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

Measure what is measurable, and make measurable what is not so! This timeless quotation by Galileo forms the basis of all quantitative understanding, including asymmetric organocatalysis. We report pKa values for 15 chiral Brønsted acid catalysts in DMSO solutions (see scheme): the strongest acids were bis‐sulfonyl/sulfuryl imides (pKa=1.7–1.9) followed by phosphoric acids/amides (pKa=2.4–4.2). A new class of chiral Brønsted acids, tetrakis‐perfluoroalkyl analogues of TADDOLs (TEFDDOLs) have pKas from 2.4 to 5.7.

Laser-based thermal desorption facilitates the first gas-phase study of ultrafast non-adiabatic dynamics operating in 5,6-dihydroxyinole following ultraviolet excitation.

Cascade reaction sequences incorporating N-heterocyclic carbene-based organocatalysis have been developed that allow the direct preparation of a range of (+/-)-4-phenoxycarbonylazlactones in good isolated yields (66-84%) from the corresponding N-p-anisoyl amino acids.