Alpha Substitution and Condensations of Enols and Enolate Ions. Alpha Substitution

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1 Alpha Substitution and ondensations of Enols and Enolate Ions hap 23 W: 27, 28, 30, 31, 37, 39, 42-44, 47, 51, Alpha Substitution Replacement of a hydrogen on the carbon adjacent to the carbonyl, =. Slide 2 1

2 ondensation with Aldehyde or Ketone Enolate ion attacks a = and the alkoxide is protonated. The net result is addition. Slide 3 ondensation with Esters Loss of alkoxide ion results in nucleophilic acyl substitution. Slide 4 2

3 Keto-Enol Tautomers Tautomers are isomers which differ in the placement of a hydrogen. ne may be converted to the other. In base: Slide 5 Keto-Enol Tautomers (2) Tautomerism is also catalyzed by acid. In acid: Slide 6 3

4 Equilibrium Amounts For aldehydes and ketones, the keto form is greatly favored at equilibrium. An enantiomer with an enolizable hydrogen can form a racemic mixture. Slide 7 Acidity of α-ydrogens pk a for α- of aldehyde or ketone ~20. Much more acidic than alkane or alkene (pk a > 40) or alkyne (pk a = 25). Less acidic than water (pk a = 15.7) or alcohol (pk a = 16-19). In the presence of hydroxide or alkoxide ions, only a small amount of enolate ion is present at equilibrium. Slide 8 4

5 Enolate Reaction As enolate ion reacts with the electrophile, the equilibrium shifts to produce more. Slide 9 Acid-Base Reaction to Form Enolate Very strong base is required for complete reaction. Example: Slide 10 5

6 α alogenation Base-promoted halogenation of ketone. Base is consumed. ther products are water and chloride ion. l l l Slide 11 Multiple alogenations The α-halo ketone produced is more reactive than ketone. Enolate ion stabilized by e - -withdrawing halogen. l l _ 2, 2 l l l l l l l l l Slide 12 6

7 aloform Reaction Methyl ketones replace all three s with halogen. The trihalo ketone then reacts with hydroxide ion to give carboxylic acid. 3 excess I 2 - I I 3 - I 3 Iodoform, yellow ppt. Slide 13 Positive Iodoform for Alcohols If the iodine oxidizes the alcohol to a methyl ketone, the alcohol will give a positive iodoform test. Slide 14 7

8 Acid atalyzed alogenation of Ketones an halogenate only one or two α- s. Use acetic acid as solvent and catalyst. Slide 15 Aldehydes and alogens alogens are good oxidizing agents and aldehydes are easily oxidized. + Br Br Slide 16 8

9 The VZ Reaction The ell-volhard-zelinsky reaction replaces the α - of a carboxylic acid with Br. Slide 17 Alkylation Enolate ion can be a nucleophile. Reacts with unhindered halide or tosylate via S N 2 mechanism. (i-pr) 2 N - Li + 3 Br 3 Slide 18 9

10 Stork Reaction Milder alkylation method than using LDA. Ketone + 2 amine enamine. Enamine is α-alkylated, then hydrolyzed. N + + N N + N N 3 Br + N Br N + Slide 19 Acylation via Enamines Product is a β-diketone. Slide 20 10

11 Aldol ondensation Enolate ion adds to = of aldehyde or ketone. Product is a β-hydroxy aldehyde or ketone. Aldol may lose water to form =. Slide 21 Mechanism for Aldol ondensation Also catalyzed by acid. Slide 22 11

12 Dehydration of Aldol reates a new = bond. Slide 23 rossed Aldol ondensations Two different carbonyl compounds. nly one should have an alpha. Slide 24 12

13 Aldol yclizations 1,4-diketone forms cyclopentenone. 1,5-diketone forms cyclohexenone. Slide 25 Planning Aldol Syntheses Slide 26 13

14 laisen ondensation Two esters combine to form a β-keto ester. 3 2 R base 3 R pka = 24 enolate ion R 2 3 R 2 3 R 3 R 3 3 R _ 2 R 3 R pka = 11 2 R _ 3 Slide 27 Dieckmann ondensation A 1,6 diester cyclic (5) β-keto ester. A 1,7 diester cyclic (6) β-keto ester. Slide 28 14

15 rossed laisen Two different esters can be used, but one ester should have no α hydrogens. Useful esters are benzoates, formates, carbonates, and oxalates. Ketones (pk a = 20) may also react with an ester to form a β-diketone. Slide 29 β-dicarbonyl ompounds More acidic than alcohols. Easily deprotonated by alkoxide ions and alkylated or acylated. At the end of the synthesis, hydrolysis removes one of the carboxyl groups malonic ester, pk a = 13 acetoacetic ester, pk a =11 Slide 30 15

16 Malonic Ester Synthesis Deprotonate, then alkylate with good S N 2 substrate. (May do twice.) Decarboxylation then produces a mono- or di-substituted acetic acid. Slide 31 Acetoacetic Acid Synthesis Product is mono- or di-substituted ketone. Slide 32 16

17 onjugate Additions When = is conjugated with =, 1,2-addition or 1,4-addition may occur. A 1,4-addition of an enolate ion is called the Michael reaction. Slide 33 Michael Reagents Michael donors: enolate ions stabilized by two electron-withdrawing groups. β-diketone, β-keto ester, enamine, β-keto nitrile, α-nitro ketone. Michael acceptors: = conjugated with carbonyl, cyano, or nitro group. conjugated aldehyde, ketone, ester, amide, nitrile, or a nitroethylene. Slide 34 17

18 A Michael Reaction Enolates can react with α,β-unsaturated compounds to give a 1,5-diketo product _ _ heat δ-keto acid Slide 35 Robinson Annulation A Michael reaction to form a 1,5-diketone followed by an intramolecular aldol condensation to form a cyclohexenone _ 3 Slide 36 18

19 Mechanism for Robinson Annulation (1) Slide 37 Mechanism for Robinson Annulation (2) Slide 38 19

20 End of hapter 23 Slide 39 20

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