Radical species biphenyl anion

First, we examined the efficiency of the initiation process. A solution of buthyllithium was added to a THF solution of 7 at -70°C. The color of the solution turned to red immediately and a strong ESR signal was observed with a well separated hyperfme structure. The observed radical species was identified as the anion radical of 2-butyl-l,l,2,2-tetramethyldisilanyl-substituted biphenyl by computational simulation as well as by comparison with the spectra of a model compound. The anion radical should be a product of a single electron transfer (SET) process from buthyllithium to the monomer. Since no polymeric product was obtained under the above-mentioned conditions, the SET process is an undesired side reaction of the initiation and one of the reasons why more higher molecular weight polymer was observed than expected. ... 

Electron tunneling between organic species was first detected, by direct kinetic experiments, for reactions of the biphenyl anion radical with naphthalene and pyrene [11] and triphenylethylene [12], As is known, upon irradiating vitreous solutions containing biphenyl or pyrene, Py, these acceptors react with electrons to form Ph2 and Py with characteristic optical spectra [13]. Ph2 particles have been found [11] to enter into the electron exchange reactions at 77 K with naphthalene, Nh, and pyrene molecules in mixtures of ethyl alcohol and diethyl ether (2 1). 

The high yield reduction of 1,2-dihalides to produce olefins has been employed to advantage to prepare reactive olefins. Electron transfer in electrochemistry is proportional to the diffusion coefficient, which is related in a much less sensitive way to temperature changes than is chemical reactivity. Thus it may become possible to synthesize and study electro-chemically species whose chemical reactivity is high by working at low temperatures. Electroreduction of 1,2-dibromobenzocyclobutene (144) in acetonitrile or butyroni-trile/TEAP or chemical reduction using the biphenyl radical anion resulted in the formation of benzocyclobutadiene (145)128. Efforts to observe the electrochemically generated anion radical or dianion of benzocyclobutadiene indicated that dimerization to 146 was faster than further reduction (equation 84). 

The reduction of PhsB is known to result in a radical anion PhsB- that is in equilibrium with the dimeric species [PhsB BPhs] ". However, if bulky substituents as in MessB are present and thus prevent attack of nucleophiles or dimerization, a blue-colored stable radical anion can readily be identified. While arylboranes are also known to form purple-colored dianions, the second reduction of simple triarylboranes is typically irreversible. In a recent study, Okada and Oda reported the formation of purple-colored solutions of dimesifylphenylborane dianions [Mes2BAr] (146 Ar = Ph, d-MesSiCeUi, biphenyl) upon extended reaction of Mes2BAr with Na K alloy in THE (Scheme 21). Formation of dianions was confirmed by multinuclear NMR specfroscopy. While the bulky mesityl groups provide chemical sfabihty to the dianions and thus prevent further chemical reactions, the presence of the unsubstituted phenyl group is beheved to allow for effective tt-interactions with the / -orbital on boron. 

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. 

The one-electron reduction of triphenylamine by sodium or potassium gives the biphenyl radical-anion (Iwaisumi and Isobe, 1965). In contrast, triphenylphosphine and potassium give a radical-anion which is thought to be Ph2P—K the same species is formed from diphenyl-phosphine with an excess of potassium (Britt and Kaiser, 1965). More recently, Cowley and Hnoosh (1966) have found that the reduction of triphenylphosphine oxide with sodium in dimethoxyethane gives the biphenyl radical-anion, whereas reduction with potassium gives, in tetrahydrofuran, PhsPO" and, in dimethoxyethane, a spectrum very similar to that reported by Britt and Kaiser but which is attributed to Ph2P(0)K-. 

It has been considered that a coordinated nitrene may give the same reaction and work in our group has shown that this is indeed possible in at least one case [260] (see also Chapter 5). However, it should be also noted that we now know that most reactions of nitro- and nitroso compounds with a metal complex occur through an intermediate electron transfer to the organic compound vide supra). Thus the formation of the radical anion of o-nitro- and/or o-nitrosobiphenyl should be considered as probable during the reaction. The reactivity of such radicals is virtually unknown and it cannot be excluded that these species, and not a nitrene intermediate, are responsible for the carbazole formation. The validity of carbazole formation as an indication for nitrene intermediates has been questioned very early [261]. It was shown that other reactions (including oxidation of 2-amino-biphenyl) can afford carbazole. Thus the results of this test should be taken with caution. 

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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