Reducing biphenyl radical anions

Thiophene is not reducible by direct electroreduction, but by indirect reduction (Chapter 29) in DMF using biphenyl radical anion as electron transfer agent it is possible to reduce thiophene to 2,5-dihydrothiophene and tetrahydrothiophene in high yield [192] thiophenes substituted at C-2 with carboxyl, however, may be reduced by direct electrolysis to the 2,5-dihydro derivatives [193]. 

The trimethylsilyl radical produced either rapidly dimerizes or reacts with solvent so that very clean ESR spectra of the radical anion, with minimum interactions with the counterion, can be obtained (116). Further reduction to dianions is very slow, and exhaustive reduction to anion radicals minimizes problems associated with exchange between anion radicals and unreduced substrate (115). It now appears that the solvent HMPA greatly facilitates the one-electron reduction, not only for trimethylsilylsodium, but also for organolithium and magnesium reagents (110). It was found that 0.1F solutions of methyl-, n-butyl-, or f-butyllithium or benzylmagnesium chloride will quantitatively reduce biphenyl to its radical anion in less than 10 minutes (110). 

The previously discussed characters will influence the electron transfer rates implying anions. One of the simplest examples was given by the rate constant difference observed in reactions in which pyrene (Py) reacts with an electron or an electron-cation pair [44,45]. The same type of difference was measured in the exchange between the radical anion of biphenyl (B) and pyrene (Py) [46]. The reduced reactivity is the consequence of the cation proximity in the ion pair. 

In contrast, C-substituted benzenes like biphenyl 7.96 are reduced to 3-substituted cyclohexa-1,4-dienes 7.99, and this too fits the analysis. The Hiickel coefficients for the SOMO of the radical anion 7.97 also reflect the total 7r-electron distribution, since the other three filled orbitals lead to a more or less even distribution of 7i-electron population. So, regardless of whether it is the Coulombic or the frontier orbital term that is more important, both contributions lead to protonation at C-4 to give the radical 7.98. Reduction and protonation of this intermediate (or possibly a mixture with the 1-protonated isomer) leads to the observed product 7.99. Further reduction of this molecule then takes place, but now the benzene ring is an X-substituted one. The major final product, accordingly, is the hydrocarbon 7.100, which has been reduced 1,4 in one ring and 2,5 in the other. 

Addition of electron acceptors to acetonitrile confirmed the presence of a transient reducing species through the formation of radical anions of solutes such as biphenyl [21a, 22], pyrene, trans-stilbene, and so forth [21b]. This reducing species has a broad absorption with Amax at 1450 nm, and was shown to exist in the monomeric and dimeric forms (see Eq. 36) from the effect of temperature on the absorbance at - max [21b]. The monomeric form is responsible for the 1450 nm peak, whilst the dimeric form exhibits a weak maximum at 550 nm superimposed on the tail of the monomeric band. The enthalpy change accompanying the reaction of Eq. 36 has been measured to be —34.9 kJ moE [21b] so that CH3CN is the dominant reducing species at room temperature. 

Hoijtink et al. [27] also developed an alternative method of generating anionic species, which was improved by Szwarc et al. [28]. The technique involves potentiometric titration of aromatic compounds with a standard solution of Na-biphenylide. The extremely negative reduction potential of biphenyl assures that most of the common aromatics can be reduced to at least their respective radical anions. The values of the thermodynamic reduction potentials are generally obtained from the potentiometric titration curve. As all experiments are usually carried out in ethereal solutions, such as tetrahydrofuran (THF) or dimethoxyethane, problems of follow-up processes are less severe. Later, Gross and Schindewolf [29] reported on the potentiometric titration of aromatics using solvated electrons in liquid ammonia. 

By contrast, C-substituted benzenes are reduced to 3-substituted cyclohexa-1,4-dienes,343 and this too fits nicely into a frontier orbital analysis. We can use biphenyl (430) as an example. The Hiickel coefficients for the SOMO of the radical anion are shown on 431. Since this molecule does not have heteroatoms... 

DCA sensitisation of tri-1-naphthyl phosphate or di-l-naphthyl methyl-phosphonate results in formation of l,r-binaphthyl. The reaction occurs only in compounds with at least two naphthyl substituents linked by an 0-P(0)-0 chain. Reaction involves intramolecular face-to-face dimerisation of the two naphthyl units within the radical cation (445), followed by elimination of the 1,1 -binaphthyl radical cation, which is subsequently reduced by the DCA radical anion. In a related reaction DCA-sensitisation of bis(3,4-methylenedioxy-phenyl) methylphosphonate in acetonitrile gives 2-(3,4-methylenedioxyphenyl)-4,5-methylenedioxyphenyl methylphosphonate whereas direct irradiation in methanol gives bis(3,4-methylenedioxy)biphenyl as an additional product. " ... 

The electronic absorption spectra of unsubstituted and substituted p-oligophenyls have been reported [66-68]. Each oligomer shows an intense absorption in the ultraviolet region. The band is assigned to the iT-ir transition. The absorption maxima of biphenyl, p-terphenyl, p-quaterphenyl, p-quinquephenyl, and p-sexiphenyl are listed in Table V. The electronic absorption spectra of p-oligophenyls in the radical-anion and dianion states have been studied [68-75]. Furukawa et al. [73] showed that neutral terphenyl is stepwise reduced to its radical anion and dianion in tetahydro-furan by using sodium (Na). In these reduction processes, the reactions... 

PA anion radical rapidly reduced 4-bromobiphenyl (4-BB) to biphenyl in 0.1 H CTAB with an enhanced rate compared to isotropic solvent (Table 1). Quantitative bulk electrolytic reduction of 0.02 mmol of 4-BB in 25 mb 0.1 M CTAB was effected on stirred mercury pool electrodes in 2.5 h with 20 % decomposition of the catalyst. Time for complete conversion to biphenyl and amount of catalyst decomposed were significantly smaller compared to similar experiments in surfactant-free N,N-dimethyl-formamide (DMF) . Diffusion controlled CV and chronocoulometric data for 0.2 mM 9-PA in 0.1 H CTAB were used to obtain an apparent diffusion coefficient (D ) of lO cm s-. This is much too large to attribute to a diffusing micelle-bound species. Furthermore, at scan rates (v) below 5 mV s i, CV s for the 9-PA anion radical were not diffusion controlled as at higher v, but had a symmetric peak shape attributable to a thick surfactant layer at the surface of the electrode. Thus, at the potential required (-2.2 V vs SCE) to reduce 9-PA in 0.1 M CTAB, the catalytic reduction of 4-BB takes place in a thick, spontaneously organized surfactant film on the electrode surface. In addition to voltammetric results , support for existence of a thick film comes from differential capacitance, ellipsometry , and reflectance infrared spectroscopy . 

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