Palladium Catalyzed Allylic Alkylations of Nonstabilized Enolates

U.      Kazmaier

Abstract

Palladium catalyzed allylic alkylations are important reactions in organic synthesis. In general stabilized carbanions are used as soft C-nucleophiles. Nonstabilized enolates from ketones and esters often cause problems, and the developments and improvements made with these interesting nucleophiles are summarized in this review. With respect to ketone enolates, best results are obtained with the corresponding tin derivatives, but enoxy-borates also can be used. With ketone enolates α-allylated products are obtained. In the presence of suitable chiral ligands, the optically active substitution products are obtained with high stereoselectivity. In contrast, sterically hindered ester enolates do not provide allylation products, but give rise to cyclopropane deriva-tives. This can be explained by an attack of the enolate at the central carbon atom of the intermediatly formed π-allyl complex. In contrast, if silylketene acetals are used as nucleophiles, or vinylepoxides as ally-lic substrates, the α-allylated products are obtained. This is also true with more or less stabilized enolates derived from pyrazinones or azlactones, giving rise to α-alkylated amino acid derivates. “Normal” unsatu-rated amino acids can be obtained by using chelated amino acid ester enolates as nucleophiles. This enolates show a high reactivity and therefore these reactions can be carried out under very mild conditions, condi-tions under which the π-σ-π-isomerization of the π-allyl intermediates can be suppressed. This opens up totally new synthetic posibilities. In the presence of chiral ligands optically active amino acids are obtaine

Keywords: allylic alkylation, nonstabilized enolates, stabilized carbanions, soft c-nucleophiles, ketone enolates, enoxyborates

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