Radical anions, continued reduction

Reactions of PhCH2Cl and m-02NC6H4CH2Cl with 9 and 10 failed to produce the a-alkylated ketones under similar reaction conditions. The failure of a chain reaction with PhCH2Cl reflects the increased reduction potential of the alkyl halide while, in the case of the latter, the radical anion formed by the facile reduction does not readily undergo the fragmentation step required to continue the chain process. 

On continuous cycling of the potential through the two reduction processes, the magnitude of the current associated with the first reduction process gradually drops whereas that due to the second process increases. This behaviour is expected for an ECE process in which the p-iodonitrobenzene is reduced irreversibly to the nitrobenzene radical anion. Further, the addition of iodide anions to the solution decreases the rate of formation of nitrobenzene as evidenced by noting that the rate at which the peak current associated with the p-iodonitrobenzene reduction decreases less rapidly on successive scans through both reduction processes. The dependence on the iodide concentration in bulk solution suggests that the C step of the ECE mechanism actually consisted of processes (51) and (52),... 

Figure 27. Application of flow cell and UV spectroscopy to study the reduction of aromatic compounds in iV,iV-dimethylformamide/0.1 M BU4NBF4 a) Plot of absorbance at = 556 nm and 732 nm, of the products obtained in the reduction of anthraquinone (T) and anthracene ( ), respectively, as the galvanostatic current to the flow cell is increased and a continuous flow of 5 mL min is maintained. The substrate concentrations are both 0.1 mM and the light path is 1 cm b) and c) The absorption spectra of the product obtained from reduction of anthraquinone and anthracene, respectively, when the galvanostatic current is increased above the maximum required for generating the radical anion. The current is increased from 2.0 to 2.8 mA in steps of 0.2 mA and the development in the spectra is indicated with arrows. Isosbestic points are also indicated. For anthraquinone, the spectra of the radical anion and the dianion could be resolved whereas for anthracene the dianion is protonated and spectra of the radical anion and 9,10-dihydroanthracen-9-ide could be resolved [65].

In nonaqueous aprotic solvents, such as dimethoxyethane [25] or acetonitrile [26,27], the reduction product from tertiary nitroalkanes is the radical anion. Cyclic voltammetric data of 2-nitro-2-methylpropane showed that the electrochemical rate constant was rather low and depended on the size of the supporting electrolyte cation the electrochemical transfer coefficient a was found to be potential dependent [28]. The nitro-t-butyl radical anion is rather unstable (half-life of 0.66s) and decomposes into nitrite ion and t-butyl radical. Continued electrolysis results in the formatrion of di-t-alkyl nitroxide radical [25,27]. 

The path for the formation of the Grignard reagent is believed to involve transfer of an electron from the sea of electrons present in the metal into the lowest unoccupied molecular orbital (LUMO) of the alkyl (aryl) halide. The radical anion/ surface-metal cation can continue with the transfer of a second electron or, depending on steric and electronic circumstances, an alkyl (aryl radical) and halogen anion can be produced. Coupling and elimination products occasionally accompany reduction and those products can be rationalized as being derived from free radicals. The halide anion remains associated with the surface as it does with most reactions in solvents that cannot support ionic materials. The alkyl radical would be free from that constraint. 

It has been found that at 293K the short illumination of Ryiridis RCs by continuous light at low redox potential (prereduced quinones and cytochrome) causes the reduction of HL and partially HM [19] (Fig.IB). Further illumination of RCs at 77K is accompanied by the reduction of HM and partial reduction of BL (or BM) since the bleachings of HM and BL (BM) bands and the developing the characteristic band of radical anion of BChl b at 1060 nm [23] are observed. The illumination of R.viridis RCs frozen in the dark at low redox potential down to 77K induces the selective reduction of HL (Fig.lA) [19]. 

---