One electron transfer

The reaction of perfluoroalkyl iodides with alkenes affords the perfluoro-alkylated alkyl iodides 931. Q.a-Difluoro-functionalized phosphonates are prepared by the addition of the iododifluoromethylphosphonate (932) at room temperature[778], A one-electron transfer-initiated radical mechanism has been proposed for the addition reaction. Addition to alkynes affords 1-perfluoro-alkyl-2-iodoalkenes (933)[779-781]. The fluorine-containing oxirane 934 is obtained by the reaction of allyl aicohol[782]. Under a CO atmosphere, the carbocarbonylation of the alkenol 935 and the alkynol 937 takes place with perfluoroalkyl iodides to give the fluorine-containing lactones 936 and 938[783]. 

The formation of a Gngnard reagent is analogous to that of organohthium reagents except that each magnesium atom can participate m two separate one electron transfer steps... 

With most transition metals, eg, Cu, Co, and Mn, both valence states react with hydroperoxides via one electron transfer (eqs. 11 andl2). Thus, a small amount of transition-metal ion can decompose a large amount of hydroperoxide and, consequendy, inadvertent contamination of hydroperoxides with traces of transition-metal impurities should be avoided. 

Apparently the alkoxy radical, R O , abstracts a hydrogen from the substrate, H, and the resulting radical, R" , is oxidized by Cu " (one-electron transfer) to form a carbonium ion that reacts with the carboxylate ion, RCO - The overall process is a chain reaction in which copper ion cycles between + 1 and +2 oxidation states. Suitable substrates include olefins, alcohols, mercaptans, ethers, dienes, sulfides, amines, amides, and various active methylene compounds (44). This reaction can also be used with tert-huty peroxycarbamates to introduce carbamoyloxy groups to these substrates (243). 

Metal-Catalyzed Oxidation. Trace quantities of transition metal ions catalyze the decomposition of hydroperoxides to radical species and greatiy accelerate the rate of oxidation. Most effective are those metal ions that undergo one-electron transfer reactions, eg, copper, iron, cobalt, and manganese ions (9). The metal catalyst is an active hydroperoxide decomposer in both its higher and its lower oxidation states. In the overall reaction, two molecules of hydroperoxide decompose to peroxy and alkoxy radicals (eq. 5). 

The detailed mechanism of battery electrode reactions often involves a series of chemical and electrochemical or charge-transfer steps. Electrode reaction sequences can also include diffusion steps on the electrode surface. Because of the high activation energy required to transfer two electrons at one time, the charge-transfer reactions are beheved to occur by a series of one electron-transfer steps illustrated by the reactions of the 2inc electrode in strongly alkaline medium (41). 

Oxidation—Reduction. Redox or oxidation—reduction reactions are often governed by the hard—soft base rule. For example, a metal in a low oxidation state (relatively soft) can be oxidized more easily if surrounded by hard ligands or a hard solvent. Metals tend toward hard-acid behavior on oxidation. Redox rates are often limited by substitution rates of the reactant so that direct electron transfer can occur (16). If substitution is very slow, an outer sphere or tunneling reaction may occur. One-electron transfers are normally favored over multielectron processes, especially when three or more species must aggregate prior to reaction. However, oxidative addition... 

Some instances of incomplete debromination of 5,6-dibromo compounds may be due to the presence of 5j5,6a-isomer of wrong stereochemistry for anti-coplanar elimination. The higher temperature afforded by replacing acetone with refluxing cyclohexanone has proved advantageous in some cases. There is evidence that both the zinc and lithium aluminum hydride reductions of vicinal dihalides also proceed faster with diaxial isomers (ref. 266, cf. ref. 215, p. 136, ref. 265). The chromous reduction of vicinal dihalides appears to involve free radical intermediates produced by one electron transfer, and is not stereospecific but favors tra 5-elimination in the case of vic-di-bromides. Chromous ion complexed with ethylene diamine is more reactive than the uncomplexed ion in reduction of -substituted halides and epoxides to olefins. ... 

The radical X is formed by homolysis of the X—R bond either thermally or photolytically. In the reactions of alcohols with lead tetraacetate evidence suggests that the X—R bond (X = 0, R = Pb(OAc)3) has ionic character. In this case the oxy radical is formed by a one electron transfer (thermally or photochemically induced) from oxygen to lead. 

The reactivities of the substrate and the nucleophilic reagent change vyhen fluorine atoms are introduced into their structures This perturbation becomes more impor tant when the number of atoms of this element increases A striking example is the reactivity of alkyl halides S l and mechanisms operate when few fluorine atoms are incorporated in the aliphatic chain, but perfluoroalkyl halides are usually resistant to these classical processes However, formal substitution at carbon can arise from other mecharasms For example nucleophilic attack at chlorine, bromine, or iodine (halogenophilic reaction, occurring either by a direct electron-pair transfer or by two successive one-electron transfers) gives carbanions These intermediates can then decompose to carbenes or olefins, which react further (see equations 15 and 47) Single-electron transfer (SET) from the nucleophile to the halide can produce intermediate radicals that react by an SrnI process (see equation 57) When these chain mechanisms can occur, they allow reactions that were previously unknown Perfluoroalkylation, which used to be very rare, can now be accomplished by new methods (see for example equations 48-56, 65-70, 79, 107-108, 110, 113-135, 138-141, and 145-146)... 

Flavin coenzymes can exist in any of three different redox states. Fully oxidized flavin is converted to a semiqulnone by a one-electron transfer, as shown in Figure 18.22. At physiological pH, the semiqulnone is a neutral radical, blue in color, with a A ax of 570 nm. The semiqulnone possesses a pAl of about 8.4. When it loses a proton at higher pH values, it becomes a radical anion, displaying a red color with a A ax of 490 nm. The semiqulnone radical is particularly stable, owing to extensive delocalization of the unpaired electron across the 77-electron system of the isoalloxazine. A second one-electron transfer converts the semiqulnone to the completely reduced dihydroflavin as shown in Figure 18.22. 

Access to three different redox states allows flavin coenzymes to participate in one-electron transfer and two-electron transfer reactions. Partly because of this, flavoproteins catalyze many different reactions in biological systems and work together with many different electron acceptors and donors. These include two-electron acceptor/donors, such as NAD and NADP, one- or two-elec-... 

The second step involves the transfer of electrons from the reduced [FMNHg] to a series of Fe-S proteins, including both 2Fe-2S and 4Fe-4S clusters (see Figures 20.8 and 20.16). The unique redox properties of the flavin group of FMN are probably important here. NADH is a two-electron donor, whereas the Fe-S proteins are one-electron transfer agents. The flavin of FMN has three redox states—the oxidized, semiquinone, and reduced states. It can act as either a one-electron or a two-electron transfer agent and may serve as a critical link between NADH and the Fe-S proteins. 

Cytochrome c oxidase contains two heme centers (cytochromes a and %) as well as two copper atoms (Figure 21.17). The copper sites, Cu and Cug, are associated with cytochromes a and respectively. The copper sites participate in electron transfer by cycling between the reduced (cuprous) Cu state and the oxidized (cupric) Cu state. (Remember, the cytochromes and copper sites are one-electron transfer agents.) Reduction of one oxygen molecule requires passage of four electrons through these carriers—one at a time (Figure... 

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. ... 

From the foregoing it can be seen that the nitro group can be activated for C-C bond formation in various ways. Classically the nitro group facilitates the Henry reaction, Michael addition, and Diels-Alder reaction. Komblum and Russell have introduced a new substitution reaction, which proceeds via a one electron-transfer process The Spj l reactions have... 

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) ... 

A number of metal chelates containing transition metals in their higher oxidation states are known to decompose by one electron transfer process to generate free radical species, which may initiate graft copolymerization reactions. Different transition metals, such as Zn, Fe, V, Co, Cr, Al, etc., have been used in the preparation of metal acetyl acetonates and other diketonates. Several studies demonstrated earlier that metal acetyl acetonates can be used as initiators for vinyl polymeriza-... 

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. 

This statement does not mean, however, that the mechanism of diazotization was completely elucidated with that breakthrough. More recently it was possible to test the hypothesis that, in the reaction between the nitrosyl ion and an aromatic amine, a radical cation and the nitric oxide radical (NO ) are first formed by a one-electron transfer from the amine to NO+. Stability considerations imply that such a primary step is feasible, because NO is a stable radical and an aromatic amine will form a radical cation relatively easily, especially if electron-donating substituents are present. As discussed briefly in Section 2.6, Morkovnik et al. (1988) found that the radical cations of 4-dimethylamino- and 4-7V-morpholinoaniline form the corresponding diazonium ions with the nitric oxide radical (Scheme 2-39). 

Diffusion-controlled one-electron transfers can be observed in sulfolane (Elofson and Gadallah, 1967), in nitromethane (Bottcher et al., 1973), and in some other apolar solvents (Janderka and Cejpek, 1989), but unfortunately not in aqueous acid, where four electrons are involved (Elofson, 1958). For that process Orange et al. (1981) proposed a complex mechanism ending in arenehydra-zinium ions. 

The last three columns contain the parameters which give the most reliable information on the nucleophilic character of the solvent for a wide range of solvents, namely Koppel and Paju s -values (1974), Schleyer s iVBS-values (Schadt et al., 1976), and Taft s / -values (Kamlet et al., 1983). The columns showing the type of product demonstrate roughly that increasing nucleophilicity favors the formation of products of homolytic intermediates. The fact that there is no exact correlation with nucleophilicity is likely to be due to the (one-) electron transfer capabilities of these solvents, which do not parallel the nucleophilicity (see Sec. 8.6). 

A single report concerning the kinetics of the oxidation of dimethyl sulphoxide by tervalent manganese has been published143. This reaction proceeds by one-electron transfer forming the sulphoxide radical cation which, in the presence of a monomer such as acrylonitrile, may be used to initiate polymerization. Equation (48) has been proposed to describe the reaction. 

A more detailed study of the biological oxidation of sulphoxides to sulphones has been reported165. In this study cytochrome P-450 was obtained in a purified form from rabbit cells and was found to promote the oxidation of a series of sulphoxides to sulphones by NADPH and oxygen (equation 56). Kinetic measurements showed that the process proceeds by a one-electron transfer to the activated enzymatic intermediate [an oxenoid represented by (FeO)3+] according to equation (57). 

At this point, special mention37 should be made of the behaviour of highly conjugated ethylenic sulphones in weakly acidic media. For example, in the case when R1 =Ph (Z isomer), a fairly stable anion radical was obtained in dry DMF. However, either in aprotic (consecutive two one-electron transfer) or in protic media (ECE process, occurrence of the protonation step on anion radical), C—S bond cleavage is observed. The formation of the corresponding olefins by C—S bond cleavage may occur in high yield, and is nearly quantitative when R1 = H and R2 = Ph for an electrolysis conducted in... 

A particular case of a [3C+2S] cycloaddition is that described by Sierra et al. related to the tail-to-tail dimerisation of alkynylcarbenes by reaction of these complexes with C8K (potassium graphite) at low temperature and further acid hydrolysis [69] (Scheme 24). In fact, this process should be considered as a [3C+2C] cycloaddition as two molecules of the carbene complex are involved in the reaction. Remarkable features of this reaction are (i) the formation of radical anion complexes by one-electron transfer from the potassium to the carbene complex, (ii) the tail-to-tail dimerisation to form a biscarbene anion intermediate and finally (iii) the protonation with a strong acid to produce the... 

We postulated a reaction mechanism with participation of an aromatic radical cation which was formed by one electron transfer from an aromatic hydrocarbon to copper(II) chloride. Activated alumina has electron-acceptor properties, and formation of a radical cation of an aromatic hydrocarbon adsorbed on alumina has been observed by ESR (ref. 13). Therefore, it seemed to us that alumina as a support facilitates the generation of the radical cation of the aromatic hydrocarbon. 

These reactions are postulated to proceed by electro-transfer to give the radical cation of alkoxynaphtalene, which either undergoes reaction with copper(II) bromide or dimerizes (ref. 15). That is, one-electron transfer from the electron-rich alkoxynaphtalene to Cu(II) results in generation of the corresponding radical cation. The radical cation reacts with bromide anion leading to the brominated compound, whereas the radical cation undergoes reaction with another alkoxynaphtalene leading to the binaphtyl (eqns. 2-4). 

In the presence of bromide ion the slow one-electron transfer oxidation of the ArCH3 substrate is replaced by the rapid one-electron oxidation of bromide ion by cobalt(III) to afford a bromine atom. The latter, or rather its adduct with bromide ion, Br2 acts as the chain transfer agent in the reaction with the ArCH3 substrate (Fig. 10). 

Molecules have some occupied and some unoccupied orbitals. There occur diverse interactions (Scheme 1) when molecules undergo reactions. According to the frontier orbital theory (Sect 3 in Chapter Elements of a Chemical Orbital Theory by Inagaki in this volume), the HOMO d) of an electron donor (D) and the LUMO (fl ) of an electron acceptor (A) play a predominant role in the chemical reactions (delocalization band in Scheme 2). The electron configuration D A where one electron transfers from dio a significantly mixes into the ground configuration DA where... 

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