Contribution of the intramolecular hydrogen bond to the shift of the pKa value and the oxidation potential of phenols and phenolate anions

Abstract

Intramolecularly OHO[double bond, length as m-dash]C hydrogen bonded phenols, 2-HO-C6H2-3,5-(t-Bu)2-CONH-t-Bu (1-OH), 2-HO-C6H2-5-t-Bu-1,3-(CONH-t-Bu)2 (2-OH) and 2-HO-C6H2-3,5-(t-Bu)2-NHCO-t-Bu (4-OH), were synthesized and their phenolate anions were prepared as tetraethylammonium salts (-1O-(NEt4+), 2-O-(NEt4+) and 4-O-(NEt4+)) with intramolecular NHO(oxyanion) hydrogen bonds. 4-HO-C(6)H(2)-3,5-t-Bu(2)-CONH-t-Bu (3-OH) and its phenolate anion, 3-O-(NEt4+), were synthesized as non-hydrogen bonded references. The presence of intramolecular hydrogen bonds was established through the crystallographic analysis and/or (1)H NMR spectroscopic results. Intramolecular NHO(phenol) hydrogen bonds shift the pK(a) of the phenol to a more acidic value. The results of cyclic voltammetry show that the intramolecular OH...O=C hydrogen bond negatively shifts the oxidation potential of the phenol. In contrast, the intramolecular NHO(oxyanion) hydrogen bond positively shifts the oxidation potential of the phenolate anion, preventing oxidation. These contributions of the hydrogen bond to the pKa value and the oxidation potentials probably play an important role in the formation of a tyrosyl radical in photosystem II.

Persistent Hydrogen‐Bonded and Non‐Hydrogen‐Bonded Phenoxyl Radicals

“…[17,18,21] /salicylate [22] compounds. The bond lengths and angles within both structures are as expected for phenolate compounds, [18] with a C À O bond length of 1.303 Table 2). The 3 CN each display a NÀH resonance at a much lower field (i.e.…”

“…at d = 13.90 and 13.20 ppm, respectively) than that of the corresponding parent phenol (d = 7.49 and 7.37 ppm, respectively), clearly indicating that the N À H proton is involved in an intramolecular O À ···H À N hydrogen bond, as shown in Scheme 2. [18] Moreover, the IR spectra of NHOH L À and NHMe L À , in CH 3 CN solution, exhibit an NÀH stretching at a much lower frequency (3005 and 3009 cm À1 , respectively) than that of the corresponding phenol (3411 and 3413 cm À1 ), which is typically indicative of a relatively strongly Hbonded N À H bond. [19] Thus, these data clearly demonstrate that the O À ···H À N H-bonding identified in the X-ray structure of NHOH …”

Monocyclopentadienyl Phenoxido–Amino and Phenoxido–Amido Titanium Complexes: Synthesis, Characterisation, and Reactivity of Asymmetric Metal Centre Derivatives

“…In the solid state, intramolecular hydrogen bonding OH···N is observed for the three compounds, in agreement with the behaviour deduced in solution from NMR spectroscopic studies. Although there are no crystallographic data for Mannich organic homologue molecules with a "CH 2 NR 2 " pendant arm, the molecular structures of analogous benzamide derivatives have been described, [25] which present similarities in the structural parameters of the phenolic skeleton and the N-alkylamino ends with that observed for 2a, 2d and 2e. SiMe 2 Cl, RЈ = C 6 H 5 4a) (Cy = cyclohexyl) (Scheme 3), which exhibit an asymmetric titanium centre.…”

Conformational Switching Between Acids and their Anions by Hydrogen Bonding

“…The pK a values for phenol [13][14][15] and carboxylic acid derivatives [16] show a similar tendency to the prelocated amide NH in aqueous micellar solution. Singly and doubly hydrogen-bonded phenol and benzoic acid derivatives exhibit a clear pK a shift.…”

“…The NH group begins to interact with the O atom of the phenol under the transition state for deprotonation ( Figure 27.3(a)). This effect contributes to the pK a shift through deprotonation [13]. Thus, only the prelocated neighbouring amide NH affects the pK a shift.…”