A DFT study on the NHC catalysed Michael addition of enols to α,β-unsaturated acyl-azoliums. A base catalysed C-C bond-formation step

The NHC catalysed nucleophilic additions of enols to α,β-unsaturated acyl-azolium intermediates have been investigated using DFT methods at the MPWB1K/6-31G** computational level. In the direct and the conjugate additions, formation of a hydrogen bond (HB) with the carboxyl oxygen is not sufficient...

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Published in:Organic & biomolecular chemistry 2014-02, Vol.12 (6), p.895-904
Main Authors: Domingo, Luis R, Sáez, José A, Arnó, Manuel
Format: Article
Language:English
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Summary:The NHC catalysed nucleophilic additions of enols to α,β-unsaturated acyl-azolium intermediates have been investigated using DFT methods at the MPWB1K/6-31G** computational level. In the direct and the conjugate additions, formation of a hydrogen bond (HB) with the carboxyl oxygen is not sufficient to favour the C-C bond formation as a consequence of the low nucleophilic character of enols. Interestingly, when enols form a HB with the chloride counterion, the activation energies associated with the conjugate addition decrease as a consequence of the increased nucleophilic character of enols and the increased electrophilic character of the 'acyl-azolium + Cl' ion pair. Analysis of the DFT reactivity indices allows establishing a base catalysed C-C bond-formation step promoted by the chloride counterion.
ISSN:1477-0520
1477-0539
DOI:10.1039/c3ob41924j