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Theoretical study on NHC-catalysed enantioselective cycloaddition of ketenes and 3-aroylcoumarins: Mechanism and enantioselectivity

Abstract

The NHC-catalysed enantioselective cycloaddition of ketenes to 3-aroylcoumarins to yield dihydrocoumarin-fused dihydropyranones has been investigated using DFT methods at the B3LYP/6-31G* and MPWB1K/6-311G** computational levels. Two plausible mechanisms have been studied: the “ketene-first” mechanism A and the “coumarin-first” mechanism B. An analysis of the activation Gibbs free energies involved in the two competitive pathways makes it possible to rule out the pathway associated with the “coumarin-first” mechanism B. The first step of the “ketene-first” mechanism A is the formation of zwitterionic intermediate IN1-Z via nucleophilic attack of NHC 1 to ketene 2. [4+2] cycloaddition through nucleophilic attack of enolate IN1-Z to the conjugate double bond of benzoyl group of the coumarin 3, via TS3-SS-a2 or TS3-RR-a2, yield IN3. Finally the extrusion of catalyst through TS5 leads to the final products either 4-SS or 4-RR. Enantioselectivity observed in the experimental results is determined in the transition states TS3-SS-a2/ TS3-RR-a2. In this path, the intramolecular hydrogen-bonding between the hydroxyl group of the IN1-Z adduct and the carbonyl oxygen of the original ketene group directs the final stereochemistry throughout the entire process.

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Publication details

The article was received on 03 May 2018, accepted on 11 Jul 2018 and first published on 11 Jul 2018


Article type: Paper
DOI: 10.1039/C8OB01035H
Citation: Org. Biomol. Chem., 2018, Accepted Manuscript
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    Theoretical study on NHC-catalysed enantioselective cycloaddition of ketenes and 3-aroylcoumarins: Mechanism and enantioselectivity

    R. J. Zaragoza, M. J. Aurell and M. A. González-Cardenete, Org. Biomol. Chem., 2018, Accepted Manuscript , DOI: 10.1039/C8OB01035H

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