Aromatic electron transfer reactions

Electron transfer reactions involving alkali metals are heterogeneous, and for many purposes it is desirable to deal with a homogeneous electron transfer system. It was noticed by Scott39 that sodium and other alkali metals react rapidly with aromatic hydrocarbons like diphenyl, naphthalene, anthracene, etc., giving intensely colored complexes of a 1 to 1 ratio of sodium to hydro-... 

Recently, Weissman and his colleagues52 showed that the product is paramagnetic indicating that it results from an electron transfer process giving one unpaired electron to the hydrocarbon ion. Furthermore, they demonstrated30 that electron transfer reactions easily proceed in systems containing aromatic" ions and neutral aromatic hydrocarbon molecules, e.g., naphthalene" + phenathrene - naphthalene -j- phenanthrene". 

Ebenau-Jehle, M Boll, G Fuchs (2003) 2-oxoglutarate NADP oxidoreductase m Azoarcus evansii properties and function in electron transfer reactions in aromatic ring reduction. J Bacterial 185 6119-6129. 

Aromatic diazo compounds can be reduced in water via a radical process (Scheme 11.5).108 The reduction mechanism of arenediazo-nium salts by hydroquinone was studied in detail.109 Arenediazonium tetrafluoroborate salts undergo facile electron-transfer reactions with hydroquinone in aqueous phosphate-buffered solution containing the hydrogen donor solvent acetonitrile. Reaction rates are first order in a... 

Chemiluminescence is defined as the production of light by chemical reactions. This light is cold , which means that it is not caused by vibrations of atoms and/or molecules involved in the reaction but by direct transformation of chemical into electronic energy. For earlier discussions of this problem, see 7 9h Recent approaches towards a general theory of chemiluminescence are based on the relatively simple electron-transfer reactions occurring in aromatic radical-ion chemiluminescence reactions 10> and on considerations of molecular orbital symmetry as applied to 1.2-dioxetane derivatives, which very probably play a key role in a large number of organic chemiluminescence reactions 11>. 

In complex organic molecules calculations of the geometry of excited states and hence predictions of chemiluminescent reactions are very difficult however, as is well known, in polycyclic aromatic hydrocarbons there are relatively small differences in the configurations of the ground state and the excited state. Moreover, the chemiluminescence produced by the reaction of aromatic hydrocarbon radical anions and radical cations is due to simple one-electron transfer reactions, especially in cases where both radical ions are derived from the same aromatic hydrocarbon, as in the reaction between 9.10-diphenyl anthracene radical cation and anion. More complex are radical ion chemiluminescence reactions involving radical ions of different parent compounds, such as the couple naphthalene radical anion/Wurster s blue (see Section VIII. B.). 

Cyclic chain termination with aromatic amines also occurs in the oxidation of tertiary aliphatic amines (see Table 16.1). To explain this fact, a mechanism of the conversion of the aminyl radical into AmH involving the (3-C—H bonds was suggested [30]. However, its realization is hampered because this reaction due to high triplet repulsion should have high activation energy and low rate constant. Since tertiary amines have low ionization potentials and readily participate in electron transfer reactions, the cyclic mechanism in systems of this type is realized apparently as a sequence of such reactions, similar to that occurring in the systems containing transition metal complexes (see below). 

The fluorescence decrease in Figure 1 can be attributed to the consumption of the anthracene photosensitizer during the photosensitization reaction. The photosensitization proceeds by an electron transfer reaction from the anthracene to the initiator, resulting in loss of aromaticity of the of the central ring.17 Therefore, the photosensitization reaction leads to a disruption in the n electron structure of the anthracene, and the resulting molecule does not absorb at 364 nm (nor fluoresce in the 420 - 440 nm region). Hence, the steady-state fluorescence measurements allow the anthracene concentration to be monitored in situ while the photosensitization reaction takes place. 

The importance of tertiary amines in the photochemically induced electron transfer reactions has also been addressed5. Direct irradiation of aromatic or aliphatic amines often leads to the scission of C—N, N—H or C—H bonds that lead to the subsequent chemical reactions by radical pathways6. In this section, photochemical reactions of amines reported since 1978 will be considered with emphasis on photoinduced electron transfer. Photochemical reactions of inorganic and organometallic compounds will not be included unless photochemistry of amine moieties is the primary interest. 

Oxaziranes are in a real sense active oxygen compounds and exhibit many reactions grossly analogous to those of organic peroxides. Thus they undergo one electron transfer reaction with ferrous salts and on pyrolysis they are converted to amides. Oxaziranes are also useful synthetic intermediates since in appropriate cases they may be isomerized to aromatic nitrones which are a convenient source of N-alkylhydroxylamines. The reaction of oxaziranes with peracids also provides a source of nitrosoal-kanes and is in many instances the method of choice for preparation of these compounds. ... 

The cation-radicals ArH+ were detected, but they originated from the fast reaction of a one-electron transfer, which does not affect kinetic constants of the oxidation. The rate constant depends linearly on Brown s a constants of substituents (Dessau et al. 1970). All these data are in agreement with the formation of the strong polar dication of an aromatic hydrocarbon as an intermediate. Because PF salts (in particular the diacetate) are not reductants, the two-electron transfer reaction proceeds irreversibly. 

Alkali, alkaline-earth, and rare-earth metal cations also catalyze electron transfer reactions. Thus, in the pair of Co -tetraphenylporphyrin complex with BQ, no redox reaction takes place, or it takes place too slowly to be determined. The metal cations promote this reaction. For example, in the presence of 80(0104)3, the corresponding rate constant of 2.7 X 10 M s was observed. BQ transforms into benzosemiquinone under these conditions (Fukuzumi and Ohkubo 2000). Zinc perchlorate accelerates the reaction between aromatic amines and quinones (Strizhakova et al. 1985). This reaction results in the formation of charge-transfer complexes [ArNHj Q ]. The complexes dissociate in polar solvents, giving ion-radicals ... 

Last time, electron-transfer reactions were frequently performed in micellar media. Analyzing temperature effects on electron transfer from aromatic amines to coumarins in aqueous Trilon X-100 micelles, Kumbhakar et al. (2006) deduced that the two-dimensional electron-transfer (2DET) model is more suitable to explain the results obtained than the conventional electron-transfer theories. The model is detailed in the article by Kumbhakar et al. (2006) and references therein. 

Trimethylsilyl triflate (McsSiOTf) acts as an even stronger Lewis acid than Sc(OTf)3 in the photoinduced electron-transfer reactions of AcrCO in dichloro-methane. In general, such enhancement of the redox reactivity of the Lewis acid complexes leads to the efficient C—C bond formation between organosilanes and aromatic carbonyl compounds via the Lewis-acid-catalyzed photoinduced electron transfer. Formation of the radical ion pair in photoinduced electron transfer from PhCHiSiMes to the (l-NA) -Mg(C104)2 complex (Scheme 11) and the AcrCO -Sc(OTf)3 complex (Scheme 12) was confirmed by the laser flash experiments [113]. 

Figure 4.5, Potential energy diagrams for the homogeneous electron transfer reaction between an aromatic radical-anion and a second aromatic with a frangible R-X bond, (a) The situation where back electron transfer and bond cleavage have similar free energy of activation, (b) The situation where the RX radical-anicm has high energy and the R-X bond has low dissociation ertergy.

Figure 4.6. Homogeneous dissociative electron transfer reaction between aromatic radical-anions and (a) di-(4-cyanophenyl) disulphide, (b) diphenyl disulphide in dimethyl formamide. Data from ref [31J.

Various substituted cyclopropanes have been shown to undergo nucleophilic addition of alcoholic solvents. For example, the electron transfer reaction of phenylcyclopropane (43, R = H) with p-dicyanobenzene resulted in a ring-opened ether 44. This reaction also produced an aromatic substitution product (45, R = H) formed by coupling with the sensitizer anion. This reaction is the cyclopropane analog of the photo-NOCAS reaction, but preceded it by almost a decade. 

Reactions involving electron transfer. Reaction of free ion radicals, oxidation of anion radicals of aromatic and heteroaromatic hydrocarbons. Usually an energy acceptor is required to be present... 

We turn to the chemical behavior of cycloalkane holes. Several classes of reactions were observed for these holes (1) fast irreversible electron-transfer reactions with solutes that have low adiabatic IPs (ionization potentials) and vertical IPs (such as polycyclic aromatic molecules) (2) slow reversible electron-transfer reactions with solutes that have low adiabatic and high vertical IPs (3) fast proton-transfer reactions (4) slow proton-transfer reactions that occur through the formation of metastable complexes and (5) very slow reactions with high-IP, low-PA (proton affinity) solutes. 

A further significant mechanistic pathway for aromatic nitration can involve a single electron-transfer reaction to an initial radical ion intermediate ... 

The remainder of this section considers several experimental studies of reactions to which the Smoluchowski theory of diffusion-controlled chemical reaction rates may be applied. These are fluorescence quenching of aromatic molecules by the heavy atom effect or electron transfer, reactions of the solvated electron with oxidants (where no longe-range transfer is implicated), the recombination of photolytically generated radicals and the reaction of carbon monoxide with microperoxidase. 

Electron transfer reactions between various aromatic molecules in a vitreous MTHF have been investigated in ref. 79 by the same method as in ref. 12. Reactions having free energy changes from AG° = —0.01 to — 2.75 eV have been studied. For each reaction the electron transfer rate constants, k(R), for various distances, R, between the reacting particles have been found from the reaction kinetics assuming random distribution of the... 

The interest in pyrazine and related ligands has increased recently as a result of a new interest in electron-transfer reactions via ligand aromatic systems.65... 

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