Premed · Premed · Organic Chemistry 2
Lecture 13: Claisen Condensation and Related Reactions
Organic Chemistry II
Learning Objectives
By the end of this lecture, students will be able to:
- Write the mechanism of the Claisen condensation
- Predict products of Claisen, crossed Claisen, and Dieckmann condensations
- Describe the malonic ester synthesis and acetoacetic ester synthesis
- Apply the Michael reaction (conjugate addition) to enolate chemistry
- Describe the Robinson annulation as a combination of Michael and aldol reactions
- Use retrosynthetic analysis to identify enolate-based disconnections
Lecture Content
I. The Claisen Condensation
The Claisen condensation is the self-condensation of an ester possessing alpha hydrogens in the presence of a strong base, producing a beta-keto ester. The overall transformation converts two equivalents of an ester (RCOOR') into a beta-keto ester (RC(=O)CH2COOR') plus one equivalent of the alcohol R'OH.
The mechanism involves four steps. First, the base (typically NaOR', matching the alkoxide of the ester) removes an alpha hydrogen from the ester to generate the ester enolate. Second, this enolate acts as a nucleophile and attacks the carbonyl carbon of a second ester molecule in a nucleophilic acyl substitution. Third, the resulting tetrahedral intermediate collapses, expelling the alkoxide leaving group (R'O-) and forming the beta-keto ester. Fourth, and critically, the expelled alkoxide deprotonates the beta-keto ester at the highly acidic position between the two carbonyl groups (pKa approximately 11). This final deprotonation is thermodynamically favorable and drives the entire equilibrium forward. Without this step -- if the product lacked an alpha hydrogen between the two carbonyls -- the Claisen condensation would not proceed because the equilibrium would be unfavorable.
The classic example is the condensation of ethyl acetate with sodium ethoxide to give ethyl acetoacetate (ethyl 3-oxobutanoate), one of the most widely used building blocks in organic synthesis.
<image>Complete mechanism of the Claisen condensation of ethyl acetate. Step 1: NaOEt removes alpha-H to form the ester enolate. Step 2: Enolate attacks the carbonyl of a second ethyl acetate molecule (curved arrows shown). Step 3: Tetrahedral intermediate collapses with loss of ethoxide. Step 4: The beta-keto ester product is deprotonated at the doubly activated alpha position (between two carbonyls) by ethoxide, forming the resonance-stabilized enolate of ethyl acetoacetate. This final deprotonation drives the equilibrium to completion. Acidic workup (H3O+) re-protonates to give neutral ethyl acetoacetate.</image>
II. Crossed Claisen Condensations
Mixing two different esters with base risks generating a mixture of four products, just as in the crossed aldol reaction. The solution is to use one ester that lacks alpha hydrogens and can therefore serve only as the electrophilic partner. Examples of such esters include ethyl formate (HCOOEt), ethyl benzoate (PhCOOEt), diethyl carbonate ((EtO)2CO), and diethyl oxalate (EtOOC-COOEt). The ester with alpha hydrogens provides the enolate, and only a single crossed product forms.
Crossed Claisen condensations can also be performed between a ketone enolate and an ester to produce a 1,3-diketone. Because ketone alpha hydrogens (pKa around 20) are more acidic than ester alpha hydrogens (pKa around 25), a base such as NaH or LDA can be used to selectively generate the ketone enolate.
III. Dieckmann Condensation (Intramolecular Claisen)
The Dieckmann condensation is simply an intramolecular Claisen condensation. When a diester has a chain long enough to form a five- or six-membered ring, treatment with base causes one ester enolate end to attack the carbonyl of the other ester end, cyclizing to give a cyclic beta-keto ester. The mechanism is identical to the intermolecular Claisen condensation, and the critical final deprotonation of the product drives the reaction forward. Five- and six-membered rings are strongly favored according to Baldwin's rules. For example, diethyl adipate (a six-carbon diester) undergoes Dieckmann condensation with sodium ethoxide to give ethyl 2-oxocyclopentanecarboxylate.
IV. The Acetoacetic Ester Synthesis
The acetoacetic ester synthesis is a reliable method for synthesizing substituted methyl ketones starting from ethyl acetoacetate. The sequence begins by deprotonating ethyl acetoacetate with sodium ethoxide at the highly acidic methylene position (pKa approximately 11). The resulting stabilized enolate is then alkylated with an alkyl halide via SN2. A second alkylation can be performed if desired by repeating the deprotonation and alkylation steps. Finally, hydrolysis of the ester under acidic or basic conditions gives a beta-keto acid, which undergoes thermal decarboxylation through a six-membered cyclic transition state, losing CO2 and producing the substituted methyl ketone.
In retrosynthetic terms, any methyl ketone RCOCH3 in which R contains up to two branch points at the alpha position can potentially be traced back to ethyl acetoacetate. The principal limitation is that the alkylation step requires SN2-compatible electrophiles: methyl, primary, and sometimes secondary halides work, but tertiary halides give elimination.
V. The Malonic Ester Synthesis
The malonic ester synthesis parallels the acetoacetic ester synthesis but produces substituted carboxylic acids instead of ketones. Diethyl malonate is deprotonated with sodium ethoxide (pKa approximately 13), alkylated with an alkyl halide by SN2, and optionally alkylated a second time. Hydrolysis of both ester groups gives a substituted malonic acid, which loses one molecule of CO2 upon heating to yield the substituted acetic acid.
Both syntheses employ the same four-step strategy: activate (deprotonate), alkylate, hydrolyze, and decarboxylate. The acetoacetic ester synthesis produces ketones; the malonic ester synthesis produces carboxylic acids.
<image>Side-by-side comparison of the acetoacetic ester synthesis (left) and malonic ester synthesis (right). Each shows the four key steps: (1) deprotonation with NaOEt, (2) alkylation with R-X, (3) ester hydrolysis with H3O+/heat, and (4) decarboxylation (loss of CO2). The acetoacetic ester synthesis starts from ethyl acetoacetate and ends with a substituted methyl ketone (RCOCH3). The malonic ester synthesis starts from diethyl malonate and ends with a substituted acetic acid (RCH2COOH). The decarboxylation step shows the six-membered cyclic transition state for each.</image>
VI. The Michael Reaction (Conjugate Addition of Enolates)
The Michael reaction is the 1,4-conjugate addition of a stabilized carbanion (the Michael donor) to an alpha,beta-unsaturated carbonyl compound (the Michael acceptor). The Michael donor is typically an enolate derived from a 1,3-dicarbonyl compound, a nitro compound, or another species with a highly acidic alpha hydrogen. The Michael acceptor is an alpha,beta-unsaturated electrophile such as an enone, enal, or alpha,beta-unsaturated ester or nitrile.
The mechanism involves base-generated enolate formation, nucleophilic attack by the enolate at the beta carbon of the acceptor (1,4-addition), and protonation of the resulting enolate intermediate. The product is a 1,5-dicarbonyl compound, a structural motif that is the hallmark of Michael addition. Stabilized (soft) nucleophiles preferentially undergo 1,4-addition under thermodynamic control, while strong, hard nucleophiles such as RLi and RMgBr often add directly to the carbonyl in 1,2-fashion.
VII. The Robinson Annulation
The Robinson annulation combines a Michael reaction with an intramolecular aldol condensation to construct a new cyclohexenone ring in a single sequence. In the classic example, 2-methylcyclohexanone reacts with methyl vinyl ketone (MVK). The Michael addition produces a 1,5-diketone intermediate. An intramolecular aldol cyclization then occurs, in which the enolate of one ketone group attacks the carbonyl of the other. Finally, dehydration gives the alpha,beta-unsaturated cyclohexenone product fused to the original ring.
The Robinson annulation is one of the most important methods for constructing six-membered rings in organic synthesis. It was used extensively in the total synthesis of steroids, terpenes, and many other natural products, including Woodward's landmark synthesis of cortisone.
<image>The Robinson annulation mechanism in three stages. Starting materials: 2-methylcyclohexanone and methyl vinyl ketone. Stage 1 (Michael addition): enolate of the cyclohexanone attacks the beta-carbon of MVK to form the 1,5-diketone intermediate. Stage 2 (intramolecular aldol): new enolate formation and cyclization to form the six-membered ring with a beta-hydroxy ketone. Stage 3 (dehydration): loss of water gives the bicyclic alpha,beta-unsaturated ketone (the Hajos-Parrish ketone analog). Each stage shows the relevant curved-arrow mechanism.</image>
