Kinetic Studies of Acetate Exchange in <i>trans</i>-4-Acetoxy-[<i>η</i><sup>3</sup>-(1,2,3)-cyclohexenyl]palladium Complexes. Relevance for Asymmetric 1,4-Oxidation Reactions
2002-06-29T00:00:00Z (GMT) by
The acetate/acetate-<i>d</i><sub>3</sub> exchange reaction of the ring-bonded acetate of bis(4-acetoxy-[<i>η</i><sup>3</sup>-(1,2,3)-cyclohexenyl])palladium acetate-<i>d</i><sub>3</sub> complexes <b>1a</b>−<b>c</b> was studied in acetic acid solutions using <sup>1</sup>H NMR spectroscopy. The reactions followed first-order kinetics in palladium, and the rates were highly affected by the presence of methanesulfonic acid or lithium acetate. The nature of the substituent in the 2-position of the complex was found to have a large impact on the reaction rate. Complexes <b>1a</b>−<b>c</b> are observed intermediates in the benzoquinone-assisted palladium(II)-catalyzed 1,4-diacetoxylation reaction of 1,3-dienes. Complex <b>1b</b> was treated with stoichiometric amounts of the enantiomerically pure ligand (<i>S</i>)-(+)-2-(4‘-fluorophenylsulfinyl)-1,4-benzoquinone <b>4</b> under conditions where no exchange reaction occurs. Kinetic resolution was observed, implying that the two enantiomers of <b>1b</b> reacted to <i>trans</i>-1,4-diacetoxy-2-phenyl-2-cyclohexene with different rates. Attempts to demonstrate dynamic kinetic resolution in stoichiometric reactions between <b>1b</b> and <b>4</b> were unsuccessful. The major reason for this is presumably that with lithium acetate the equilibrium reaction between the two enantiomers of <b>1b</b> is too slow compared to the chiral benzoquinone-induced attack of acetate to give the products. Under very acidic conditions the decomposition of the (π-allyl)palladium complex is faster than benzoquinone-induced product formation. This scenario is in full agreement with our observed rates.