Single electron transfer substitution

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 has been well known since the pioneering work of Bunnett59 that some nucleophilic aromatic substitutions can be catalyzed by single electron transfer. Electrochemistry was shown60,61 to be an efficient technique both for inducing reactions and for determining mechanisms and thermodynamic data concerning equilibria in the overall process. 

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. ... 

Since the publication of the review on Single Electron Transfer and Nucleophilic Substitution in this same series,1 reviews or research accounts have appeared concerning several particular points among those addressed here, namely, dynamics of dissociative electron transfer,2-6 single electron transfer and Sn2 reactions,2,7 9 and SRN1 reactions.10,11... 

Nucleophilic substitution, in phosphate esters, mechanism and catalysis of, 25,99 Nucleophilic substitution, single electron transfer and, 26, 1 Nucleophilic vinylic substitution, 7,1... 

When the nucleophile is an electron-rich molecule, RC60+ can be reduced via single electron transfer, producing a dimer (47). Thus, electrophilic aromatic substitution normally occurs with substituted benzenes (Figure 22, [A]), but the mode of the reaction is switched if the benzene is strongly activated (Figure 22, [B]). 

Saveant, J-M. Single Electron Transfer and Nucleophilic Substitution, in Advances in Physical Organic Chemistry, Bethel, D., Ed., Academic Press New York, 1990, Vol. 26, pp. 1-130. 

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... 

The SrnI reaction thus appears as a reaction in which single electron transfer plays a pre-eminent role but is by no means a single elementary step. A different problem is that of the possible involvement of single electron transfer in reactions that are not catalysed by electron injection (or removal). A typical example of such processes is another substitution reaction, namely. 

The question we address now is that of the possible role of single electron transfer in substitution reactions that, unlike SrnI reactions, are not catalysed by electron injection. The problem is twofold. One side of it consists in answering the questions do bond breaking and bond formation belong to two different and successive processes, i.e. (135) followedhy (136), or, more... 

The weird EC, CE, ECE. .. jargon of molecular electrochemists, a community well exposed to single electron transfer, draws a sharp distinction between electron-transfer ( E ) and chemical reactions ( C ). Should they now totally abandon this dichotomy and see, together with all molecular chemists, single electron transfer in every chemical reaction, particularly in every nucleophilic substitution ... 

Photo-oxidation of l,l-dialkyl-2-arylhydrazines by single-electron transfer with trimethylsilyl cyanide (TMSCN) as cyanide ion source leads to regio- and stereoselective a-hydrazino nitriles. This stereoselective cyanation of hydrazines takes place on the more substituted carbon atom compared with the results obtained with tertiary amines (Scheme 5). 

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