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A mechanistic investigation into N-heterocyclic carbene (NHC) catalyzed umpolung of ketones and benzonitriles: Is cyano group better than a classical carbonyl group for the addition of NHC?

Abstract

Density functional theory (DFT) computations have been utilized to investigate the N-heterocyclic carbene (NHC) catalytic reaction mechanism when carbonyl and cyano compounds are co-exist. The results suggested that both the carbonyl and cyano compounds can be activated by NHC, while the order of activation by the NHC depends on the substituents of the two compounds. In the model reaction of NHC catalyzed cyclocondensation of cyclohexanone with 2-aminobenzonitrile, two key activation modes were explored, the new NHC activation carbon of cyano-group (Mode II) is found to be perferred over the activation of carbonyl carbon (Mode I), this is remarkably at variance from the previous reports. More importantly, water molecule can improve the catalytic activity of NHC to some extent, especially in the Dimroth rearrangement process. The cooperative participation of NHC and H2O catalyst is identified. Moreover, the quadruple strong hydrogen bonding interactions between water/NHC and substrates are proposed to be the origin of the high activity of NHC-H2O co-catalyzed Mode II. Additionally, these interactions stabilize the generated negative charge on carbonyl oxygen and further strengthen the nucleophilicity of the –NH group in the NHC-H2O co-catalyzed key transition state structure, which is further confirmed by AIM and NCI analyses. This novel preferred umpolung of a cyano-group via NHC-H2O catalysis was proposed to be responsible for the experimentally observed high product yield.

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Publication details

The article was received on 05 Dec 2018, accepted on 03 Jan 2019 and first published on 07 Jan 2019


Article type: Research Article
DOI: 10.1039/C8QO01309H
Citation: Org. Chem. Front., 2019, Accepted Manuscript
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    A mechanistic investigation into N-heterocyclic carbene (NHC) catalyzed umpolung of ketones and benzonitriles: Is cyano group better than a classical carbonyl group for the addition of NHC?

    S. Cao, H. Yuan and J. Zhang, Org. Chem. Front., 2019, Accepted Manuscript , DOI: 10.1039/C8QO01309H

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