Kinetic Studies of Acetate Exchange in trans-4-Acetoxy-[η3-(1,2,3)-cyclohexenyl]palladium Complexes. Relevance for Asymmetric 1,4-Oxidation Reactions

The acetate/acetate-d3 exchange reaction of the ring-bonded acetate of bis(4-acetoxy-[η3-(1,2,3)-cyclohexenyl])palladium acetate-d3 complexes 1ac was studied in acetic acid solutions using 1H 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 1ac are observed intermediates in the benzoquinone-assisted palladium(II)-catalyzed 1,4-diacetoxylation reaction of 1,3-dienes. Complex 1b was treated with stoichiometric amounts of the enantiomerically pure ligand (S)-(+)-2-(4‘-fluorophenylsulfinyl)-1,4-benzoquinone 4 under conditions where no exchange reaction occurs. Kinetic resolution was observed, implying that the two enantiomers of 1b reacted to trans-1,4-diacetoxy-2-phenyl-2-cyclohexene with different rates. Attempts to demonstrate dynamic kinetic resolution in stoichiometric reactions between 1b and 4 were unsuccessful. The major reason for this is presumably that with lithium acetate the equilibrium reaction between the two enantiomers of 1b 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.