pK_as of the conjugate acids of N-heterocyclic carbenes in water

Abstract

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.

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

“…The kinetic acidities towards deprotonation of all triazolium ions by deuteroxide ion via Pathway A (k DO , Table 1) are significantly higher than for analogous imidazolium, 22,24 4,5-dihydroimidazolium 24 and thiazolium ions 27 The triazolium salts in Table 1 are also substantially more acidic than any thiazolium salt studied to date under similar reaction conditions, although the latter experiments have been limited to N-alkyl substituted examples. 27 We have measured a suggested that the more electron withdrawing N-pentafluorophenyl triazolium salts are more acidic than the corresponding N-mesityl salts.…”

“…Buffer catalysis of exchange was also not significant in previous studies of the analogous deuterium exchange reactions of imidazolium, dihydroimidazolium and trihydropyrimidinium ion precursors to carbenes 2-4. 22,24 For the triazolium salts 8a-f-BF 4 -, 9-Cl -, 13-BF 4 -, 14c-f Cl -and 16-BF 4 -studied in fully aqueous solution, values of k ex increase with pD in the region from pD = 0 to 4.5. Figure 2 shows the fit of the log k ex -pD data to eq 3, which is derived from eq 4, where k DO (M -1 s -1 ) is the second-order rate constant for deuteroxide-catalyzed exchange, K W = 10 -14.87 46 is the ion product of D 2 O at 25 o C and γ DO is the activity coefficient for deuteroxide ion under our experimental conditions.…”

“…24 Despite having two N-aryl substituents, variation of these substituents (4-chlorophenyl, mesityl, 2,6-di-…”

“…It is common in many synthetic transformations to generate the active NHC species in situ from the conjugate acid azolium ion precursor of the NHC by use of an appropriate base. 20 Remarkably, despite the widespread use of azolium ion precursors to NHCs in organocatalysis and elsewhere, there have been relatively few literature reports of the solution kinetic or thermodynamic Brønsted acidities of these precatalysts, [21][22][23][24][25][26][27][28][29][30][31][32] although there have been studies of NHC nucleophilicities 33 and gas phase proton affinities. [34][35][36][37][38][39][40][41][42] Washabaugh and Jencks reported on the C2-H/D exchange and aqueous pK a values of a range of N-alkyl thiazolium ion analogues of thiamine, that are precursors to thiazol-2-ylidenes 1.…”

“…22 We recently reported the kinetic acidities towards hydroxide ion, and the aqueous pK a values, of a broad range of conjugate acid precursors to imidazol-2-ylidenes 2, imidazolin-2-ylidenes 3 and trihydropyrimidin -2-ylidenes 4. 24 In this paper, we report kinetic acidities and pK a values in aqueous solution for a large series of synthetically relevant triazolium ion precursors to triazol-3-ylidenes 5. A remarkably diverse range of triazol-3-ylidenes has been employed in organocatalytic transformations, with significant variations in both catalytic activities and reactivities being observed within this architecture.…”