Chapter 21 Ester Enolates

1. Chapter 21Ester Enolates

2. 21.1Ester a-Hydrogens and Their pKa’s

3. Introduction • Hydrogens a to an ester carbonyl group are less acidic, pKa 24, than a of aldehydes and ketones, pKa 16-20. • The decreased acidity is due the decreased electron withdrawing ability of an ester carbonyl. • Electron delocalization decreases the positive character of the ester carbonyl group.

4. O O C C R C OR' H H Introduction • The preparation and reactions of b-dicarbonyl compounds, especially b-keto esters, is the main focus of this chapter. • A proton on the carbon flanked by the two carbonyl groups is relatively acidic, easily and quantitatively removed by alkoxide ions.

5. O O C C R C OR' H H – CH3CH2O O O C C •• R C OR' – H Introduction pKa ~ 11

6. •• •• •• •• – O O O O •• •• •• •• •• C C C C •• R C OR' R C OR' – H H Introduction • The resulting carbanion is stabilized by enolate resonance involving both carbonyl groups.

7. – •• •• O O O O •• •• •• •• •• C C C C •• R C OR' R C OR' – H H Introduction •• •• • The resulting carbanion is stabilized by enolate resonance involving both carbonyl groups.

8. 21.2The Claisen Condensation(gives -keto esters)

9. O O O 2RCH2COR' RCH2CCHCOR' R The Claisen Condensation 1. NaOR' • b-Keto esters are made by the reaction shown, which is called the Claisen condensation. • Ethyl esters are typically used, with sodium ethoxide as the base. + R'OH 2. H3O+

10. O O O 2CH3COCH2CH3 CH3CCH2COCH2CH3 Example 1. NaOCH2CH3 • Product from ethyl acetate is called ethylacetoacetateor acetoaceticester. 2. H3O+ (75%)

11. •• O •• – •• CH3CH2 CH2 O H COCH2CH3 •• •• Mechanism Step 1:

12. •• O •• – •• CH3CH2 CH2 O H COCH2CH3 •• •• O •• CH3CH2 O H •• Mechanism Step 1: •• •• – CH2 COCH2CH3 ••

13. – •• O •• •• CH2 COCH2CH3 •• O •• – CH2 COCH2CH3 •• Mechanism Step 1: • Anion produced is stabilized by electron delocalization; it is the enolate of an ester.

14. •• O O •• – CH2 COCH2CH3 •• Mechanism Step 2: •• •• CH3COCH2CH3

15. •• – •• O O •• •• •• CH3C CH2 COCH2CH3 •• O O •• – CH2 COCH2CH3 •• Mechanism Step 2: OCH2CH3 •• •• •• •• CH3COCH2CH3

16. •• – •• O O •• •• •• CH3C CH2 COCH2CH3 OCH2CH3 •• •• Mechanism Step 2:

17. •• – •• O O •• •• •• CH3C CH2 COCH2CH3 OCH2CH3 •• •• •• •• O O •• •• – •• CH3C CH2 OCH2CH3 COCH2CH3 •• •• Mechanism Step 3: +

18. •• •• O O •• •• – •• CH3C CH2 OCH2CH3 COCH2CH3 •• •• Mechanism Step 3: • The product at this point is ethyl acetoacetate. • However, were nothing else to happen, the yield of ethyl acetoacetate would be small because the equilibrium constant for its formation is small. • Something else does happen. Ethoxide abstracts a proton from the CH2 group to give a stabilized anion. The equilibrium constant for this reaction is favorable. +

19. •• •• O O •• •• •• – OCH2CH3 CH3C CH H COCH2CH3 •• •• •• •• O O •• •• – •• CH3C CH2 OCH2CH3 COCH2CH3 •• •• Mechanism Step 4: + +

20. •• O O •• CH3C CH Mechanism Step 5: •• • In a separate operation, the reaction mixture is acidified. This converts the anion to the isolated product, ethyl acetoacetate. – COCH2CH3 ••

21. •• H O O •• + O CH3C CH H •• H •• •• O H O •• •• CH3C CH O COCH2CH3 •• •• H H Mechanism Step 5: •• – COCH2CH3 •• +

22. O 1. NaOCH2CH3 2. H3O+ O O CH3CH2CCHCOCH2CH3 CH3 Another example 2 CH3CH2COCH2CH3 • Reaction involves bond formation between the a-carbon atom of one ethyl propanoate molecule and the carbonyl carbon of the other. (81%)

23. 21.3Intramolecular Claisen Condensation:The Dieckmann Reaction

24. O O 1. NaOCH2CH3 2. H3O+ O O COCH2CH3 Example CH3CH2OCCH2CH2CH2CH2COCH2CH3 (74-81%)

25. •• O O •• •• •• O O •• •• – CH3CH2OCCH2CH2CH2CHCOCH2CH3 •• via CH3CH2OCCH2CH2CH2CH2COCH2CH3 NaOCH2CH3

26. •• •• O O •• •• – CH3CH2OCCH2CH2CH2CHCOCH2CH3 •• via

27. •• •• – O CH3CH2O •• •• •• •• O •• C CHCOCH2CH3 H2C H2C CH2 •• •• O O •• •• – CH3CH2OCCH2CH2CH2CHCOCH2CH3 •• via

28. •• •• – O CH3CH2O •• •• •• •• O •• C CHCOCH2CH3 H2C H2C CH2 via

29. •• •• – O CH3CH2O •• •• •• •• O •• C CHCOCH2CH3 H2C H2C CH2 •• O •• •• O •• C •• – CH3CH2O CHCOCH2CH3 H2C •• •• H2C CH2 via +

30. 21.4Mixed Claisen Condensations

31. Mixed Claisen Condensations • As with mixed aldol condensations, mixedClaisen condensations are best carried outwhen the reaction mixture contains one compound that can form an enolate and another that cannot.

32. O O O O HCOR ROCOR ROC COR O COR Mixed Claisen Condensations • These types of esters cannot form an enolate.

33. O O COCH3 CH3CH2COCH3 O O CCHCOCH3 CH3 Example + 1. NaOCH3 2. H3O+ (60%)

34. 21.5Acylation of Ketones with Esters

35. Acylation of Ketones with Esters • Esters that cannot form an enolate can be used to acylate ketone enolates.

36. O O CH3CH2OCOCH2CH3 O O COCH2CH3 Example + 1. NaH 2. H3O+ (60%)

37. O O COCH2CH3 CH3C O CCH2C Example + 1. NaOCH2CH3 2. H3O+ O (62-71%)

38. O O CH3CH2CCH2CH2COCH2CH3 O O CH3 Example 1. NaOCH3 2. H3O+ (70-71%)

39. 21.6Ketone Synthesis via b-Keto Esters

40. O O O RCH2CCHCOH R Ketone Synthesis • b-Keto acids decarboxylate readily to give ketones (Section 19.17). + RCH2CCH2R CO2

41. O O O O RCH2CCHCOR' RCH2CCHCOH R R Ketone Synthesis H2O • b-Keto acids decarboxylate readily to give ketones (Section 19.17). • b-Keto acids are available by hydrolysis of b-keto esters. + R'OH

42. O O O 2RCH2COR' RCH2CCHCOR' R Ketone Synthesis 1. NaOR' • b-Keto acids decarboxylate readily to give ketones (Section 19.17). • b-Keto acids are available by hydrolysis of b-keto esters. • b-Keto esters can be prepared by the Claisen condensation. + R'OH 2. H3O+

43. O 2 CH3CH2CH2CH2COCH2CH3 O O CH3CH2CH2CH2CCHCOCH2CH3 CH2CH2CH3 Example 1. NaOCH2CH3 2. H3O+ (80%)

44. O O CH3CH2CH2CH2CCHCOH CH2CH2CH3 O O CH3CH2CH2CH2CCHCOCH2CH3 CH2CH2CH3 Example 1. KOH, H2O, 70-80°C 2. H3O+

45. O O CH3CH2CH2CH2CCHCOH CH2CH2CH3 Example 70-80°C O CH3CH2CH2CH2CCH2CH2CH2CH3 (81%)