Kinetic and structure–activity studies of the triazolium ion-catalysed benzoin condensation. Issue 2 (22nd December 2020)

• Record Type: Journal Article
• Authors: Massey, Richard S.; Murray, Jacob; Collett, Christopher J.; Zhu, Jiayun; Smith, Andrew D.; O'Donoghue, AnnMarie C.
• Is Part Of: Organic & biomolecular chemistry. Volume 19:Issue 2(2020)
• Page Start: 387
• Page End: 393
• Publication Date: 2020-12-22
• Subjects: Chemistry, Organic -- Periodicals; Bioorganic chemistry -- Periodicals; Chemistry, Physical organic -- Periodicals; 547
• DOI: 10.1039/d0ob02207a
• Languages: English
• ISSNs: 1477-0520

Abstract

Abstract : An initial rate evaluation of the triazolium-catalysed benzoin condensation permitted a Hammett structure–activity analysis providing insight into the rate-limiting step. 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 ν max at high aldehyde concentrations for all triazolium catalysts. Values for ν max 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 ν max 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.