One of the reactants has no hydrogen and therefore

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Unformatted text preview: d aldol reactions are successful if: 1. one of the reactants has no α-hydrogen and, therefore, cannot form an enolate anion and 2. the other reactant has a more reactive carbonyl group, namely an aldehyde 10 Polyalkylation of Enolates: Organic Organic Lecture Series 11 Organic Lecture Series 12 Organic Organic Lecture Series Crossed Enolate Reactions using LDA • With a strong enough base, enolate anion formation can be driven to completion. • The base most commonly used for this purpose is lithium diisopropylamide, LDA. • LDA is prepared by dissolving diisopropylamine in THF and treating the solution with butyllithium. 13 Organic Lecture Series Crossed Enolate Reactions using LDA • Using a molar equivalent of LDA converts an aldehyde, ketone, or ester completely to its corresponding enolate anion. 14 Organic Organic Lecture Series Crossed Enolate Reactions using LDA • The crossed aldol reaction between acetone and an aldehyde can be carried out successfully by adding acetone to one equivalent of LDA to preform its enolate anion: • which is then treated with the aldehyde. 15 Organic Lecture Series Crossed Enolate Reactions using LDA • For ketones with two sets of nonequivalent α-hydrogens, is formation of the enolate anion regioselective? – The answer is that a high degree of regioselectivity exists and that it depends on experimental conditions. 16 Organic Organic Lecture Series Crossed Enolate Reactions using LDA – When 2-methylcyclohexanone is treated with a slight excess of LDA, the enolate is almost entirely the less substituted enolate anion. 17 Organic Lecture Series Crossed Enolate Reactions using LDA • When 2-methylcyclohexanone is treated with LDA under conditions in which the ketone is in slight excess, the product is richer in the more substituted enolate. 18 Organic Organic Lecture Series Crossed Enolate Reactions using LDA • The most important factor determining the composition of the enolate anion mixture is whether the reaction is under kinetic...
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