7t-radical anion

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. 

In the case of dissociative electron transfer to aromatic compounds, electron transfer is not necessarily concerted with bond dissociation. The substrate 7t-radical-anion may be an intermediate whose existence can be demonstrated by fast scan cyclic voltammetry in aptotic solvents. At fast scan rates, reversible electron transfer occurs. At slower scan rates, die anodic peak height falls and a second reversible electron transfer step appears due to formation of the radical-anion of the compound formed by replacement of the substituent by hydrogen. Cleavage of the... 

The voltammograms of ETRPyP exhibit reversible waves at Ey2 = 1.00 and —0.65 V in DMF solution, which were assigned to the Ru(III/II) redox pair and to the monoelectronic reduction of the porphyrin ring to the 7t-radical anion, respectively, and a shoulder at = — 1.04 V attributed to the formation of the respective porphyrin dianion (Fig. 38). The monoelectronic reduction of a 5-Clphen ligand on each peripheral ruthenium complex occurs at E = — 1.11 V. Its anodic wave is broad, more intense, and shifted to positive potentials because of the overlap with the anodic wave corresponding to the reoxidation of the porphyrin dianion. 

These two values are in reasonable agreement and confirm unmistakably the following electron transfer from the one-electron reduced metalloporphyrin 7t-radical anion to the fullerene (Eq. 5) ... 

A powerful technique to detect paramagnetic radical species, such as the one-electron reduced 7t-radical anions (Ceo ) and oxidized 7t-radical cation (Ceo ) is electron spin resonance (ESR) (9,10). Studies, regarding the characterization of fullerene intermediates via employing the ESR technique are, however, still somewhat controversial. Absorption spectrophotometry, on the other hand, is been successfully employed in matrix irradiation, silver mirror reduction in tetrahydrofuran (THF), and detection of transients in time-resolved laser photolysis and pulse radiolysis (10,11). 

Addition of base or acid had, however, a significant impact on the stability and yield of the characteristic 1080 nm absorption band. A semilogarithmic correlation between the proton concentration and the intensity of the fullerene 7t-radical anion band (1080 nm) is observed in anaerobic aqueous solutions. This observation has been ascribed to a reversible protonation of Ceo /Y-CD. Experimental proof for this assumption was brought forward by the fact that the 1080 nm absorption, in an alkaline solution (pH 10), diminished upon acidifying (pH 3) and was completely restored upon addition of base (pH 10). The reversible protonation process gives rise to a pKa of 4.5. 

Addition of various concentrations of [60]fullerene, for example, to a ZnTPP solution, resulted in an accelerated decay of the 7t-radical anion (ZnTPP "). The observed rate was linearly dependent on the [60]fullerene concentration, which, in turn, has led to the assumption that the ZnTPP tt-radical anion reacts with [60]fullerene. To confirm a probable electron transfer, the formation of the characteristic C60 absorption in the NIR ( ax = 1080 run) was also monitored. The grow-in rate of the C o " absorption at various wavelengths in the 980-1060 nm range was nearly identical to the decay rate of the MP absorption at 650-750 nm. For example, in the case of ZnTPP 7t-radical anion (ZnTPP "), a bimolecular rate constant of (2.5+1.0) x 10 M s was derived from the ZnTPP " decay (720 nm) and (1.4 1.0) x 10 M s from the Ceo formation (970 nm). These two values are in reasonable agreement and confirm unmistakably the electron transfer from the one-electron reduced metalloporphyrin (ZnTPP) to the singlet ground state of the fullerene ... 

Tin-(IV) porphyrins (Sn P) are very easily reduced to their long-lived 7t-radical anions. Because their reduction potential is only slightly more negative than that of [60]fullerene, it was expected that the electron transfer between these two species, if sufficiently rapid as compared with the decay of the radicals, may lead to the observation of equilibrium conditions. Indeed, we found such an equilibrium with a tin-(IV) porphyrin (Sn" (Ph)3(Py)P). Reduction of this porphyrin resulted in formation of the characteristic 7t-radical anion with absorption in the 700-800 nm range. 

C6oC(COOEt)2 (1) and C6o(C4H9N) (2) Pulse irradiation of an oxygen-free toluene, acetone and 2-propanol solution (8 1 1 v/v) containing C6oC(COOEt)2 resulted in the formation of a distinct absorption pattern in the NIR. This band is ascribed to the 7t-radical anion formed in the general reaction (59,60) ... 

The differential absorption spectrum obtained upon pulse radiolysis of C6oC(COOEt)2 exhibits a maximum at 1040 nm, hypsochromicly shifted by 40 nm relative to the 1080 nm 7t-radical anion band of [60]fullerene. A corresponding blue-shift was also observed for the 7t-radical anion of C6o(C4H9N) (61). This reflects the... 

Pulse radiolytic reductions were conducted without employing a hydrophilic host molecule. Radical-induced reduction of e-C6o[C(COO )2]2 in N2-purged aqueous solution showed that the expected formation of the diagnostic NIR transition band occurs synchronously with the decay of the electron absorption. The fullerene 7t-radical anion, absorbs at 1055 nm, which is 5 nm blue-shifted relative to the analogous reduced ester derivative. 

Obviously the structures and yields of Birch reduction products are determined at the two protonation stages. The ring positions at which both protonations occur are determined kinetically the first protonation or 7t-complex collapse is rate determining and irreversible, and the second protonation normally is irreversible under the reaction conditions. In theory, the radical-anion could protonate at any one of the six carbon atoms of the ring and each of the possible cyclohexadienyl carbanions formed subsequently could protonate at any one of three positions. Undoubtedly the steric and electronic factors discussed above determine the kinetically favored positions of protonation, but at present it is difficult to evaluate the importance of each factor in specific cases. A brief summary of some empirical and theoretical data regarding the favored positions of protonation follows. 

Electrons are transferred singly to any species in solution and not in pairs. Organic electrochemical reactions therefore involve radical intermediates. Electron transfer between the electrode and a n-system, leads to the formation of a radical-ion. Arenes, for example are oxidised to a radical-cation and reduced to a radical-anion and in both of these intermediates the free electron is delocalised along the 7t system. Under some conditions, where the intermediate has sufficient lifetime, these electron transfer steps are reversible and a standard electrode potential for the process can be measured. The final products from an electrochemical reaction result from a cascade of chemical and electron transfer steps. 

When the molecule has an especially weak bond together with a higher energy 7t -otbital, the potential energy scheme for bond dissociation can resemble Figure 4.5(b). The approaching radical-anion electron donor interacts directly with the... 

Reduction of benzenoid hydrocarbons with solvated electrons generated by the solution of an alkali metal in liquid ammonia, the Birch reaction [34], involves homogeneous electron addition to the lowest unoccupied 7t-molecular orbital. Protonation of the radical-anion leads to a radical intermediate, which accepts a further electron. Protonation of the delocalised carbanion then occurs at the point of highest charge density and a non-conjugated cyclohexadiene 6 is formed by reduction of the benzene ring. An alcohol is usually added to the reaction mixture and acts as a proton source. The non-conjugated cyclohexadiene is stable in the presence of... 

For the series of trimethylsilylaryl radical anions, Sipe and West found that the silicon-methyl proton ESR splitting, although not related to the 7t density on silicon alone or to the density on the aromatic carbon to which silicon is bonded, may be related to a combination of the spin densities by the relation ... 

From a synthetic objective it is unfortunate that attempts to produce the dianion of OFCOT by various reduction procedures have not resulted in a stable dianion. Although the organic decomposition products resulting from reduction by alkali metals or sodium naphthalenide are unknown, fluoride ion is produced (126). However, the nine-7t-electron radical anion has been produced at low temperatures by y irradiation of OFCOT, and electron spin resonance (ESR) spectroscopy indicates that it possesses the anticipated planar delocalized D8b structure 55 (127). The unavailability of the dianion... 

Simple olefins do not react with eaq at an appreciable rate, but compounds with an extended 7t-system such as butadiene can also accommodate an additional electron (k = 8 x 109 dm3 mol-1 s 1 Hart et al. 1964). However, as in the case of benzene, the rate is often below diffusion controlled [reaction (23) k = 7.2 x 106 dm3 mol 1 s 1 (Gordon et al. 1977) in THF, the reaction of the solvated electron with benzene is even reversible (Marasas et al. 2003)], and the resulting radical anion is rapidly protonated by water [reaction (24)]. 

Dodecamethylcyclohexasilane gives a radical anion when treated with sodium-potassium alloy in dimethoxyethane-tetrahydrofuran at — 95° C (86). By ESR analysis, it was concluded that the unpaired electron is delocalized equally over all six silicon atoms and so contacts equally all of the protons on the 12 methyl groups of the molecule. Recently, similar radical anions [Si(CH3)2]5T and [Si(CH3)2]7T have been obtained from the five- and seven-membered ring cyclopolysilanes (21,205a). 

In fact, it was possible to prove the formation of the radical ion pair state by transient absorption spectroscopy. Particularly, at the expense of the vanishing H2P/ZnP singlet absorption new features with maxima in the 600-700 nm range as well as at 480 nm grow in. These maxima correspond to the one-electron oxidized 7t-radical cations of H2P (H2P +) and ZnP (ZnP +). Additionally, in the near-infrared region the spectral signatures of the one-electron reduced anion of Ceo are discernible at 1000 nm (Fig. 9.23). 

One of the solvated electrons is transferred into an antibonding 7t -orbital of the aromatic compound, and a radical anion of type C is formed (Figure 17.82). The alcohol protonates this radical anion in the rate-determining step with high regioselectivity. In the case under scrutiny, and starting from other donor-substituted benzenes as well, the protonation occurs in the ortho position relative to the donor substituent. On the other hand, the protonation of the radical anion intermediate of the Birch reduction of acceptor-substituted benzenes occurs in the para-position relative to the acceptor substituent. 

Disubstitution products are obtained when dihalobenzenes (Cl, Br, I) react with aliphatic ketone enolate anions. Conversely, the reactions of o-iodohalobenzenes (X = I, Br, Cl) with the enolate anions of aromatic ketones, such as acetophenone, propiophenone and 2-naphthyl methyl ketone in DMSO yield mainly monosubstitution with the retention of one halogen (Scheme 10.7). The extent of dehalogenation is explained in terms of the energetics of the intramolecular ET from the ArCO-7t-system to the C—X bond in the monosubstituted radical anions proposed as intermediates [19]. 

Within the area of SET-promoted di-7t-methane reactions, recent studies have shown that irradiation of 1-aza-1,4-dienes, such as 32, and the 1,4-diene 34, using AW-dimethylaniline (DMA) as electron-donor sensitizer, leads to production of the corresponding cyclopropane derivatives 33 and 35 resulting from 1-ADPM and DPM rearrangements, respectively, in reactions that take place via radical-anion intermediates (Sch. 11) [25]. 

The sodium donates an electron to the LUMO of the triple bond (one of the two orthogonal 7t orbitals). The resulting radical anion can pick up a proton from the ammonia solution to give a vinyl radical. A second electron, supplied again by the sodium, gives an anion that adopts the more stable trans geometry. A final proton quench by a second molecule of ammonia or by an added proton source (t-butanol is often used, as in the Birch reduction) forms the E-alkene. 

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