Massey, Richard S. and Murray, Jacob and Collett, Christopher J. and Zhu, Jiayun and Smith, Andrew D. and O'Donoghue, AnnMarie C. (2021) 'Kinetic and structure–activity studies of the triazolium ion-catalysed benzoin condensation.', Organic & biomolecular chemistry., 19 (2). pp. 387-393.
Steady-state kinetic and structure–activity studies of a series of six triazolium-ion pre-catalysts 2a–2f were investigated for the benzoin condensation. These data provide quantitative insight into the role of triazolium N-aryl substitution under synthetically relevant 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.
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|Publisher Web site:||https://doi.org/10.1039/D0OB02207A|
|Publisher statement:||This article is licensed under a Creative Commons Attribution 3.0 Unported Licence|
|Date accepted:||11 December 2020|
|Date deposited:||06 January 2021|
|Date of first online publication:||14 December 2020|
|Date first made open access:||06 January 2021|
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