Enantioselective Catalysis in Alkylations and Allylations of Enolates

The synthesis of enantiomerically pure a-alkylated carbonyl compounds was based mainly on the chiral-auxiliary approach, outlined in Section 4.1, that was particularly fruitful for the enolates of alkali metals, mainly lithium. The enantioselective alkylation and allylation of prochiral enolates mediated by chiral catalysts have been developed only in recent years, after seminal contributions were made since the turn of the century. The main problem this concept encounters is the high reactivity of these enolates that react with alkylating agents in a noncatalyzed and necessarily nonstereoselective manner [1]. 

A modified protocol was elaborated that starts from the corresponding silyl enol ether that is cleaved into the lithium enolate by methyl lithium in the presence of lithium bromide and the free amine 2 [2a]. Both procedures, however, suffer from the fact that either the lithium amide base 1 or the chiral amine 2 has to be applied in stoichiometric amounts. Fortunately, the presence of 1 equiv. of lithium bromide and 2 equiv. of the additive AfAfdV W -tetramethylpropylenediamme permitted to reduce the amount of the valuable chiral amine 2b to 5mol% 

Modern Enolate Chemistry From Pr aration to Applications in Asymmetric Synthesis, First Edition. Manfred Braun. 

R CHjX = H2C=CHCH2Br, H2C=CHCH2l, PhCH2Br, ICH2C02Et, Me3SiC=CCH2Br 

However, after boron and tin enolates and their ate complexes had been successfully applied in the palladium-catalyzed allylic alkylation [9, 11], Trost and Schroeder reported in 1999 [12a] the first enantioselective catalytic variant wherein the tin enolate 13a derived from 2-methyl tetralone was reacted with allyl acetate. The use of the Cj-symmetric ligand (5, 5 )-14 ( Trost s ligand ) 

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