pK(a) values of 19.8-28.2 are reported for the conjugate acids of a large series of NHCs in water. The effects of ring size, N-substituent and C(4)-C(5) saturation on pK(a) are discussed.

Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Abstract:Second order rate constants have been determined for deuteroxide ion-catalyzed exchange of the C(3)-proton for deuterium, k DO (M -1 s -1 ), of a series of twenty triazolium salts in aqueous solution at 25 °C and ionic strength I = 1.0 (KCl). Evidence is presented that the rate constant for the reverse protonation of the triazol-3-ylidenes by solvent water is close to that for dielectric relaxation of solvent (10 11 s -1 ). This data enabled the calculation of carbon acid pK a values in the range 16.6-18.5 for the twenty triazolium salts. pD-rate profiles for deuterium exchange of the triazolium salts reveal that protonation at nitrogen to give dicationic triazolium species occurs under acidic conditions, with estimates of pK a N1 = -0.2-0.5.2

The in situ observation, isolation and reversible formation of intermediate 3-(hydroxybenzyl)azolium salts derived from NHC addition to a range of substituted benzaldehydes is probed. Equilibrium constants for the formation of these 3-(hydroxybenzyl)azolium salts, as well as rate constants of hydrogen-deuterium exchange (k ex) at C(a) of these intermediates for a range of N-aryl triazolinylidenes is reported. These combined studies give insight into the preference of N-pentafluorophenyl NHCs to participate in benzoin and Stetter reaction processes.

Rate and equilibrium constants for the reaction between N-aryl triazolium N-heterocyclic carbene (NHC) precatalysts and substituted benzaldehyde derivatives to form 3-(hydroxybenzyl)azolium adducts under both catalytic and stoichiometric conditions have been measured. Kinetic analysis and reaction profile fitting of both the forward and reverse reactions, plus onwards reaction to the Breslow intermediate, demonstrate the remarkable effect of the benzaldehyde 2-substituent in these reactions and provide insight into the chemoselectivity of cross-benzoin reactions.

No abstract