One-Electron Reduction of Nitrobenzenes by α-Hydroxyalkyl Radicals via Addition/Elimination. An Example of an Organic Inner-Sphere Electron-Transfer Reaction

Article abstract of DOI:10.1021/ja00334a015

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 k_s (from < 10² 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 <= 1 μs if the substituent R is less elecron donating than OH.However, if R=NH2, an adduct is formed which undergoes spontaneous heterolysis with k_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 k_s=1.5*10⁴ s^-1 for R=CN as compared to >10⁶ s^-1 in water.