Enantioselective 1,3-Dipolar Cycloaddition Reactions of C -Carboxy Ketonitrones and Enals with MacMillan Catalysts: Evidence of a Nonconcerted Mechanism
| Autor: | Souhir Abid, Arnaud Martel, Mathieu Y. Laurent, Gilles Dujardin, Khalid B. Selim, Kawther Ben Ayed |
|---|---|
| Rok vydání: | 2017 |
| Předmět: |
chemistry.chemical_classification
Reaction mechanism 010405 organic chemistry Concerted reaction Stereochemistry Chemistry Organic Chemistry Enantioselective synthesis Iminium 010402 general chemistry 01 natural sciences Cycloaddition 0104 chemical sciences Nitrone Organocatalysis 1 3-Dipolar cycloaddition Physical and Theoretical Chemistry |
| Zdroj: | European Journal of Organic Chemistry. 2017:6763-6774 |
| ISSN: | 1434-193X |
| Popis: | Highly diastereo- and enantioselective 1,3-dipolar cycloadditions between functional ketonitrones and β-substituted enals are promoted by organocatalysis with the imidazolidinium catalyst of MacMillan. Study of the scope of the reaction shows that high selectivities are conserved by varying the N-protecting group or the ester function. However it is sensitive to sterical interaction with the C-substituent of the nitrone. Reaction proceeds in all cases with a high exo selectivity. In most cases, a third diastereomer, not compatible with a concerted mechanism, was observed, although in minute amount. DFT calculations evidence that the cycloaddition proceeds in a non-concerted fashion by a first oxa Michael-type addition of the nitrone to the double bond followed by a cyclization. This mechanism explains the formation of the observed minor diastereomers. In addition, the diastereo- and enantioselectivities of the reaction were shown to be intermediately thermodynamically controlled and the diastereomeric ratio is modulated by the kinetics of iminium hydrolysis. |
| Databáze: | OpenAIRE |
| Externí odkaz: |
|