NATURE OF THE INTERACTIONS BETWEEN POLAR BETA-SUBSTITUENTS AND PALLADIUM IN ETA(3)-ALLYLPALLADIUM COMPLEXES - A COMBINED EXPERIMENTAL AND THEORETICAL-STUDY

Deuteromethanolysis of six beta-methoxy-substituted eta(3)-allylpallad ium complexes (1-5) was studied under the same reaction conditions. Th e reaction rates depend on the ring size of the cyclic complexes, on t he sigma-donor/pi-acceptor nature of the ancillary ligand, on the conf iguration of the allylic moiety, and on the position of the beta-subst ituent with respect to the palladium atom. Replacement of the methoxy group proceeds about 1000 times faster in the trans-beta-substituted c ycloheptylallyl palladium complex 2 than in the cyclooctyl analogue 3, this indicates that the C-O bond strength is a function of the ring a nd substituent conformations. A theoretical analysis of the structure and stability of slightly simplified model compounds 6-10 was performe d with density functional theory at the B 3 PW 91 level in order to el ucidate the relationship between the rate of deuteromethanolysis and t he electronic interactions between the beta-methoxy substituent and th e palladium atom. It was concluded that the strength of these beta-sub stituent effects critically depends on the relative position of the Pd -C3 and C4-O bonds. In cyclohexyl and cycloheptylallyl complexes 6 and 7, the trans-beta-methoxy substituent adopts a conformation that is p articularly favoured by the beta-substituent effect. Since the eta(3)- allylpalladium complexes studied and their derivatives are key interme diates of important palladium-catalysed transformations, the implicati ons of the beta-substituent effect for the regio- and chemoselectivity of nucleophilic attack are also discussed.