Radical anions cleavage

The formation of RSSR from RS and RS species is particularly relevant in the present context because it is the reverse of the electro-induced radical anion cleavage (equation 76). Actually, the formation of RSSR from reaction (79) is as well studied as the reaction between aryl carbon radicals and anionic nucleophiles, the fundamental step of the SrnI. Equilibrium constants in the range 10 -10 M for reaction (79) were determined for a number of alkyl-type systems in water, although the corresponding values for aryl-type systems are smaller. The rate constants... 

In another study, the reductive C-F bond cleavage of fluoromethylarenes has been investigated in liquid ammonia and DMF by CV and/or redox catalysis [299]. Within a series of 4-cyanotoluenes where the a-carbon bears one, two or three fluorine atoms, the rate of radical anion cleavage increases on going from the trifluoro to the monofluoro derivative, a behavior which reflects the decrease in both C-F BDE and standard potential, °rx/rx.-, as the number of fluorine atoms is diminished (Table 14). 

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). 

NMR and visible spectra have established that a number of S-N anions are present in such solutions.The primary reduction products are polysulfides Sx, which dissociate to polysulfur radical anions, especially the deep blue 83 ion (/Imax 620nm). In a IM solution the major S-N anion detected by NMR spectroscopy is cycZo-[S7N] with smaller amounts of the [SSNSS] ion and a trace of [SSNS]. The formation of the acyclic anion 5.23 from the decomposition of cyclo-Sjl is well established from chemical investigations (Section 5.4.3). The acyclic anions 5.22 and 5.23 have been detected by their characteristic visible and Raman spectra. It has also been suggested that a Raman band at 858 cm and a visible absorption band at 390 nm may be attributed to the [SaN] anion formed by cleavage of a S-S bond in [SSNS]. ° However, this anion cannot be obtained as a stable species when [SsN] is treated with one equivalent of PPhs. 

Electrochemical reductions of sulphones have been reviewed212, and have been discussed at intervals213-215. There is evidence that the cathodic reduction reaction proceeds via a radical anion, followed by a cleavage reaction, as outlined in equation (91)212,213. 

The cleavage of alkyl aryl sulfones by sodium amalgam and alcohols65 probably proceeds also through the intermediacy of a radical anion, followed by splitting to the arylsulfinate anion and an alkyl radical. Both the sulfinate anion and the disproportionation products of the radical have been observed. 

The rigidity of the hexacyclic cage structure of koumine (18) renders some of its chemical behavior quite unusual, for instance, the resistance to Hofmann degradation shown by /Va-acetyldihydrokoumine methyl hydroxide (27). However, owing to the presence of a /J-aromatic imino system in 18, reductive cleavage by sodium-alcohol to yield dihydrokouminol (39) proceeds smoothly. This reaction has been considered to occur through a radical-anion mechanism as indicated in Scheme 12 (27). 

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