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. These complexes are used as probes to determine whether nucleophilic attack occurs at the central or terminal carbon of the pi-allyl group. Attack at the central carbon leads to substitution of chloride via a palladacyclobutane intermediate. The regiochemistry of the reaction (central 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 direct the attack to the terminal carbons (C-1 or C-3). It was found that 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 performed on (pi-allyl)palladium complexes as well as on the postulated palladacyclobutane. The calculations support the experimental results, and for the pi-allyl complexes with the sigma-donor ligands the LUMO from the calculations is the symmetrical orbital with a large coefficient at the central carbon.