Coupling of aromatic hydrocarbons

That the reaction with a lower rate constant is taking place preferentially and that the rate increases during the reaction are phenomena that can also occur with parallel reactions. As an example, Wauquier and Jungers (48), when studying competitive hydrogenation of a series of couples of aromatic hydrocarbons on Raney-nickel, have observed these phenomena for the couple tetraline-p-xylene (Table I). The experimental result was... 

Representative couplings of aromatic hydrocarbons are summarized in Table 10. Alkyl-substituted aromatic hydrocarbons can be coupled to diphenyls and/or diphenylmethanes depending on their substitution pattern (Table 10, numbers 1-6). Reactions occur according to Scheme 9, paths (a) and (c). 

Nitromethane is one of the few solvents useful for anodic reactions, among them anodic coupling of aromatic hydrocarbons. Its application for cathodic reactions is limited, but in some cases in which its unreactivity toward certain active halogen compounds is valuable it may be used. The liquid range of nitromethane is —29 to 101 °C its dielectric constant is 37, but the dissociation of salts is not as high as could be expected. 

Representative couplings of aromatic hydrocarbons are summarized in Table 1. Alkyl-substituted aromatic hydrocarbons can be coupled to diphenyls and/or diphenylmethanes depending on their substitution pattern (Table 1, numbers 1-6). The initially formed radical cation I [Eq. (3)] reacts with the starting compound to the diphenyl (II) (Eq. (3), path a] or loses a proton to form a benzyl radical [Eq. (3), path b], which after oxidation to the cation undergoes an electrophilic aromatic substitution at the starting compound to form the diphenylmethane (III). A low charge density on an unsubstituted carbon atom of I favors path a, whereas a low charge density on a substituted carbon atom favors path b[4]. 

MO) with the protons in the nodal plane. The mechanism of coupling (discussed below) requires contact between the unpaired electron and the proton, an apparent impossibility for n electrons that have a nodal plane at the position of an attached proton. A third, pleasant, surprise was the ratio of the magnitudes of the two couplings, 5.01 G/1.79 G = 2.80. This ratio is remarkably close to the ratio of spin densities at the a and (3 positions, 2.62, predicted by simple Hiickel MO theory for an electron placed in the lowest unoccupied MO (LUMO) of naphthalene (see Table 2.1). This result led to Hiickel MO theory being used extensively in the semi-quantitative interpretation of ESR spectra of aromatic hydrocarbon anion and cation radicals. 

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.). 

Cyanation of aromatic hydrocarbons, also a carbon-carbon coupling reaction, is achieved in the case of anthracene in MeCN-Et4NCN to yield 54% 9,10-dicyanoanthracene [165]. The cyanation is simplified when it is carried out in an emulsion system (aqueous sodium cyanide, dichloromethane, and TBAHSO4). Its synthetic utility in this mode has been demonstrated for the preparation of 4-alkoxy-4-cyanobiphenyls, a class of liquid crystals [166]. 

Radical ion pairs also react by proton, atom, or group transfer. We illustrate proton transfer in reactions of aromatic hydrocarbons with tertiary amines. These reactions cause reduction or reductive coupling. In the reduction of naphthalene, the initial ET is followed by H" transfer from cation to anion, forming 67 paired with an aminoalkyl radical the pair combines to generate... 

Tertiary amines have also been employed in electron transfer reactions with a variety of different acceptors, including enones, aromatic hydrocarbons, cyanoaro-matics, and stilbene derivatives. These reactions also provide convincing evidence for the intermediacy of aminoalkyl radicals. For example, the photoinduced electron transfer reactions of aromatic hydrocarbons, viz. naphthalene, with tertiary amines result in the reduction of the hydrocarbon as well as reductive coupling [183, 184]. Vinyl-dialkylamines can be envisaged as the complementary dehydrogenation products their formation was confirmed by CIDNP experiments [185]. 

Cation (anion) radicals of aromatic hydrocarbons should in principle be strong 7r-acceptors (ir-donors) and it is interesting to speculate that CT complexes between substrate and radical ion might play a role in coupling reactions, such as the biaryl coupling mechanism shown in eqn (57). A CT complex of the [ArH—ArH] + type would certainly be more difficult to oxidize than ArH. Thus ArH might be protected from oxidation, and coupling within the complex would take place instead. 

Reductive coupling, in aprotic solvents, of aromatic hydrocarbons with CO2 has long been known. Reduction of naphthalene in DMF in the presence of COo leads to 1,4-dicarboxy-1,4-dihydronaphthalene ( 50%) [246]. Under similar conditions, phenanthrene gives fra725-9,10-dicarboxy-9,10-dihydrophenanthrene ( 30%) [246]. Carboxylation of aromatic halides can be achieved in aprotic solvents either by using transition metal catalysts or by direct reduction using sacrificial anodes in undivided cells (cf. Chapter 8). 

The vibronic coupling in the radical and radical cation of aromatic hydrocarbons is studied by photoionizing the corresponding anion and neutral molecules, respectively. The vibronic Hamiltonian of the final states of the ionized species is constructed in terms of the dimensionless normal coordinates of the electronic ground state of the corresponding (reference) anion or neutral species. The mass-weighted normal coordinates ) are obtained by diagonalizing the force field and are converted into the dimensionless form by [68]... 

A Majcherczyk, C Johannes. Radical mediated indirect oxidation of a PEG-coupled polycyclic aromatic hydrocarbon (PAH) model compound by fungal laccase. Biochim. Biophys. Acta-General Subjects 1474(2) 157-162, 2000. 

Hydrocarbons — Up until now, degradation of aromatic hydrocarbons has been limited to monocyclic representatives — particularly toluene, and there is only circumstantial evidence so far for the degradation of naphthalene and phenanthrene coupled to sulfate reduction under anaerobic conditions (Coates et al. 1996a) and the degradation of naphthalene, phenanthrene, and pyrene under denitrifying conditions (McNally et bal. 1998). This issue has been discussed more fully in Chapter 6, Section 6.7.3.2, and in the context of BTEX remediation in Section 8.2.6.I. 

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