Oxypalladation and the Wacker oxidation
This combination of reagents has been used to oxidize terminal vinyl groups to methyl ketones and is known as the Wacker oxidation. The nucleophile is simply water, which attacks the activated alkene at the more substituted end in an oxypalladation step. β-Hydride elimination from the resulting σ-alkyl palladium complex releases the enol, which is rapidly converted into the more stable keto form. Overall, the reaction is a hydration of a terminal alkene that can tolerate a range of functional groups.
A related reaction is the oxidation of silyl enol ethers to enones. This requires stoichiometric palladium (II), although reoxidation of Pd (0) with benzoquinone can cut that down to about half an equivalent. The reaction provides a valuable way of turning regioselective methods for making silyl enol ethers (Chapter 20) into regioselective methods for oxidizing ketones to enones. The fi rst step is again oxypalladation and β elimination puts the alkene in conjugation with the ketone: there are no β hydrogens on the other side.
An example of catalytic oxypalladation is the rearrangement of allylic acetates with Pd(II). The reaction starts with oxypalladation of the alkene and it is the acetate already present in the molecule that provides the nucleophile to attack the alkene. The intermediate can reverse the oxypalladation in either direction and the product is whichever allylic acetate has the more substituted alkene. In this case, trisubstituted beats monosubstituted easily.
The reaction is E-selective, which means that a simple synthesis of an E,Z-diene is possible from the symmetrical acetate with two Z-allylic alkenes. The one that rearranges goes E and the one that stays behind remains Z. The driving force for this rearrangement, from one disubstituted alkene to another, is establishment of conjugation.
