Stereoelectronic Control on the Kinetic Stability of β-Acetoxy-Substituted (η3-Allyl)palladium Complexes in a Mild Acidic Medium

Deuteromethanolysis of trans - and cis -β-acetoxy-substituted (η 3 -cyclohexylallyl)palladium complexes ( 1 and 2 ) were studied under mild acidic conditions. The trans -β-acetoxy-substituted complex ( 1 ) reacted about 200 times faster than its cis -substituted counterpart ( 2 ). A theoretical analysis of the structure and stability of slightly simplified model compounds ( 4 − 7 ) was performed employing density functional theory at the B3PW91 level in order to elucidate the relationship between the rate of deuteromethanolysis and the electronic interactions between the β-acetoxy substituent and the palladium atom. In the trans complex, the stereoelectronic requirements of the conjugative interactions between the palladium atom and the C−O(Ac) bond are fulfilled, which facilitates the C−O(Ac) bond cleavage in the deuteromethanolysis reaction. Since the substituent geometry in the cis complex is different, these conjugative interactions are suppressed, providing a greater kinetic stability for the cis complex under the reaction conditions applied. Since the (η 3 -allyl)palladium complexes studied and their derivatives are key intermediates of important palladium catalyzed transformations, the implications of the β-substituent effects for the regio- and chemoselectivity of the nucleophilic attack have also been discussed.