Palladium , monomer-dimer equilibrium

Powell et al. (113) have utilized NMR to aid in the detection of a tri-haptoidimexy-pentahaptoijaonovaeT) equilibrium for a series of hepta-2,6-dienylpalladium acetate complexes in chloroform solution. The chemical shifts for C(l) and C(2) in IV (W = H, X = O2CCF3) are compatible with a coordinated olefin, but for IV (W = Cl, X = O2CCF3) these shifts are almost identical to those of III (W = Cl, X = hfacac) in which the olefin C(l) = C(2) is not coordinated to palladium. This evidence together with molecular weight data led the authors to conclude that IV existed in an /t (dimer)-A (monomer) equilibrium the position of equilibrium being dependent on both the olefinic substituent W and the carboxylate substituent R. 

Ea = 41.5 kJmol. The palladium(ii) hepta-2,6-dienyl complexes (64) exist in solution at normal temperatures as a rapidly equilibrating mixture of the A -dimer and A -monomer forms (Scheme 9). The position of equilibrium is seen to be affected... 

Solvent effects on, and products from, reaction of styrene with ethylene in the presence of di-)ti-chloro-dichlorobis(styrene)dipalladium(n), [Pd-(Ph CH—CH2)Cl2]2, indicate a mechanism similar to (i)->(iv) above, with the addition of a preliminary equilibrium between the dimer and solvated monomers. The mechanism of reaction of styrene with vinyl compounds, catalysed by the same chloride-bridged dipalladium complex, has been studied using isotopic tracer (H, D) experiments. Palladium-acetate-catalysed reaction of styrene with benzene, also investigated using deuterium tracer experiments, involves no hydride shift, in contrast to the rather closely related Wacker process. The importance of intermediates with palladium-carbon n-bonds in palladium(ii)-catalysed alkylation and arylation of alkenes has been demonstrated. 

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