Electron transfer reagents

Because anions of nitro compounds are good electron-transfer reagents, they can serve as reducing agents in radical type eliminations of vicinal dinitro compounds. In fact, N- azolyl- sub-... 

This statement applies only to the comparison of I with Br-, Cl-, and F. For other electron transfer reagents containing iodine, compare Secs. 10.6 and 10.11. 

Several other electron-transfer reagents have been tested with arenediazonium ions, for example, A-benzyl-l,4-dihydronicotinamide, which is a model for biochemical reductions by NAD(P)H, the reduced form of NADP+ (nicotinamide adenine cfinucleotide phosphate) (Yasui et al., 1984). 

Fig. 8-7. Dependence of the yield in chloro-de-diazoniations on the redox potential of electron transfer reagents (from Galli, 1988 Fc = ferrocene).

Table 10-1. Yield of chlorobenzene in chloro-de-diazoniation of benzenediazonium sulfate as a function of electron transfer reagent and ligand transfer reagent. Experiments 1-5 from Galli, 1981a), experiment 6 from Daasbjerg and Lund, 1992b).

Exp. no. Electron transfer reagent Ligand transfer reagent Yield ArCl (Vo)... 

A number of approaches have been tried for modified halo-de-diazoniations using l-aryl-3,3-dialkyltriazenes, which form diazonium ions in an acid-catalyzed hydrolysis (see Sec. 13.4). Treatment of such triazenes with trimethylsilyl halides in acetonitrile at 60 °C resulted in the rapid evolution of nitrogen and in the formation of aryl halides (Ku and Barrio, 1981) without an electron transfer reagent or another catalyst. Yields with silyl bromide and with silyl iodide were 60-95%. The authors explain the reaction as shown in (Scheme 10-30). The formation of the intermediate is indicated by higher yields if electron-withdrawing substituents (X = CN, COCH3) are present. In the opinion of the present author, it is likely that the dissociation of this intermediate is not a concerted reaction, but that the dissociation of the A-aryl bond to form an aryl cation is followed by the addition of the halide. The reaction is therefore mechanistically not related to the homolytic halo-de-diazoniations. 

The classical syntheses of phenanthrene and fluorenone fit well into the electron transfer scheme discussed in Section 8.6 and in this chapter. The aryl radical is formed by electron transfer from a Cu1 ion, iodide ion, pyridine, hypophosphorous acid, or by electrochemical transfer. The aryl radical attacks the neighboring phenyl ring, and the oxidized electron transfer reagent (e. g., Cu11) reduces the hexadienyl radical to the arenium ion, which is finally deprotonated by the solvent (Scheme 10-76). 

The proximity of the reaction centre to the second phenyl ring makes the aryl cation, formed by heterolytic dediazoniation, a serious competitor to the aryl radical. This is evident in Table 10-6 from various examples where the yield obtained in aqueous mineral acid (varying from 0.1 m to 50% H2S04) is higher than in the presence of an electron-transfer reagent. This competition was studied in three types of product analyses by Cohen s group (Lewin and Cohen, 1967 Cohen et al., 1977), by Huisgen and Zahler (1963 a, 1963 b), and by Bolton et al. (1986). 

By combining these reactions, hydroxyl radicals, generated with the photocatalyst and the electron transfer reagent, should react with methane to produce m yl radicals. In our... 

The proposed mechanism includes a reductive epoxide opening, trapping of the intermediate radical by a second equivalent of the chromium(II) reagent, and subsequent (3-elimination of a chromium oxide species to yield the alkene. The highly potent electron-transfer reagent samarium diiodide has also been used for deoxygenations, as shown in Scheme 12.3 [8]. 

The (3-metaloxy radical was first exploited for synthetic purposes in C—H and C—C bond-forming reactions by Nugent and RajanBabu through the use of titanocene(III) chloride as an electron-transfer reagent [5]. They established that the (3-titaniumoxy radicals formed after electron transfer can be reduced by hydrogen atom donors, e. g. 1,4-cy-clohexadiene or tert-butyl thiol, that they add to a,(3-unsaturated carbonyl compounds, and that they can react intramolecularly with olefins in 5-exo cyclizations. 

The Fe-protein has the protein fold and nucleotide-binding domain of the G-protein family of nucleotide-dependent switch proteins, which are able to change their conformation dependent on whether a nucleoside diphosphate (such as GDP or ADP) is bound instead of the corresponding triphosphate (GTP or ATP). However, nucleotide analogues, which induce the conformational switch of the Fe-protein, do not allow substrate reduction by the MoFe-protein, nor does reduction of the MoFe-protein by other electron-transfer reagents (whether small proteins or redox dyes) drive substrate reduction. Only the Fe-protein can reduce the MoFe-protein to a level that allows it to reduce substrates such as... 

As a particular example, reactions of diphenylbis(phenylthiomethyl)silane (155f) are shown. While the reaction of 155f with four equivalents of LiCioHg at —40 °C yields the dimetalated bis(lithiomethyl)diphenylsilane (101), a selective monolithiation (compound 158) can be achieved by using only two equivalents of the electron transfer reagent at —60°C. A side reaction of the monolithiated silane 158 is observed, when the reaction temperature rises above —30°C, giving [lithio(phenylthio)methyl]methyldiphenylsilane (159) (Scheme 57). This problem, also observed for other (phenylthiomethyl)element systems, can mostly be avoided by an exact control of the reaction temperature . 

On addition of S04 to the triple bond in the lO-member cycloalkyne 24 and cyclo-aUcynone 27, a nonchain, and anionic, self-terminating radical cyclization cascade is induced. In the former reaction (equation 22) the bicyclic ketones 25 and 26 are formed, and in the latter reaction (equation 23) the a,/3-epoxy ketones 28 and 29 are formed in good yields. Because of the difficulty of oxidizing isolated triple bonds, 804 does not react as an electron-transfer reagent in these reactions but acts as a donor of atomic oxygen. 

Sakurai and co-workers (115, 116) have found that trimethylsilylso-dium is an excellent one-electron transfer reagent in HMPA or other highly polar po I vents ... 

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