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Lithium enolates of carbonyl compounds

المؤلف:  Jonathan Clayden , Nick Greeves , Stuart Warren

المصدر:  ORGANIC CHEMISTRY

الجزء والصفحة:  ص587-588

2025-06-26

677

The problem of self-condensation of carbonyl compounds (that is, enolate reacting with un enolized carbonyl) under basic conditions does not exist if there is absolutely no unenolized carbonyl compound present. One way to achieve this is to use a base sufficiently strong (pKa at least 3 or 4 units higher than pKa of the carbonyl compound) to ensure that all of the starting carbonyl is converted into the corresponding enolate. This will work only if the resulting enolate is sufficiently stable to survive until the alkylation is complete.

lithium enolates are stable, and are among the best enolate equivalents for use in alkylation reactions. The best base for making lithium enolates is usually LDA, made from diisopropylamine (i-Pr₂NH) and BuLi. LDA will deprotonate virtually all ketones and esters that have an acidic proton to form the corresponding lithium enolates rapidly, completely, and irreversibly even at the low temperatures (about ^–78°C) required for some of these reactive species to survive. Deprotonation occurs through a cyclic mechanism, which is illustrated below for ketones and esters. The basic nitrogen anion removes the proton as the lithium is delivered to the forming oxyanion.