Single-Electron Transfer SET Reactions

A similar SET mechanism involving cation-radical intermediates 30 has also been confirmed for the reactions of phenolic ethers with diaryliodonium salts in hexafluoroisopropanol [215], The use of fluoroal-cohols as solvents in these reactions is explained by their unique ability to stabilize the aromatic cation-radicals [107], 

The SET mechanism was also proposed for some oxidations involving X -iodanes. In particular, mechanistic studies involving isotope labeling, kinetic studies, cyclic voltammetry measurements and NMR spectroscopic analysis confirm that SET is a rate-determining step in the IBX-promoted oxidative cyclization of unsaturated anilides in THE-DMSO solutions [216], The analogous mechanism was proposed for the oxidation of alkylbenzenes at the benzylic position under similar conditions [217]. 

(1999) Chemistry of Hypervalent Compounds, Wiley-VCH Verlag GmbH, Weinheim. 

Greenwood, N.N. and Eamshaw, A. (1997) Chemistry of the Elements, Butterworth-Heinemann, Oxford. 

Mellor, J.W. (1922) A Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. 2, Longmans, Green and Co, London. 

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There are two types of reaction involving metals (1) in which the metal is a reagent and is consumed in the process and (2) in which the metal functions as a catalyst. While it is certainly true that any cleansing of metallic surfaces will enhance their chemical reactivity, in many cases it would seem that this effect alone is not sufficient to explain the extent of the sonochemically enhanced reactivity. In such cases it is thought that sonication serves to sweep reactive intermediates, or products, clear of the metal surface and thus present renewed clean surfaces for reaction. Other ideas include the possibility of enhanced single electron transfer (SET) reactions at the surface. 

The reaction of trifluoromethyl iodide with arene thiolates provides trifluoromethyl aryl sulfides via a single electron transfer (SET) reaction rather than the SN2 reaction, which is the only formal mechanism (Scheme 2.31). In general, perfluoroalkyl (Rf—X), ferf-alkyl, and vinyl and aromatic halides are strongly deactivated for the replacement of halogens with... 

With the growth of PTC, various new technologies have been developed where PTC has been combined with other methods of rate enhancement. In some cases, rate enhancements much greater than the sum of the individual effects are observed. Primary systems studied involving the use of PTC with other rate enhancement techniques include the use of metal co-catalysts, sonochemistry, microwaves, electrochemistry, microphases, photochemistry, PTC in single electron transfer (SET) reactions and free radical reactions, and PTC reactions carried out in a supercritical fluid. Applications involving the use of a co-catalyst include co-catalysis by surfactants (Dolling, 1986), alcohols and other weak acids in hydroxide transfer reactions (Dehmlow et al., 1985,1988), use of iodide (traditionally considered a catalyst poison, Hwu et... 

Kaim has shown that the single electron transfer (SET) reactions of AIH3 and AID3 in THE with pyrazine (1), quinoxaline (2), and phenazine (3) can be studied by ESR spectroscopy, which fully characterizes persistent radical complexes formed as escape products besides the conventional diamagnetic reduction products <84JA1712>. The same author was able to isolate unusually stable... 

The conversion of 24a into intermediate 25a is given as a double single-electron transfer (SET) reaction. Alternatively, a Diels-Alder reaction of 21a could produce 19a. 

Topics reviewed during the year include the photochemistry of indoles, sulfoxides, pyrazoles and isothiazoles, (S-hetero)cyclic unsaturated carbonyl compounds, photoinduced single electron transfer (SET) reactions of amines and of azo compounds, SET reactions of organosilanes and organostannanes with Qo and ketones, photochromic polypeptides and di(hetero)arylethenes, processes in chromophore sequences on a-helical polypeptides,aryl-aryl coupling in furans, thiophenes and pyrroles," [3+2]cycloaddition of aromatic nitriles (and esters) with alkenes, and reactions of benzylsilane derivatives. ... 

Some Sn2 reactions have electron transfer characteristics. Such single-electron transfer (SET) reactions start with an electron transfer from the nucleophile to the electrophile. 

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)... 

C-Methylation products, o-nitrotoluene and p-nitrotoluene, were obtained when nitrobenzene was treated with dimethylsulfoxonium methylide (I)." The ratio for the ortho and para-methylation products was about 10-15 1 for the aromatic nucleophilic substitution reaction. The reaction appeared to proceed via the single-electron transfer (SET) mechanism according to ESR studies. 

This first single electron transfer (SET) is followed, due to the chemical and electrochemical instability of R by at least one of the following reactions ... 

Although single-electron-transfer (SET) processes would be expected to be important in reactions that use metals as reagents, this type of process has also been recognized in the reduction of carbonyl groups that involve 1,4-dihydronicotinamide derivatives . Recent work by Oae and coworkers" has shown that an SET process is operative in the reduction of dibenzothiophene S-oxide by l-benzyl-l,4-dihydronicotinamide when the reaction is catalyzed by metalloporphins. The reaction is outlined in equation (18), but the study gave results of much more mechanistic than synthetic value. This type of study is relevant to understanding biochemical mechanisms since it is known that methionine sulphoxide is reduced to methionine by NADPH when the reaction is catalyzed by an enzyme isolated from certain yeasts . 

However, there is evidence that reactions of aluminium hydride produced in situ involve single-electron-transfer (SET) processesThe reactions described by Trost and Ghadiri have most likely not been studied in sufficient detail to permit an adequate description of the reaction mechanism to be given at this stage. It is, however, quite likely that the Grignard reactions catalyzed by copper(II) and nickel(II) complexes , as developed by julia - and by Masaki , do involve SET processes, although, if this is so, the preservation of stereochemistry in some of the examples described by these workers is quite remarkable. (In this context, the reader s attention is drawn to Reference 196, end of this section.)... 

In an anionic/radical domino process an interim single-electron transfer (SET) from the intermediate of the first anionic reaction must occur. Thus, a radical is generated which can enter into subsequent reactions. Although a SET corresponds to a formal change of the oxidation state, the transformations will be treated as typical radical reactions. To date, only a few true anionic/radical domino transformations have been reported in the literature. However, some interesting examples of related one-pot procedures have been established where formation of the radical occurs after the anionic step by addition of TEMPO or Bu3SnH. A reason for the latter approach are the problems associated with the switch between anionic and radical reaction patterns, which often do not permit the presence of a radical generator until the initial anionic reaction step is finished. 

First, we examined the efficiency of the initiation process. A solution of buthyllithium was added to a THF solution of 7 at -70°C. The color of the solution turned to red immediately and a strong ESR signal was observed with a well separated hyperfme structure. The observed radical species was identified as the anion radical of 2-butyl-l,l,2,2-tetramethyldisilanyl-substituted biphenyl by computational simulation as well as by comparison with the spectra of a model compound. The anion radical should be a product of a single electron transfer (SET) process from buthyllithium to the monomer. Since no polymeric product was obtained under the above-mentioned conditions, the SET process is an undesired side reaction of the initiation and one of the reasons why more higher molecular weight polymer was observed than expected. ... 

Reductive Cross-Coupling of Nitrones Recently, reductive coupling of nitrones with various cyclic and acyclic ketones has been carried out electrochem-ically with a tin electrode in 2-propanol (527-529). The reaction mechanism is supposed to include the initial formation of a ketyl radical anion (294), resulting from a single electron transfer (SET) process, with its successive addition to the C=N nitrone bond (Scheme 2.112) (Table 2.9). 

Single Electron Transfer A single electron transfer (SET) mechanism is often difficult to distinguish from an SN2 reaction because the principal product of these two pathways is the same, apart from the stereochemistry at carbon (race-mization instead of inversion). The radicals formed can recombine rapidly in a solvent cage (inner-sphere ET) [2, 193, 194]. The [HFe(CO)5] -catalyzed deiodina-tion of iodobenzene may serve as an example [179] (Eq. (13)). 

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