Salts electron transfer

Macrae and Wright (96) demonstrated that visible light irradiation of xanthene dyes (eosin, erythrosin, rhodamine B, or RB) in ethanolic solutions of 4-(N,N-diethylamino)benzene-diazonium chloride (as the zinc chloride double salt) resulted in decomposition of the diazonium salt. Electron transfer from the dye excited state(s) to the diazonium salt was postulated and dye-diazonium salt ion pair formation in the ground state was shown to be important. Similar dyes and diazonium salts were claimed by Cerwonka (97) in a photopolymerization process in which vinyl monomers (vinylpyrrolidone, bis(acrylamide)) were crosslinked by visible light. Initiation occurs by the sequence of reactions in eqs. 40-42 ... 

SCHEME 12.7 Photolysis of diaryliodonium salts—electron transfer. 

Borg, R.M., Heuckenroth, R.O., Lan, A.J.Y., Quillen, S.L., and Mariano, P.S., Arene-iminium salt electron transfer photochemistry. Mechanistically interesting photoaddition processes, /. Am. Chem. Soc., 109, 2728,1987. 

Mariano, P.S., Stavinoha, J., and Bay, E., Photochemistry of iminium salt. Electron transfer mechanism for singlet quenching and photoaddition of n-donating ethers and alcohols. Tetrahedron, 37, 3385,1981. 

Using the electron transfer definition, many more reactions can be identified as redox (reduction-oxidation) reactions. An example is the displacement of a metal from its salt by a more reactive metal. Consider the reaction between zinc and a solution of copper(If) sulphate, which can be represented by the equation... 

The preparation of 5-azothiazoles uses the nucleophilic character of C-5 carbon in reaction with the appropriate diazonium salt (402, 586). These 5-azothia2oles form 1 1 complexes with Ag (587). 2-Amino-4-methyl-5-arylazothiazoles give reduction waves involving two-electron transfer the Ej/ values correlate to the angle between the thiazole and phenyl rings (588). 

Fig. 20. Proposed photochemical mechanisms for the generation of acid from sulfonium salt photolysis. Shown ate examples illustrating photon absorption by the onium salt (direct irradiation) as well as electron transfer sensitization, initiated by irradiation of an aromatic hydrocarbon.

Hydroperoxides are more widely used as initiators in low temperature appHcations (at or below room temperature) where transition-metal (M) salts are employed as activators. The activation reaction involves electron-transfer (redox) mechanisms ... 

Iron(II) ediylenediaminetetraacetic acid [15651 -72-6] Fe(EDTA) or A/,Ar-l,2-ethaiiediylbis[A[-(carboxymethyl)glyciQato]ferrate(2—), is a colorless, air-sensitive anion. It is a good reducing agent, having E° = —0.1171, and has been used as a probe of outer sphere electron-transfer mechanisms. It can be prepared by addition of an equivalent amount of the disodium salt, Na2H2EDTA, to a solution of iron(II) in hydrochloric acid. Diammonium [56174-59-5] and disodium [14729-89-6] salts of Fe(EDTA) 2— are known. 

The pale blue tris(2,2 -bipyridine)iron(3+) ion [18661-69-3] [Fe(bipy)2], can be obtained by oxidation of [Fe(bipy)2]. It cannot be prepared directiy from iron(III) salts. Addition of 2,2 -bipyridine to aqueous iron(III) chloride solutions precipitates the doubly hydroxy-bridged species [(bipy)2Fe(. t-OH)2Fe(bipy)2]Cl4 [74930-87-3]. [Fe(bipy)2] has an absorption maximum at 610 nm, an absorptivity of 330 (Mem), and a formation constant of 10. In mildly acidic to alkaline aqueous solutions the ion is reduced to the iron(II) complex. [Fe(bipy)2] is frequentiy used in studies of electron-transfer mechanisms. The triperchlorate salt [15388-50-8] is isolated most commonly. 

Photopolymerization. In many cases polymerization is initiated by ittadiation of a sensitizer with ultraviolet or visible light. The excited state of the sensitizer may dissociate directiy to form active free radicals, or it may first undergo a bimoleculat electron-transfer reaction, the products of which initiate polymerization (14). TriphenylaLkylborate salts of polymethines such as (23) ate photoinitiators of free-radical polymerization. The sensitivity of these salts throughout the entire visible spectral region is the result of an intra-ion pair electron-transfer reaction (101). 

Photochromism Based on Redox Reactions. Although the exact mechanism of the reversible electron transfer is often not defined, several viologen salts (pyridinium ions) exhibit a photochromic response to uv radiation in the crystalline state or in a polar polymeric matrix, for example,... 

Photopolymerization reactions are widely used for printing and photoresist appHcations (55). Spectral sensitization of cationic polymerization has utilized electron transfer from heteroaromatics, ketones, or dyes to initiators like iodonium or sulfonium salts (60). However, sensitized free-radical polymerization has been the main technology of choice (55). Spectral sensitizers over the wavelength region 300—700 nm are effective. AcryUc monomer polymerization, for example, is sensitized by xanthene, thiazine, acridine, cyanine, and merocyanine dyes. The required free-radical formation via these dyes may be achieved by hydrogen atom-transfer, electron-transfer, or exciplex formation with other initiator components of the photopolymer system. 

The simplest electroplating baths consist of a solution of a soluble metal salt. Electrons ate suppHed to the conductive metal surface, where electron transfer to and reduction of the dissolved metal ions occur. Such simple electroplating baths ate rarely satisfactory, and additives ate requited to control conductivity, pH, crystal stmcture, throwing power, and other conditions. 

One-electron oxidation of carboxylate ions generates acyloxy radicals, which undergo decarboxylation. Such electron-transfer reactions can be effected by strong one-electron oxidants, such as Mn(HI), Ag(II), Ce(IV), and Pb(IV) These metal ions are also capable of oxidizing the radical intermediate, so the products are those expected from carbocations. The oxidative decarboxylation by Pb(IV) in the presence of halide salts leads to alkyl halides. For example, oxidation of pentanoic acid with lead tetraacetate in the presence of lithium chloride gives 1-chlorobutane in 71% yield ... 

Both the Af-fluorosulfonamides and the A -fluoroammonium salts are very effective in the fluormation of enol acetates, enamines, silyl enol ethers, and enolates (Table 2) The reactions are thought to proceed through a mechanism which involves Sf 2 attack on the fluorine atom, but contributions from electron-transfer pathways also exist [65, 68, 73, 75, 76, 79, 80, 81, 82]... 

The measured emfs of concentration cells of mercury(I) salts are only explicable on the assumption that a 2-electron transfer is involved. This would not be the case if Hg+ were involved [E = (2.303RT/nF) logai/a2 where n = 2 for Hg2 + and n = 1 for Hg+]. 

Reaction of 2-chloromethyl-4//-pyrido[l,2-u]pyrimidine-4-one 162 with various nitronate anions (4 equiv) under phase-transfer conditions with BU4NOH in H2O and CH2CI2 under photo-stimulation gave 2-ethylenic derivatives 164 (01H(55)535). These alkenes 164 were formed by single electron transfer C-alkylation and base-promoted HNO2 elimination from 163. When the ethylenic derivative 164 (R = R ) was unsymmetrical, only the E isomer was isolated. Compound 162 was treated with S-nucleophiles (sodium salt of benzyl mercaptan and benzenesulfinic acid) and the lithium salt of 4-hydroxycoumarin to give compounds 165-167, respectively. 

The acetylide anion 3 is likely to form an alkynyl-copper complex by reaction with the cupric salt. By electron transfer the copper-II ion is reduced, while the acetylenic ligands dimerize to yield the -acetylene 2 ... 

The ability of a nltro group in the substrate to bring about electron-transfer free radical chain nucleophilic subsdnidon fSpj li at a saniratedcarbon atom is well documented. Such electron transfer reacdons are one of the characterisdc feanires of nltro compounds. Komblum and Russell have established ihe Spj l reaction independently the details of the early history have been well reviewed by them. The reacdon of -nitrobenzyl chloride v/ith a salt of nitro ilkane is in sharp contrast to the general behavior of the ilkyladon of the carbanions derived from nitro ilkanes here, carbon ilkyladon is predominant. The carbon ilkyladon process proceeds via a chain reacdon involving anion radicals and free radicals, as shovmin Eq. 5.24 and Scheme 5.4 fSpj l reacdoni. 

In 1970, a new reacdon, the displacement of a nitro group from ct-nitro esters, ct-nitro nitnles, ct-nitro ketones, iind ct,ct-dinitro compounds by nitroalkiine salts, was described. These displacements, which are exemplified by the reacdon presented in Eq. 7.1, take place at room temperanire iind give excellent yields of pure products. The reacdon proceeds via a radiciil chain mechanism involving one electron-transfer processes as shovmin Scheme 7.1 the details of the mechanism are described in a review. ... 

The initiating radicals are assumed to be SCN, ONO or N3 free radicals. Tris oxalate-ferrate-amine anion salt complexes have been studied as photoinitiators (A = 436 nm) of acrylamide polymer [48]. In this initiating system it is proposed that the CO2 radical anion found in the primary photolytic process reacts with iodonium salt (usually diphenyl iodonium chloride salt) by an electron transfer mechanism to give photoactive initiating phenyl radicals by the following reaction machanism ... 

Meanwhile, it was found by Asai and colleagues [48] that tetraphenylphosphonium salts having such anions as Cl, Br , and Bp4 work as photoinitiators for radical polymerization. Based on the initiation effects of changing counteranions, they proposed that a one-electron transfer mechanism is reasonable in these initiation reactions. However, in the case of tetraphenylphosphonium tetrafluoroborate, it cannot be ruled out that direct homolysis of the p-phenyl bond gives the phenyl radical as the initiating species since BF4 is not an easily pho-tooxidizable anion [49]. Therefore, it was assumed that a similar photoexcitable moiety exists in both tetraphenyl phosphonium salts and triphenylphosphonium ylide, which can be written as the following resonance hybrid [17] (Scheme 21) ... 

The salts of alkyl xanthates, A/,A/ -di-substituted dithio-carbamates and dialkyidithiophosphates [26] are effective peroxide decomposers. Since no active hydrogen is present in these compounds, an electron-transfer mechanism was suggested. The peroxide radical is capable of abstracting an electron from the electron-rich sulfur atom and is converted into a peroxy anion as illustrated below for zinc dialkyl dithiocarbamate [27] ... 

Following the discovery of Haber and Weiss [78] that Fe(II) salts re ct with H2O2 by one electron transfer process to give OH, Baxandale et al. [79] used the Fe - H2O2 system for effecting polymerization of vinyl monomers. 

A salt bridge serves as an ionconducting connection between the two half-cells. When the external circuit is closed, the oxidation reaction starts with the dissolution of the zinc electrode and the formation of zinc ions in half-cell I. In half-cell II copper ions are reduced and metallic copper is deposited. The sulfate ions remain unchanged in the aqueous solution. The overall cell reaction consists of an electron transfer between zinc and copper ions ... 

It lias also been suggested that photoexcited benzoyl peroxide is somewhat more susceptible to induced decomposition processes involving electron transfer than the ground state molecule. Rosenthal et c//.15 reported on redox reactions with certain salts (including benzoate ion) and neutral molecules (e.g. alcohols). 

Certain transition metal salts can be used as radical traps (Scheme 3.89, Scheme 3.90).486 These include various cupric (e.g. Cu(OAc)2, CuCl , Cu(SCN)i),l8 1<,8 J< 3 432 487 ferric (e.g. FeCli),316 488 and titanotis salts (eg. TiCL,).379 These traps react with radicals by ligand- or electron-transfer to give products which can be determined by conventional analytical techniques. 

Transition metal salts trap carbon-centered radicals by electron transfer or by ligand transfer. These reagents often show high specificity for reaction with specific radicals and the rates of trapping may be correlated with the nucleophilicity of the radical (Table 5.6). For example, PS radicals are much more reactive towards ferric chloride than acrylic propagating species."07... 

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