Trans hydroxypalladation

In another study [34] bis-[(cis-l,2-dideuterioethylene)dichloropalladium] was reacted with carbon monoxide in aqueous acetonitrile to form trans-2,3-dideuterio- 6-propiolactone according to eq. (25) via a suggested trans-hydroxypalladation intermediate. 

The point has been made that the conditions of jS-chloroethanol formation are not the same as used for the Wacker oxidation. Cu -Pd chlorine-bridged dimers are likely reactants under higher [Cl ] reaction conditions, which may lead to a different reaction mechanism. However, a second stereochemical study also obtained results consistent with trans hydroxypalladation. When c7s-l,2-dideuteroethene is oxidized in water with PdCl2 under a CO atmosphere, the product is tran5-2,3-dideutero-/3-propiolactone (Scheme 37). The reaction conditions were, once again, not identical with standard Wacker process conditions, since the solvent was acetonitrile-water, the temperature was —25°C, the bis-ethene PdCl2 complex was used, and there was no excess CU present. Nevertheless, it is clear that, under many reaction conditions, a trans addition of water onto ethene coordinated to Pd is the favored reaction stereochemistry. 

In the case of certain diolefins, the palladium-carbon sigma-bonded complexes can be isolated and the stereochemistry of the addition with a variety of nucleophiles is trans (4, 5, 6). The stereochemistry of the addition-elimination reactions in the case of the monoolefins, because of the instability of the intermediate sigma-bonded complex, is not clear. It has been argued (7, 8, 9) that the chelating diolefins are atypical, and the stereochemical results cannot be extended to monoolefins since approach of an external nucleophile from the cis side presents steric problems. The trans stereochemistry has also been attributed either to the inability of the chelating diolefins to rotate 90° from the position perpendicular to the square plane of the metal complex to a position which would favor cis addition by metal and a ligand attached to it (10), or to the fact that methanol (nucleophile) does not coordinate to the metal prior to addition (11). In the Wacker Process, the kinetics of oxidation of olefins suggest, but do not require, the cis hydroxypalladation of olefins (12,13,14). The acetoxypalladation of a simple monoolefin, cyclohexene, proceeds by trans addition (15, 16). 

The kinetics indicated that, in fact, the hydroxypalladation occurred by trans attack of water on the dimeric n complex 121) ... 

In a stereochemical study f-Cethylenel-da (C2H2D2) [32] was reacted with palladium chloride and cupric chloride under extreme conditions, i. e., extremely high chloride ion concentration as cupric and lithium chlorides. Under such conditions 2-chloroethanol was formed as the main product from ethylene, besides some acetaldehyde [33] (see Section 2.4.1.5.1) this is not the normal product of the Wacker reaction. In the above study the formation of cis-, 2-dideuterioethylene oxide, evidently via 1,2-dideuterio-2-chloroethanoI, suggests trans addition of water (nnh-hydroxypalladation). 

Hydroxypalladation of ethylene with H2O to give /1-hydroxyethylpalladium intermediate is a key step involved in the Wacker process, an industrial oxidation of ethylene to acetaldehyde (Scheme 8.27). How the C-0 bond formation takes place has been a matter of controversy for a long time. Occurrence of trans attack of water at the Pd-bound ethylene ligand was suggested by a stereochemical... 

Assuming the same intermediate for both products, the oxypalladation step should be trans-orientated. The authors of these studies claim the mechanism of the Wacker reaction, although the conditions and the products are different from it. This is also the case with the hydroxypalladation of cyclooctadiene [26] where the chelated Pd complex is rigid and the rotation of the complexed olefin is prevented to enable the cfs-interaction. 

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