Nucleophilic substitution electron transfer

There is great interest in understanding the preference of different transition metals for cis or trans isomers or the way in which these complexes undergo geometrical isomerization. Indeed, the course of many reactions of these species, such as nucleophilic substitution, electron transfer, oxidative addition, reductive elimination, thermal decomposition, interaction with molecules of biological interest, and so on, is dictated by the geometry of these compounds. 

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. 

The symmetric series provides functional cyclohexadienes, whereas the non-symmetric one serves to build deuterated and/or functional arenes and tentacled compounds. In both series, several oxidation states can be used as precursors and provide different types of activation. The complexes bearing a number of valence, electrons over 18 react primarily by electron-transfer (ET). The ability of the sandwich structure to stabilize several oxidation states [21] also allows us to use them as ET reagents in stoichiometric and catalytic ET processes [18, 21, 22]. The last well-developed type of reactions is the nucleophilic substitution of one or two chlorine atoms in the FeCp+ complexes of mono- and o-dichlorobenzene. This chemistry is at least as rich as with the Cr(CO)3 activating group and more facile since FeCp+ activator is stronger than Cr(CO) 3. 

How deeply one wishes to query the mechanism depends on the detail sought. In one sense, the quest is never done a finer and finer resolution of the mechanism may be obtained with further study. For example, the rates and mechanisms of electron transfer reactions have been studied experimentally and theoretically since the 1950s. but the research continues unabated as issues of ever finer detail and broader import are examined. The same can be said of other reactions—nucleophilic substitution, hydrolysis, etc. 

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. 

Kattenberg and coworkers54 studied the chlorination of a-lithiated sulfones with hexachloroethane. These compounds may react as nucleophiles in a nucleophilic substitution on halogen (path a, Scheme 5) or in an electron transfer reaction (path b, Scheme 5) leading to the radical anions. The absence of proof for radical intermediates (in particular, no sulfone dimers detected) is interpreted by these authors in favour of a SN substitution on X. 

This cycle involves, first, a monoelectronic transfer from the nickel (0) complex to the aryl halide affording a Ni(I) complex and then an oxidative addition affording a 16 electron-nickel (II) which undergoes a nucleophilic substitution of Nu-, then a monoelectronic transfer occurs once again with a second aryl halide, and, last, a reductive elimination of the arylated nucleophile regenerates the active Ni(I) species. 

In certain reactions where nucleophilic substitutions would seem obviously indicated, there is evidence that radicals and/or radical ions are actually involved. The first step in such a process is transfer of an electron from the nucleophile to the substrate to form a radical anion ... 

Organomagnesium and organolithium compounds are strongly basic and nucleophilic. Despite their potential to react as nucleophiles in SN2 substitution reactions, this reaction is of limited utility in synthesis. One limitation on alkylation reactions is competition from electron transfer processes, which can lead to radical reactions. Methyl and other primary iodides usually give the best results in alkylation reactions. 

The ability of a nitro group in the substrate to bring about electron-transfer free radical chain nucleophilic substitution (SrnI) at a saturated carbon atom is well documented.39 Such electron transfer reactions are one of the characteristic features of nitro compounds. Komblum and Russell have established the SrnI reaction independently the details of the early history have been well reviewed by them.39 The reaction of p-nitrobenzyl chloride with a salt of nitroalkane is in sharp contrast to the general behavior of the alkylation of the carbanions derived from nitroalkanes here, carbon alkylation is predominant. The carbon alkylation process proceeds via a chain reaction involving anion radicals and free radicals, as shown in Eq. 5.24 and Scheme... 

PMR) trends that correspond to relative rates.179 From an examination of the displacement of chloride from l-chloro-5-nitrofuran by potassium iodide in acetic acid or by sodium sulfide in water it was concluded that the substitution need not be a true nucleophilic substitution. Initially there could be a transfer of one electron from the nucleophile to the furan nucleus the resultant radical anion loses chloride to form a furyl radical and product.179... 

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... 

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