Radical anions chemistry

Ozone, 17 63 depletion, 46 109-110 fluoride, see Trioxygen difluoride Ozonide radical anion, chemistry, 33 76... 

Although cycloadditions have frequently been observed in radical-cation chemistry, this reaction mode is apparently very rare in radical-anion chemistry because of the electron repulsion term. Few examples are known of Diels-Alder dimerizations [355], [2 -I- 2] cycloadditions [356], retro-[2 - - 2] cycloadditions [357], and cyclo-trimerizations [358]. Equally, little is known about electrocyclic reactions, despite their interesting stereochemical course [359]. 

Unlike radical cations, the quantum of chemistry originating from PET-gener-ated radical anions is still limited possibly due to the impending development of a suitable photosystem to initiate photosensitized one-elecron redox reactions in wide array of functionalities. Nevertheless, the radical anion chemistry follows, more or less, the analogous pattern of bond dissociation and addition (electrophilic/radical) reactions as observed for the radical cations. As there are not many examples to describe the separate categories, this section is subdivided... 

Timmons, C.L., and Hess, D.W. Electrochemical cleaning of post-plasma etch fluorocarbon residues using reductive radical anion chemistry. Electrochemical and Solid-State Letters, 7, 302-305, 2004. 

One aspect that reflects the electronic configuration of fullerenes relates to the electrochemically induced reduction and oxidation processes in solution. In good agreement with the tlireefold degenerate LUMO, the redox chemistry of [60]fullerene, investigated primarily with cyclic voltammetry and Osteryoung square wave voltammetry, unravels six reversible, one-electron reduction steps with potentials that are equally separated from each other. The separation between any two successive reduction steps is -450 50 mV. The low reduction potential (only -0.44 V versus SCE) of the process, that corresponds to the generation of the rt-radical anion 131,109,110,111 and 1121, deserves special attention. 

Some of the reactions in this chapter operate by still other mechanisms, among them an addition-elimination mechanism (see 13-15). A new mechanism has been reported in aromatic chemistry, a reductively activated polar nucleophilic aromatic substitution. The reaction of phenoxide with p-dinitrobenzene in DMF shows radical features that cannot be attributed to a radical anion, and it is not Srn2. The new designation was proposed to account for these results. 

The chemistry of polysulfide radical anions S (n = 2-4) was reviewed by Chivers [12] in 1977, including a historical discussion describing the difficult route to the final identification of these ubiquitous and highly colored species. However, since that time considerable progress has been made. Only the species 82, 83, and S6 have been experimentally characterized in detail while the existence of 84 has only been suspected. The nature of the color centers in ultramarine-type solids (82 , 83 ) has been reviewed by Re-inen and Lindner [115]. 

The redox chemistry of several phosphaferrocenes,31,50 l,l -diarsaferro-cene (7),13 the complete series of 2,2, 5,5 -tetramethyl-l,r-diheteroferro-cenes (89, 26, 29, 32),22 and octamethyl-1,1 -diheteroferrocenes (90, 44, 48, 49)22 has been investigated by cyclic voltammetry. These compounds undergo quasi-reversible one-electron oxidations (0/+) to their radical cations and irreversible one-electron reductions (0/-) to their radical anions. The data are summarized in Table VI. 

In contrast to the allyl system, where the reduction of an isolated double bond is investigated, the reduction of extensively delocalized aromatic systems has been in the focus of interest for some time. Reduction of the systems with alkali metals in aprotic solvents under addition of effective cation-solvation agents affords initially radical anions that have found extensive use as reducing agents in synthetic chemistry. Further reduction is possible under formation of dianions, etc. Like many of the compounds mentioned in this article, the anions are extremely reactive, and their intensive studies were made possible by the advancement of low temperature X-ray crystallographic methods (including crystal mounting techniques) and advanced synthetic capabilities. 

Other selected examples include tris(tetramethylethylene diamine-sodium)-9,9-dianthryl 143,154 alkali metal salts of 9,10-bis(diisopropylsilyl)anthracene 144,155 as well as the closely related naked 9,10-bis(trimethylsilyl)anthra-cene radical anion 145.156 This chemistry is further extended to the solvent-shared and solvent-separated alkali metal salts of perylene radical anions and dianions 146, 147,156 while other examples focus on alkali metal salts of 1,2-diphenylbenzene and tetraphenylethylene derivatives, where reduction with potassium in diglyme afforded contact molecules with extensive 7r-bonding, [l,2-Ph2C6H4K(diglyme)] 148.157 Extensive 7r-coordination is also observed in (1,1,4,4 tetraphenylbutadiene-2,3-diyl)tetracesiumbis(diglyme)bis(methoxyethanolate) 149.158... 

Fundamental knowledge on the structures and properties of the ladder polysilanes has accumulated in our research for the past 15 years. Some results were unpredictable, including the silicon double helix structure, the domino oxidation, the formation of persistent radical anions, the Diels-Alder reactions at the 1,4-positions of anthracene, etc. These results let us recognize that the construction of novel structures will open the new chemistry. 

The redox chemistry of [4]radialenes shows similarities as well as differences with respect to [3]radialenes (see elsewhere1 for a more detailed comparison). The simplest [4]radialene for which a redox chemistry in solution is known appears to be octa-methyl[4]radialene (94). It has been converted into the radical anion 94 (with potassium, [2.2.2]cryptand, THF, 200 K) and into the radical cation 94 + (with AICI3/CH2CI2, 180 K)82. Both species are kinetically unstable, but the radical cation is less stable than the radical anion and disappears even at 180 K within 2 hours, probably by polymerization. For the success of the oxidation of 94 with the one-electron transfer system... 

The redox chemistry of stannyl metalcarbonyl clusters has been studied in one case177. CV of the cluster PhSnCo3(CO)i2 shows an irreversible reduction peak —0.74 V vs Ag wire, at 100 mV s 1. Faster scans were not reported. It is concluded that the radical anion [PhSnCo3(CO)i2r is unstable, in contrast to the silicon cluster PhSiCo3(CO)n which gives a stable radical anion (reported lifetime of 2.3 s) at —0.26 V (200 mV s 1). 

The acyl anion chemistry of acylzirconocene chlorides has also been applied to the stereoselective preparation of ( )-a,(3-unsaturated selenoesters and telluroesters (Scheme 5.35) [38]. Although no carbon—carbon bond was formed, this reaction reflects the synthetic interest in ( )-a,(3-unsaturated selenoesters and telluroesters, which are well-known precursors of acyl radicals and acyl anions, respectively. 

The coordination chemistry of phenoxyls in many aspects resembles that of other coordinated radical anions and, in particular, that of semiquinones. We refer... 

Polymer cation radicals, anion radicals and excited state species are all very reactive, so that further chemistry will generally take place. Polymer cation radicals are usually reactive even at temperatures below 77K, and often decompose to produce a polymer radical and a cation, which is often H. ... 

Spin trapping of the superoxide radical anion, as well as that of hydroperoxyl and hydroxyl radicals and related species will be considered later in connection with biological chemistry (pp. 52-54). 

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