Palladium(II)-catalyzed exchange and isomerization reactions .16. The kinetics and stereochemistry of the oxidation and isomerization of hexafluoro allylic alcohols in aqueous solution catalyzed by PdCl3(pyridine)(-)

Further mechanistic studies on the PdCl3(pyridine)(-) catalytic system in aqueous solution are described using the tetrasubstituted allylic alcohol, (E)-2-methyl-d(3)-4-methyl-1,1,1,5,5,5-hexafluoro-3-penten-2-ol, 3a, and the trisubstituted allylic alcohol, (E)-4-Methyl-1,1,1,5,5,5-hexafluoro-3-penten-2-ol, 6, as substrates. At low [Cl-] the PdCl42- catalyzed isomerization of 3a, which can only undergo isomerization into its allylic isomer, was previously found to obey the Wacker rate expression: k(i)[PdCl42-][3a]/[H+][Cl-](2). In contrast, the rate expression for isomerization of 3a by [PdCl3(Py)(-)] at low [Cl-] was found to be: rate(i) = k(i)[PdCl3(Py)(-)][3a]/[Cl-]. This rate expression is of the same form as that previously found for the isomerization of 3a by PdCl42- at high [Cl-]. This result strongly suggests that the hydroxypalladation by PdCl3(Py)(-) at low [Cl-] is a trans process as opposed to a cis process with PdCl42-. This expectation was confirmed by stereochemical studies with chiral 3a. The stereochemistry of addition for PdCl3(Py)(-) was identical to that for PdCl42- at high [Cl-]. Independent stereochemical studies have shown this addition to be trans. With PdCl3(Py)(-) there are two possible routes for olefin oxidation. A cis process similar to that found for PdCl42- or a trails process analogous to that previously proposed to explain the traits stereochemistry found at high [Cl-]. Stereochemical studies with 6, which can undergo oxidation, showed that both processes are operative with PdCl3(Py)(-) at [Cl-] = 0.05 M. Thus addition of a pyridine to the coordination sphere of Pd(II) causes a profound change in reactivity.