51546-73-7Relevant academic research and scientific papers
One-Electron Reduction of Nitrobenzenes by OH and H Radical Adducts to 6-Methyluracil and 6-Methylisocytosine via Electron Transfer and Addition/Elimination. Effect of Substituents on Rates and Activation Parameters for Formation and Heterolysis of Nitroxyl-Type Thetrahedral Intermediate
Jagannadham, V.,Steenken, S.
, p. 111 - 118 (2007/10/02)
The radicals formed by OH radical addition to C-5 of 6-methyluracil or of 6-methylisocytosine, i.e., 5-hydroxy-5,6-dihydro-6-methyluracil-6-yl or 5-hydroxy-5,6-dihydro-6-methylisocytosin-6-yl, or that produced by H addition to C-5 of 6-methyluracil (or by H abstraction from C-6 of 5,6-dihydro-6-methyluracil), i.e., 5,6-dihydro-6-methyluracil-6-yl, reacts in aqueous solution with para-substituted nitrobenzenes to give both nitrobenzene radical anions and nitroxyl-type radicals with rate constants that vary from ca. 7E7 to (2-5)E9 M-1s-1, depending on the pyrimidine radical and on the nitrobenzene.The nitroxyl radicals undergo a spontaneous unimolecular heterolysis to yield (additional) nitrobenzene radical anion and oxidized pyrimidine with rate constants of 1E3 to 5E5 s-1 depending on the structure of the pyrimidine and of the nitrobenzene.This reaction is characterized by activation enthalpies of 30-40 kJ mol-1 and by activation entropies of -7 to -89 J mol-1 K-1 (entropy control).The addition/elimination sequence constitutes a case of inner-sphere electron transfer.The rate constants for the heterolysis reaction are a measure of the reducing power of 5,6-dihydro-6-methylpyrimidin-6-yl radicals.On this basis, the cytosine radicals are better reductants than the corresponding uracil radicals, and the radicals derived by hydrogen atom addition to pyrimidines are stronger reductants than those formed by OH radical addition.
One-Electron Reduction of Nitrobenzenes by α-Hydroxyalkyl Radicals via Addition/Elimination. An Example of an Organic Inner-Sphere Electron-Transfer Reaction
Jagannadham, V.,Steenken, S.
, p. 6542 - 6551 (2007/10/02)
The reaction in aqueous solution of α-hydroxyalkyl radicals with para-substituted nitrobenzenes were studied by using product analysis, electron spin resonance, and pulse radiolysis techniques.At neutral pH the α-hydroxyalkyl radicals are quantitatively oxidized to yield the corresponding ketones or aldehydes and H+, and the nitrobenzenes are reduced to the radical anions.The mechanism of this redox reaction depends strongly on the substituents on the α-hydroxyalkyl radical (the electron donor) and on the nitrobenzene (the electron acceptor).In case of α-hydroxymethyl radical, the reaction proceeds by addition to the nitro group to produce an alkoxynitroxyl radical which can undergo an OH--catalyzed heterolysis to give formaldehyde and the radical anion of the nitrobenzene.With the α-hydroxyethyl radical, both addition and "electron transfer" take place, the fraction of electron transfer increasing with increasing electron-withdrawing power of the substituent.The nitroxyl-type adducts undergo a spontaneous unimolecular heterolysis to give acetaldehyde, H+, and nitrobenzene radical anion.The rate constants ks (from 2 to 5*104 s-1) for this heterolysis increase with increasing electron-withdrawing strength of the substituent if it is on the benzene, and they decrease if the substituent is on the methyl carbon of the nitroxyl.The heterolysis reaction is characterized by low (5-10 kcal/mol) activation enthalpies and strongly negative (-5 to -25 eu) activation entropies, which originate from hydration of a proton in the transition state.From the effect on the activation parameters exerted by substituents on the electron acceptor and on the electron donor parts of the nitroxyl radical it is concluded that the heterolysis reaction proceeds by a push-pull mechanism and is entropy controlled.In the α-hydroxyprop-2-yl radical with substituted nitrobenzenes, the lifetimes of potential adducts of the nitroxyl type are s=2.1*103 s-1.The heterolysis reaction can also be slowed down by making the solvent less polar than water: in 95percent propan-2-ol/5percent water ks=1.5*104 s-1 for R=CN as compared to >106 s-1 in water.
