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J Org Chem. 2011 May 6;76(9):3005-15. doi: 10.1021/jo200431v. Epub 2011 Mar 29.

NMR investigations on the proline-catalyzed aldehyde self-condensation: Mannich mechanism, dienamine detection, and erosion of the aldol addition selectivity.

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  • 1Institut für Organische Chemie, Universität Regensburg, Universitätsstr. 31, D-93053 Regensburg, Germany.

Abstract

The proline-catalyzed self-condensation of aliphatic aldehydes in DMSO with varying amounts of catalyst was studied by in situ NMR spectroscopy. The reaction profiles and intermediates observed as well as deuteration studies reveal that the proline-catalyzed aldol addition and condensation are competing, but not consecutive, reaction pathways. In addition, the rate-determining step of the condensation is suggested to be the C-C bond formation. Our findings indicate the involvement of two catalyst molecules in the C-C bond formation of the aldol condensation, presumably by the activation of both the aldol acceptor and donor in a Mannich-type pathway. This mechanism is shown to be operative also in the oligomerization of acetaldehyde with high proline amounts, for which the first in situ detection of a proline-derived dienamine was accomplished. In addition, the diastereoselectivity of the aldol addition is evidenced to be time-dependent since it is undermined by the retro-aldolization and the competing irreversible aldol condensation; here NMR reaction profiles can be used as a tool for reaction optimization.

© 2011 American Chemical Society

PMID:
21446689
[PubMed - indexed for MEDLINE]
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