Schmid, Markus B. and Zeitler, Kirsten and Gschwind, Ruth M. (2011) Formation and Stability of Prolinol and Prolinol Ether Enamines by NMR: Delicate Selectivity and Reactivity Balances and Parasitic Equilibria. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 133 (18). pp. 7065-7074. ISSN 0002-7863,
Full text not available from this repository. (Request a copy)Abstract
Enamine key intermediates in organocatalysis, derived from aldehydes and prolinol or Jorgensen-Hayashitype prolinol ether catalysts, were generated in different solvents and investigated by NMR spectroscopy. Depending on the catalyst structure, trends for their formation and amounts are elucidated. For prolinol catalysts, the first enamine detection in situ is presented and the rapid cyclization of the enamine to the oxazolidine ("parasitic equilibrium") is monitored. In the case of diphenylprolinol, this equilibrium is fully shifted to the endo-oxazolidine ("dead end") by the two geminal phenyl rings, most probably because of the Thorpe-Ingold effect. With bulkier and electron-withdrawing aryl rings, however, the enamine is stabilized relative to the oxazolidine, allowing for the parallel detection of the enamine and the oxazolidine. In the case of prolinol ethers, the enamine amounts decrease with increasing sizes of the aryl meta-substituents and the O-protecting group. In addition, for small aldehyde alkyl chains, Z-configured enamines are observed for the first time in solution. Prolinol silyl ether enamines are evidenced to undergo slow desilylation and subsequent rapid oxazolidine formation in DMSO. For unfortunate combinations of aldehydes, catalysts, solvents, and additives, the enamine formation is drastically decelerated but can be screened for by a rapid and facile NMR approach. Altogether, especially by clarifying the delicate balances of catalyst selectivity and reactivity, our NMR spectroscopic findings can be expected to substantially aid synthetically working organic chemists in the optimization of organocatalytic reaction conditions and of prolinol (ether) substitution patterns for enamine catalysis.
| Item Type: | Article |
|---|---|
| Uncontrolled Keywords: | BOND-FORMING REACTIONS; ASYMMETRIC MICHAEL ADDITION; ALDOL REACTIONS; ALPHA-ALKYLATION; C-C; DIARYLPROLINOL ETHERS; ALDEHYDES; CATALYSIS; CASCADE; 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: | 22 Jun 2020 05:13 |
| Last Modified: | 22 Jun 2020 05:13 |
| URI: | https://pred.uni-regensburg.de/id/eprint/20830 |
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