Bonding modes in palladium(II) enolates:: Consequences for dynamic behavior and reactivity

The behavior of palladium C-bound enolates [Pd(CH2C(O)CR3)Cl(PPh3)(2)] (R = H, 1; R = Me, 2) and [Pd(CH2C(O)CR3)(PPh3)(2)(NCMe)](BF4) (R = H, 5; R = Me, 6) has been studied. Dimeric-species with bridging enolate moieties are formed in solution when a coordination site on the metal is made available, either with Pd-2{mu-kappa(2)-C,O-CH2C(O)CR3}(2) or with mixed Pd-2{mu-kappa(2)-C, O-CH2C(O)CR3}(mu-X) (X = Cl, OH) bridges. It is proposed that pi back-donation is important to stabilize oxygen banding. Complexes 1 and 2 undergo exchange between free and coordinated phosphine in solution. Kinetic experiments support an intramolecular associative mechanism which could involve an oxoallyl-like transition state. The reactivity of the complexes has been explored. Some reactions typical of Pd-alkyls have been observed such as insertion of CO to give CR3C(O)CH2COOH, Electrophilic attack on oxygen is very important: the hydrolysis of the enolate complexes has been studied and also the reaction with ClSiMe3 to give silyl enol ethers.