Chapter 20 Enolates / other Carbanions Organic Chemistry II Fall 1999 Chapter 20 Enolates / other Carbanions tautomer equilibrium:  859 Table 20.1 acidic/basic mechanism:  860-1 Figure 20.1-2 acidity of a-protons:  861 middle & 861 top bases to produce enolates:  861 bot. & NaOEt practice:  861 Problem 20.1 OrgChem-Chap20 Chapter 16

Alkylation of Enolates Organic Chemistry II Fall 1999 Alkylation of Enolates leaving group (L-) nucleophile (Nu:-) electrophile (R-L) (Nu-R) d+ d- SN2 reactions SN2 reactions:  864 bottom & 865 bottom use of bulky bases (LDA): no addition to carbonyls practice:  866 Problem 20.5 OrgChem-Chap20 Chapter 16

Halogenation of the a-Carbon Organic Chemistry II Fall 1999 Halogenation of the a-Carbon a-substitution: neutral,  862 bottom faster with acid:  862 bot. & 863 top basic conditions:  863 middle haloform reaction: one-carbon elimination;  863 bottom & 864 top terminal groups: CH3C(O)- & CH3CH(OH)- OrgChem-Chap20 Chapter 16

More Stabilized Enolates Organic Chemistry II Fall 1999 More Stabilized Enolates better Nu: & easier to generate the anions synthetic equivalents: acetone anion ( 868), acetic acid anion ( 869) dialkylation & other reactions:  870  acetoacetate malonate p. 351 p. 364 OrgChem-Chap20 Chapter 16

Aldol Condensation self-condensation: b-hydroxy aldehyde Organic Chemistry II Fall 1999 Aldol Condensation self-condensation: b-hydroxy aldehyde addition to carbonyl:  873 Figure 20.3 dehydration: under vigorous conditions stable a,b-unsaturated aldehyde:  874 diketones & mixed aldols:  876 & 877 reverse aldol in vivo:  880-1 Focus On practice:  879 Problem 20.13 OrgChem-Chap20 Chapter 16

Chapters 18-19 Additions to the Carbonyls Organic Chemistry II Fall 1999 Chapters 18-19 Additions to the Carbonyls OrgChem-Chap20 Chapter 16

Ester Condensation Claisen condensation: b-ketoester Organic Chemistry II Fall 1999 Ester Condensation Claisen condensation: b-ketoester equilibrium controlled:  882 Figure 20.4 a,a-dialkyester: strong base only,  882 & 883 Dieckmann condensation:  883 middle mixed Claisen condensation:  884 Cannizzaro reaction: no a-Hs;  888 Figure 20.5 practice:  886 Problem 20.18 OrgChem-Chap20 Chapter 16

Other Carbon Nucleophiles (I) Organic Chemistry II Fall 1999 Other Carbon Nucleophiles (I) enamines: weak enolate equivalent preparation: regioselectivity, 890 &  889 bot. reactive E+ required:  890 top &  890 bottom anion of dithianes: acyl anion equivalent preparation & acidity: pKa = 31,  891 umpolung (polarity inversion): removal,  892 OrgChem-Chap20 Chapter 16

Other Carbon Nucleophiles (II) Organic Chemistry II Fall 1999 Other Carbon Nucleophiles (II) dianions: two anions with different reactivity preparation: strong bases,  892 bottom - 893 top practice: 893 Problem 20.24 conjugate addition: a,b-unsaturated carbonyls 1,4-addition (Michael reaction): Figure 20.6 &  894 Robinson annulation: Michael / aldol,  896 bottom 2-cyclohexenones OrgChem-Chap20 Chapter 16

Synthesis & Retrosynthesis Organic Chemistry II Fall 1999 Synthesis & Retrosynthesis disconnection approach: retrosynthetic analysis retrosynthetic arrow: target starting recognition of functional groups: carbonyls alkyl groups (enolates) mono/dicarbonyls:  898-9 b-hydroxy/a,b-unsaturated carbonyls aldol:  899 middle 1,3-dicarbonyls Claisen/aklyation:  899 middle 1,5-dicarbonyls Michael addition:  899 bottom 2-cyclohexenone Robinson anuulation:  900 top practice:  900-901 & 901 Problem 20.29 OrgChem-Chap20 Chapter 16