Question

3-Amino-2-oxindole catalyzes the decarboxylation of -keto acids.

a. Propose a mechanism for the catalyzed reaction.

b. Would 3-aminoindole be equally effective as a catalyst?

Short answer

a. A mechanism for the catalyzed reaction can be given as:

b. 3-Aminoindole is not an effective catalyst as the electrons left behind when CO₂is eliminated cannot be delocalized onto an electronegative atom.

Step-by-step solution

Decarboxylation

Decarboxylation constitutes a process that removes a carboxyl group and gives out carbon dioxide. The -keto acid decarboxylation can be oxidative or non-oxidative.

Proposing a mechanism for the catalyzed reaction

a. 3-Amino-2-oxindole catalyzes the decarboxylation of an -keto acid by forming an imine in the beginning. It then loses a proton from the alpha-carbon.

Now the compound can be decarboxylated, as the electrons left behind can be delocalized onto an oxygen atom. It further undergoes enolization followed by imine hydrolysis to generate an aldehyde.

Proposing a mechanism for the catalyzed reaction

b. When 3-Aminoindole is added to alpha-keto acid, a new bond is generated between the nitrogen atom of the NH₂ group in the catalyst and the carbon atom in the carboxyl group forming an imine. The electrons left behind are delocalized onto oxygen.

In the case of 3-Aminoindole, there is no oxygen atom for delocalization of electrons that are left behind after losing a proton from the carbon of the imine.