4- -nitrobenzenes, electron-transfer reactions

Electron affinities for 35 substituted nitrobenzenes have been reported and provided a comprehensive data set for the examination of substituent effects38. The data were used to derive Taft gas-phase substituent parameters and discussed qualitatively based on frontier orbital molecular theory38. The rate constants for the exo-energetic electron-transfer reactions were found to be close to those predicted by the ADO (average dipole orientation) theory38. 

Romanian scientists compared one-electron transfer reactions from triphenylmethyl or 2-methyl benzoyl chloride to nitrobenzene in thermal (210°C) conditions and on ultrasonic stimulation at 50°C (lancu et al. 1992, Vinatoru et al. 1994, Chivu et al. 2006). In the first step, the chloride cation-radical and the nitrobenzene anion-radicals are formed. In the thermal and acoustic variants, the reactions lead to the same set of products with one important exception The thermal reaction results in the formation of HCl, whereas ultrasonic stimulation results in CI2 evolution. At present, it is difficult to elucidate the mechanisms behind these two reactions. As an important conclusion, the sonochemical process goes through the inner-sphere electron transfer. The outer-sphere electron transfer mechanism is operative in the thermally induced process. 

They can be isolated in good yields by reduction of the nitrobenzene in aqueous ethanolic sodium acetate under reflux, passing around 10% excess electric charge [103]. Any hydrazobenzene formed is rapidly oxidised back to the azobenzene by air during work-up. Azoxybenzene is formed first and then reduced to azobenzene and finally hydrazobenzene at the cathode. A solution electron transfer reaction between azoxybenzene and the hydrazobenzene reforms azobenzene. 

The presence of HC104 in MeCN significantly accelerates the electron transfer from [Ru(bpy)3]2+ to the nitrobenzene derivatives [44]. In this case the ktl values increase parabolically with an increase in the HC104 concentration, as shown in Figure 5. Such a second-order dependence of ktl on [HC104] corresponds to the experimental conditions where K2[H +] 1. Kt[H +] 1 and M = H+ in Eq. 6, when two protons are involved in the photoinduced electron transfer reaction ... 

The ke[ values of photoinduced electron transfer reactions from [Ru(bpy)3]2 + to various nitrobenzene derivatives in the presence of 2.0 mol dm-3 HC104 are listed in Table 1, where the substituent effect is rather small irrespective of electron-withdrawing or donating substituents. A similar insensitivity to the substituent effect is also observed in the acid-catalyzed photoinduced electron transfer from [Ru(bpy)3]2+ to acetophenone derivatives [46,47]. The stronger the electron acceptor ability is, the weaker is the protonation ability, and vice versa. Thus, the reactivity of substrates in the acid-catalyzed electron transfer may be determined by two reverse effects, i.e., the proton and electron acceptor abilities, and they are largely canceled out. Such an insensitive substituent effect shows a sharp contrast with the substituent effect on the acid-catalyzed hydride transfer reactions from Et3SiH to carbonyl compounds, in which the reactivity of substrates is determined mainly by the protonation ability rather than the electron acceptor ability. 

The carbon dioxide anion radical was used for one-electron reductions of nitrobenzene diazonium cations, nitrobenzene itself, quinones, aliphatic nitro compounds, acetaldehyde, acetone and other carbonyl compounds, maleimide, riboflavin, and certain dyes (Morkovnik Okhlobystin 1979). This anion radical reduces organic complexes of Com and Rum into appropriate complexes of the metals in the valence 2 state (Morkovnik Okhlobystin 1979). In the case of the pentammino-p-nitrobenzoato-cobalt(III) complex, the electron-transfer reaction passes a stage of the formation of the Co(III) complex with the p-nitrophenyl anion radical fragment. This intermediate complex transforms into the final Co(II) complex with the p-nitrobenzoate ligand as a result of an intramolecular electron transfer. Scheme 1-89 illustrates this sequence of transformations ... 

Ito T, Shinohara H, Hatta H, Nishimoto S-l (1999) Radiation-induced and photosensitized splitting of C5-C5 -linked dihydrothymine dimers product and laser flash photolysis studies on the oxidative splitting mechanism. J Phys Chem A 103 8413-8420 ItoT, Shinohara H, Hatta H, Fujita S-l, Nishimoto S-l (2000) Radiation-induced and photosensitized splitting of C5-C5 -linked dihydrothymine dimers. 2. Conformational effects on the reductive splitting mechanism. J Phys Chem A 104 2886-2893 ItoT, Shinohara H, Hatta H, Nishimoto S-l (2002) Stereoisomeric C5-C5 -linked dehydrothymine dimers produced by radiolytic one-electron reduction of thymine derivatives in anoxic solution structural characteristics in reference to cyclobutane photodimers. J Org Chem 64 5100-5108 Jagannadham V, Steenken S (1984) One-electron reduction of nitrobenzenes by a-hydroxyalkyl radicals via addition/elimination. An example of an organic inner-sphere electron-transfer reaction. J Am Chem Soc 106 6542-6551... 

The nse of polysnlfide complexes in catalysis has been discnssed. Two major classes of reactions are apparent (1) hydrogen activation and (2) electron transfers. For example, [CpMo(S)(SH)]2 catalyzes the conversion of nitrobenzene to aniline at room temperature, while (CpMo(S))2S2CH2 catalyzes a number of reactions snch as the conversion of bromoethylbenzene to ethylbenzene and the rednction of acetyl chloride, as well as the rednction of alkynes to the corresponding cw-alkenes. Electron transfer reactions see Electron Transfer in Coordination Compounds) have been studied because of their relevance to biological processes (in, for example, ferrodoxins), and these cluster compounds are dealt with in Iron-Sulfur Proteins. Other studies include the use of metal polysulfide complexes as catalysts for the photolytic reduction of water by THF and copper compounds for the hydration of acetylene to acetaldehyde. ... 

In the work by Samec et al. [147, 184, 185], the electron transfer reaction between ferrocene (Fc) in nitrobenzene and Fe(CN)g in water was demonstrated for the first time. Since then, a number of redox reactions taking place at a water-organic solvent interface have been reported [26, 186-190], though kinetic data are few. 

Figure 4. Plots of logfcei for photoinduced electron transfer reactions from [Ru(bpy)3] to nitrobenzene derivatives (Nos, 1-16) in the absence of HCIO4 and acetophenone derivatives (Nos. 17-21) in the presence of HCIO4 (0.10 mol dm ) in MeCN vs. the free energy change of electron transfer, AG°et = the difference between the one-electron redox potentials of [Ru(bpy)3] + and the electron acceptors in the absence of HCIO4 in MeCN, E°ox - red [77].

Acid-catalyzed electron transfer plays an important role in reduction of not only carbonyl compounds but also other substrates such as O2 [90, 91], N02 [92], nitrobenzene derivatives [93, 94], nitrosobenzene derivatives [93, 94] and sulfoxides [95, 96], The ket value for the photoinduced electron transfer from [Ru(bpy)3] + to nitrobenzene increases parabolically with increase in [HCIO4] [94]. This indicates that PhN02 is doubly protonated in the photoinduced electron transfer reaction to give PhN02H2 + (Eq. 9) ... 

Quantitative approaches to describing reactions in micelles differ markedly from treatments of reactions in homogeneous solution primarily because discrete statistical distributions of reactants among the micelles must be used in place of conventional concentrations [74], Further, the kinetic approach for bimolecular reactions will depend on how the reactants partition between micelles and bulk solution, and where they are located within the microphase region. Distinct microphase environments have been sensed by NMR spectrometry for hydrophobic molecules such as pyrene, cyclohexane and isopropylbenzene, which are thought to lie within a hydrophobic core , and less hydrophobic molecules such as nitrobenzene and N,N-dimethylaniline, which are preferentially located at the micelle-water interface [75]. Despite these complexities, relatively simple kinetic equations for electron-transfer reactions can be derived for cases where both donors and acceptors are uniformly distributed inside the micelle or on its surface. 

V. Jagannadham and S. Steenken, One-electron reduction of nitrobenzenes by a-hydroxyalkyl radicals via addition/elimination. An example of an organic inner-sphere electron-transfer reaction, J. Am. Chem. Soc., 106 (1984) 6542. 

Carbon dioxide radical anions, C02 , are commonly used in aqueous chemistry as a reducing agent for metalloporphyrins or as intermediate in the formation of superoxide anion. COf has been reported to undergo efficient electron transfer reactions with methyl violo-gen, quinones, alkyl halides, fumarates, nitro and nitrosobenzenes and chlorinated benzaldehydes. With nitrobenzenes and chlorinated benzaldehydes, electron attachment occurs on the nitro and aldehyde groups, respectively. CO2 radicals have also been reported to add to some unsaturated compounds such as acrylamide and pyridin-3-ol. Efficient hydrogen abstraction from mercaptobenzenes have also been reported. 

One of the few systematic investigations of this matter is a study of the electron-transfer reactions between various acceptors and excited pyrene molecules [13,d], in a series of straight-chain hydrocarbons from n-heptane (C7H10) to squalane (C30H62) the diffusion coefficients of the reactants were measured as well as the rate constants in each solvent. The rate constants deviated markedly from Equation (2.3), based on viscosity, by a factor of up to 10, but much less from Equation (2.1) based on diffusion coefficients. With nitrobenzene as acceptor the ratio of the observed values of k to the Smoluchowski value 4 r was constant within a few per cent. 

These processes are quite rapid with an apparent rate constant which exceeds lO" cm s" [5,6] The only example of electron transfer reaction which has been observed was between the hydrophobic ferro-cinium - ferrocene redox couple in nitrobenzene and the hydrophilic hexacyanoferrate redox couple in water [9]. A more complex mechanism is involved in the case of ion transfer facilitated by an iono-phore[10]. This is the case, for example in the transfer of the alkali and alkaline earth metal cations across a water/nitrobenzene interface facilitated by synthetic neutral cyclic or acyclic iono-phores derived from 3,6-dioxaoctanedicarboxylic acid [11]. 

These reactions were further studied at miCTO-ITIES by Quinn et al. who found the reactions to be reversible [204]. In the early days, most electron-transfer reactions were considered heterogeneous, but, as discussed, the locus of the ET step may occur in one of the adjacent phases. More recently, Sugihara et al. showed that the oxidation of ascorbic acid and chlorogenic acid in water by a zinc porphyrin (5,10,15,20-tetraphenylporphirinato zinc(II)) in nitrobenzene occurs on the organic side of the interface, accompanied by a back proton transfer reaction [205]. According to Osakai et al., one case of truly heterogeneous ET reactions was observed with a cadmium tetraphenylporphyrin in nitrobenzene and ferricyanide in water [206]. 

Osakai, T., S. Ichikawa, H. Hotta, and H. Nagatani, A true electron-transfer reaction between 5,10,15,20-tetraphenylporphyrinato cadmium(II) and the hexacyanoferrate couple at the nitrobenzene/water interface. Anal Sci, Vol. 20, (2004) p. 1567. 

C-Methylation products, o-nitrotoluene and p-nitrotoluene, were obtained when nitrobenzene was treated with dimethylsulfoxonium methylide (I)." The ratio for the ortho and para-methylation products was about 10-15 1 for the aromatic nucleophilic substitution reaction. The reaction appeared to proceed via the single-electron transfer (SET) mechanism according to ESR studies. 

It was mentioned earlier (Sec. 8.6) that for iodo-de-diazoniation no catalyst is necessary because the redox potential of the iodide ion (E° = 1.3 V) is sufficient for an electron transfer to the arenediazonium ion. The reaction was actually observed by Griess (1864 c). Four iodo-de-diazoniation procedures are described in Organic Syntheses. For the syntheses of iodobenzene and 4-iodophenol (Lucas and Kennedy, 1943, and Daines and Eberly, 1943, respectively) KI is used in equimolar quantity and in 1.2 molar excess. However, for 2-bromoiodobenzene and for 1,3,4-triiodo-5-nitrobenzene (replacement of a diazonio group in the 4-position by iodine), up to... 

Heterogeneous electron reactions at liquid liquid interfaces occur in many chemical and biological systems. The interfaces between two immiscible solutions in water-nitrobenzene and water 1,2-dichloroethane are broadly used for modeling studies of kinetics of electron transfer between redox couples present in both media. The basic scheme of such a reaction is... 

This reaction was found to be accelerated by the addition of electron acceptors such as nitrobenzene and m-trifluoromethylnitrobenzene. These electron acceptors accelerate the electron transfer from the carbanion to dioxygen. 

An Iranian group described the synthesis of some [l,3,4]thiadiazolo[2,3-c][l,2,4]triazinones 88 <2002PS2399> and in the course of the synthetic pathway the dihydro derivative 87 was first obtained. These authors found that microwave irradiation of 87 on montmorillonite in the presence of nitrobenzene allowed to accomplish the final oxidative step and yielded the fully conjugated end-product in good yields (50-62%). The reaction as proceeding was interpreted by electron transfer to 89 caused by the microwave irradiation followed by the formation of the intermediate radical 90. 

The mechanism of electrochemical reduction of nitrosobenzene to phenylhydroxylamine in aqueous medium has been examined in the pH range from 0.4 to 13, by polaro-graphic and cyclic voltametry. The two-electron process has been explained in terms of a nine-membered square scheme involving protonations and electron transfer steps565. This process is part of the overall reduction of nitrobenzene to phenylhydroxylamine, shown in reaction 37 (Section VI.B.2). Nitrosobenzene undergoes spontaneous reaction at pH > 13, yielding azoxybenzene471. 

Electron transfer [Eq. (1)] would occur at a rate near the diffusion limit if it were exothermic. However, a close estimate of the energetics including solvation effects has not been made yet. Recent support of the intermediacy of a charge transfer complex such as [Ph—NOf, CP] comes from the observation of a transient (Amax f 440 nm, t =2.7 0.5 ms) upon flashing (80 J, 40 ps pulse) a degassed solution (50% 2-propanol in water, 4 X 10 4 M in nitrobenzene, 6 moles 1 HCl) 15). The absorption spectrum of the transient is in satisfactory agreement with that of Ph—NO2H, which in turn arises from rapid protonation of Ph—NOf under the reaction conditions ... 

It should be mentioned that irradiation of nitrobenzene in aqueous (no alcohol added) hydrochloric acid at room temperature also 5uelds 44—62% 2,4,6-trichloro-and 10% 2,4-dichloroaniline in an undoubtedly complicated reaction 35) very likely also initiated by electron transfer [Eq. (1)]. 

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