Reduction of aromatic hydrocarbons

Silver sulfate has been described as a catalyst for the reduction of aromatic hydrocarbons to cyclohexane derivatives (69). It is also a catalyst for oxidation reactions, and as such has long been recommended for the oxidation of organic materials during the deterrnination of the COD of wastewater samples (70,71) (see WASTES, INDUSTRIAL WATER, INDUSTRIAL WATERTTEATI NT). 

Peter C, Suslick KS (2000) Ultrasound-enhanced reactivity of calcium in the reduction of aromatic hydrocarbons. Ultrason Sonochem 7 53-61... 

Benkeser, R. A. etal., Tetrahedron Lett., 1984, 25, 2089-2092 The use of calcium in 1,2-diaminoethane as a safer substitute for sodium or lithium in liquid ammonia for the improved Birch reduction of aromatic hydrocarbons is described in detail. 

TABLE 3. ORGANOSILANE REDUCTION OF AROMATIC HYDROCARBONS (Continued)... 

Supplemental Reference for Table 3. Organosilane Reduction of Aromatic Hydrocarbons... 

Coming back to solvent reorganization, the reduction of aromatic hydrocarbons in an aprotic solvent such as DMF provides a series of data that may be used for testing the Marcus-Hush model of solvent reorganization13,61-63... 

FIGURE 1.22. Solvent reorganization energies derived from the standard rate constants of the electrochemical reduction of aromatic hydrocarbons in DMF (with n-Bu4N+ as the cation of the supporting electrolyte) uncorrected from double-layer effects. Variation with the equivalent hard-sphere radii. Dotted line, Hush s prediction. Adapted from Figure 4 in reference 13, with permission from the American Chemical Society. 

Peover, M. E. Oxidation and Reduction of Aromatic Hydrocarbon Molecules at Electrodes, in Reactions of Molecules at Electrodes, Hush, N. S., Ed., Wiley New York, 1971, pp. 259-281. 

Cathodic reduction of aromatic hydrocarbons gives 7T-radical anions, which are possible EGBs. However, the PBs normally have low solubilities in polar aprotic solvents, relatively low reduction potentials. 

Fig. 3 Electrochemical and homogeneous standard free energies of activation for self-exchange in the reduction of aromatic hydrocarbons in iV.A -dimethylformamide as a function of their equivalent hard sphere radius, a. 1, Benzonitrile 2, 4-cyanopyridine 3, o-toluonitrile 4, w-toluonitrile 5, p-toluonitrile 6, phthalonitrile 7, terephthalonitrile 8, nitrobenzene 9, w-dinitrobenzene 10, p-dinitrobenzene 11, w-nitrobenzonitrile 12, dibenzofuran 13, dibenzothiophene 14, p-naphthoquinone 15, anthracene 16, perylene 17, naphthalene 18, tra 5-stilbene. Solid lines denote theoretical predictions. (Adapted from Kojima and Bard, 1975.)...

Aptotic solvents can be used for the reduction of aromatic hydrocarbons, particularly the condensed ring systems. Solvents used for the conversion of benzene to cyclohexa-1,4-diene at a mercury cathode under constant current conditions include dimethoxyethanc [45] and N-medtylpyrrolidone [46]. Each solvent contained water as a proton source and tetraethylammonium bromide as supporting electro-... 

Several examples of the use of microelectrodes in highly resistive media exist. The first reported measurements were an examination of the reduction of aromatic hydrocarbons such as perylene in benzene containing tetrahexylammonium perchlorate [57]. Although this electrolyte is presumably in a quite associated state in benzene (or toluene [58]), it does impart sufficient conductivity for electrochemistry to be observed. In subsequent work, this result was confirmed and extended to other low-dielectric-constant solvents [59]. Even voltammetry in hexane has been shown to be possible with a microelectrode [60]. In this sol-... 

In order to understand features of oxidative one-electron transfer, it is reasonable to compare average energies of formation between cation-radicals and anion-radicals. One-electron addition to a molecule is usually accompanied by energy decrease. The amount of energy reduced corresponds to molecule s electron affinity. For instance, one-electron reduction of aromatic hydrocarbons can result in the energy revenue from 10 to 100 kJ mol-1 (Baizer Lund 1983). If a molecule detaches one electron, energy absorption mostly takes place. The needed amount of energy consumed is determined by molecule s ionization potential. In particular, ionization potentials of aromatic hydrocarbons vary from 700 to 1,000 kJ-mol 1 (Baizer Lund 1983). 

A classification of solvents can be developed on the basis of the stability of the radial anion produced by reduction of aromatic hydrocarbons, such as naphthalene and anthracene. The solvent reactions of such anions have been widely studied2 and have generally been found to go by a sequence of reactions in either a protic solvent or in the presence of a proton donor in an aprotic solvent 3... 

More recently, it has been suggested that the mechanism of reduction of aromatic hydrocarbons proceeds via the reaction of Eq. (7.3b) rather than that of Eq. (7.3a),4 but this will have no effect on a classification that is based on the stability of the radical anion produced in the reaction of Eq. (7.1). 

The half-wave potentials (corrected for changes in liquid-junction potential) for the one-electron reduction of aromatic hydrocarbons generally become more positive (the reduction is easier) as the dielectric constant of the solvent increases.44 This is in accord with the direction of the variation in solvation energy of the radical anions that is predicted by the simple Bom theory... 

The effect of proton donors on the reduction of aromatic hydrocarbons has been discussed earlier in this chapter. The importance of potential-pH relations to the understanding and utilization of the redox behavior of these systems has long been recognized extensive potentiometric data have been obtained for only a limited number of organic compounds.50... 

DMF has been widely used as an electrochemical solvent, especially for the reduction of aromatic hydrocarbons.88 The polarography of a number of metal ions in DMF also has been reviewed.89 In general, die voltage range attained in reductions is comparable to acetonitrile and dimethyl sulfoxide, but DMF is less suitable for the study of oxidations. It has been suggested that the cyclic amide, iV-methylpyrrolidone, may have most of the favorable properties of DMF, but with less tendency to hydrolyze.90,91 However, it is less available and more expensive. 

The organosamarium(II) complex Sm( / 5-CsMc5)2 has the bent-metallocene structure, which can be attributed to polarization effects its THF solvate Sm( / 5-C5Mc5)2(TI If )2 exhibits the expected pseudo-tetrahedral coordination geometry. Sm( j5-C5Me5 )i] is a very powerful reductant, and its notable reactions include the reduction of aromatic hydrocarbons such as anthracene and of dinitrogen, as illustrated in the following scheme ... 

Experiments designed to elucidate the role of S in cathodic reduction tend to be just as ambiguous as their anodic counterparts, unless certain precautions are taken. The possible intervention of S in the reduction of aromatic hydrocarbons (Asahara et al., 1968 Benkeser and Kaiser, 1963 Benkeser et al., 1964 Sternberg et al., 1963, 1966, 1967, 1969) in SSEs made up of amines or HMPA (to which up to 65% ethanol can be added without impairing the stability of HMPA too much) as compared to the possible direct processes taking part in protic solvents illustrates the problem. 

The mechanism of the reduction of aromatic hydrocarbons was actually established early by Hoijtink and his co-workers (Hoijtink, 1970) as an ECE mechanism (p. 25) (see also Given and Peover, 1960 Santhanam and Bard, 1966). The two one-electron waves due to formation of anion radical and dianion in an aprotic medium change in a characteristic way upon addition of incremental amounts of a proton donor the height of the first wave increases at the expense of the second one until at sufficiently high concentration only a single two-electron wave is obtained. This behaviour in combination with the HMO calculations referred to above clearly show that the radical anion is protonated to a neutral radioed which is reducible at a less negative potential than the substrate [reaction... 

Reduction ofarenes. Calcium in a mixed solvent composed of methylamine and cthylenediamine is comparable to lithium and an alkylamine for reduction of aromatic hydrocarbons to monoenes. Thus naphthalene is reduced by this newer system to A -octalin (82%) and A -octalin (18%). The method is more efficient than one using calcium and liquid ammonia, although the latter method is considerably improved in rate and selectivity by small amounts of HMPT. 

Reduction of aromatic hydrocarbons. Anthracene is reduced quantitatively to 9,10-dihydroanthraccncby sodium in HMPT with THF as cosolvent. The reaction has been extended to benzanthracene, tetracene, and 9-alkyl- and 9,10-dialkylanthraocnes to give the corresponding mejo-dihydro derivatives. Water or methanol diluted in THF is used as the proton source. ... 

The Birch reduction of aromatic hydrocarbons and ethers to the 2,5-dihydro derivatives proceeds most satisfactorily when the substitution pattern allows the addition of hydrogen to two unsubstituted positions in a para relationship. If this requirement is satisfied, better yields are obtained from more highly substituted aromatic rings than from (say) anisole itself, which affords a substantial amount (20%) of 1-methoxycyclohexene (c/. Scheme 1). Extra substitution presumably hinders protonation at the terminus of the dienyl anion (which would lead to a conjugated diene and overreduction). The utilization of anisole moieties as precursors to cyclohexenones has been of very limited value with many 1,2,3-substitution patterns and more densely substituted derivatives. Compounds (23) to (26), for example, have only been reduced by employing massive excesses (200-600 equiv.) of lithium metal,2 while the aromatic ring in (28) is completely resistant to reduction. ... 

The early applications of fast CV mainly focused on the measurement of the peak-potential separation, AEp, for the reduction or oxidation process of aromatic compounds, to obtain the pertinent standard heterogeneous rate constant k° from the relationship given in Table 2 [22]. The largest k° values of about 4 cm s were found for the reduction of aromatic hydrocarbons such as anthracene at a gold electrode in acetonitrile. The peak-potential separation increased from the 58 mV expected for a reversible process at low v to about 100 mV on going to v values of 10 kV s . This also shows that there is no real need for employing extremely large sweep rates in the determination of k° for the majority of compounds. Rather, it is important to ensure that the measurements at the lower sweep rates are not hampered by the contribution from spherical diffusion if a too small UME is used. 

The Birch reduction of aromatic hydrocarbons or of double bonds with alkali metals in liquid ammonia or amines [37] resembles in yield and selectivity the cathodic reduction with lithium halide as supporting electrolyte (for example, Ref. 38). 

The reduction of aromatic hydrocarbons in DMF to their radical anions has been used to show the reductive properties of a number of TBA metals [16,17,19] and interstitial compounds of graphite [23,24]. 

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