Stereoelectronic control on the kinetic stability of beta-acetoxy-substituted (eta(3)-allyl)palladium complexes in a mild acidic medium

Deuteromethanolysis of trans-and cis-beta-acetoxy-substituted (eta(3)-cyclohexylallyl)palladium complexes (1 and 2) were studied under mild acidic conditions. The trans-beta-acetoxysubstituted 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 beta-acetoxy substituent and the palladium atom. In the traits 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 (eta(3)-allyl)palladium complexes studied and their derivatives are key intermediates of important palladium catalyzed transformations, the implications of the beta-substituent effects for the regio-and chemoselectivity of the nucleophilic attack have also been discussed.