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18.11 Effects of Conjugation in a,b -Unsaturated Aldehydes and Ketones

18.11 Effects of Conjugation in a,b -Unsaturated Aldehydes and Ketones. Relative Stability. aldehydes and ketones that contain a carbon-carbon double bond are more stable when the double bond is conjugated with the carbonyl group than when it is not

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18.11 Effects of Conjugation in a,b -Unsaturated Aldehydes and Ketones

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  1. 18.11Effects of Conjugation in a,b-Unsaturated Aldehydes and Ketones

  2. Relative Stability • aldehydes and ketones that contain a carbon-carbon double bond are more stable when the double bond is conjugated with the carbonyl group than when it is not • compounds of this type are referred to as a,b unsaturated aldehydes and ketones

  3. O g b a CH3CH CHCH2CCH3 O CHCCH3 CH3CH2CH g b a Relative Stability (17%) K = 4.8 (83%)

  4. O H2C CHCH Acrolein

  5. O H2C CHCH Acrolein

  6. O H2C CHCH Acrolein

  7. O H2C CHCH Acrolein

  8. – •• •• O O C C •• •• •• C C C C + Resonance Description

  9. – •• •• O O C C •• •• •• C C C C + – •• O C •• •• C C + Resonance Description

  10. Properties • a,b-Unsaturated aldehydes and ketones are more polar than simple aldehydes and ketones. • a,b-Unsaturated aldehydes and ketones contain two possible sites for nucleophiles to attack • carbonyl carbon • b-carbon

  11. •• O C •• C C Properties • a,b-Unsaturated aldehydes and ketones are more polar than simple aldehydes and ketones. • a,b-Unsaturated aldehydes and ketones contain two possible sites for nucleophiles to attack • carbonyl carbon • b-carbon b

  12. O O Dipole Moments d– d– • greater separation of positive and negative charge d+ d+ d+ m = 2.7 D m = 3.7 D Butanal trans-2-Butenal

  13. 18.12Conjugate Addition to a,b-Unsaturated Carbonyl Compounds

  14. Nucleophilic Addition to a,b-Unsaturated Aldehydes and Ketones • 1,2-addition (direct addition) • nucleophile attacks carbon of C=O • 1,4-addition (conjugate addition) • nucleophile attacks b-carbon

  15. Kinetic versus Thermodynamic Control • attack is faster at C=O • attack at b-carbon gives the more stable product

  16. O C C C H Y H O Y C C C + • formed faster • major product under conditions of kinetic control (i.e. when addition is not readily reversible) 1,2-addition

  17. O C C C H Y H O C Y C C + • enol • goes to keto form under reaction conditions 1,4-addition

  18. O C C C H Y O C Y H C C + • keto form is isolated product of 1,4-addition • is more stable than 1,2-addition product 1,4-addition

  19. O C C C H Y O H O C Y C Y H C C C C + 1,2-addition 1,4-addition C=O is stronger than C=C

  20. 1,2-Addition • observed with strongly basic nucleophiles • Grignard reagents • LiAlH4 • NaBH4 • Sodium acetylide • strongly basic nucleophiles add irreversibly

  21. O + CHCH CH3CH HC CMgBr OH CHCHC CH3CH CH Example 1. THF2. H3O+ (84%)

  22. 1,4-Addition • observed with weakly basic nucleophiles • cyanide ion (CN–) • thiolate ions (RS–) • ammonia and amines • azide ion (N3–) • weakly basic nucleophiles add reversibly

  23. O C6H5CH CHCC6H5 ethanol,acetic acid KCN O C6H5CHCH2CC6H5 CN Example (93-96%)

  24. •• O O •• – CH C6H5CH CC6H5 C6H5CH CHCC6H5 •• CN ethanol,acetic acid KCN •• – O O •• •• C6H5CHCH2CC6H5 CC6H5 C6H5CH CH CN CN Example via (93-96%)

  25. CH3 O CH3 SCH2C6H5 Example O C6H5CH2SH HO–, H2O (58%)

  26. •• O •• – •• CH3 SCH2C6H5 CH3 •• – O O •• •• CH3 CH3 SCH2C6H5 SCH2C6H5 Example via O C6H5CH2SH HO–, H2O (58%)

  27. 18.13Addition of Carbanions toa,b-Unsaturated Carbonyl Compounds:The Michael Reaction

  28. Michael Addition • Stabilized carbanions, such as those derived from b-diketones undergo conjugateaddition to a,b-unsaturated ketones.

  29. O O CH3 H2C CHCCH3 O O O CH3 CH2CH2CCH3 O Example + KOH, methanol (85%)

  30. Michael Addition • The Michael reaction is a useful method forforming carbon-carbon bonds. • It is also useful in that the product of the reaction can undergo an intramolecularaldol condensation to form a six-membered ring. One such application is called the Robinsonannulation.

  31. O O CH3 O CH3 CH2CH2CCH3 O O OH Example NaOHheat not isolated;dehydrates under reaction conditions

  32. O O CH3 O CH3 CH2CH2CCH3 O O OH O CH3 O Example NaOHheat (85%)

  33. 18.14Conjugate Addition of Organocopper Reagents to a,b-Unsaturated Carbonyl Compounds

  34. Addition of Organocopper Reagents toa,b-Unsaturated Aldehydes and Ketones • The main use of organocopper reagents is toform carbon-carbon bonds by conjugate addition to a,b-unsaturated ketones.

  35. O CH3 1. diethyl ether 2. H2O O CH3 CH3 Example + LiCu(CH3)2 (98%)

  36. 18.15Alkylation of Enolate Anions

  37. Enolate Ions in SN2 Reactions • Enolate ions are nucleophiles and react withalkyl halides. • However, alkylation of simple enolates does not work well. • Enolates derived from b-diketones can bealkylated efficiently.

  38. O O O O CH3CCH2CCH3 CH3 Example K2CO3 + CH3I CH3CCHCCH3 (75-77%)

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