Crossed Aldol Reaction Using Weak Bases

Recall that an efficient crossed aldol reaction requires one of the two reacting unique carbonyl compounds to lack an α hydrogen, as illustrated in the reaction of formaldehyde with another aldehyde containing an α-hydrogen.

The self-condensation of the aldehyde comprising an α hydrogen is prevented by adding it slowly to the reaction mixture of formaldehyde and a weak base.

Recollect that hydroxides and alkoxides with relatively smaller pK_a values function as weak bases.

In this reaction, the hydroxide ion deprotonates the α hydrogen of the aldehyde to form a nucleophilic enolate, which attacks the formaldehyde to produce a single crossed aldol product.

Similarly, in a solution of a β-ketoester and a ketone, the β-ketoester with a lower pK_a is more acidic. Hence, sodium ethoxide, a weak base, preferentially deprotonates the β-ketoester, which then reacts with the ketone to form a single product in good yield.