24. Enolate Chemistry: Reactions at the Alpha-Carbon

Draw the products of the following reactions:

b.

Draw the products of the following reactions:

a.

Alkylation of the following compound with methyl iodide under two different conditions forms two different ketoesters (A and B). Each ketoester forms a cyclic diketone (C and D) when treated with methoxide ion in methanol. a. Draw the structures of A and B, and indicate the conditions used in the alkylation reaction that cause that ketoester to be formed.

2. Indicate which compounds would be more than 99% deprotonated by a solution of sodium ethoxide in ethanol.

For each molecule shown below,

2. draw the important resonance contributors of the anion that results from removal of the most acidic hydrogen.

(a)

(b)

(c)

1. Rank the following compounds in order of increasing acidity.

Give the important resonance forms for the possible enolate ions of the following:

(a) acetone

(b) cyclopentanone

In Chapter 20, we studied the aldol reaction. Although not discussed at the time, this reaction is stereospecific, proceeding through the Zimmerman–Traxler transition state shown here.

(a) Show an arrow-pushing mechanism for this concerted reaction.

(b) Why is this a favorable mechanism?

LDA can be used to form enolates on esters and nitriles. Predict the product of these alkylation reactions.

(a)

Identify the enolate(s) that would form on treatment of each of the following carbonyls with base. [When there are two possibilities, draw both.]

(a)

Show the resonance forms for the enolate ions that result when the following compounds are treated with a strong base.

(c) ethyl α-cyanoacetate

Show the resonance forms for the enolate ions that result when the following compounds are treated with a strong base.

(d) nitroacetone

For each molecule shown below,

1. indicate the most acidic hydrogens.

2. draw the important resonance contributors of the anion that results from removal of the most acidic hydrogen.

(g)

For each molecule shown below,

1. indicate the most acidic hydrogens.

2. draw the important resonance contributors of the anion that results from removal of the most acidic hydrogen.

(h)

Give the important resonance forms for the possible enolate ions of the following:

(c) pentane-2,4-dione