0215-Ch19 - Chapter 19 Enolate Anions and Enamines 1 Ch-19...

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Unformatted text preview: Chapter 19 Enolate Anions and Enamines 1 Ch-19 CH 2 =C-H O CH 2-C-H O An enolate anion Formation of an Enolate Anion Enolate anions are formed by treating an aldehyde, ketone, or ester, each of which has at least one -hydrogen , with base . CH 3-C- H O NaOH H C C-H O H H C C-H O Na + H H 2 O + + An enolate anion Most of the negative charge in an enolate anion is on oxygen. 2 Ch-19 Enolate Anions Enolate anions are nucleophiles in S N 2 reactions and carbonyl addition reactions. An enolate anion nucleophilic addition A ketone A tetrahedral carbonyl addition intermediate R R R O + R' R' O O R R R O R' R' An enolate anion nucleophilic substitution A 1 haloalkane or sulfonate R R R O + R' Br O R R R S N 2 R' + Br 3 Ch-19 The Aldol Reaction The most important reaction of enolate anions is nucleophilic addition to the carbonyl group of another molecule of the same or different compound. Although these reactions may be catalyzed by either acid or base , base catalysis is more common (see next slide). 4 Ch-19 The Aldol Reaction The product of an aldol reaction is: a -hydroxyaldehyde . O CH 3-C-CH 3 CH 3-C-CH 3 O CH 2-C-CH 3 O H Ba(OH) 2 Ba(OH) 2 OH CH 3 CH 3-C-CH 2-C-CH 3 CH 3 CH 3-C-CH 2-C-CH 3 O Acetone + 4-Hydroxy-4-methyl-2-pentanone (a -hydroxyketone) + Acetone CH 3-C-H O CH 2- C-H H O NaOH CH 3-CH-CH 2-C-H OH O + Acetaldehyde Acetaldehyde 3-Hydroxybutanal (a -hydroxyaldehyde; racemic ) 5 Ch-19 or a -hydroxyketone . The Aldol Reaction: Base Catalysis Base-catalyzed aldol reaction: Step 1 : Formation of a resonance-stabilized enolate anion ( A/B ). CH 3-C-H O CH 2-C-H O CH 3-CH-CH 2-C-H O O - A tetrahedal carbonyl addition intermediate + CH 2 = C-H O CH 2- C-H O H- O-H + H- CH 2- C-H O + H-O An enolate anion p K a 20 (weaker acid) p K a 15.7 (stronger acid) 6 Ch-19 Step 3 : Proton transfer (A/B) from H 2 O to -alkoxide O completes the aldol reaction and regenerates the catalyst, Step 2 : Addition of enolate to carbonyl (:Nu/El; CC bond formation) gives a TCAI. The Aldol Reaction: Acid Catalysis Acid-catalyzed aldol reaction: Step 1 : Acid-catalyzed equilibration of keto and enol forms. O OH CH 3- C-H CH 2 = C- H HA CH 3-C-H O H A O CH 3-C-H H A + + 7 Ch-19 Step 2 : Proton transfer (A/B) from HA to the carbonyl group of a second molecule of aldehyde or ketone. Step 3 : Attack ( addition ) of the enol of one molecule on the protonated carbonyl group of another molecule ( :Nu/ El, CC bond formation). O CH 3-C-H H CH 2 =C-H H O :A- CH 3-CH-CH 2-C-H OH O H-A + + + (racemic) 8 Ch-19 The Aldol Reaction: Acid Catalysis Step 4 : Proton transfer from the carbonyl oxonium ion to A (A/B) completes the reaction and regenerates the catalyst HA. The Aldol Products: H 2 O Aldol products are very easily dehydrated to give , -unsaturated aldehydes or ketones ....
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This note was uploaded on 04/17/2011 for the course ENY 3005 taught by Professor Staff during the Fall '08 term at University of Florida.

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0215-Ch19 - Chapter 19 Enolate Anions and Enamines 1 Ch-19...

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