Kinetic and structure-activity studies of the triazolium ion-catalysed benzoin condensation
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Steady-state kinetic and structure-activity studies of a series of six triazolium-ion pre-catalysts 2a–2f were investigated for the benzoin condensation under catalytic conditions in a polar solvent environment. Kinetic behaviour was significantly different to that previously reported for a related thiazolium-ion pre-catalyst 1, with the observed levelling of initial rate constants to νmax at high aldehyde concentrations for all triazolium catalysts. Values for νmax for 2a–2f increase with electron withdrawing N-aryl substituents, in agreement with reported optimal synthetic outcomes under catalytic conditions, and vary by 75-fold across the series. The levelling of rate constants supports a change in rate-limiting step and evidence supports the assignment of the Breslow-intermediate forming step to the plateau region. Correlation of νmax reaction data yielded a positive Hammett ρ-value (ρ = +1.66) supporting the build up of electron density adjacent to the triazolium N-Ar in the rate-limiting step favoured by electron withdrawing N-aryl substituents. At lower concentrations of aldehyde, both Breslow-intermediate and benzoin formation are partially rate-limiting.
Massey , R , Murray , J , Collett , C J , Zhu , J , Smith , A D & O'Donoghue , A 2020 , ' Kinetic and structure-activity studies of the triazolium ion-catalysed benzoin condensation ' , Organic & Biomolecular Chemistry , vol. Advance article . https://doi.org/10.1039/D0OB02207A
Organic & Biomolecular Chemistry
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