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O. O. C. C. R. C. OR'. H. H. Chapter 21: Ester Enolates. The preparation and reactions of  -dicarbonyl compounds, especially  -keto esters , is the main focus of this chapter. O. O. ••. ••. ••. ••. –. O. O. C. C. O. O. • •. • •. • •. • •. • •. R. C. OR'. C. C.

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Chapter 21: Ester Enolates

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Chapter 21 ester enolates l.jpg

O

O

C

C

R

C

OR'

H

H

Chapter 21: Ester Enolates

  • The preparation and reactions of -dicarbonyl compounds, especially -keto esters, is the main focus of this chapter.


B h is acidic carbanion is stabilized by enolate resonance involving both carbonyl groups l.jpg

O

O

••

••

••

••

O

O

C

C

O

O

••

••

••

••

••

R

C

OR'

C

C

C

C

••

H

H

R

C

OR'

R

C

OR'

CH3CH2O

H

H

b-H is Acidic carbanion is stabilized by enolate resonance involving both carbonyl groups.

pKa ~ 11


The claisen condensation synthesis of b keto esters l.jpg

O

O

O

O

O

O

1. NaOCH2CH3

2CH3COCH2CH3

2RCH2COR'

RCH2CCHCOR'

CH3CCH2COCH2CH3

2. H3O+

(75%)

R

The Claisen CondensationSynthesis of b-Keto esters

1. NaOR'

+

R'OH

2. H3O+

b-Keto esters

ethylacetoacetateor acetoaceticester.


Mechanism l.jpg

••

O

••

••

CH3CH2

CH2

O

H

COCH2CH3

••

••

••

O

••

••

CH3CH2

CH2

O

H

COCH2CH3

••

••

Mechanism

Step 1: Form nucleophile by removing acidic a-H.


Mechanism5 l.jpg

••

••

O

O

••

••

••

CH3C

CH2

COCH2CH3

OCH2CH3

••

••

••

••

O

O

••

••

CH2

CH3COCH2CH3

COCH2CH3

••

Mechanism

Step 2: Nucleophile formed from

one ester attacks carbon of

a second ester molecule


Mechanism6 l.jpg

••

••

O

O

••

••

••

CH3C

CH2

COCH2CH3

OCH2CH3

••

••

••

••

O

O

••

••

••

+

CH3C

CH2

OCH2CH3

COCH2CH3

••

••

Mechanism

Step 3:

Leaving

group

leaves

Acid

Base


Slide7 l.jpg

••

••

O

O

••

••

••

OCH2CH3

CH3C

CH

H

COCH2CH3

••

••

••

••

O

O

••

••

••

+

CH3C

CH2

OCH2CH3

COCH2CH3

••

••

Mechanism

Step 4: Acid-base reaction

+

Step 5: Add acid to protonate negative ion.


Another example l.jpg

O

2CH3CH2COCH2CH3

1. NaOCH2CH3

2. H3O+

O

O

CH3CH2CCHCOCH2CH3

CH3

Another example

  • Reaction involves bond formation between the -carbon atom of one ethyl propanoate molecule and the carbonyl carbon of the other.

(81%)


Intramolecular claisen condensation the dieckmann reaction l.jpg

O

O

1. NaOCH2CH3

2. H3O+

O

O

COCH2CH3

(74-81%)

Intramolecular Claisen Condensation:The Dieckmann Reaction

CH3CH2OCCH2CH2CH2CH2COCH2CH3


Step 1 form nucleophile by abstracting acidic b h l.jpg

••

O

O

••

••

••

O

O

••

••

CH3CH2OCCH2CH2CH2CHCOCH2CH3

••

Step 1; Form Nucleophile by abstracting acidic b-H

CH3CH2OCCH2CH2CH2CH2COCH2CH3

NaOCH2CH3


Step 2 intramolecular reaction nu attacks electrophilic carbon l.jpg

••

••

O

CH3CH2O

••

••

••

••

O

••

C

CHCOCH2CH3

H2C

H2C

CH2

••

••

O

O

••

••

CH3CH2OCCH2CH2CH2CHCOCH2CH3

••

Step 2: Intramolecular reaction; Nu attacks electrophilic carbon


Step 3 reform carbonyl group leaving group leaves l.jpg

••

••

O

CH3CH2O

••

••

••

••

O

••

C

CHCOCH2CH3

H2C

H2C

CH2

••

O

••

••

O

••

C

••

+

CH3CH2O

CHCOCH2CH3

H2C

••

••

H2C

CH2

Step 3; Reform carbonyl group; leaving group leaves


Mixed claisen condensations l.jpg

O

O

O

O

HCOR

ROCOR

ROC

COR

O

COR

Mixed Claisen Condensations

  • work best when the reaction mixture contains one compound that can form an enolate and another that cannot.

These esters

cannot form

an enolate.


Example l.jpg

O

O

COCH3

CH3CH2COCH3

O

O

(60%)

CCHCOCH3

CH3

Example

+

1. NaOCH3

2. H3O+


Acylation of ketones with esters l.jpg

O

O

CH3CH2OCOCH2CH3

1. NaH

2. H3O+

O

O

COCH2CH3

(60%)

Acylation of Ketones with Esters

+

Esters that cannot

form an enolate can

be used to acylate

ketone enolates.


Example16 l.jpg

O

O

COCH2CH3

CH3C

1. NaOCH2CH3

2. H3O+

O

O

CCH2C

(62-71%)

Example

+


Example17 l.jpg

O

O

CH3CH2CCH2CH2COCH2CH3

1. NaOCH3

2. H3O+

O

O

CH3

(70-71%)

Example


Ketone synthesis via keto esters l.jpg

O

O

1. NaOR'

heat

2RCH2COR'

2. H3O+

RCH2CCHH

R

Ketone Synthesis via -Keto Esters

3. H2O

  • Form b-keto ester from two esters.

  • Add water to hydrolyze ester and form b-keto acid.

  • Decarboxylate (-CO2) b-ketoester.


Slide19 l.jpg

O

O

O

2RCH2COR'

RCH2CCHCOR'

R

1. NaOR'

+

R'OH

2. H3O+

Step 1: Form b-keto ester from two esters.


Slide20 l.jpg

O

O

O

O

RCH2CCHCOR'

RCH2CCHCOH

R

R

Step 2: Hydrolysis of Ester to form b-keto acid.

H+

+ H2O

+ R’OH


Slide21 l.jpg

O

O

O

RCH2CCHCOH

R

RCH2CCHH

R

Step 3: Decarboxylation (-CO2) to form Ketone

heat

+ CO2


Example step 1 formation of b ketoester l.jpg

O

2 CH3CH2CH2CH2COCH2CH3

O

O

CH3CH2CH2CH2CCHCOCH2CH3

CH2CH2CH3

Example; Step 1- Formation of b-ketoester

1. NaOCH2CH3

2. H3O+

(80%)


Example cont step 2 hydrolysis to form b keto acid l.jpg

O

O

CH3CH2CH2CH2CCHCOH

CH2CH2CH3

O

O

CH3CH2CH2CH2CCHCOCH2CH3

CH2CH2CH3

Example(cont): step 2: Hydrolysis to form b-keto acid

1. KOH, H2O, 70-80°C

2. H3O+


Example cont step 3 decarboxylation to form ketone l.jpg

O

O

CH3CH2CH2CH2CCHCOH

CH2CH2CH3

O

CH3CH2CH2CH2CCH2CH2CH2CH3

Example (cont): Step 3: Decarboxylation to form ketone

70-80°C

(81%)


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