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  •  » Kinetic versus thermodynamic enolate formation.

#1 2011-12-09 12:09:48

orgopete
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Kinetic versus thermodynamic enolate formation.

A question was posted asking why products A and B formed in the following reaction.

     http://www.curvedarrow.com/chem/ZZ234C540F.jpg

This is question of why different enolates should form under the different conditions. The top conditions give the thermodynamic enolate. That is, the enolate can form and revert back to the ketone. The enolate with the greatest stability is the enolate that is the most substituted. This is consistent with alkene stability, the more substituted, the more stable. (It is a little more complicated than that, such as a trans alkene is more stable than a cis, etc.)

If an excess of LDA is used in THF, a kinetic enolate is formed. By keeping the temperature low, an aprotic solvent, and avoiding an excess of ketone (a proton source), the equilibrium to the more stable enolate can be avoided. One can anticipate formation of a kinetic enolate under these conditions. It is frequently suggested that the hindered base LDA will remove the a proton from the CH3 group due to steric hindrance. I prefer a more general explanation based upon acidity. Since carbon is an electron donor, CH acidity decreases with increasing substitution. On that basis, the CH3 hydrogens should be the most acidic and kinetically the fastest to react.

    http://www.curvedarrow.com/chem/ZZ0B494D48.jpg

A simple alkylation will complete the reaction.

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