Radical anions from 1,4-quinone

Electrochemical oxidation and reduction [104] can be a simple, clean method for the generation of a wide variety of radicals, for example, radical anions from quinones and nitrocompounds. 

As we mentioned previously, photoinduced electron transfer at the polarizable liquid I liquid junction manifests itself by photocurrent responses under potentiostatic conditions. The nature of the photoelectrochemical processes is reflected in the basic features of the photocurrent transient. For instance, a homogeneous photochemical reaction followed by the transfer of the products is characterized by a slow increase in the photocurrent on illumination. A typical example can be extracted from the work of Kotov and Kuzmin shown in Fig. 5(a) [64-66]. In this case, protoporphyrin is located in the organic phase in the presence of benzoquinone. On illumination, the quinone is reduced and the radical anion transfers to the water phase. The increasing photocurrent is connected with the flux of the radical anion from DCE to water. 

The micellar residence time of Br was calculated to be 1.5 x 10 s . In like manner, pulse radiolysis of Ag2S04 in sodium laurylsulphate micelles gives Ag2, and the various decay processes may be analysed. Electron-transfer between quinone radical anions and quinones (the former generated from eaq reduction) may occur at the micelle surface in sodium laurylsulphate solution. The encounter rate constant for radical anion plus anionic micelle is an order of magnitude lower than for neutral molecule-micelle interactions because of charge-charge repulsions. " ... 

Weak to moderate chemiluminescence has been reported from a large number of other Hquid-phase oxidation reactions (1,128,136). The Hst includes reactions of carbenes with oxygen (137), phenanthrene quinone with oxygen in alkaline ethanol (138), coumarin derivatives with hydrogen peroxide in acetic acid (139), nitriles with alkaline hydrogen peroxide (140), and reactions that produce electron-accepting radicals such as HO in the presence of carbonate ions (141). In the latter, exemplified by the reaction of h on(II) with H2O2 and KHCO, the carbonate radical anion is probably a key intermediate and may account for many observations of weak chemiluminescence in oxidation reactions. 

Allyl (27, 60, 119-125) and benzyl (26, 27, 60, 121, 125-133) radicals have been studied intensively. Other theoretical studies have concerned pentadienyl (60,124), triphenylmethyl-type radicals (27), odd polyenes and odd a,w-diphenylpolyenes (60), radicals of the benzyl and phenalenyl types (60), cyclohexadienyl and a-hydronaphthyl (134), radical ions of nonalternant hydrocarbons (11, 135), radical anions derived from nitroso- and nitrobenzene, benzonitrile, and four polycyanobenzenes (10), anilino and phenoxyl radicals (130), tetramethyl-p-phenylenediamine radical cation (56), tetracyanoquinodi-methane radical anion (62), perfluoro-2,l,3-benzoselenadiazole radical anion (136), 0-protonated neutral aromatic ketyl radicals (137), benzene cation (138), benzene anion (139-141), paracyclophane radical anion (141), sulfur-containing conjugated radicals (142), nitrogen-containing violenes (143), and p-semi-quinones (17, 144, 145). Some representative results are presented in Figure 12. 

EPR techniques were used to show (Polyakov et al. 2001a) that one-electron transfer reactions occur between carotenoids and the quinones, 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ), and tetrachlorobenzoquinone (CA). A charge-transfer complex (CTC) is formed with a -values of 2.0066 and exists in equilibrium with an ion-radical pair (Car Q ). Increasing the temperature from 77 K gave rise to a new five-line signal with g=2.0052 and hyperfine couplings of 0.6 G due to the DDQ radical anions. At room temperature a stable radical with y=2.0049 was detected, its... 

S. Sinnecker, E. Reijerse, F. Neese and W. Lubitz, Hydrogen bond geometries from paramagnetic resonance and electron-nuclear double resonance parameters Density functional study of quinone radical anion-solvent interactions, J. Am. Chem. Soc., 2004, 126, 3280. 

Nirofuran compounds are also effective anti-parasitic drugs. Nifurtimox, for example, is used to treat Chagas disease (caused by Trypansoma cruzi) but has side effects. In exploring the use of alternatives to nifurtimox, Olea-Azar et al. have examined radical formation from two analogues. Radical anions were observed upon electrolytic reduction of the compounds and a nitroxide, believed to be the glutathionyl radical-adduct, was detected upon electrolysis in the presence of DMPO and GSH. Radical adducts were also detected upon incubation of one of the analogues with microsomes from T. Cruzi.m A novel endo-peroxide reductase has been isolated from T. Cruzi. Whereas the flavoenzyme was found to reduce quinones to their semiquinones, nifurtimox underwent a direct, two-electron reduction, without the formation of radicals.129... 

In the electroreduction of aromatic hydrocarbons, nitro compounds, and quinones in aptotic solvents, the first step is the transfer of an electron from the electrode to form a radical anion. Once the radical anion is formed, electron repulsion will decrease the facility with which a second electron transfer occurs. But solvation and ion pairing diminish the effect of electron repulsion and tend to shift the reduction potential for the addition of the second electron to more... 

In a previous study we have found that, at low temperature, PS-I electron transfer is largely blocked away from A, and that the state (P-700+, A, ) decays with a half-time of 130us. Analysis of the absorption spectrum of that state showed that A, is presumably a quinone radical anion (Brettel et al, 1986). Chemical analysis, following separation by HPLC, has shown that phylloquinone (a naphthoquinone also named vitamin Kj) is the only quinone present in PS-I. We have found 2 moles of phylloquinone per PS-I. Extraction with dry hexane does not change the electron transfer reactions this treatment only extracts only one phylloquinone per PS-I (Biggins and Mathis, 1987). 

Kulikov A.V., Bogatyrenko, V R., Melnikov, A.V., Syrtzova, LA. and Likhtenshtein, G.I. (1979) Determination of distance between cation radical of bacteriochlorophyl dimer and anion of quinone in photosynthetic reaction center from R. rub mm. Biofisika 24, 178-185. 

Hydro quinone transforms in the presence of irradiated nitrite to yield ben-zoquinone and hydroxybenzoquinone [78,79]. At the irradiation wavelength adopted in the cited works (365 nm), hydroquinone direct photolysis should be limited and benzoquinone most likely forms upon reaction between hydroquinone and hydroxyl (reactions 44 and 45 hydroquinone absorbs radiation at A, < 320 nm). Hydroxybenzoquinone is likely to be a product of benzoquinone photolysis. No nitration or nitrosation intermediates of hydroquinone were observed in the presence of nitrite under irradiation, differently from the cases of resorcinol and catechol [78,79]. The reaction between hydroquinone and nitrogen dioxide is, however, quite rapid [106,115], as confirmed by the marked inhibition of phenol nitration upon nitrite photolysis by added hydroquinone [62], The point is that the reaction between hydroquinone and NO2 mainly yields benzoquinone [62], Another interesting feature in the case of hydroquinone is the formation of the fairly stable semiquinone radical anion upon reaction between benzoquinone and depro-tonated hydroquinone. The spectrum of the resulting solution shows the typical absorption bands of the semiquinone at 308, 315, 403, and 430 nm [79]. 

If the original semiquinone radical QH is polarized, the semi-quinone radical anion derived from eq. 52 can be expected to retain much of the initial polarization. Thus in the CIDEP studies of the photoreduct ion of quinones in triethylamine solution, the primary photochemical process was thought to involve the possible exciplexes (42) ... 

Trimethylsilylphenyltelluride could also be used to efficiently bis-silylate quinones to the corresponding bis-protected hydroquinones (Scheme 62) [ 173]. The reaction required two equivalents of the silyltelluride and diphenylditel-luride was also isolated. The proposed mechanism is slightly different from above, featuring an initial single electron-transfer to form the quinone radical-anion, which was presumably silylated to form phenoxyl radical 192. Subsequent reaction with trimethylsilylphenyltelluride delivered 193 and diphenylditelluride. 

The radical-pair state P I is also formed if unreduced reaction centers are excited with a short flash, but it then decays with a time constant of about 200 ps [54,70-74]. The rapid decay of the transient state presumably reflects electron transfer from I to (Fig. 1), because it is prevented if the quinone is already reduced or is extracted from the reaction centers. The transient absorption changes suggest that I is a BPh 7r-radical anion, which interacts with a nearby BChl [75-77], The absorbance changes in a band associated with the BChl decay with somewhat different kinetics from those in bands associated with the BPh or BPh , perhaps because they reflect nuclear motions in the electron carriers or the surrounding protein [75]. The possible role of the BChl in the initial transfer of an electron from P to the BPh will be discussed below. 

---