Proton Transfer Reactions of Triazol-3-ylidenes: Kinetic Acidities and Carbon Acid pK_a Values for Twenty Triazolium Salts in Aqueous Solution

Abstract

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

Covalently crosslinked 1,2,4-triazolium-containing polyester networks prepared by Michael addition polymerization

“…Thermal analysis of the imidazolium-containing network revealed that the polymer exhibited a slightly lower T g (À18.2 C) than the corresponding 1,2,4-triazolium-based polymer 10b (À12.6 C) while the T d5% values, as determined by TGA, were within experimental error (Table 4). It could be argued that the increased Lewis acidity of the 1,2,4-triazolium ring might led to stronger cation-anion bonding within the polymer network, thus leading to the small observed increase in the T g [54,55].…”

Asymmetric catalysis with N-heterocyclic carbenes as non-covalent chiral templates

“…Relative to amines, NHCs are significantly stronger Brønsted bases, and their conjugate acids have pKa values in the range of 17-25, similar to alkoxides [42][43][44] . In the case of a Brønsted base-catalysed conjugate addition reaction, the regeneration of the free NHC catalyst in the final step of the catalytic cycle would require a very basic product enolate.…”

“…Those reports primarily used N-alkyl imidazolium-derived NHCs because they are strongly basic. We suggest that such NHCs can fully deprotonate a diketone substrate, which triggers the first nucleophilic addition cycle, and that the reaction stalls at the imidazolium stage because the conjugate imidazolium salt has a high pKa and cannot be redeprotonated [42][43][44] . When a C 2 -symmetric imidazolium salt with two identical chiral alkyl chains on both nitrogen atoms was used stoichiometrically, no ee was observed for the product, suggesting that the diketone enolate and imidazolium cation ion pair is rather loose and flexible or that the reaction is reversible due to slow protonation of the product anion.…”

Proton transfer reactions of a bridged bis‐propyl bis‐imidazolium salt

“…1) in aqueous solution, which include imidazol-2-ylidenes 5, [11,12] thiazol-2-ylidenes 6, [13] imidazolin-2-ylidenes 7 [12] and triazol-3-ylidenes 8. [14,15] The carbenes thus formed have found many applications as organic catalysts [16] and as ligands in organometallic catalysis.…”