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Condensations and Alpha Substitutions of Carbonyl Compounds

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1 Condensations and Alpha Substitutions of Carbonyl Compounds
Organic Chemistry, 7th Edition L. G. Wade, Jr. Chapter 22 Condensations and Alpha Substitutions of Carbonyl Compounds Copyright © 2010 Pearson Education, Inc.

2 Alpha Substitution Alpha substitution is the substitution of one of the hydrogens attached to the alpha-carbon for an electrophile. The reaction occurs through an enolate ion intermediate. Chapter 22

3 Condensation with an Aldehyde or Ketone
The enolate ion attacks the carbonyl group to form an alkoxide. Protonation of the alkoxide gives the addition product: a b-hydroxy carbonyl compound. Chapter 22

4 Condensation with Esters
The enolate adds to the ester to form a tetrahedral intermediate. Elimination of the leaving group (alkoxide) gives the substitution product (a b-carbonyl compound). Chapter 22

5 Keto–Enol Tautomers Tautomerization is an interconversion of isomers that occur through the migration of a proton and the movement of a double bond. Tautomers are not resonance form. Chapter 22

6 Base–Catalyzed Tautomerism
In the presence of strong bases, ketones and aldehydes act as weak proton acids. A proton on the a carbon is abstracted to form a resonance-stabilized enolate ion with the negative charge spread over a carbon atom and an oxygen atom. The equilibrium favors the keto form over the enolate ion. Chapter 22

7 Acid-Catalyzed Tautomerism
In acid, a proton is moved from the a-carbon to oxygen by first protonating oxygen and then removing a proton from the carbon. Chapter 22

8 Racemization For aldehydes and ketones, the keto form is greatly favored at equilibrium. If a chiral carbon has an enolizable hydrogen atom, a trace of acid or base allows that carbon to invert its configuration, with the enol serving as the intermediate. This is called racemization. Chapter 22

9 Acidity of  Hydrogens pKa for  H of aldehyde or ketone ~20.
Much more acidic than alkane or alkene (pKa > 40) or alkyne (pKa = 25). Less acidic than water (pKa = 15.7) or alcohol (pKa = 16–19). Only a small amount of enolate ion is present at equilibrium. Chapter 22

10 Formation and Stability of Enolate Ions
The equilibrium mixture contains only a small fraction of the deprotonated, enolate form. Chapter 22

11 Energy Diagram of Enolate Reaction
Even though the keto–enol tautomerism equilibrium favors the keto form, addition of an electrophile shifts the equilibrium toward the formation of more enol. Chapter 22

12 Synthesis of Lithium Diisopropylamine (LDA)
LDA is made by using an alkyllithium reagent to deprotonate diisopropylamine. Chapter 22

13 Enolate of Cyclohexanone
When LDA reacts with a ketone, it abstracts the a-proton to form the lithium salt of the enolate. Chapter 22

14 The a Halogenation of Ketones
When a ketone is treated with a halogen and a base, an ahalogenation reaction occurs. The reaction is called base-promoted, rather than base-catalyzed, because a full equivalent of the base is consumed in the reaction. Chapter 22

15 Base-Promoted Halogenation Mechanism
The base-promoted halogenation takes place by a nucleophilic attack of an enolate ion on the electrophilic halogen molecule. The products are the halogenated ketone and a halide ion. Chapter 22

16 Multiple Halogenations
C l 2 , H _ The -haloketone produced is more reactive than ketone because the enolate ion is stabilized by the electron-withdrawing halogen. The second halogenation occurs faster than the first. Because of the tendency for multiple halogenations this base-promoted halogenation is not widely used to prepare monohalogenated ketones. Chapter 22

17 Bromoform Reaction A methyl ketone reacts with a halogen under strongly basic conditions to give a carboxylate ion and a molecule of haloform. The trihalomethyl intermediate is not isolated. Chapter 22

18 Mechanism of Haloform Formation
The trihalomethyl ketone reacts with hydroxide ion to give a carboxylic acid. A fast proton exchange gives a carboxylate ion and a haloform. When Cl2 is used, chloroform is formed; Br2 forms bromoform ; and I2 forms iodoform. Chapter 22

19 Positive Iodoform Test for Alcohols
The iodine oxidizes the alcohol to a methyl ketone and it will give a positive iodoform test. Iodoform (CHI3) is a yellow solid that will precipitate out of solution. Chapter 22

20 Solved Problem 1 Solution
Propose a mechanism for the reaction of 3-pentanone with sodium hydroxide and bromine to give 2-bromo-3-pentanone. Solution In the presence of sodium hydroxide, a small amount of 3-pentanone is present as its enolate. The enolate reacts with bromine to give the observed product. Copyright © 2006 Pearson Prentice Hall, Inc. Chapter 22

21 Acid-Catalyzed α Halogenation
Ketones also undergo acid-catalyzed a halogenation. Acidic halogenation may replace one or more alpha hydrogens depending on how much halogen is used. Acetic acid serves as both the solvent and the acid catalyst. Chapter 22

22 Mechanism of Acid-Catalyzed α Halogenation
The mechanism of acid-catalyzed halogenation involves attack of the enol form of the ketone on the electrophile halogen molecule. Loss of a proton gives the haloketone and the hydrogen halide. Chapter 22

23 Solved Problem 2 Solution
Propose a mechanism for the acid-catalyzed conversion of cyclohexanone to 2-chlorocyclohexanone. Solution Under acid catalysis, the ketone is in equilibrium with its enol form. Copyright © 2006 Pearson Prentice Hall, Inc. The enol acts as a weak nucleophile, attacking chlorine to give a resonance-stabilized intermediate. Loss of a proton gives the product. Chapter 22

24 Hell–Volhard–Zelinsky (HVZ) Reaction
The HVZ reaction replaces a hydrogen atom with a bromine atom on the alpha-carbon of a carboxylic acid (a-bromoacid). The acid is treated with bromine and phosphorus tribromide, followed by hydrolysis. Chapter 22

25 Hell–Volhard–Zelinski Reaction: Step 1
The enol form of the acyl bromide serves as a nucleophilic intermediate. The first step is the formation of acyl bromide, which enolizes more easily than does the acid. Chapter 22

26 Hell–Volhard–Zelinski Reaction: Step 2
The enol is nucleophilic, so it attacks bromine to give the alpha-brominated acyl bromide. In the last step of the reaction, the acyl bromide is hydrolyzed by water to the carboxylic acid. Chapter 22

27 Alkylation of Enolate Ions
Because the enolate has two nucleophilic sites (the oxygen and the a carbon), it can react at either of these sites. The reaction usually takes place primarily at the acarbon, forming a new C—C bond. Chapter 22

28 a Alkylation of Enolate Ions
LDA forms the enolate. The enolate acts as the nucleophile and attacks the partially positive carbon of the alkyl halide, displacing the halide and forming a C—C bond. Chapter 22

29 Enamine Formation Ketones or aldehydes react with a secondary amine to form enamines. The enamine has a nucleophilic a-carbon, which can be used to attack electrophiles. Chapter 22

30 Mechanism of Enolate Formation
An enamine results from the reaction of a ketone or aldehyde with a secondary amine. Chapter 22

31 Electrostatic Potential Map of an Enamine
The electrostatic potential map (EPM) of a simple enamine shows a high negative electrostatic potential (red) near the a-carbon atom of the double bond. This is the nucleophilic carbon atom of the enamine. Chapter 22

32 Alkylation of an Enamine
Enamines displace halides from reactive alkyl halides, giving alkylated iminium salts. The alkylated iminium salt can be hydrolyzed to the ketone under acidic conditions. Chapter 22

33 Acylation of Enamines The enamine attacks the acyl halide, forming an acyl iminium salt. Hydrolysis of the iminium salt produces the b-diketone as the final product. Chapter 22

34 Aldol Condensation Under basic conditions, the aldol condensation involves the nucleophilic addition of an enolate ion to another carbonyl group. When the reaction is carried out at low temperatures, the b-hydroxy carbonyl compound can be isolated. Heating will dehydrate the aldol product to the a-b unsaturated compound. Chapter 22

35 Base-Catalyzed Aldol Condensation: Step 1
During Step 1, the base removes the a-proton, forming the enolate ion. The enolate ion has a nucleophilic a-carbon. Chapter 22

36 Base-Catalyzed Aldol Condensation: Step 2
The enolate attacks the carbonyl carbon of a second molecule of carbonyl compound. Chapter 22

37 Base-Catalyzed Aldol Condensation: Step 3
Protonation of the alkoxide gives the aldol product. Chapter 22

38 Dehydration of Aldol Products
Heating a basic or acidic aldol dehydration of the alcohol functional group. The product is a a,b-unsaturated conjugated aldehyde or ketone. An Aldol condensation, followed by dehydration, forms a new carbon–carbon double bond. Chapter 22

39 Crossed Aldol Condensations
Chapter 22

40 Successful Crossed Aldol Condensations
Chapter 22

41 Solved Problem 3 Solution
Propose a mechanism for the base-catalyzed aldol condensation of acetone (Figure 22-2). Solution The first step is formation of the enolate to serve as a nucleophile. Copyright © 2006 Pearson Prentice Hall, Inc. The second step is a nucleophilic attack by the enolate on another molecule of acetone. Protonation gives the aldol product. Chapter 22

42 Aldol Cyclization Intramolecular aldol reactions of diketones are often used for making five- and six-membered rings. Rings smaller or larger than five or six members are not favored due to ring strain or entropy. Chapter 22

43 Retrosynthesis of Aldol Condensation
Chapter 22

44 Claisen Condensation The Claisen condensation results when an ester molecule undergoes nucleophilic acyl substitution by an enolate. Chapter 22

45 Dieckman Condensation
Chapter 22

46 Crossed Claisen Two different esters can be used, but one ester should have no  hydrogens. Useful esters are benzoates, formates, carbonates, and oxalates. Ketones (pKa = 20) may also react with an ester to form a -diketone. Chapter 22

47 Crossed Claisen Condensation
In a crossed Claisen condensation, an ester without a hydrogens serves as the electrophilic component. Chapter 22

48 Crossed Claisen Condensation with Ketones and Esters
Crossed Claisen condensation between ketones and esters are also possible. Ketones are more acidic than esters, and the ketone component is more likely to deprotonate and serve as the enolate component in the condensation. Chapter 22

49 Crossed Claisen Mechanism
The ketone enolate attacks the ester, which undergoes nucleophilic acyl substitution, and thereby, acylates the ketone. Chapter 22

50 Solved Problem 4 Solution
Propose a mechanism for the self-condensation of ethyl acetate to give ethyl acetoacetate. Solution The first step is formation of the ester enolate. The equilibrium for this step lies far to the left; ethoxide deprotonates only a small fraction of the ester. The enolate ion attacks another molecule of the ester; expulsion of ethoxide ion gives ethyl acetoacetate. Copyright © 2006 Pearson Prentice Hall, Inc. Chapter 22

51 Solved Problem 4 (Continued)
Solution (Continued) In the presence of ethoxide ion, ethyl acetoacetate is deprotonated to give its enolate. This exothermic deprotonation helps to drive the reaction to completion. When the reaction is complete, the enolate ion is reprotonated to give ethyl acetoacetate. Copyright © 2006 Pearson Prentice Hall, Inc. Chapter 22

52 Solved Problem 5 Solution
Show what ester would undergo Claisen condensation to give the following b-keto ester. Solution First, break the structure apart at the a, b bond (a, b to the ester carbonyl). This is the bond formed in the Claisen condensation. Copyright © 2006 Pearson Prentice Hall, Inc. Chapter 22

53 Solved Problem 5 (Continued)
Solution (Continued) Next, replace the a proton that was lost, and replace the alkoxy group that was lost from the carbonyl. Two molecules of methyl 3-phenylpropionate result. Now draw out the reaction. Sodium methoxide is used as the base because the reactants are methyl esters. Copyright © 2006 Pearson Prentice Hall, Inc. Chapter 22

54 Chapter 22

55 Malonic Ester Synthesis
The malonic ester synthesis makes substituted derivatives of acetic acids. Malonic ester is alkylated or acylated on the carbon that is alpha to both carbonyl groups, and the resulting derivative is hydrolyzed and allowed to decarboxylate. Chapter 22

56 Decarboxylation of the Alkylmalonic Acid
Decarboxylation takes place through a cyclic transition state, initially giving an enol form that quickly tautomerizes to the product. Chapter 22

57 Example of the Malonic Synthesis
Chapter 22

58 Dialkylation of Malonic Ester
Chapter 22

59 Solved Problem 6 Solution
Show how the malonic ester synthesis is used to prepare 2-benzylbutanoic acid. Solution 2-Benzylbutanoic acid is a substituted acetic acid having the substituents Ph–CH2– and CH3CH2–. Adding these substituents to the enolate of malonic ester eventually gives the correct product. Copyright © 2006 Pearson Prentice Hall, Inc. Chapter 22

60 Acetoacetic Ester Synthesis
The acetoacetic ester synthesis is similar to the malonic ester synthesis, but the final products are ketones. Chapter 22

61 Alkylation of Acetoacetic Ester
Ethoxide ion completely deprotonates acetoacetic ester. The resulting enolate is alkylated by an unhindered alkyl halide or tosylate to give an alkylacetoacetic ester. Chapter 22

62 Hydrolysis of Alkylacetoacetic Ester
Acidic hydrolysis of the alkylacetoacetic ester initially gives an alkylacetoacetic acid, which is a b-keto acid. The keto group in the b-position promotes decarboxylation to form a substituted version of acetone. Chapter 22

63 Solved Problem 7 Solution
Show how the acetoacetic ester synthesis is used to make 3-propylhex-5-en-2-one. Solution The target compound is acetone with an n-propyl group and an allyl group as substituents: Copyright © 2006 Pearson Prentice Hall, Inc. Chapter 22

64 Solved Problem 7 (Continued)
Solution (Continued) With an n-propyl halide and an allyl halide as the alkylating agents, the acetoacetic ester synthesis should produce 3-propyl-5-hexen-2-one. Two alkylation steps give the required substitution: Hydrolysis proceeds with decarboxylation to give the disubstituted acetone product. Copyright © 2006 Pearson Prentice Hall, Inc. Chapter 22

65 Conjugate Additions: The Michael Reaction
a,b-unsaturated carbonyl compounds have unusually electrophilic double bonds. The b-carbon is electrophilic because it shares the partial positive charge of the carbonyl carbon through resonance. Chapter 22

66 1,2-Addition and 1,4-Addition
When attack occurs at the carbonyl group, protonation of the oxygen leads to a 1,2-addition. When attack occurs at the β-position, the oxygen atom is the fourth atom counting from the nucleophile, and the addition is called a 1,4-addition. Chapter 22

67 Donors and Acceptors Chapter 22 Figure: 22_04-22UNT.jpg Title:
Michael Donors and Acceptors Caption: Some Common Michael Donors and Michael Acceptors Notes: Chapter 22

68 1,4-Addition of an Enolate to Methyl Vinyl Ketone (MVK)
An enolate will do a 1,4-attack on the a,b-unsaturated ketone (MVK). Chapter 22

69 Solved Problem 8 Solution
Show how the following diketone might be synthesized using a Michael addition. Solution A Michael addition would have formed a new bond at the b carbon of the acceptor. Therefore, we break this molecule apart at the b,g bond. Copyright © 2006 Pearson Prentice Hall, Inc. Chapter 22

70 Robinson Annulation With enough base, the product of the Michael reaction undergoes a spontaneous intramolecular aldol condensation, usually with dehydration, to give a six-membered ring—a conjugated cyclohexenone. Chapter 22

71 Robinson Mechanism Chapter 22

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