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Chapter 18: Enols and Enolates

Chapter 18: Enols and Enolates. Overview. Enols : Enolates :. 18.1 – The a Hydrogen and Its p Ka. 18.2 – The Aldol Condensation. When appropriate base is used, both aldehyde and enolate present in solution, this leads to the aldol reaction. 18.2 – The Aldol Condensation (loss of H 2 O).

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Chapter 18: Enols and Enolates

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  1. Chapter 18: Enols and Enolates Overview Enols : Enolates :

  2. 18.1 – The a Hydrogen and Its pKa

  3. 18.2 – The Aldol Condensation When appropriate base is used, both aldehyde and enolate present in solution, this leads to the aldol reaction

  4. 18.2 – The Aldol Condensation (loss of H2O) When the reaction is run at low temperature the aldol product may be isolated; at elevated temperature the elimination occurs

  5. 18.3 – Mixed (Crossed) Aldol Reactions 2 enolizable substrates leads to multiple products all present in solution at same time

  6. 18.3 – Mixed (Crossed) Aldol Reactions Usually use 1 non-enolizable substrate: Elimination of H2O common when conjugation results, can also be induced by heating the reaction mixture

  7. 18.4 – Alkylation of Enolate Ions

  8. Staphylococcus aureus (MRSA, VRSA) • Greek staphyle meaning “a bunch of grapes” • Greek kokkos meaning “berry” • aureus = yellow

  9. Gram-positive, cluster-forming coccus • Cause food poisoning, endocarditis, osteomyelitis • Can cause septiceamia, infections on implants • Becoming increasingly resistant to antibiotics • MRSA strains appeared in 1961 • VRSA first reported in the USA in 2002

  10. Microcapsule (carbohydrate) – defends against phagocytosis

  11. Bacterial Capsular Polysaccharides – serotypes

  12. 18.5 – Enolization and Enol Content Tautomers – two structures that differ by placement of an atom or a group The enol formis usually in low concentration since the C=O is more stable Tautomerismis acid-catalyzed and base-catalyzed

  13. 18.5 – Enolization and Enol Content

  14. 18.5 – Base-Catalyzed Enolization Enolate resonance forms – not tautomers

  15. 18.5 – Acid-Catalyzed Enolization Sequential proton transfers

  16. 18.6 – Stabilized Enols

  17. 18.7 – a-Halogenation of Aldehydes and Ketones • The reaction overall is a substitution • The reaction is regiospecific – only a-H is replaced

  18. 18.8 – Mechanism of a-Halogenation of Aldehydes and Ketones Very good cation generated (hetero-atom stabilized)

  19. 18.9, 18.10 – The Haloform Reaction and a-Deuteration Iodoform reaction iodoform A methyl ketone a-Deuteration

  20. 18.11 – Conjugation in a,b-Unsaturated Systems Carbonyl carbon andb-carbon have positive character Acrolein Figure 18.2 Pi system is completely delocalized

  21. 18.12 – Conjugate Addition to a,b-Unsaturated Systems More reactive nucleophiles (e.g. RMgX) attack the most +ve C i.e. the carbonyl carbon. These reactions are usually irreversible. Softer nucleophiles undergo reversible addition and lead to the more stable (thermodynamically favoured) 1,4-addition product.

  22. 18.13 – The Michael Reaction H alpha to two C=O will be more acidic than next to one C=O, deprotonation leads to a soft nucleophile that adds 1,4-

  23. 18.13 – Michael Addition – Robinson Annulation Sequence Application:

  24. 18.14 – Conjugate Addition of Organocopper Reagents

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