Enantioselective 1,3-Dipolar Cycloaddition Reactions of C -Carboxy Ketonitrones and Enals with MacMillan Catalysts: Evidence of a Nonconcerted Mechanism - Archive ouverte HAL Accéder directement au contenu
Article Dans Une Revue European Journal of Organic Chemistry Année : 2017

Enantioselective 1,3-Dipolar Cycloaddition Reactions of C -Carboxy Ketonitrones and Enals with MacMillan Catalysts: Evidence of a Nonconcerted Mechanism

Résumé

Highly diastereo- and enantioselective 1,3-dipolar cycloadditions between functional ketonitrones and β-substituted enals are promoted by MacMillan imidazolidinium organocatalysts. A study of the reaction scope shows that high selectivities are conserved if the N-protecting group or the ester function is varied. However, the reaction is sensitive to steric interactions with the C substituent of the nitrone. In all cases, the reaction proceeds with high exo selectivity. In most cases, a third diastereomer, incompatible with a concerted mechanism, was also observed, albeit in minute amounts. DFT calculations suggest that the cycloaddition proceeds in a nonconcerted fashion by an initial 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.

Domaines

Chimie organique
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Dates et versions

hal-02150288 , version 1 (07-06-2019)

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Citer

Kawther Ben Ayed, Arnaud Martel, Mathieu Y. Laurent, Khalid Selim, Souhir Abid, et al.. Enantioselective 1,3-Dipolar Cycloaddition Reactions of C -Carboxy Ketonitrones and Enals with MacMillan Catalysts: Evidence of a Nonconcerted Mechanism. European Journal of Organic Chemistry, 2017, 2017 (45), pp.6763-6774. ⟨10.1002/ejoc.201701307⟩. ⟨hal-02150288⟩
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