TY - JOUR
T1 - Triflic Acid Mediated Dealkylative Lactonisation via NMR-Observable Alkyloxonium Intermediates
AU - Munoz, M. Paz
AU - Lloyd-Jones, Guy C.
PY - 2009/2
Y1 - 2009/2
N2 - Trifluoromethanesulfonic acid (TfOH) efficiently induces the dealkylative cyclisation of pent-4-enoates to generate gamma-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 H-1 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 approximate to k(obsd.)[TfOH](2)[ester](1)), possibly via the dimer (TfOH)(2). The oxonium triflate intermediate then evolves to the gamma-lactone according to unimolecular kinetics, liberating MeOTf in an S(N)i reaction. H-2-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. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)
AB - Trifluoromethanesulfonic acid (TfOH) efficiently induces the dealkylative cyclisation of pent-4-enoates to generate gamma-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 H-1 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 approximate to k(obsd.)[TfOH](2)[ester](1)), possibly via the dimer (TfOH)(2). The oxonium triflate intermediate then evolves to the gamma-lactone according to unimolecular kinetics, liberating MeOTf in an S(N)i reaction. H-2-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. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)
U2 - 10.1002/ejoc.200800970
DO - 10.1002/ejoc.200800970
M3 - Article (Academic Journal)
SN - 1434-193X
SP - 516
EP - 524
JO - European Journal of Organic Chemistry
JF - European Journal of Organic Chemistry
IS - 4
ER -