Triflic acid mediated dealkylative lactonisation via NMR-observable alkyloxonium intermediates

Munoz-Herranz, Maria ORCID: and Lloyd-Jones, Guy C. (2009) Triflic acid mediated dealkylative lactonisation via NMR-observable alkyloxonium intermediates. European Journal of Organic Chemistry, 2009 (4). pp. 516-524. ISSN 1434-193X

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Trifluoromethanesulfonic acid (TfOH) efficiently induces the dealkylative cyclisation of pent-4-enoates to generate ?-lactones with high selectivity. For primary alkyl esters bearing an additional alkene, only monolactonisation occurs, even in the presence of excess TfOH. The kinetics of the reaction have been studied by 1H NMR spectroscopy, which reveal that the TfOH acid undergoes a self-catalysed reaction with the pent-4-enoate to generate an oxonium triflate intermediate (rate ˜ kobsd.[TfOH]2[ester]1), possibly via the dimer (TfOH)2. The oxonium triflate intermediate then evolves to the ?-lactone according to unimolecular kinetics, liberating MeOTf in an SNi reaction. 2H-labelling experiments with TfOD suggest that the acid protonates the carbonyl moiety of the ester, with subsequent intramolecular delivery of D+ to the alkene. The resulting carbocation is transient, being rapidly captured intramolecularly to generate the oxonium species. Reversibility in this step mediates equilibration of diastereomeric oxonium intermediates via the carbocation.

Item Type: Article
Faculty \ School: Faculty of Science > School of Chemistry
UEA Research Groups: Faculty of Science > Research Groups > Synthetic Chemistry (former - to 2017)
Faculty of Science > Research Groups > Chemistry of Materials and Catalysis
Depositing User: Rachel Smith
Date Deposited: 14 Feb 2012 16:37
Last Modified: 11 Jan 2023 10:33
DOI: 10.1002/ejoc.200800970

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