Palladium(II)-catalyzed exchange and isomerization reactions. XVI The kinetics and stereochemistry of the oxidation and isomerization of hexafluoro allylic alcohols in aqueous solution catalyzed by PdCl 3(pyridine) −

Abstract

Further mechanistic studies on the PdCl 3(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 PdCl 2− 4 catalyzed isomerization of 3a, which can only undergo isomerization into its allylic isomer, was previously found to obey the Wacker rate expression: k i[PdCl 2− 4][ 3a]/[H +][Cl −] 2. In contrast, the rate expression for isomerization of 3a by [PdCl 3(Py) −] at low [Cl −] was found to be: rate i = k i[PdCl 3(Py) −][ 3a]/[Cl −]. This rate expression is of the same form as that previously found for the isomerization of 3a by PdCl 2− 4 at high [Cl −]. This result strongly suggests that the hydroxypalladation by PdCl 3(Py) − at low [Cl −] is a trans process as opposed to a cis process with PdCl 2− 4. This expectation was confirmed by stereochemical studies with chiral 3a. The stereochemistry of addition for PdCl 3(Py) − was identical to that for PdCl 2− 4 at high [Cl −]. Independent stereochemical studies have shown this addition to be trans. With PdCl 3(Py) − there are two possible routes for olefin oxidation. A cis process similar to that found for PdCl 2− 4 or a trans process analogous to that previously proposed to explain the trans stereochemistry found at high [Cl −]. Stereochemical studies with 6, which can undergo oxidation, showed that both processes are operative with PdCl 3(Py) − at [Cl −] = 0.05 M. Thus addition of a pyridine to the coordination sphere of Pd(II) causes a profound change in reactivity.