Vinyl epoxides provide their own alkoxide base

Vinyl epoxides and allylic carbonates are especially useful electrophiles because under the influence of palladium (0) they generate an alkoxide base, so no added base is required with these substrates. The overall reaction proceeds under almost neutral conditions—ideal with complex and sensitive substrates. The relief of strain in the three-membered ring drives the reaction with palladium (0) to produce the zwitterionic intermediate. Proton transfer activates the nucleophile, and attack at the less hindered end of the π-allyl palladium intermediate preferentially leads to overall 1,4-addition of NuH.

Retention of stereochemistry is demonstrated by the reaction of a substituted malonate with epoxycyclopentadiene. Palladium adds to the side opposite the epoxide so the nucleophile is forced to add from the same side as the OH group. This, no doubt, helps 1,4-regioselectivity.

Allylic carbonates produce the required alkoxide by decarboxylation of the carbonate anion that is displaced in the formation of the π-allyl palladium intermediate. Deprotonation activates the nucleophile, which rapidly traps the π-allyl palladium complex to give the allylated product, regenerating the palladium(0) catalyst.

Trost and his group have used both of these palladium-catalysed alkylations in a synthesis of aristeromycin from epoxycyclopentadiene. The cis stereochemistry of this carbocyclic nucleotide analogue is of paramount importance and was completely controlled by retention of configuration in both substitutions. The first reaction is between epoxycyclopentadiene and adenine, one of the heterocyclic building blocks of nucleic acids, and follows the mechanism we have just described to give a cis-1,4-disubstituted cyclopentene.

The alcohol is then activated by conversion into the carbonate, which reacts with phenyl sulfonylnitromethane, and could later be converted into an alcohol. Once again, retention of stereochemistry during the palladium-catalysed substitution gives the cis product.

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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

Allylic electrophiles are activated by palladium (0)

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