PALLADIUM(II)-CATALYZED EXCHANGE AND ISOMERIZATION-REACTIONS .15. KINETICS AND STEREOCHEMISTRY OF THE ISOMERIZATION OF 2-(METHYL-D3)-4-METHYL-1,1,1,5,5,5-HEXAFLUORO-3-PENTEN-2-OL IN AQUEOUS-SOLUTION CATALYZED BY PDCL42- AT HIGH CHLORIDE CONCENTRATIONS

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 spectroscopy under conditions of high chloride (>2.0 M) concentration used previously in the presence Of CuCl2 to determine the stereochemistry of hydroxypalladation. The rate expression under these conditions is rate = k(i)[PdCl42-][2a]/[Cl-], with k(i) = 1.1 x 10(-3)s-1. This rate expression at high chloride concentrations is identical to the rate expression found for the nonoxidative isomerization of allyl alcohol under the same reaction conditions and is consistent with an equilibrium pi-complex formation followed by trans attack of water to give the oxypalladation intermediate, which reverses the process to give exchange. The fact that the attack is from outside the coordination sphere of the palladium(II) explains the single-chloride inhibition. Stereochemical studies were conducted with chiral (E)-2a. The observed result was the formation of chiral 2b with the same configuration as the initial 2a but with the Z geometric configuration. This result is also consistent only with trans hydroxypalladation; thus, both the kinetic and stereochemical studies give the same result. This result also agrees with earlier stereochemical studies at high chloride concentrations which used quite a different technique. The important point is that since the exchange stereochemical studies carried out in the previous paper of this series showed the hydroxypalladation to have stereochemistry opposite from that at high chloride concentrations, the previous stereochemical studies at high [Cl-] are not a valid indication of the stereochemistry at low [Cl-].