Bonding Modes in Palladium(II) Enolates:  Consequences for Dynamic Behavior and Reactivity

The behavior of palladium C-bound enolates [Pd(CH<INF>2</INF>C(O)CR<INF>3</INF>)Cl(PPh<INF>3</INF>)<INF>2</INF>] (R = H, <BO>1</BO>; R = Me, <BO>2</BO>) and [Pd(CH<INF>2</INF>C(O)CR<INF>3</INF>)(PPh<INF>3</INF>)<INF>2</INF>(NCMe)](BF<INF>4</INF>) (R = H, <BO>5</BO>; R = Me, <BO>6</BO>) 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<INF>2</INF>{μ-κ²-C,O-CH<INF>2</INF>C(O)CR<INF>3</INF>}<INF>2</INF> or with mixed Pd<INF>2</INF>{μ-κ²-C,O-CH<INF>2</INF>C(O)CR<INF>3</INF>}(μ-X) (X = Cl, OH) bridges. It is proposed that π back-donation is important to stabilize oxygen bonding. Complexes <BO>1</BO> and <BO>2</BO> 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 CR<INF>3</INF>C(O)CH<INF>2</INF>COOH. Electrophilic attack on oxygen is very important:  the hydrolysis of the enolate complexes has been studied and also the reaction with ClSiMe<INF>3</INF> to give silyl enol ethers.