Mechanism of the palladium-catalyzed carbohydroxylation of allene-substituted conjugated dienes: rationalization of the recently observed nucleophilic attack by water on a (pi-allyl)palladium intermediate

The mechanism of the palladium-catalyzed oxidative carbohydroxylation of allene-substituted 1,3-cyclohexadiene was studied by DFT calculations. All intermediates and transition states of the reaction were identified and their structures were calculated. The calculations confirm the mechanism previously proposed and show that the C--C bond-forming step occurs via insertion of one of the double bonds of 1,3-cyclohexadiene into a Pd--vinyl bond of a vinylpalladium intermediate. This reaction leads to a (pi-allyl)palladium intermediate, and coordination of benzoquinone and a double bond in the molecule to Pd creates a highly reactive cationic pi-allyl complex, which is readily attacked by water according to the calculations.