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24 Carbonyl Condensation Reactions 24.1 The aldol reaction 24.2 Crossed aldol reactions 24.3 Directed aldol reactions 24.4 Intramolecular aldol reactions 24.5 The Claisen reaction 24.6 The crossed Claisen and related reactions 24.7 The Dieckmann reaction 24.8 The Michael reaction 24.9 The Robinson annulation Ibuprofen is the generic name for the pain reliever known by the trade names of Motrin and Advil. Like aspirin, ibuprofen acts as an anti-infl ammatory agent by blocking the synthesis of prostaglandins from arachidonic acid. One step in a commercial synthesis of ibuprofen involves the reaction of a nucleophilic enolate with an electrophilic carbonyl group. In Chap- ter 24, we learn about the carbon–carbon bond-forming reactions of enolates with carbonyl electrophiles. 916 smi75625_ch24_916-948.indd 916 11/12/09 12:12:52 PM 24.1 The Aldol Reaction 917 In Chapter 24, we examine carbonyl condensations—that is, reactions between two car- bonyl compounds—a second type of reaction that occurs at the α carbon of a carbonyl group. Much of what is presented in Chapter 24 applies principles you have already learned. Many of the reactions may look more complicated than those in previous chapters, but they are fundamen- tally the same. Nucleophiles attack electrophilic carbonyl groups to form the products of nucleo- philic addition or substitution, depending on the structure of the carbonyl starting material. Every reaction in Chapter 24 forms a new carbon–carbon bond at the ` carbon to a carbonyl group, so these reactions are extremely useful in the synthesis of complex natural products. 24.1 The Aldol Reaction Chapter 24 concentrates on the second general reaction of enolates—reaction with other car- bonyl compounds. In these reactions, one carbonyl component serves as the nucleophile and one serves as the electrophile, and a new carbon–carbon bond is formed. Reaction of enolates with O O other carbonyl compounds – δ– α C + C+ O C – C δ C C O enolate second carbonyl component new C C bond nucleophile electrophile The presence or absence of a leaving group on the electrophilic carbonyl carbon determines the structure of the product. Even though they appear somewhat more complicated, these reactions are often reminiscent of the nucleophilic addition and nucleophilic acyl substitution reactions of Chapters 21 and 22. Four types of reactions are examined: • Aldol reaction (Sections 24.1–24.4) • Claisen reaction (Sections 24.5–24.7) • Michael reaction (Section 24.8) • Robinson annulation (Section 24.9) 24.1A General Features of the Aldol Reaction In the aldol reaction, two molecules of an aldehyde or ketone react with each other in the pres- ence of base to form a a-hydroxy carbonyl compound. For example, treatment of acetaldehyde with aqueous –OH forms 3-hydroxybutanal, a a-hydroxy aldehyde. O – OH H O OH, H2O β a-hydroxy carbonyl Many aldol products contain The aldol reaction 2 CH3 C 3 CCH C C compound an aldehyde and an alcohol— H H H H hence the name aldol. acetaldehyde new C–C bond 3-hydroxybutanal The mechanism of the aldol reaction has three steps, as shown in Mechanism 24.1. Carbon– carbon bond formation occurs in Step [2], when the nucleophilic enolate reacts with the electro- philic carbonyl carbon. smi75625_ch24_916-948.indd 917 11/12/09 12:12:53 PM 918 Chapter 24 Carbonyl Condensation Reactions Mechanism 24.1 The Aldol Reaction Step [1] Formation of a nucleophilic enolate – O [1] O O • In Step [1], the base removes a – α H CH2 C CH2 C CH2 C + H2O proton from the carbon to form a – α HO H H H resonance-stabilized enolate. resonance-stabilized enolate Steps [2]–[3] Nucleophilic addition and protonation HHO • In Step [2], the nucleophilic enolate O – attacks the electrophilic carbonyl O O O OHα O C + – [2] [3] β carbon of another molecule of CH H CH C CH C CH C CH C CH C 3 2 3 2 3 2 aldehyde, thus forming a new carbon– H H H H H This joins the carbon – carbon bond. ` nucleophilic attack new C–C bond + OH of one aldehyde to the carbonyl carbon of a second aldehyde. • Protonation of the alkoxide in Step [3] forms the a-hydroxy aldehyde. The aldol reaction is a reversible equilibrium, so the position of the equilibrium depends on the base and the carbonyl compound. –OH is the base typically used in an aldol reaction. Recall from Section 23.3B that only a small amount of enolate forms with –OH. In this case, that’s appropriate because the starting aldehyde is needed to react with the enolate in the second step of the mechanism. Aldol reactions can be carried out with either aldehydes or ketones. With aldehydes, the equilibrium usually favors the products, but with ketones the equilibrium favors the starting materials. There are ways of driving this equilibrium to the right, however, so we will write aldol products whether the substrate is an aldehyde or a ketone. • The characteristic reaction of aldehydes and ketones is nucleophilic addition (Section 21.7). An aldol reaction is a nucleophilic addition in which an enolate is the nucleophile. See the comparison in Figure 24.1. A second example of an aldol reaction is shown with propanal as starting material. The two molecules of the aldehyde that participate in the aldol reaction react in opposite ways. • One molecule of propanal becomes an enolate—an electron-rich nucleophile. • One molecule of propanal serves as the electrophile because its carbonyl carbon is electron defi cient. – O O Figure 24.1 Nucleophilic addition— – The aldol reaction—An General reaction C + Nu CH3 C Nu The enolate is CH H the nucleophile. example of nucleophilic 3 H addition nucleophile – Aldol reaction— O O O O – An example C + CH2 C CH3 C CH2 C CH H 3 H H H • Aldehydes and ketones react by nucleophilic addition. In an aldol reaction, an enolate is the nucleophile that adds to the carbonyl group. smi75625_ch24_916-948.indd 918 11/12/09 12:12:53 PM 24.1 The Aldol Reaction 919 α O H CH C – HO H CH3 propanal HHO – O O O O OH α O – C + CH C CH3CH2 CHC C CH3CH2 C CH C CH CH H 3 2 CH H H CH H H CH H propanal 3 3 3 electrophile nucleophile new C–C bond These two examples illustrate the general features of the aldol reaction. The ` carbon of one carbonyl component becomes bonded to the carbonyl carbon of the other component. O α O – OH O General aldol OH, H2O reaction C + CH2 C RCH2 C CH C RCH H 2 R H H R H Join these 2 C’s together. Problem 24.1 Draw the aldol product formed from each compound. O CH2CHO O a. b. (CH3)3CCH2CHO c. C d. CH3 CH3 – Problem 24.2 Which carbonyl compounds do not undergo an aldol reaction when treated with OH in H2O? O CHO O O CHO a. b. c. C d. C e. (CH3)3C H (CH3)3C CH3 24.1B Dehydration of the Aldol Product The β-hydroxy carbonyl compounds formed in the aldol reaction dehydrate more readily than other alcohols. In fact, under the basic reaction conditions, the initial aldol product is often not isolated. Instead, it loses the elements of H2O from the ` and a carbons to form an `,a-unsaturated carbonyl compound. aldol dehydration conjugated product All alcohols—including β O – OH H O β α O -hydroxy carbonyl OH, H2O β α –OH compounds—dehydrate [1] 2 CH3 C CH3 C C C CH3CH CH C –H2O in the presence of acid. H H H H H acetaldehyde β-hydroxy aldehyde Only β-hydroxy carbonyl α,β-unsaturated carbonyl compounds dehydrate in the compound presence of base. O O CH O An aldol reaction is often HO CH3 3 C CCα CCα called an aldol condensation, – – CH3 OH, H2O β C OH β C because the β-hydroxy 2[2] H H –H O H carbonyl compound that is 2 β initially formed loses H2O by acetophenone -hydroxy ketone (E and Z isomers dehydration. A condensation (not isolated) can form.) reaction is one in which a It may or may not be possible to isolate the β-hydroxy carbonyl compound under the conditions small molecule, in this case of the aldol reaction. When the α,β-unsaturated carbonyl compound is further conjugated with a H2O, is eliminated during a reaction. carbon–carbon double bond or a benzene ring, as in the case of Reaction [2], elimination of H2O is spontaneous and the β-hydroxy carbonyl compound cannot be isolated. smi75625_ch24_916-948.indd 919 11/12/09 12:12:53 PM 920 Chapter 24 Carbonyl Condensation Reactions The mechanism of dehydration consists of two steps: deprotonation followed by loss of –OH, as shown in Mechanism 24.2. Mechanism 24.2 Dehydration of a-Hydroxy Carbonyl Compounds with Base – OH new π bond OH H O OH O O [1] – [2] • In Step [1], base removes a proton from the α CH3 C C C CH3 C C C CH3CH CH C carbon, thus forming a resonance-stabilized H H H H H H H – enolate. + OH + H2O • In Step [2], the electron pair of the enolate forms the π bond as –OH is eliminated. – OH O CH3 C C C H H H resonance-stabilized enolate This elimination mechanism, called the E1cB mechanism, differs from the two more general Like E1 elimination, E1cB mechanisms of elimination, E1 and E2, which were discussed in Chapter 8.
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