enolsg - Copyright, Arizona State University Acidities of...

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Enols and Enolates 1 Copyright, Arizona State University Copyright, Arizona State University Enols and Enolates Acidities of Carbonyls (more. ..) Fundamental Property we are concerned with: R C OH CH 2 enol C O R CH 2 C O R CH 2 enolate anion nucleophilic carbon LA LA • Enolate anions, and their related enols (seen before) are Lewis bases, and more importantly have NUCLEOPHILIC carbon atoms Enols come from Aldehydes/Ketones R C O CH 3 R C OH CH 2 acid or base catalyzed seen before enol Enolates come from Aldehydes/Ketones Base Enolate Anion Stabilized by resonance O H α− β− γ -positions H α β γ O H O H • H atoms on carbons atoms alpha- to the C=O group in carbonyl compounds are unusually acidic considering the fact that they are on sp3 hybridized carbon atoms, the conjugate base ENOLATE anions are resonance stabilized. • alpha, beta, gamma terminology refers to relative position, alpha means next to, beta means one position further away etc., in THIS CONTEXT, alpha means the carbon next to the C=O bond, beta is the next carbon away etc. • hydrogens that are on carbons alpha- to carbonyls are unusually acidic, they are ENOLIZABLE 1 Enolizable Hydrogens: Hydrogens alpha- to a Carbonyl keto-enol tautomerization is catalyzed by acid OR base Example ; There are FOUR (alpha) enolizable hydrogens in cyclohexanone, these can exchange in presence acid • The mechanism for exchange can be revealed using deuterated (in effect labeled) acidic water, D3O+, involves reversible formation if an enol, ALL enolizable H atoms can be exchanged this way O H OD D 3 O + D O D D H O D H O D O D D D O D D O D D O D D O D D O D α β O D D D D
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Enols and Enolates 2 Copyright, Arizona State University • There are FOUR enolizable hydrogens in the molecule below, exchange of the hydrogen at the asymmetric center can result in inversion of configuration, i.e., formation of a racemic mixture. The base catalyzed mechanism is shown that proceeds via the enolate anion H 3 C C O C CH 3 Ph H H 3 C C O C CH 3 Ph H 3 C C O C Ph CH 3 H OH/H 2 O Racemization OH H 3 C C O C CH 3 Ph OH/H 2 O O H H (±) Which bases can be used to make an enolate anion ? CH 3 H 3 C O CH 2 H 3 C O + OH + H 2 O pKa ~19 pka ~15 • water is stronger acid, therefore equilibrium lies mostly on ketone side Recall Na + – NH 2 (a base strong enough to deprotonate a terminal alkyne) CH 3 C H 3 C O pKa ~19 CH 2 C H 3 C O N H N pKa ~40 100% in enolate form new strong base + irreversible + • ketone is much stronger acid, therefore equilibrium lies essentially completely on enolate side • where does this base come from?? Li N lithium diisopropylamide (LDA) N H + n-BuLi + butane (g) REALLY strong Base • LDA is like NaNH 2 , but is BULKY BASE, thus less nucleophilic, stronger base than tertiary butoxide Summary • Use OH if you don't care about forming enolate reversibly • Use LDA if you want to form the enolate irreversibly, using LDA, ALL of the carbonyl is consumed!
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enolsg - Copyright, Arizona State University Acidities of...

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