Reducing radical anions

Stirring the neutral precursors R2MMR2 [M = Al or Ga R = —CH(SiMe3)2 [20,53] or —C6H2-2,4,6-z-Pr3 (Trip) [4, 54] with alkali metal afforded singly reduced, radical anions [R2MMR2] which could be crystallized as solvent... 

Figure 7 Complete set of 45 bands required to fill the absorption and MCD band envelopes of the phthalocyanine-rmg-reduced radical anion species [ZnPc(—3)] . The absorption spectrum was recorded at 77K, and the MCD spectrum at 40K. The bands were fitted using Gaussian-shaped bands with the identical band centers and bandwidths for pairs of absorption and MCD bands. A weak, Faraday term located at 14 860cm is due to a residual 1% impurity of neutral ZnPc(—2). Experimental data (sohd fine) fitted data (broken line). (Reproduced with permission from J. Mack, Y. Asano, N. Kobayashi, M. J. Stilhnan (2005) J. Am. Chem. Soc. 127 17697-17711. 2005 American Chemical Society)...

Although boranes still produce exciting new molecular species like water-stable B(C6Cl5)3 and associated one-electron reduced radical anions, most work in electroanalytical mechanisms is focused on boronie acid derivatives with a more or less strong interaction with a redox active reaction centre. The most literature-abundant redox active boronie acid is ferrocene-boronic acid (as synthesised first by Nesmeyanov ) with a range of selective redox label applications. 

Firstly, the Ru(bpy)3 + is oxidized at the electrode to the Ru(bpy)3 + cation. This species is then capable of oxidizing the oxalate ( 204 ) in the diffusion layer close to the electrode surface to form an oxalate radical anion ( 204 ). This breaks down to form a highly reducing radical anion (C02 , E° = —1.9 V vs. NHE (126)) and carbon dioxide. The reducing intermediate then either reduces the Ru(bpy)3 complex back to the parent complex in an excited state, or reduces Ru(bpy)3 to form Ru(bpy)3+ that reacts with Ru(bpy)3 + to generate the excited state Ru(bpy)3 , which emits light with -620 nm. 

The electron donors able to drive Equation 8.9 are, for example, anion radicals, Red = M , of aromatic moieties, M, such as the anion radical of naphthalene, and can be generated at various electrodes in different organic solvents (DMF, acetonitrile, tetrahydrofuran). The knowledge of the homogeneous reduction kinetics and thermodynamics of bromoalkanes allows one to predict that a surface-anchored bromoalkyl chain might be reduced and therefore debrominated by reducing radical anions such as that of 2,2 -dipyridyl (E = -2.10 V vs. SCE). 

Most enones are reduced to anion radicals by organo cuprates. It is likely, that this reaction is connected with the alkylation. Both the formation of anion radicals and of conjugate adducts are not observed, when the redox potential of the enone becomes too negative (H.O. House, 1976). 

Water-soluble peroxide salts, such as ammonium or sodium persulfate, are the usual initiators. The initiating species is the sulfate radical anion generated from either the thermal or redox cleavage of the persulfate anion. The thermal dissociation of the persulfate anion, which is a first-order process at constant temperature (106), can be greatly accelerated by the addition of certain reducing agents or small amounts of polyvalent metal salts, or both (87). By using redox initiator systems, rapid polymerizations are possible at much lower temperatures (25—60°C) than are practical with a thermally initiated system (75—90°C). 

Various other observations of Krapcho and Bothner-By are accommodated by the radical-anion reduction mechanism. Thus, the position of the initial equilibrium [Eq. (3g)] would be expected to be determined by the reduction potential of the metal and the oxidation potential of the aromatic compound. In spite of small differences in their reduction potentials, lithium, sodium, potassium and calcium afford sufficiently high concentrations of the radical-anion so that all four metals can effect Birch reductions. The few compounds for which comparative data are available are reduced in nearly identical yields by the four metals. However, lithium ion can coordinate strongly with the radical-anion, unlike sodium and potassium ions, and consequently equilibrium (3g) for lithium is shifted considerably... 

When saturated steroidal ketones are reduced in ammonia, an alcohol is usually present to act as a proton donor and high yields of steroidal alcohols are obtained. Under these conditions, reduction probably proceeds by protonation of the radical-anion (or ketyl) (61), which results from a one electron addition to the carbonyl group, followed by addition of a second electron and proton. Barton has proposed that reduction proceeds via protonation of the dianion (62) arising from addition of two electrons to the carbonyl group. This proposal implies that the ketyl (61) undergoes addition of a second electron in preference to undergoing protonation by the... 

Reductive dunenzation to form fluorinated benzopinacols proceeds m the partly fluormated case either with zinc or by photolysis but is not observed with perfluorobenzophenone [651 (equation 53). Trifluoroacetophenone is reduced electrochemically in dimethylformamide to a stable radical anion, which, m the presence ot lithium ion, rapidly dunerizes to pinacol in higher yield than that available by photoreduction [66] (equation 54)... 

Sulfur diimides may be reduced chemically (by alkali metals) or electrochemically to the corresponding radical anions [S(NR)2] , which... 

Flavin coenzymes can exist in any of three different redox states. Fully oxidized flavin is converted to a semiqulnone by a one-electron transfer, as shown in Figure 18.22. At physiological pH, the semiqulnone is a neutral radical, blue in color, with a A ax of 570 nm. The semiqulnone possesses a pAl of about 8.4. When it loses a proton at higher pH values, it becomes a radical anion, displaying a red color with a A ax of 490 nm. The semiqulnone radical is particularly stable, owing to extensive delocalization of the unpaired electron across the 77-electron system of the isoalloxazine. A second one-electron transfer converts the semiqulnone to the completely reduced dihydroflavin as shown in Figure 18.22. 

Alkali metals in liquid ammonia can transfer an electron to the solvent, leading to so-called solvated electrons. These can add to the aromatic substrate 1 to give a reduced species, the radical anion 3 ... 

Sulfur is known to be easily reducible in nonaqueous solvents and its reduction products exist at various levels of reduction of polysulfide radical anions (S . ) and dianions (Sm2 ) 173], Recently Be-senhard and co-workers [74] have examined the effect of the addition of polysulfide to LiC104-PC. Lithium is cycled on an Ni substrate with Qc=2.7 C cm 2 and cycling currents of 1 mA cm. The cycling efficiency in PC with polysulfide is higher than that without an additive. The lithium deposition morphology is compact and smooth in PC with added polysulfide, whereas it is dendritic in PC alone. 

Packer et al. (1981) found that y-irradiation reduces arenediazonium tetrafluoro-borates to aryl radicals. Packer and Taylor (1985) investigated the y-irradiation of 4-chlorobenzenediazonium tetrafluoroborate by a 60Co source in the presence of 1 alone or I- +13 . The major product in the presence of iodide was 4,4 -dichloroazo-benzene. With I- + 1 ", however, it was 4-chloroiodobenzene. Two other investigations of the reactivity of aryl radicals with iodine-containing species are important for the understanding of the chain process of iodo-de-diazoniation that starts after formation of the aryl radical. Kryger et al. (1977) showed that, in the thermal decomposition of phenylazotriphenylmethane, the rate of iodine abstraction from I2 is extremely fast (see also Ando, 1978, p. 341). Furthermore, evidence for the formation of the radical anion V2 was reported by Beckwith and Meijs (1987) and by Abey-wickrema and Beckwith (1987) (see Sec. 10.11). 

Many anodic oxidations involve an ECE pathway. For example, the neurotransmitter epinephrine can be oxidized to its quinone, which proceeds via cyclization to leukoadrenochrome. The latter can rapidly undergo electron transfer to form adrenochrome (5). The electrochemical oxidation of aniline is another classical example of an ECE pathway (6). The cation radical thus formed rapidly undergoes a dimerization reaction to yield an easily oxidized p-aminodiphenylamine product. Another example (of industrial relevance) is the reductive coupling of activated olefins to yield a radical anion, which reacts with the parent olefin to give a reducible dimer (7). If the chemical step is very fast (in comparison to the electron-transfer process), the system will behave as an EE mechanism (of two successive charge-transfer steps). Table 2-1 summarizes common electrochemical mechanisms involving coupled chemical reactions. Powerful cyclic voltammetric computational simulators, exploring the behavior of virtually any user-specific mechanism, have... 

After addition of an excess of a proton donor, a single two-electron wave is observed. This is because the radical anion protonates to give a radical which is more readily reduced than the parent hydrocarbon... 

---