Chapter 16 Enolate and Other Carbon Nucleophiles

Chapter 16 Enolate and Other Carbon Nucleophiles - 1...

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Unformatted text preview: 1 Chapter 16 Enolate and Other Carbon Nucleophiles 16.1 Introduction The acidity of hydrogen: H 3 CR H 2 C=CR 2 > > CR HC > H O R-C-C-enol form keto form OH R-C=C-H O R-C-C-H H O R-C=C O R-C-C B R-C-C O H + + enolate ion enolate ion jD 2 The order of acidity for the Hs of substituted methyl group: 16.2 Enols and enolate anions R-C-C O H R-C=C OH H O O C C C O O C CH 2 C H -dicarbonyl compounds 6-membered ring enolization: O O > CH 3 CH=CH 2 > CH 3 CH CH 3 NO 2 > CH 3 CH , CH 3 CR > CH 3 COOR > CH 3 CN > CH 3 C tautomerization tautomerization 3 O OH H O O 16.3 Halogenation and alkylation of enolate anions a. Racemization of enolate anions OH C=C OH H 2 O O C=C O C-C H 2 O OH O H C-C enol form enolate ion keto form Base-catalyzed enolization: enol tautomer enol tautomer 4 OH C=C H 2 O OH C-C H H O H C-C + + Acid-catalyzed enolization: Racemization via enolization: S + C-C-R Et O H Me C-C-R Et O H R Me H R OH Me Et Me C=C H R H O Et C-C-R + + + H keto form chiral enol form racemer no racemization occurs H Pr C-C-R Et O Me + no -H 5 b. Halogenation fast fast O X C-C- HX X X C-C-OH- X X-X OH C=C slow H O H C-C + + C-CH 2 Cl CH 3 O CH 2 Cl C-CH 3 O Cl C-CH 3 CH 3 O Cl 2 /H Cl 2 h Cl 2 /FeCl 3 O CH 3 C-CH 3 + a comparison: a comparison: 6 c. Haloform (CHX 3 ) reaction ( R-C-CX 3 O step 3 step 2 1) B 2) X 2 O X H R-C-C-X 2) X 2 1) B O R-C-CH 2 X X-X O R-C step 1 B O RC-CH 3 CH 2 General reaction: The acidity of the -Hs on the methyl group: step 1 < step 2 < step 3 The rate of halogenation: O O O RCCH 3 < RCCH 2 X < RCCHX 2 7 e.g. e.g. + HCX 3 R-C-O O + O R-C-OH OH O RC-CX 3 OH O RC-CX 3 CX 3 haloform proton transfer Iodoform test: ( Iodoform test: ( jD 6 ) ) + HCX 3 O R-C-OH- 3NaX 3NaOH + 3X 2 O R-C-CH 3 H 3 O + H 3 O + + CHI 3 O RC-OH 3NaOH 3I 2 R-C-CH 3 O + 3NaI + 2H 2 O yellow H 3 O + RC-OH O + CHI 3 I 2 /OH O R-C-CH 3 I 2 /OH OH R-CH-CH 3 + 3NaI + 2H 2 O oxidation 8 d. Alkylation R' R' R-C-Y R'X LDA R' RCHY S N 2 R'X RCHY B RCH 2-Y R-C-Y R' Y = PhC-, -COOR', -CN and other e-withdrawing groups O or NaH B = NaNH 2 , NaOR, NaH, LiNR 2 , (LiN(i-Pr) 2 ) (LDA: lithium diisopropylamide) R' = CH 3 , 1 R X = Br, I 9 16.4 Alkylation of more stabilized anions O O O O O O O O O-SR, -SR , -SOR-NO 2 , N Y,Y': -CR, -CH, -C-OR, -CNR 2 , -C Y-CH 2-Y' Active methylene compounds: stabilized anion B = OCH 3 , OC 2 H 5 , N(C 2 H 5 ) 2 Y-CH-Y'- HB B Y-CH 2-Y' 10 3 kinds of important reactions of active methylene compounds: alkylation( 6 ), acylation( 6 ), and Knoevenagel condensation Y' Y R' R C=C- H 2 O H , Y R' O R-C-CH-Y' or R = H O RCR' CH-Y' aprotic polar solvent R, R' = CH 3 , 1 X = Br, I M = K, Na R' R Y-C-Y' 2) R'X 1) MOBu t or (RCO) 2 O O R-C-Cl...
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Chapter 16 Enolate and Other Carbon Nucleophiles - 1...

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