Role of hydrogen bonding in the oxidation potential of enols

Abstract

epoc ABSTRACT: Three stable ,-dimesityl enols with heteroaromatic rings in the -position were synthesized to study the effect of OH Á Á ÁN hydrogen bonding on the oxidation potentials of enols. In contrast to its solid-state structure, enol E1 exists predominantly as intramolecularly hydrogen-bonded species in solution. For enol E2 an intermolecular hydrogen bond and for E3 a partial proton transfer were established based on NMR, dilution experiments, solvent dependence and UV-visible spectroscopic studies. Cyclic voltammetric investigations revealed that OH Á Á ÁN hydrogen bonding may shift the oxidation potentials of enols by up to 510 mV cathodically. RESULTS AND DISCUSSIONEnols E1-E3 were prepared by the reaction of ,dimesitylketene 8 with the corresponding heteroaryl lithium reagent, in yields ranging from 24 to 40%. They were fully characterized by 1 H and 13 C NMR, IR and elemental analysis. Moreover, the solid-state structure of E1 (see below) confirms the enol structure of E1-E3.

A comprehensive picture of the one-electron oxidation chemistry of enols, enolates and α-carbonyl radicals: oxidation potentials and characterization of radical intermediates

“…For enols E8, E10, E11, and E14 a linear relationship was observed between the calculated IP a and the experimentally determined oxidation potentials (E pa ). 19 As anticipated, the calculated IP a as well as the oxidation potentials decreased with increasing electron-donating ability of the a-aryl ring. Moreover, within a small set of enols, i.e.…”

“…The use of electron-withdrawing groups in the b-position has been utilized for the first time by Rappoport et al 19 to generate enols derived from amides. The high enol content (>90%) in nonpolar solvents enabled us to investigate their electrochemical properties (Fig.…”

“…Also model studies suggest that hydrogen bonding to enols may shift the oxidation potentials of enols by up to 500 mV. 19 Generation of an enolate not only generates a better electron donor, it also allows the system to act as either one-or two-electron transfer reagent, with the second transfer involving the a-carbonyl radical as reducing agent.…”

Synthesis and Cyclic Voltammetry Studies of 3,4-Methylenedioxymethamphetamine (MDMA) Human Metabolites

“…Probably, the most obvious reason for this event may be the difference in the pK a of 4, a primary amine, versus 3, a secondary amine, bearing higher basicity. 41) It's known from literature 42,43) that OH-N hydrogen bonding can have a profound effect in altering the oxidation potential of enols to lower values. At the pH of 7.4 used for the cyclic voltammetry experiments, 3 should exist preferentially as its hydrochloride salt and 4, partially in its free amine form, could be involved in intermolecular hydrogen bonding responsible for lowering the E p .…”

Spectroscopic and theoretical approaches for studying radical reactions in class I ribonucleotide reductase

“…In homogeneous solution, protonation of the hydroxy group will only occur in rather acidic media. Moreover, enol radical cations are rather strong oxidants, irrespective of hydrogen bonding interactions (Lal et al, 2003); thus, the intermediate could also trigger undesired electron-transfer oxidation of the C225 thiolate that would be strongly exothermic.…”