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Kinetic and structure-activity studies of the triazolium ion-catalysed benzoin condensation

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Author(s)

Richard Massey, Jacob Murray, Christopher John Collett, Jiayun Zhu, Andrew D. Smith, AnnMarie O'Donoghue

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Abstract

Steady-state kinetic and structure-activity studies of a series of six triazolium-ion pre-catalysts 2a2f 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 2a2f 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.
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Original languageEnglish
Number of pages7
JournalOrganic & Biomolecular Chemistry
VolumeAdvance article
Early online date14 Dec 2020
DOIs
Publication statusE-pub ahead of print - 14 Dec 2020

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