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Chapter 23

Carbonyl Condensation Notes - Lecture and Textbook Notes Chapter 23

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Cecilia Kutas

CHM247H1 Jasmyn Lee Chapter 23: Carbonyl Condensation Reactions  In carbonyl condensation reactions – the nucleophile and the electrophile are both carbonyl compounds that form a new C-C bond at the α carbon  Carbonyl condensations are: o Reaction between 2 carbonyl compounds (eg/ aldol) o Always form new C-C bond at α carbon o Useful for making complex natural products like steroids and hormones  In condensation reactions, enolate anions react with C=O groups 23.1 Carbonyl Condensation: The Aldol Reaction  Aldehydes and ketones with an α-hydrogen atom undergo an aldol reaction, a base-catalyzed carbonyl condensation reaction  Aldol product is favored with unhindered aldehydes  Reactant is favored with ketones due to steric favors General Aldol Mechanism 23.2 Carbonyl Condensation vs. Alpha Substitution  Both take place under basic conditions and involve enolate ion intermediates  Alpha-substitution reactions o Require a full equivalent of strong base o Normally carried out so that the carbonyl compound is rapidly and completely converted into its enolate ion at a low temperature o An electrophile is then quickly added t ensure that the reactive enolate ion is quenches quickly  Carbonyl Condensation o Require only a catalytic amount of a relatively weak base, so that small amount of enolate ion is generated in the presence of unreacted carbonyl compound o Once a condensation has occurred, the basic catalyst is regenerated 1 CHM247H1 Jasmyn Lee 23.3 Dehydration of Aldol Products: Synthesis of Enones  Β-hydroxy aldehydes or ketones formed in aldol reactions can be easily dehydrated to yield α, β-unsaturated products, or conjugated enones  Stability of conjugated product drives the dehydration  Spontaneous when product is multiply conjugated (eg/ dibenzalacetone) o Alcohols are normally resistant to dehydration by base because –OH is bad leaving group o Aldol products dehydrate easily due to carbonyl group  Even if the initial aldol favors reactants, subsequent dehydration step pushed the reaction to completion 23.5 “Crossed” or “Mixed” Aldol Condensations  In general: a mixed aldol reaction between two similar aldehyde or ketone partners leads to a mixture of four possible products  If both components have α-Hydrogen’s – can potentially get 4 different products  Condense different aldehydes and ketones in good yield if o One reactant has no α-hydrogens – thus can’t form enolate ion to become a donor o The same reactant has unhindered carbonyl group – thus is a good acceptor of nucleophiles 23.6 Intramolecular Aldol Condensation  If both carbonyl groups are in the same molecule, an intramolecular reaction can occur if a 5 or 6 membered ring results 2 CHM247H1 Jasmyn Lee  Practice: Expand the blue ring to make progesterone in 2 steps O O H H H H H H Progesterone O  5- or 6- membered rings favored – C-C-C bond angle is close to the tetrahedral angle o 3-, 4-, and 7- membered rings are less common due to angle strain 23.7 Claisen Condensation  Similar to Aldol condensation reaction  Two esters react instead of aldehydes/ketones  Similar to aldol, but there is a leaving group, alkoxide ion (RO ), and a tetrahedral intermediate  So the product is a β-dicarbonyl  Use alkoxide to remove α-H because HO would hydrolyze the ester  All steps reversible  Full equivalent of base needed, not just catalytic amount 23.8 Crossed/Mixed Claisen Condensation  Analogous to crossed Aldol  Only one of the esters can have an α-H, and thus cant form enolate ion (and can’t serve as donors; can act as electrophilic acceptor)  Can use ketone to form enolate and react with ester o Gives best yield when no α-H on ester o All Claisen products have a 1,3-dicarbonyl (β-dicarbonyl) relationship 3 CHM247H1 Jasmyn Lee 23.9 Dieckmann Condensation (Intramolecular Claisen)  Intramolecular Claisen condensation reactions; carried out with diesters (analogous to intramolecular aldol condensation with diketones)  When two ester groups are in the same molecule cyclization occurs  Mechanism of Dieckmann Condensation  1,7-diester has formed a cyclohexane β-ketoester  Similarly, a 1,6-diester would form a cyclopentane  Sequence i. Dieckmann cyclization ii. Β-keto ester alkylation iii. Decarboxylation 23.10 Conjugate Carbonyl Additions: The Michael Reaction Reaction of enolate anions with conjugated systems  Consider an α, β-unsaturated carbonyl compound  Nucleophile can add to the carbonyl or the alkene  Addition to the alkene is called conjugated addition  When nucleophile is enolate, it is called Michael addition  Michael addition gives good yield between an α
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