Reactions with Enolates and Their Derivatives

Nonstabilized enolates from ketones and esters often cause problems, and the developments and improvements made with these interesting nucleophiles are summarized in a little more detail [52, 54, 99]. 

The effect of the countercation was carefuUy investigated. Good results are also obtained with boron and zinc enolates, while a wide range of other counterions 

Another popular method is the use of enamines as ketone or aldehyde equivalents [109]. Mukherjee and List have shown that chiral Br0nsted acids can be used as catalysts for asymmetric allylations of aldehydes 87 with allylamines 88 

2 Ester Enolates Quite different is the situation when ester enolates are used as nucleophiles. The yields obtained are generally low, and it is assumed that nonstabilized carbanions attack the metal in preference to the aUyl group [110], resulting in reduction of the complex rather than alkylation. However, addition of hexamethyl phosphoric acid triamide (HMPA) to this reaction completely suppresses the reduction and permits the alkylation to proceed with nonstabilized carbanions as well [111]. For example, treatment of tt-allylpalladium chloride with 

Interestingly, CO has a positive effect on the yield of the reaction, although it is not incorporated [112]. It is observed, that the combination TMEDA/CO (tetramethylethylenediamine) is superior to HMPA/NEtj under the same reaction conditions. Under these modified conditions, not only sterically hindered ester enolates, but also deprotonated amides, lactams, ketones, sulfones, and the Evans enolates can be transformed [113]. Tertiary anions give the best results. 

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