Radical anion generation

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). 

The ESR spectrum of the pyridazine radical anion, generated by the action of sodium or potassium, has been reported, and oxidation of 6-hydroxypyridazin-3(2//)-one with cerium(IV) sulfate in sulfuric acid results in an intense ESR spectrum (79TL2821). The self-diffusion coefficient and activation energy, the half-wave potential (-2.16 eV) magnetic susceptibility and room temperature fluorescence in-solution (Amax = 23 800cm life time 2.6 X 10 s) are reported. 

Commercially available anhydrous diethyl ether is distilled under nitrogen from a solution of the sodium benzophenone radical anion generated by treating a solution of 10 g of benzophenone and 1 L of ether with 10 g of sodium ribbon until a dark blue or purple color persists. 

Scheme 12.7. Carbon Alkylation via Nitroalkane Radical Anions Generated by...

Tile ESR spectra of the radical anions, generated by one-electron reduction of the a-oxothioketone 173 and the dithiete 172, were determined, and spin densities were calculated from the coupling constants and, especially, from the anisotropic values (87CB575). 

We have seen similar radical anions generated from ketones in pinacol reduction with sodium or magnesium (p. 218), and also from esters with sodium in the acyloin condensation (p.218). 

Simple 1,2,3-thiadiazoles show three absorption bands in the ultraviolet (UV) 211-217 (emax 4380-5300), 249-253 (1460-2100), 290-294 (195-245) nm <1996CHEC-II(4)289>. The ESR spectrum for the radical anion generated by the electrochemical reduction of the 1,2,3-thiadiazolium ion 8 has been reported. A number of 5-substituted derivatives were also examined and the splitting constants in the ESR spectrum were analyzed <1998MRC8>. 

Reduction of the aromatic nucleus in AjjV-dimethylbenza-mide occurs by an initial single electron transfer to give a radical anion. Protonation of the radical anion generates a radical and a second electron transfer gives the amide enolate 1. Protonation of the cross-conjugated trienolate moiety in 1 occurs carbonyl group to give the cyclohexa-1,4-diene 2. ... 

Similarly, radical 128 couples with dicyanobenzene radical anion generating the substitution product 133 after loss of cyanide ion. 

Lloyd D, Pedersen JZ (1985) Metronidazole anion radical anion generation in vivo in Trichomonas vaginalis oxygen quenching is enhanced in a drug-resistant strain. J Gen Microbiol 131 87-92... 

Bipyridyl (20) (63AHC(2)179) is synthesized by oxidative dimerization of pyridine over hot Raney nickel, while 4,4 -bipyridyl (21) (B-79MI10705) is made by free radical coupling of the pyridine radical anion generated by sodium in liquid ammonia, followed by air oxidation (Scheme 6). Quaternization of (21) with methyl chloride gives paraquat while reaction of (20) with 1,2-dibromoethane gives diquat. 

Presumably the radical anion generated by the primary electron transfer fragments to a radical anion pair. Back electron transfer to the sensitizer cation radical generates the allyl cation from which geometric isomerization and structural rearrangement can be easily accomplished. 

If a radical anion generated by electron transfer isomerizes before back electron transfer, this photoreaction provides an additional route for geometric isomerization, eq. 80 (242) ... 

Radical anions generated from ketones (or aldehydes) are referred to as ketyl anions. Common methods for generation of >C=0 are summarized in Figure 1. 

In 1990, Tanko and Drumright51 presented evidence that the ring opening of the radical anion generated from phenyl cyclopropyl ketone (Scheme 15) is reversible, with an... 

The electrocarboxylation of aldehydes and ketones leads to the corresponding a-hydroxycarboxylic acids that can easily be converted into carboxylic acids via a hydrogenation reaction [7]. It has been reported that the electrocarboxylation of aromatic ketones occurs through the reaction of C02 onto the activated carbon atom of the carbonyl group of the ketyl radical anion generated upon electron transfer to the ketone [7]. Otherwise, the aforementioned intermediate is likely to be a resonance hybrid (see Equation 12.23), and its electrophilic reaction with C02 may take place both at the carbon or the oxygen atom [42, 43]. 

Direct formation of excited states by reaction of SOi,- with radical anions generated at the electrode does not appear to be an important step in the reductive oxidation ECL of polyaromatic fluorophores. 

FIGLfRE 15. ESR spectrum of the radical anion generated from the reaction of 5-nitrothiophene-2-carboxaldehyde with germylhthiums.l2-crown-4. From Reference 197 with permission from Gordon and Breach Publishing, copyright Taylor Francis... 

The chemistry of radical anions, generated by PET processes, is less developed than that of radical cations, possibly due to the nonavailability of suitable photosystems to initiate photosensitized one-electron redox reactions. 

Fast-scan cyclic voltammetry (10 mVs-1 to 106 Vs-1) was used to measure the rate constants of C—X cleavage, which are extremely fast. The technique was applied to measure rate constants of the order of submicrosecond half-lives. For example, the radical anions generated at the electrode surface were determined to have a half-life ranging from less than 100 ns in the case ofp-bromoacetophenone to 70 ms for m-nitrobenzyl chloride189. The method complements the redox catalytic method developed by Saveant and co workers190. 

These various processes have been recently reviewed (Davidson, 1979 Wasserman and Murray, 1979). The discovery that the reactions of excited states can occur by an electron transfer raises the question as to whether these reactions participate in the Type I reaction by providing a source of radicals or whether the radical anion generated in the reaction can reduce oxygen giving the superoxide ion which reacts with the radical cation to give products as indicated in (36) and (37). 

The rate of the reaction of diazodiphenylmethane radical anion generated in acetonitrile at a mercury electrode"... 

Ueda et al. reported a tandem radical addition-cycUzation reaction in aqueous media [184]. This reaction was initiated by single-electron transfer from indium to an alkyl iodide. Fragmentation of the iso-propyl iodide radical anion generated the iso-propyl radical, which triggered the addition/cyclization tandem. Final SET and in situ hydrolysis delivered cyclic sulfonamides in good yield but low stereoselectivity. 

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