310_14 - Organic Lecture Series CH 310 N LECTURE 14...

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Organic Lecture Series 1 CH 310 N LECTURE 14 Textbook Assignment: Chapter 19 Enolates & Enamines Homework (for credit): Problem Set 5-Posted Today’s Topics: Enolates-the Aldol Rxn (& variations) Notice & Announcements: EXAM 1: Pick up papers-Dr C’s office hrs Organic Lecture Series 2 Enolate Enolate Anions Anions and and Enamines Enamines Chapter 19
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Organic Lecture Series 3 • Esters also form enolate anions which participate in nucleophilic acyl substitution the product of a Claisen condensation is a β -ketoester: C C CCO R O O A β -ketoester β α Claisen Claisen Condensation Condensation Organic Lecture Series 4 – Claisen condensation of ethyl propanoate gives this β -ketoester OEt O O 1. EtO - Na + 2. H 2 O, HCl O O Et OH + Ethyl propanoate Ethyl 2-methyl-3- oxopentanoate (racemic) + propanoate Claisen Claisen Condensation Condensation Nota bene : the base should be the alkoxide of the ester group (This will overcome trans-esterification.)
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Organic Lecture Series 5 Step 1: formation of an enolate anion Step 2: attack of the enolate anion on a carbonyl carbon gives a TCAI Et O - CH 2 -COEt H O Et OH 2 O O - 2 =COEt p K a = 22 (weaker acid) p K a 15.9 (stronger acid) Resonance-stabilized enolate anion - + + 3 -C-OEt O O 2 - O OEt 3 -C-CH 2 + A tetrahedral carbonyl addition intermediate Claisen Claisen Condensation Condensation Organic Lecture Series Step 3: collapse of the TCAI gives a β -ketoester and an alkoxide ion: Step 4: an acid-base reaction drives the reaction to completion: Et O 3 -C-CH 2 OO O 3 -C-CH 2 -C-OEt O + H 3 -C-CH-C-OEt 3 O O p K a 15.9 p K a 10.7 (stronger acid) + + Claisen Claisen Condensation Condensation
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Organic Chemistry Mechanism Worksheet 6 Claisen Condensation tertahedral intermediate -ketoester anion -ketoester EtO EtO H H 3 O + C O H 3 C C O OEt C H H
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Organic Lecture Series 7 • An intramolecular Claisen condensation + Diethyl hexanedioate (Diethyl adipate) Ethyl 2-oxocyclo-
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310_14 - Organic Lecture Series CH 310 N LECTURE 14...

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