1084 CHAPTER THE CHEMITRY F ENLATE IN, ENL, AND a,b-unaturated CARBNYL CMPUND H H CA CL CoA + enol form of acetyl-coa _ C N NH acetyl-coa carboxylase H H R H carboxybiotin HN NH _ LC LCH LCLCoA + H H malonyl-coa R H biotin Provide a curved-arrow mechanism for this reaction, using B3 as a base (which is part of the enzyme) and BH as its conjugate acid..7 ALKYLATIN F ETER ENLATE IN ections.4.6 described reactions in which enolate ions react as nucleophiles at the carbonyl carbon atom. This section considers two reactions in which enolate ions are used as nucleophiles in N reactions. A. Malonic Ester ynthesis Diethyl malonate (malonic ester), like many other b-dicarbonyl compounds, has unusually acidic a-hydrogens. (Why?) Consequently, its conjugate-base enolate ion can be formed nearly completely with alkoxide bases such as sodium ethoxide. _ Et3_ + Et LC LCH LC LEt Et L H + Et LC LCH LC LEt enolate ion of diethyl malonate diethyl malonate pk a = 1.9 (.64a) The conjugate-base anion of diethyl malonate is nucleophilic, and it reacts with alkyl halides and sulfonate esters in typical N reactions. uch reactions can be used to introduce alkyl groups at the a-position of malonic ester. Na CH CH _ 3CH(C Et) + CH L Br CH + Na Br _ EtH 3 CHCH(C Et) (83% yield) (.64b) Further Exploration. Malonic Ester Alkylation As this example shows, even secondary halides can be used in this reaction. (ee Further Exploration..) The importance of this reaction is that it can be extended to the preparation of carboxylic acids. aponification (ec. 1.7A) of the diester and acidification of the resulting solution gives a substituted malonic acid derivative. Recall that heating any malonic acid derivative causes it to decarboxylate (ec. 0.11). The result of the alkylation, saponification, and decar-
.7 ALKYLATIN F ETER ENLATE IN 1085 boxylation sequence is a carboxylic acid that conceptually is a substituted acetic acid an acetic acid molecule with an alkyl group on its a-carbon. protonation decarboxylation (ec. 0.11) CH CHCH(C Et) NaH H CH CHCH(C _ Na ) H 3 CH CHCH(C H) heat ester saponification (ec. 1.7A) CH CHCH C H + C The overall sequence of ionization, alkylation, saponification and decarboxylation starting from diethyl malonate (Eqs..64a c) is called the malonic ester synthesis. Notice that the alkylation step of the malonic ester synthesis (Eq..64b) results in the formation of a new carbon carbon bond. The anion of malonic ester can be alkylated twice in two successive reactions with different alkyl halides (if desired) to give, after hydrolysis and decarboxylation, a disubstituted acetic acid. This possibility allows us to think of any disubstituted acetic acid in terms of diethyl malonate and two alkyl halides, as follows (X = halogen): acetic acid unit a substituted acetic acid (.64c) R L CH L C H R R LC(C Et) R CH (C Et), R LX, R L X (.65) If the alkyl halides RLX and R9LX are among those that will undergo the N reaction, then the target carboxylic acid can in principle be prepared by the malonic ester synthesis. This analysis is illustrated in tudy Problem.4. tudy Problem.4 utline a malonic ester synthesis of the following carboxylic acid: (CH ) 4 CH L C H -methylheptanoic acid olution Using the analysis in the text, identify the acetic acid unit in the carboxylic acid. The two alkyl groups in this case, a methyl group and a pentyl group are derived from alkyl halides. derived from I (CH ) 4 L CH L C H derived from (CH ) 4 Br substituted acetic acid
1086 CHAPTER THE CHEMITRY F ENLATE IN, ENL, AND a,b-unaturated CARBNYL CMPUND This analysis leads to the following synthesis: formation of the enolate ion formation of the enolate ion introduction of the second alkyl group NaEt CH CH (C Et) 3 (CH ) 3 CH Br NaEt H (CH ) 3 CH CH(C Et) 3 C L I EtH EtH diethyl malonate introduction of the first alkyl group (CH ) 3 CH C(C Et) + NaI (80% yield) (.66) Ester saponification, acidification, and decarboxylation, as in Eq..64c, give the desired product. The two enolate-forming and alkylation reactions must be performed as separate steps. Adding two different alkyl halides and two equivalents of NaEt to malonic ester at the same time would not give the desired product. (Why?) PRBLEM.33 Indicate whether each of the following compounds could be prepared by a malonic ester synthesis. If so, outline a preparation from diethyl malonate and any other reagents. If not, explain why. (a) 3-phenylpropanoic acid (b) -ethylbutanoic acid (c) 3,3-dimethylbutanoic acid.34 Give the product of the following reaction sequence and explain your answer. NaEt NaH HCl CH (C Et) + BrCH CH CH Cl (C 5 H 8 ) EtH heat.35 (a) When the conjugate-base enolate of diethyl malonate is treated with bromobenzene, no diethyl phenylmalonate is formed. Explain why bromobenzene is inert...ch(cet) +CH(CEt) Br + Br diethyl phenylmalonate (b) When the same enolate ion is treated with bromobenzene and a catalytic amount of Pd[P(t-Bu) 3 ] 4, diethyl phenylmalonate is formed in excellent yield. Explain the role of the catalyst with a mechanism. B. Direct Alkylation of Enolate Ions Derived from Monoesters In the synthesis of carboxylic acids by malonic ester alkylation, a LC Et group is wasted because it is later removed. Why not avoid this altogether and alkylate directly the enolate ion of an acetic acid ester? B 3 + LR (a base) _ H C LCLR + BL H CH CH CH L I CH CH CH LCH L C LR + I _ (.67)
.7 ALKYLATIN F ETER ENLATE IN 1087 At one time this idea could not be used in practice because enolate ions derived from esters, once formed, undergo another, faster reaction: Claisen condensation with the parent ester (ec..5a). The direct alkylation shown in Eq..67 is so attractive, however, that chemists continued efforts to find conditions under which it would work. It was discovered in the early 1970s that a family of very strong, highly branched nitrogen bases, such as the following two examples, can be used to form stable enolate ions rapidly at -78 C from esters. Li _ 3 N Li _ 3 N lithium diisopropylamide (LDA) lithium cyclohexylisopropylamide (LCHIA) pk a of conjugate acids: 35 (Do not confuse the term amide in the names of these bases with the carboxylic acid derivative. This term has a double usage. As used here, an amide is the conjugate-base anion of an amine.) The conjugate acids of these bases are amines, which have pk a values near 35. Because esters have pk a values near 5, these amide bases are strong enough to convert esters completely into their conjugate-base enolate ions. The ester enolate anions formed with these bases can be alkylated directly with alkyl halides. Notice that esters with quaternary a-carbon atoms can be prepared by this method. (These compounds cannot be prepared by the malonic ester synthesis. Why?) a quaternary a-carbon C LEt H -78 C LCHIA THF < 15 min Li C LEt.. + H 3 C L I DM H 3 C L C LL C L Et + LiI ethyl -methylpropanoate NH ethyl,-dimethylpropanoate (ethyl pivalate) (87% yield) (.68) The nitrogen bases themselves are generated from the corresponding amines and butyllithium (a commercially available organolithium reagent) at -78 C in tetrahydrofuran (THF) solvent. -78 C THF N LH + CH CH CH LLi N 3 _ Li + CH CH (.69) This method of ester alkylation is considerably more expensive than the malonic ester synthesis. It also requires special inert-atmosphere techniques because the strong bases that are used react vigorously with both oxygen and water. For these reasons, the malonic ester syn-
1088 CHAPTER THE CHEMITRY F ENLATE IN, ENL, AND a,b-unaturated CARBNYL CMPUND thesis remains very useful, particularly for large-scale syntheses. However, for the preparation of laboratory samples, or for the preparation of compounds that are unavailable from the malonic ester synthesis, the preparation and alkylation of enolate ions with amide bases is particularly valuable. The possibility of the Claisen condensation as a side reaction was noted in the discussion of Eq..67. The use of a very strong amide base avoids the Claisen condensation for the following reason. The reaction is run by adding the ester to the base. When a molecule of ester enters the solution, it can react either with the strong base to form an enolate ion or with a molecule of already formed enolate ion in the Claisen condensation. The reaction of esters with strong amide bases is so much faster at -78 C than the Claisen condensation that the enolate ion is formed instantly and never has a chance to undergo the Claisen condensation. In other words, the Claisen condensation is avoided because the ester and its enolate ion are never present simultaneously (except for an instant) in the reaction flask. Another potential side reaction is the nucleophilic reaction of the amide base (or even its conjugate acid amine, which is, after all, still a base) at the ester carbonyl group. Because amines react with esters to give products of aminolysis (ec. 1.8C), it might be reasonable to expect the conjugate bases of amines very strong bases indeed to react even more rapidly with esters. That this does not happen is once again the result of a competition. When an amide base reacts with the ester, it can either remove a proton or react at the carbonyl carbon. A reaction at the carbonyl carbon is retarded by van der Waals repulsions between groups on the carbonyl compound and the large branched groups on the bases. (These van der Waals repulsions have been aptly termed F-strain, or front strain. ) For such a branched amide base to react at the carbonyl carbon is somewhat like trying to put a dinner plate into the coin slot of a vending machine. If the amide base could be in contact with the ester long enough, it would eventually react at the carbonyl carbon; but the base instead reacts more rapidly a different way: It abstracts an a-proton. Reaction with a tiny hydrogen does not involve the van der Waals repulsions that would occur if the base were to react at the carbonyl carbon. Hence, the amide base takes the path of least resistance: It forms the enolate ion. Notice that van der Waals repulsions are used productively in this example to avoid an undesired reaction. PRBLEM.36 utline a synthesis of each of the following compounds from either diethyl malonate or ethyl acetate. Because the branched amide bases are relatively expensive, you may use them in only one reaction. (a) % (b) CH CH (c) C H 5 CH L C H $ CH L C H C H 5 L C L C Et ) CH CH valproic acid (used in treatment of epilepsy) %.37 The reactions of ester enolate ions are not restricted to simple alkylations. With this in mind, suggest the structure of the product formed when the enolate ion formed by the reaction of tert-butyl acetate with LCHIA reacts with each of the following compounds at -78 C followed by dilute HCl. (a) acetone (b) benzaldehyde.38 Predict the product formed when the conjugate-base enolate ion of ethyl -methylpropanoate (shown in Eq..68) is treated with bromobenzene and a catalytic amount of Pd[P(t-Bu) 3 ] 4, and explain the role of the catalyst.
.7 ALKYLATIN F ETER ENLATE IN 1089 C. Acetoacetic Ester ynthesis Recall that b-keto esters, like malonic esters, are substantially more acidic than ordinary esters (Eq..5c, p. 1074) and are completely ionized by alkoxide bases. Et _ LCH LC LEt LH L LCH 3 + Et + H 3 C C LC LEt ethyl acetoacetate pk a = 10.7 ethanol pk a = 16 (.70) The enolate ions derived from b-keto esters, like those from malonate ester derivatives, can be alkylated by primary or unbranched secondary alkyl halides or sulfonate esters. _ LCH LC LEt + 3Br LCH CH CH H + Na Br _ 3 C LC LCH LC LEt 3 3 Na 1-bromobutane CH CH CH ethyl -acetylhexanoate (70% yield) (.71) Dialkylation of b-keto esters is also possible. LEt NaEt (1 equiv.) LC LCH _ H 3 C LC LEt (CH ) 3 I Claisen condensation LCH LC LEt (CH ) 3 NaEt first alkylation H 3 CL I second alkylation LCL C L Et (CH ) 3 (.7) Alkylation of a Dieckmann condensation product is the same type of reaction: L H L C Et NaEt Br CH CH CH CH L L C Et (.73) (from a Dieckmann condensation) ethyl -oxo-1-propylcyclopentanecarboxylate (85% yield) Like esters of substituted malonic acids, the alkylated derivatives of ethyl acetoacetate can be hydrolyzed and decarboxylated to give ketones. Ester saponification and protonation gives a substituted b-keto acid; and b-keto acids spontaneously decarboxylate at room temperature (ec. 0.11). This series of reactions is illustrated as carried out on the product of Eq..71:
1090 CHAPTER THE CHEMITRY F ENLATE IN, ENL, AND a,b-unaturated CARBNYL CMPUND LCH LC LEt CH CH CH NaH, H ester saponification H, H 3, heat protonation and decarboxylation LCH CH CH CH + C + EtH (.74) The alkylation of ethyl acetoacetate followed by saponification, protonation, and decarboxylation to give a ketone is called the acetoacetic ester synthesis. The alkylation part of this sequence, like the alkylation of diethyl malonate, involves the construction of new carbon carbon bonds. Whether a target ketone can be prepared by the acetoacetic ester synthesis can be determined by mentally reversing the synthesis. R R R LC LC L H R LC LC L C Et R R replace with L C Et R LC LCH L C Et, R LBr, R L Br R R LC LCH L C Et, R L Br R R LC LCH L C Et, R L Br (.75) TUDY GUIDE LINK.6 Further Analysis of the Claisen Condensation This analysis involves replacing an a-hydrogen of the target ketone with a LC Et group. This process unveils the b-keto ester required for the synthesis. The b-keto ester, in turn, can either be prepared directly by a Claisen condensation or can be prepared from other b-keto esters by alkylation or dialkylation with appropriate alkyl halides, as indicated by the possibilities in Eq..75. tudy Problem.5 utline a preparation of -methyl-3-pentanone by a reaction sequence that involves at least one Claisen condensation. olution The discussion in the text leads to the following analysis: H CH C L C L -methyl-3-pentanone C Et CH C L C L A where the symbol, as usual, means implies as a starting material. The b-keto ester A cannot be prepared directly by a Claisen condensation because it would require a crossed Claisen condensation (see Eq..61, p. 1079), and because the reaction could not be made irreversible by deprotonation. A second option is to provide one of the methyl groups by alkylation of the enolate ion derived from b-keto ester B:
.7 ALKYLATIN F ETER ENLATE IN 1091 C Et CH C L C L A C Et CH C L CH L,H 3 C LI B The enolate ion of compound B, in turn, can be prepared directly by the Claisen condensation of ethyl propionate. (This follows from the analysis shown in Eq..61, p. 1079.) CH C Et ethyl propionate NaEt (1 equiv.) EtH C Et H L CH C L C L CH 3 C I 3 A _ enolate ion of B aponifying A and acidifying the solution will give the b-keto acid, which will decarboxylate spontaneously under the acidic reaction conditions to give the desired ketone. C Et CH C L C L A C_ NaH H 3 CH L C L CH 3 CH C 3 -C H CH C L C L target molecule Further Exploration.3 Alkylation of Enolate Ions Derived from Ketones Do not let the large number of reactions in this chapter obscure a very important central theme: Enolate ions are nucleophiles, and they do many of the things that other nucleophiles do, such as addition to carbonyl groups, nucleophilic acyl substitution, N reactionswithalkyl halides, and so on. The reactions of enolate ions presented here are only a small fraction of those that are known. Yet if you grasp the central idea that enolate ions are nucleophiles, and if you understand the other reactions of nucleophiles, you should have little difficulty understanding (and perhaps even predicting) other reactions of enolate ions. PRBLEM.39 utline a synthesis of each of the following compounds from ethyl acetoacetate and any other reagents. (a) 5-methyl--hexanone (b) 4-phenyl--butanone.40 utline a synthesis of each of the following compounds from a b-keto ester; then show how the b-keto ester itself can be prepared. (a) (b) PhCH CH L C L CH PhCH L C L CH Ph.41 Predict the outcome of the following reaction by identifying A, then B, then the final product. (Hint: How do nucleophiles react with epoxides under basic conditions?) H 3 C $ $ A + C L CH B (C 9 H 14 4 ) EtH H 3 C $ $ diethyl malonate NaEt EtH A