Research output: Contribution to Journal/Magazine › Journal article › peer-review
Research output: Contribution to Journal/Magazine › Journal article › peer-review
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TY - JOUR
T1 - Triflic acid mediated dealkylative lactonisation via NMR-observable alkyloxonium intermediates
AU - Munoz-Herranz, Maria
AU - Lloyd-Jones, Guy C.
PY - 2009/1/13
Y1 - 2009/1/13
N2 - 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.
AB - 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.
U2 - 10.1002/ejoc.200800970
DO - 10.1002/ejoc.200800970
M3 - Journal article
VL - 2009
SP - 516
EP - 524
JO - European Journal of Organic Chemistry
JF - European Journal of Organic Chemistry
SN - 1434-193X
IS - 4
ER -