Enolate Generation using MeLi

Enantioselective alkylation of achiral enolates can provide an alternative to desymmetrization processes for the preparation of optically active Ca-substituted ketones. Alkylation of enolate 190 thus furnishes ketone 193 in 96 % ee and 76 % yield in the presence of chiral tetraamine 191 (Equation 14) (107). The reaction protocol prescribes the use of MeLi-LiBr for the generation of enolate 190 from enolsilane 189. The presence of lithium bromide was important, as it had a pronounced effect both on the subsequent rate of alkylation and on the stereoselectivity. Interestingly, the use of LiBr in combination with excess N,N,N, N -tetramethylpropylenediamine (192) enables the use of substoichiometric amounts of the chiral tetramine 191 (5mol%) (108). It has been suggested that diamine 192 functions as a trapping agent for LiBr, which otherwise complexes with and deactivates 191 in the course of the reaction. 

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