Allylic electrophiles are activated by palladium (0)

Allylic compounds with good leaving groups, such as bromide and iodide, are excellent allylating agents but they suffer from loss of regiochemistry due to competition between the direct SN2 and SN2′ reactions. In contrast, π-allyl cation complexes of palladium allow both the stereochemistry and regiochemistry of nucleophilic displacement reactions to be controlled.

In addition, leaving groups (X) that are usually regarded as rather unreactive still work, which makes the purification and handling of the starting materials easier. Acetate (X=OAc) is the most commonly used leaving group, but a range of other functional groups (X=OCO2R, OPO(OR)2, Cl, Br, OPh) will perform a similar role. The full catalytic cycle is shown below, with the intermediate π-allyl complex in equilibrium between the neutral version, which has the leaving group coordinated to palladium, and the cationic π-allyl complex.

Soft nucleophiles generally give the best results: stabilized enolates such as malonates, or cyanide, are best for carbon–carbon bond formation, but for C–X (X=O, N, S) bond formation the reaction is successful with alkoxides, amines, and thiolates (RS−). In the example below an amine nucleophile attacks the allyl system to generate the more stable product with the double bond within the ring.

The intramolecular reaction works well to give heterocyclic rings—the regioselectivity is usually determined by the length of the chain and how far it can reach. Here a 6/5 fused prod uct is preferred to a bridged product containing two seven-membered rings.

The reaction usually proceeds with retention of confi guration at the reacting centre. As in SN2 substitution reactions going with retention, this actually suggests a double inversion. Coordination of Pd to the double bond of the allylic acetate occurs on the less hindered face opposite the leaving group and we can think of the oxidative addition step as an invertive nucleophilic displacement of the leaving group by a pair of Pd electrons. The nucleophile then adds to the face of the π-allyl Pd cation complex opposite the Pd. The net result is displacement of the leaving group by the nucleophile with retention. Thereafter, the nucleophile attacks from the less hindered face of the resulting π-allyl complex (that is, away from the metal), leading to overall retention of configuration.

The reaction of this allylic acetate with the sodium salt of Meldrum’s acid demonstrates the retention of configuration in the palladium (0)-catalysed process. The tetraacetate and the intermediate π-allyl complex are symmetrical, thus removing any ambiguity in the formation or reaction of the π-allyl complex and hence in the regiochemistry of the overall reaction.

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مواضيع ذات صلة

Oxypalladation and the Wacker oxidation

Alkenes coordinated to palladium (II) are attacked by nucleophiles

Palladium-catalysed amination of aromatic rings

Intramolecular alkylations make rings

Vinyl epoxides provide their own alkoxide base

Coupling to alkynes: the Sonogashira reaction

The Suzuki coupling couples boronic acids to halides

The Suzuki coupling couples boronic acids to halides

Hydropalladation–dehydropalladation

Palladium is the most widely used metal in homogeneous catalysis

Insertion reactions are reversible

Carbon monoxide incorporation extends the carbon chain