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

Massey, Richard S.; Murray, Jacob; Collett, Christopher J.; Zhu, Jiayun; Smith, Andrew D.; O'Donoghue, AnnMarie C.

Abstract

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.

Citation

Massey, R. S., Murray, J., Collett, C. J., Zhu, J., Smith, A. D., & O'Donoghue, A. C. (2021). Kinetic and structure–activity studies of the triazolium ion-catalysed benzoin condensation. Organic and Biomolecular Chemistry, 19(2), 387-393. https://doi.org/10.1039/d0ob02207a

• Journal Article Type: Article
• Acceptance Date: Dec 11, 2020
• Online Publication Date: Dec 14, 2020
• Publication Date: 2021
• Deposit Date: Jan 6, 2021
• Publicly Available Date: Jan 6, 2020
• Journal: Organic and Biomolecular Chemistry
• Print ISSN: 1477-0520
• Electronic ISSN: 1477-0539
• Publisher: Royal Society of Chemistry
• Peer Reviewed: Peer Reviewed
• Volume: 19
• Issue: 2
• Pages: 387-393
• DOI: https://doi.org/10.1039/d0ob02207a
• Public URL: https://durham-repository.worktribe.com/output/1253944