

Fig. 5 Illustration of the oxidative trajectory for alkene exchange and oxygenation of Pd -alkene complexes...

Coordinated alkenes and alkynes undergo two kinds of reactions substitution reactions and nucleophilic additions to the coordinated ligand. While a relahvely few examples of nucleophihc attack exist for stable Pd -alkene complexes, a far larger number of examples can be inferred from the palladium-catalyzed reactions of alkenes. 

The first synthesis of the hexacyclic himandrine skeleton was achieved by L.N. Mander and co-workers. The last six-membered heterocycle was formed via an intramolecular Wacker-type oxidation in which the terminal alkene side-chain reacted with the secondary amine functionality. The oxidation was conducted in anhydrous acetonitrile to insure that the Pd-alkene complex was substituted exclusively by the internal nucleophile. The resulting six-membered enamine was then hydrogenated and the MOM protecting groups removed to give the desired final product. 

Like those of and BL, Pd -alkene complexes are very susceptible to attack by nucleophiles. In contrast to 1,2-insertion, this process exhibits anti stereospecilicity. 

The reversibility of the zwitter ion adduct formation in Eq. 8.6 also affected the rate law of the formation of amino-substituted alkylmetal complexes. Thus, kinetic studies indicated [33] that the rate of the formation of /i-aminoalkyl complex 4 in Scheme 8.20 was second-order with respect to the concentration of the amine, namely rate = [amine] [complex]. This is consistent with a reaction sequence shown in Scheme 8.20 involving a reversible formation of the zwitter ionic intermediate, followed by the rate-determining deprotonation by the second amine molecule. The observed rate constant appeared to contain contributions from both the equilibrium constant of the first step and the rate constant of the second deprotonation, so that the direct comparison of the rate of the initial nucleophilic attack at the coordinated alkene between Pd and Pt complexes was not possible. However, the higher overall reactivity (ca. 70 times) of Pd complex than Pt complex was consistent with the higher ionization potential of Pd than Pt. This difference in the ionization potential then would lead to the weaker jt basicity of Pd(II) than Pt(II) for jt back-donation to alkene jt orbital, and therefore facilitated the nucleophilic attack at the Pd-alkene complex more than that at the Pt complex. 

A very recent report from Mo and Xiao [48] disclosed the utilization of ammonium salts as efficient halide ion scavengers, providing impressive yields of branched products in short reaction times (1.5-16 h). It is believed that the qnatemary ammonium salt forms a hydrogen bond to the bromide or iodide, thus preventing possible adverse coordination to the Ar-Pd-alkene complex in the crucial insertion step (Figure 3.14). The protocol... 

The alkylidenecyclopropanes where n = 1, 2,4, reacted as expected. The 7r-allylpalla-dium complex 24 was obtained by a ring opening rearrangement of the tr-cyclopropylpal-ladium intermediate 23. This was then followed by nucleophilic attack of the anionic malonate moiety affording the new cyclic compounds 26-28. In marked contrast, the reaction of 21c (n = 3) led to the formation of a bicycUc compound 25. In this latter case the reaction must proceed via attack of the nucleophile on the Pd-alkene complex, a hitherto unknown phenomenon. This serendipitous discovery has opened up a new area of research into palladium(O) catalysis. 

Alkylpalladium derivatives formed by the attack of Pd-alkene complexes with stabilized carbanions can undergo carbon monoxide insertion, leading to the formation of 1,5-dicarbonyl derivatives. With enamides as the alkene substrates, nucleophiUc attack occurs exclusively a to N, and the carbonylation produces highly functionalized /3-amino acid derivatives, which can be cyclized to /3-lactams, as indicated by the synthesis of ( )-thienamycin shown in Scheme 19 ... 

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