Keto-enol interconversion mechanism

MECHANISM FOR ACID-CATALYZED KETO-ENOL INTERCONVERSION... 

Keto-enol interconversion (Section 18.3). The mechanisms are shown on page 858... 

The temperature-dependent n.m.r. spectrum of 3-hydroxy-2,4-dimethyl-cyclobutenone (13) in the range 30—115 °C has been reported. Of the two possible mechanisms which account for the details of the investigation, a keto-enol interconversion via 2,4-dimethylcyclobutane-l,3-dione (14) was favoured (Scheme 1). 

A ketone and an enol differ only in the location of a double bond and a hydrogen. Such isomers are called tautomers ( taw-toe-mers ). The ketone and enol are called keto-enol tautomers. Interconversion of the tautomers is called tautomerization. We will examine the mechanism of this reaction in Chapter 19. For now, the important thing to remember is that the keto and enol tautomers come to equilibrium in solution, and the keto tautomer, because it is usually much more stable than the enol tautomer, predominates at equilibrium. 

Tautomerizations involve the shift of a hydrogen atom across a tt system. The most typical tautomerization is a 1,3-shift, and the focus of this section is the interconversion of a ketone (or aldehyde) and an enol, often termed keto-enol tautomerization. The reaction can be catalyzed by acid or base, and it is technically an isomerization, a class of reactions we will cover later in this chapter. However, knowledge of the mechanism of keto-enol tautomerizations is crucial to understanding enol and enolate chemistry, and therefore we cover it here. 

The interconversion of keto and enol isomers takes place spontaneously, but slowly. It is efficiently catalyzed by acids and bases. Mechanism 20.4 shows the mechanism that operates in aqueous base. Mechanism 20.5 shows the mechanism in aqueous acid. 

Both the acid- and base-catalyzed enol-keto interconversions occur rapidly in solntion whenever there are traces of the reqnired catalysts. Remember that although the keto form (usually) predominates, the enol-to-keto conversion is reversible and the mechanisms by which the keto form equilibrates with its enol connterpart are the exact reverse of the preceding two schemes. 

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