The isomerization of 2-(methyl-d3)-4-methyl-1,1,1,5,5,5-hexafluoro-3-penten-2-ol (2a) into an equilibrium mixture of 2a and 2-methyl-4-(methyl-d3)-1,1,1,5,5,5-hexafluoro-3-penten-2-ol (2b) in aqueous solution was studied by H-1 and H-2 NMR, under the Wacker conditions of low chloride (< 1.0 M) and acid (< 0.5 M) concentrations. The rate expression under these conditions is rate = k(i)[PdCl4(2-)][2a]/[H+][Cl-]2, with k(i) = 1.05 x 10(-6) s-1. The exchange of 2a with O-18-enriched water was studied by C-13 NMR using isotope-induced shift methods and the rate of exchange was found to be the same as the rate of isomerization within experimental error. This result requires that isomerization and exchange occur by a hydroxypalladation route, rather than through palladium (IV)-pi-allyl intermediates. The rate expression for isomerization at low chloride concentrations is identical with the rate expression for the Wacker oxidation of ethene to acetaldehyde. This result is inconsistent with a proton inhibition arising from equilibrium hydroxypalladation but is consistent with proton loss from the Pd(II) coordination sphere in a preequilibrium step followed by a cis hydroxypalladation occurring from within the coordination sphere of the palladium(II). Stereochemical studied were conducted with chiral (E)-2a. The observed result was the formation of chiral 2b with the opposite configuration of the initial 2a. This result is also consistent only with cis hydroxypalladation; so both kinetic and stereochemical mechanistic probes give the same result.
