CENTRAL VERSUS TERMINAL ATTACK IN NUCLEOPHILIC-ADDITION TO (PI-ALLYL)PALLADIUM COMPLEXES - LIGAND EFFECTS AND MECHANISM

Nucleophilic addition to (eta(3)-2-chloropropenyl)palladium complexes 1 with stabilized carbanions such as dialkyl malonates was studied. Th ese complexes are used as probes to determine whether nucleophilic att ack occurs at the central or terminal carbon of the pi-allyl group. At tack at the central carbon leads to substitution of chloride via a pal ladacyclobutane intermediate. The regiochemistry of the reaction (cent ral versus terminal attack) is controlled by proper choice of ligands. Thus, sigma-donor ligands direct the attack of the nucleophile to the central carbon (C-2) of the allyl group whereas pi-acceptor ligands d irect the attack to the terminal carbons (C-1 or C-3). It was found th at there is a correlation between the relative rate of central versus terminal attack and the C-13 NMR shifts of the allyl group. The shift difference between the central and terminal carbons, C-c - C-t, can be used to predict the site of attack. Ab initio calculations were perfo rmed on (pi-allyl)palladium complexes as well as on the postulated pal ladacyclobutane. The calculations support the experimental results, an d for the pi-allyl complexes with the sigma-donor ligands the LUMO fro m the calculations is the symmetrical orbital with a large coefficient at the central carbon.