Single Electron Transfer SET Mechanism

These reactions occur easily because of the relative stability of the radicals involved.6 The single electron transfer mechanism (SET), which we have met several times (e.g., p. 307) is an important case. 

The equation does not take into account such pertubation factors as steric effects, solvent effects, and ion-pair formation. These factors, however, may be neglected when experiments are carried out in the same solvent at the same temperature and concentration for an homogeneous set of substrates. So, for a given ambident nucleophile the rate ratio kj/kj will depend on A and B, which vary with (a) the attacked electrophilic center, (b) the solvent, and (c) the counterpart cationic species of the anion. The important point in this kind of study is to change only one parameter at a time. This simple rule has not always been followed, and little systematic work has been done in this field (12) stiH widely open after the discovery of the role played by single electron transfer mechanism in ambident reactivity (1689). 

These alkylations can be looked upon as aliphatic nucleophilic substitutions, usually thoughtto proceed via SnI, Sn2, or hybrids of these mechanisms. However, in recent years more and more evidence for a single-electron transfer (SET) mechanism, represented in Eqs. (28-31), was obtained, and it was suggested that Sn2 and SET are just limiting cases of the same single-electron transfer mechanism [205, 206]. The S ET pathway involves first a transfer of an electron from the nucleophile to the electrophile followed by bond formation, whereas the Sn2 reaction involves a... 

Mechanisms that begin this way are called SET (single electron transfer) mechanisms 59 Once formed, the radical ion cleaves ... 

While the reaction is generally thought to proceed through a nucleophilic addition mechanism, sterically hindered substrates may react according to mi SET (single electron transfer) mechanism ... 

The discussion so far has been in relation to bimolecular electrophilic oxidation by hydrogen peroxide via a two-electron donor substrate. However, the oxidation process can also be envisaged as a single electron transfer process (SET). The two mechanisms are not that dissimilar152 and can be viewed as two sides of the same coin. The bimolecular mechanism can be visualized as an... 

The SET (single electron transfer) mechanism can be invoked in the transannular addition of secondary amines to 1,1-diarylalkcncs in the presence of a catalytic amount of butyllithium, sodium naphthalenide or potassium hydroxide52, since the regiochemistry which is observed is opposite to that predicted by assuming the formation of metal amides. 

Not only the nitroaromatic species, such as IV, but also some simpler compounds, which used to be considered as typical substrates of the 8 2 reactions, can be involved in multistep radical-forming nucleophilic substitutions. Evidence has been accumulating over the last years that the nucleophilic substitution with alkyl halides occurs, at least in some instances, by the single-electron transfer mechanism. It has been suggested [29,30] that the SET and Sn2 mechanisms represent the extremes of a wide spectrum of mechanistic possibilities for substitution reactions. It has been deduced on qualitative theoretical grounds that the propensity of alkyl halide R—X to react with nucleophiles via an electron-transfer step depends crucially on the stability of the three-electron bond R—X in the initially formed radical-anion species. A more electronegative R will stabilize this bond and bring about a shift in the mechanism from the Sn2 to the SET type, which has then experimentally been shown to be a correct conclusion, see Ref. [30]. 

Secondary alkyl halides with two fluorines in the -position, for example, PhCH2CH2CHXCF2Et where X = Cl, Br, or I, react with arylzinc compounds in the presence of FeCl2/dppp and TMEDA giving PhCH2CH2CHArCF2Et in moderate to good yields with very little dehalofluorination. Dehalofuorination does not occur because the reaction occurs by a SET (single electron transfer) mechanism. 

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. 

It might be mentioned that matters are much simpler for organometallic compounds with less-polar bonds. Thus Et2Hg and EtHgCl are both definite compounds, the former is a liquid and the latter is a solid. Organocalcium reagents are also known, and they are formed from alkyl halides via a single electron transfer (SET) mechanism with free-radical intermediates. "... 

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.)... 

Therefore, it has been considered that the formation of the dimer involves a mechanism different to the simple head-to-head radical coupling of the parent monomer. As suggested by the authors, it is likely that the overall mechanistic sequence is initiated by the radical-anion 472 of compound 469 formed by a single electron transfer (SET) process, which is the first stage of the bromine-lithium exchange (Scheme 68) [128],... 

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)). 

The reaction proceeds via a single electron transfer (SET) from sodium tellurolate to perfluoroalkyl halides followed by a radical chain reaction of the SRN mechanism. 

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