Thermodynamic control experimental conditions that

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Unformatted text preview: (rate) or thermodynamic (equilibrium) control. • Thermodynamic Control: Experimental conditions that permit establishment of equilibrium between two or more products of a reaction.The composition of the mixture is determined by the relative stabilities of the products. 19 Organic Lecture Series Crossed Enolate Reactions using LDA – Equilibrium among enolate anions is established when the ketone is in slight excess, a condition under which it is possible for proton-transfer reactions to occur between an enolate anion and an α-hydrogen of an unreacted ketone. Thus, equilibrium is established between alternative enolate anions. 20 Organic Organic Lecture Series Crossed Enolate Reactions using LDA • Kinetic control: Experimental conditions under which the composition of the product mixture is determined by the relative rates of formation of each product. – In the case of enolate anion formation, kinetic control refers to the relative rate of removal of alternative α-hydrogens. – With the use of a bulky base, the less hindered hydrogen is removed more rapidly, and the major product is the less substituted enolate anion. – No equilibrium among alternative structures is set up. 21 Organic Lecture Series Intramolecular Aldol Reactions intramolecular aldol reactions are most successful for formation of five- and sixmembered rings consider 2,7-octadione, which has two α-carbons 22 Organic Organic Lecture Series Claisen Condensation • Esters also form enolate anions which participate in nucleophilic acyl substitution the product of a Claisen condensation is a β-ketoester: 23 Organic Lecture Series Claisen Condensation – Claisen condensation of ethyl propanoate gives this β-ketoester Nota bene: the base should be the alkoxide of the ester group (This will overcome trans-esterification.) 24 Organic Organic Lecture Series Claisen Condensation Step 1: formation of an enolate anion Step 2: attack of the enolate anion on a carbonyl carbon gives a TCAI 25 Organic Lecture Series Claisen Condensation 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: 26 Organic Organic Lecture Series Dieckman Condensation • An intramolecular Claisen condensation 27 Organic Lecture Series Crossed Claisen Condensations Crossed Claisen condensations between two different esters, each with α-hydrogens, give mixtures of products and are not us...
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