Schmid, Markus B. and Zeitler, Kirsten and Gschwind, Ruth M. (2012) Stabilization of Proline Enamine Carboxylates by Amine Bases. CHEMISTRY-A EUROPEAN JOURNAL, 18 (11). pp. 3362-3370. ISSN 0947-6539, 1521-3765
Full text not available from this repository. (Request a copy)Abstract
As part of our ongoing studies to provide an experimental basis for the improved understanding of organocatalytic reaction mechanisms we present a study on the influence of amine bases on enamine intermediate stabilization in proline catalysis. The (partial) deprotonation of the proline acid function is displayed by characteristic shifts of certain proton resonances and is also manifested by an increase of the amount of enamine intermediate upon reaching a critical pKaH. Strong bases, such as 1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU), allow for outstanding enamine stabilization in various solvents and, hence, permit the detection of enamine species that have been inaccessible until now (illustrated by the observation of minor amounts of Z enamines). The in situ NMR detection of a prolinateDBUH+ ion pair supports the well-documented reversal of enantioselectivity of proline-catalyzed aminations in the presence of amine bases by disabling the bifunctional activity and switching to a simple stereocontrol effect (as known from the Jorgensen/Hayashi-type diarylprolinol ethers). In addition, the possibility of attractive ionic interactions between both the iminium ion and prolinate enamines available in the presence of strong amine bases suggests promotion of the Mannich pathway in aldol reactions to mainly form condensation products.
| Item Type: | Article |
|---|---|
| Uncontrolled Keywords: | ALDOL REACTIONS; ETHER ENAMINES; NMR; ORGANOCATALYSIS; ACETONITRILE; CATALYSIS; ALDEHYDES; OXAZOLIDINONES; SELECTIVITY; BASICITY; aldol reaction; enamines; ion pairs; NMR spectroscopy; organocatalysis |
| Subjects: | 500 Science > 540 Chemistry & allied sciences |
| Divisions: | Chemistry and Pharmacy > Institut für Organische Chemie > Arbeitskreis Prof. Dr. Ruth Gschwind Chemistry and Pharmacy > Institut für Organische Chemie > Alumni or Retired Professors > Arbeitskreis Dr. Kirsten Zeitler |
| Depositing User: | Dr. Gernot Deinzer |
| Date Deposited: | 18 May 2020 13:15 |
| Last Modified: | 18 May 2020 13:15 |
| URI: | https://pred.uni-regensburg.de/id/eprint/19096 |
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