Anion coupling

Generally, phenols (as the phenolate anion) couple more readily than amines, and members of the naphthalene series more readily than the members of the benzene series. 

A well-supported mechanism for a radical anion-radical anion coupling as the key step has been proposed. The relative... 

There are a few reports of poly(naphthalene) thin films. Yoshino and co-workers. used electrochemical polymerization to obtain poly(2,6-naphthalene) film from a solution of naphthalene and nitrobenzene with a composite electrolyte of copper(II) chloride and lithium hexafluoroarsenate. Zotti and co-workers prepared poly( 1,4-naphthalene) film by anionic coupling of naphthalene on. platinum or glassy carbon electrodes with tetrabutylammonium tetrafluoroborate as an electrolyte in anhydrous acetonitrile and 1,2-dichloroethane. Recently, Hara and Toshima prepared a purple-colored poly( 1,4-naphthalene) film by electrochemical polymerization of naphthalene using a mixed electrolyte of aluminum chloride and cuprous chloride. Although the film was contaminated with the electrolyte, the polymer had very high thermal stability (decomposition temperature of 546°C). The only catalyst-free poly(naphthalene) which utilized a unique chemistry, Bergman s cycloaromatization, was obtained by Tour and co-workers recently (vide infra). 

Coenzyme A is another adenine nucleotide derivative, with its primary functional group, a thiol, some distance away from the nucleotide end of the molecule. This thiol plays an important role in biochemistry via its ability to form thioesters with suitable acyl compounds (see Box 7.18). We have seen how thioesters are considerably more reactive than oxygen esters, with particular attention being paid to their improved ability to form enolate anions, coupled with thiolates being excellent leaving groups (see Box 10.8). 

Scheme 3.1. Schematic energy level diagram comparing (a) the radical-anion substrate and (b) the radical-anion radical-anion coupling routes for the clcctrodimerization process. Wavy lines indicate an electron transfer step.

The acyloin condensation is closely related to the radical anion coupling forming pinacolate anions two ester radical anions couple to form a dianion, which readily loses two alkoxide ions. The resulting diketone then is reduced by sodium, first to a semidione radical anion, then to the dianion. Finally, aqueous work-up produces the acyloin. Acyloins are convenient precursors for the generation of semidione radical anions. ... 

The electro-synthetic reactions of activated alkenes involve carbon-carbon bond formation, which, after much controversy, is now believed generally to involve radical-anion/radical anion coupling rather than the alternative radical-anion/substrate reaction. The history of this mechanistic debate is well documented168. 

Propargyl dianion (QF I ). This anion can be prepared by dilithiation of allene with BuLi in 1 1 ether/hexane. Use of THF (- 50°) or BuLi/TMEDA results in a mixture of propargylide and allenyl anions. The anion couples readily with alkyl and allyl halides to give terminal alkynes. The intermediate lithium acetylide can also react with various electrophiles.3 Example ... 

Amatore C, Thiebault A, Verpeaux J-N (1989) Unexpected single electron-transfer catalyzed cyclization of prenyl sufore dimer - evidence for radical-anion coupling in the outer-sphere oxidation of prenyl sulfone carbanion. J Chem Soc Chem Commun 20 1543-1545... 

Applying this technique to a series of four substrates, the kinetic data of Table 15 were obtained. Also included are data from homogeneous solution studies on the radical anion of 1,1-diphenyl-ethylene. For the two esters, radical anion coupling seems to be the preferred reaction mode under these conditions, whereas both mechanisms operate for the two nitriles and 1,1-diphenylethylene, k2 being 10-100 times larger than k 2 m these cases. 

Looking back at the preparative results, we are in a position to assess the validity of the mechanism (98c) originally proposed versus the radical anion coupling mechanism [eqn (98a)]. In the macroscale... 

The reverse reaction, the radical-ion coupling, is expected to have low activation energy if the cleavage reaction itself is endergonic (see above). The many known examples of SRN1 reactions [110] attest to the facility of that process. Indeed, in several cases the radical-anion coupling was shown to be diffusion limited [111]. Thus, for the purpose of kinetic analysis the reversibility of cleavage cannot be dismissed a priori. In several systems where the possibility of reversibility has been explicitly tested [29, 99, 102, 106] none was found. 

Gennett and Weaver [192] have found, when studying double-layer effects on electrochemical kinetics in nonaqueous media, that the rate constants of metallocenes M(Cp)2 °, where M = Fe, Mn and Co, were virtually independent of the double-layer structure. However, the structurally similar anionic couple Co(Cp)2 and some other metallocenes have exhibited the expected sensitivity upon changes of the double layer. These differences in behavior of cationic and anionic metallocenes were explained in terms of charge distribution between the cyclopentadienyl (Cp) ring and the metal. 

Application of the Marcus equation for electron transfer affords the electron exchange rate of the molybdenum radical/anion couple. The value is fcge = 3 X 10 L mol s. The high value argues that very little nuclear reorganization is needed to add an electron to the SOMO of the 17e radical. 

The absorbing sites have been discussed in terms of the cation-anion couples in low coordination on the surface, but no hypothesis has been made regarding the nature of the emitting sites. The emitting sites could be (i) extrinsic impurities on the surface, e.g., TMIS or organic materials (ii) point defects such as trapped electron or hole centers or (iii) sites identical or similar to the absorbing sites. These possibihties have been considered by Coluccia (13). 

As an alternative to Eq. (b), radical anions couple to give dimeric dianions ... 

However, given the properties of the films, other pathways may exist for charge transfer where the films play a more active role. As an example, the polymers of concern here contain fixed cationic sites and, as noted before, can function as anion exchange membranes. At sufficiently thick films the usual electrode response for a cationic couple in the external solution can be quenched by the film. However, anionic couples like Fe(CN)g -/3-can enter the films, arid within the films their response becomes that of an electrostatically-bound couple as noted above. In fact, the concentrating effect of the ion exchange membrane can lead to a very sensitive electroanalytical technique for anions. 

Cation-anion couples may react with an acid molecule to give ... 

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