Reductive Heck reactions, palladium compounds

Under standard reaction conditions, the mechanism of the Heck reaction is more complicated than the textbook pathway shown in Scheme 5. The active catalyst responsible for oxidative addition of the ArX substrate is an anionic Pd species, either I PdCl or L2Pd(OAc), depending on the starting palladium compound, where L is triphenylphosphine. Reduction of the starting palladium salt to a Pd species is carried out by the phosphine. A mechanism that is consistent with spectroscopic and electrochemical data is given in Scheme 13. 

In 1997, Antonietti et al. reported on catalytically active palladium nanoparticles prepared by reduction of palladium(II) compounds in inverse block copolymer micelles, namely polystyrene-ib-poly(4-vinylpyridine) (PS-b-P4VP). Activated aryl bromides were coupled reproducibly in Heck reactions [18]. Small partide sizes were a prerequisite for high conversions, as indicated by qualitative TEM investigations. Very high total turnovers were reported (0.0012 mol% palladium, 68% conversion in five days, corresponding to 56 000 TO) (Table 1). Catalyst activity was found to be dependent on the structure of the block copolymer employed, which was attributed to a better accessibility of the metal particles in smaller micelles with a high surfacer area and thinner polystyrene layer. 

The stereogenic centers were then introduced by palladium-catalyzed dynamic kinetic asymmetric transformatitHi. ITierefore, 41 was coupled with lactone 42 in the presence of chiral ligand (RJt)-43 and gave 44 in 89% yield. The synthesis of 42 is shown below in Scheme 2.6. Compound 44 was subjected to an intramolecular Heck reaction followed by acidic cleavage of the ester function 45). The intramolecular Heck reaction only produced one diastereomer, because the c/s-annelated rings are favored. Scandium(lll)-mediated cyclizatitMi and reduction of the lactone with DIB ALII yielded (-)-aflatoxin Baa (46). It was acetoxylated and then pyiolyzed to give (-)-aflatoxin Bi (1) in 1.6% overall yield and nine linear steps from catechol (40). 

Early versions of the Heck reaction employed organomercury compounds rather than organic halides. This process results in net reduction of palladium, so a stoichiometric amount was used. Other organometallic species may be used in place of organomercury compounds, such as boronic acids 5.199. In the presence of an oxidant, a catalytic process is possible, delivering Heck-like products (Scheme 5.55). ... 

The addition of an organopalladium intermediate to the double bond with subsequent cleavage of palladium hydride and restoration of unsaturation is the basis of another important process involving organopalladium compounds, the Heck reaction. In a catalytic version, the organopalladium intermediate is formed by oxidative addition to carbon-halogen or other carbon-heteroatom bonds, and the hydridopalladium complex is believed to be recycled by base-promoted reductive elimination (Scheme 5.7). 

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