Michael and Aldol CH391 December 4, 2002

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1 Michael and Aldol H391 December 4, 2002

2 RH 2 H pk a = H RHH + Enolate anions... So.a basic solution contains comparable amounts of the aldehyde and its enolate. Which gives rise to..the Aldol ondensation HH 20 pk a = 16

3 RHH RH 2 H

4 RHH RH 2 H RH 2 H RHH RH 2 H RHH H

5 RHH RH 2 H RH 2 H RHH RH 2 H H RHH 2RH 2 H NaH RH 2 H HH H R

6 Aldol ondensation RH 2 H H HH R product is called an "aldol" aldol" " because it is both an aldehyde and an alcohol

7 Aldol ondensation 2RH 2 H NaH RH 2 H HH H R heat Gives α,β unsaturated carbonyl compounds RH 2 H H R

8 Aldol ondensation 2RH 2 H NaH RH 2 H HH H R NaH heat heat RH 2 H H R

9 Dehydration of Aldol Addition Product H H dehydration of β-hydroxy aldehyde can be catalyzed by either acids or bases

10 Dehydration of Aldol Addition Product H NaH H H in base, the enolate loses hydroxide to form the α,β-unsaturated aldehyde

11 Mixed Aldols: : What is the product? H 3 H + H 3 H 2 H NaH There are 4 possibilities because the reaction mixture contains the two aldehydes plus the enolate of each aldehyde.

12 Aldehydes with no α-hydrogens H 3 H + H 3 H 3 NaH,, H 2 30 H 3 H HH 3

13 Aldol reactions of ketones 2H 3 H 3 2% 98% H H 3 H 2 H 3 H 3 The equilibrium constant for aldol addition reactions of ketones is usually unfavorable but can be driven by dehydration.

14 Intramolecular Aldol ondensation Na 2 3, H 2 heat (96%!!!) via: H

15 Enolate Anions When a ketone has two different α-hydrogens, is formation of the enolate anion regioselective? The answer depends on experimental conditions when a slight excess of LDA, a ketone is converted to its lithium enolate anion, which consists almost entirely of the less substituted enolate anion this reaction is said to be under kinetic control

16 Kinetic ontrol with slight excess of LDA slight excess - Li + - Li + + LDA (i-pr) 2 NH 2-Methylcyclohexanone 99% 1% fastest but least stable

17 Slow but most Stable Thermodynamic ontrol With slight excess of ketone slight excess - Li + - Li + + LDA (i-pr) 2 NH 10% 90%

18 Kinetic ontrol When a reaction is under kinetic control, the composition of the product mixture is determined by the relative rates of formation of each product Thermodynamic ontrol When a reaction is under thermodynamic control, the composition of the product mixture is determined by the relative stabilities of each product

19 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 α,β unsaturated aldehydes and ketones

20 Acrolein H 2 HH

21 Resonance Description +

22 Resonance Description + +

23 Properties α,β-unsaturated aldehydes and ketones are more polar than simple aldehydes and ketones. α,β-unsaturated aldehydes and ketones contain two possible sites for nucleophiles to attack carbonyl carbon β-carbon β

24 Nucleophilic Addition to α,β-unsaturated Aldehydes and Ketones 1,2-addition (direct addition) nucleophile attacks carbon of = 1,4-addition (conjugate addition) nucleophile attacks β-carbon

25 Kinetic versus Thermodynamic ontrol attack is faster at = attack at β-carbon gives the more stable product

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

27 + H Y 1,4-addition enol goes to keto form under reaction conditions Y H

28 + H Y 1,4-addition keto form is isolated product of 1,4-addition is more stable than 1,2- addition product Y H

29 + H Y 1,2-addition 1,4-addition H Y = is stronger than = Y H

30 1,2-Addition observed with strongly basic nucleophiles Grignard reagents LiAlH 4 NaBH 4 Sodium acetylide strongly basic nucleophiles add irreversibly

31 Example H 3 H HH + H MgBr 1. THF 2. H 3 + H H 3 H HH H

32 1,4-Addition observed with weakly basic nucleophiles cyanide ion (N ) thiolate ions (RS ) ammonia and amines azide ion (N 3 ) weakly basic nucleophiles add reversibly

33 Example 6 H 5 H KN H 6 H 5 ethanol, acetic acid 6 H 5 HH 2 6 H 5 N

34 Addition of arbanions to α,β-unsaturated arbonyl ompounds: The Michael Reaction

35 Michael Addition Stabilized carbanions, such as those derived from β-diketones undergo conjugate addition to α,β-unsaturated ketones.

36 Example H 3 + H 2 HH 3 KH, methanol H 3 H 2 H 2 H 3

37 Michael Addition The Michael reaction is a useful method for forming carbon-carbon bonds. Gives 1,5 dioxo derivatives! It is also useful in that the product of the reaction can undergo an intramolecular aldol condensation to form a six-membered ring. ne such application is called the Robinson annulation.

38 Example H 3 NaH H 3 H 2 H 2 H 3 heat H not isolated; dehydrates under reaction conditions

39 Example H 3 NaH H 3 H 2 H 2 H 3 heat H H 3

40 Michael Reaction Michael reaction: the 1,4-addition nucleophilic addition of an enolate anion to an α,β-unsaturated carbonyl compound!! Following are two examples in the first, the nucleophile is the enolate anion of malonic ester in the second, it is the enolate anion of acetoacetic ester

41 Michael Reaction Et H 2 + H 2 =HH 3 Et 3-Buten-2-one (Methyl vinyl ketone) Diethyl propanedioate (Diethyl malonate) Et Et - Na + EtH Et HH 2 H 2 H 3

42 Retro-synthesis of 2,6-Heptadione from acetoacetic formed in a ester Michael reaction H 3 H 2 H 2 H 2 H 3 H 3 H-H 2 H 2 H 3 lost by decarboxylation 2 H H 3 H 2 + H 2 =HH 3 2 Et Ethyl acetoacetate Methyl vinyl ketone

43 onjugate Addition of rganocopper Reagents to α,β-unsaturated arbonyl ompounds

44 Addition of rganocopper Reagents to α,β-unsaturated Aldehydes and Ketones The main use of organocopper reagents is to form carbon-carbon bonds by conjugate addition to α,β-unsaturated ketones.

45 Example + Liu(H H 3 ) 2 H 3 1. diethyl ether 2. H 2 H 3 H 3

46 Gilman Reagents Gilman reagents are unique among organometallic compounds in that they give almost exclusively 1,4- addition ther organometallic compounds, including Grignard reagents, add to the carbonyl carbon by 1,2-addition The mechanism of conjugate addition of Gilman reagents is not fully understood

47 Selectivity!! H R RMgX R 2 uli R

48 The Signature Page laisen ondensation: β-ketoesters Dieckmann: yclic β-ketoesters Aldol: α, β-unsaturated aldehydes and ketones Acetoacetic ester synthesis: decorated acetones Malonic ester synthesis: decorated acetic acids Grignard Reaction: Alcohols, etc.

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