Carboxylic acid catalysis on the imine formation versus aza‐Michael reaction in apolar aprotic solvent

• Published in: Journal of physical organic chemistry 2023-03, Vol.36 (3), p.n/a
• Main Authors: Rufino, Virginia C., Pliego, Josefredo R.
• Format: Article
• Language: English
• Subjects: Acetic acid; acid catalysis; Acids; Aldehydes; carboxylic acid additive; Carboxylic acids; Catalysis; Contaminants; Dimerization; Free energy; Imines; Ketones; Kinetics; Mathematical analysis; Michael reaction; microkinetic modeling; organocatalysis; Protons; Reaction mechanisms; Solvents; Toluene; α, β‐unsaturated imines
• ISSN: 0894-3230; 1099-1395
• DOI: 10.1002/poc.4467

The reaction between primary amine and α,β‐unsaturated aldehydes and ketones can produce α,β‐unsaturated imines or aza‐Michael products. However, a theoretical reaction mechanism with the corresponding free energy profile compatible with experimental kinetics in aprotic apolar solvents has not been reported yet. In this work, we have used theoretical calculations to investigate the mechanism behind the 1,2‐addition and 1,4‐addition of benzylamine (BnNH2) to methyl vinyl ketone (MVK) in toluene solution. The calculation of the free energy profile was followed by a microkinetic analysis. According to our results, the experimentally observed formation of the aza‐Michael product is due to more favorable kinetics of the nucleophilic attack of the BnNH2 to the β‐carbon of s‐cis conformation of MVK. Furthermore, acetic acid can be present in MVK as a stabilizer. This contaminant catalyzes the transfer of protons for the formation of the aza‐Michael product and can also catalyze the imine formation. The present analysis indicates that catalysts for proton transfer are essential for the reaction to proceed in aprotic solvent and explain the use of carboxylic acids as an additive in aza‐Michael reactions. In the case of acetic acid, this species has a powerful catalytic effect, which explains why even a trace amount is enough for catalysis. A higher concentration of acetic acid could in principle favor the imine mechanism. However, a high concentration of acetic acid leads to the dimerization of this species, producing a low concentration of the free acid and limiting its ability to produce competitive kinetics for imine formation. Carboxylic acids can work as hidden catalysts in the 1,4‐conjugate addition of amines to enones beyond of the rate‐determining step.