New insights into the mechanism of Schiff base synthesis from aromatic amines in the absence of acid catalyst or polar solvents
- Published
- Accepted
- Subject Areas
- Catalysis, Theoretical and Computational Chemistry, Physical Organic Chemistry
- Keywords
- reaction mechanism, DFT, imine, auto-protolysis
- Copyright
- © 2019 Silva
- Licence
- This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, reproduction and adaptation in any medium and for any purpose provided that it is properly attributed. For attribution, the original author(s), title, publication source (PeerJ Preprints) and either DOI or URL of the article must be cited.
- Cite this article
- 2019. New insights into the mechanism of Schiff base synthesis from aromatic amines in the absence of acid catalyst or polar solvents. PeerJ Preprints 7:e27710v1 https://doi.org/10.7287/peerj.preprints.27710v1
Abstract
Extensive computational studies of the imine synthesis from amines and aldehydes in water have shown that the large-scale structure of water is needed to afford appropriate charge delocalisation and enable sufficient transition state stabilisation. These insights cannot, however, be applied to the understanding of the reaction pathway in apolar solvents due their inability to form extensive hidrogen-bonding networks. In this work, we perform the first computational studies of this reaction in apolar conditions. This density-functional study of the reaction of benzaldehyde with four closely related aromatic amines (aniline, o-toluidine, m-toluidine and p-toluidine) shows that an additional molecule of amine may provide enough stabilization of the first transition state even in the absence of a hydrogen bonding network. Our computations also show that the second reaction step cannot take place unless an extra proton is added to the system but, crucially, that reaction rate is so high that even picomolar amounts of protonated base are enough to achieve realistic rates. Additional computations show that those minute amounts of protonated base may be obtained under reaction conditions without the addition of extraneous acid through the auto-protolysis of the amines themselves. To our knowledge, this is the first report of a role for the auto-protolysis of anilines in their extensive reactional repertoire.
Author Comment
This is a submission to PeerJ Organic Chemistry for review.
Supplemental Information
Electronic energies, ZPVE, etc. used for the computation of auto-protolysis energies
Electronic energies, ZPVE, etc. used for the computation of activation energies and reaction energies
Coordinates of all molecules studied in this work
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