Electron-transfer, single, and nucleophilic

Electron transfer, in thermal and photochemical activation of electron donor-acceptor complexes in organic and organometallic reactions, 29,185 Electron-transfer, single, and nucleophilic substitution, 26,1 Electron-transfer, spin trapping and, 31,91 Electron-transfer paradigm for organic reactivity, 35, 193... 

Electron transfer, single, and nucleophilic substitution, 26, 1 Electronically excited molecules, structure of, 1, 365... 

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 ... 

The inner salt (93) is able to react with both electrophiles (RI) and nucleophiles (RM) to give (92) and (94), respectively. Radical combination following single electron transfer from the nucleophile is believed to account for the thiophilic addition whereby the negatively charged nucleophile fails to react at the positively charged carbenium carbon. 

The solution of the riddle posed by Kornblum s dark Sj l reaction is as follows. The nucleophile does work as a single electron-transfer initiator of the chain process. However, the mechanism of initiation does not consist of a mere outer-sphere electron transfer from the nucleophile to form the anion-radical of the substrate. Rather, it involves a dissociative process in which electron transfer and bond breaking are concerted (Costentin and Saveant 2000). Scheme b at the beginning of Section 7.8 illustrates the concerted mechanism. 

Basicity is not the only general property of radical anions, anions, and dianions. Each may act as a nucleophile, a single-electron transfer agent, and as a base—sometimes all three As with conventionally generated bases, in what Baizer has dubbed secular chemistry, sterically hindered EGBs are useful because proton abstraction becomes favored over nucleophilic substitutions and additions. 

Another, related type of reaction is the halodifluoromethylation of nucleophiles by dihalodifluoromethanes (e.g. CF2Br2) [9]. This reaction is always initiated by a single electron transfer from the nucleophile to the CF2XY species (X and Y denote halogens other than fluorine). The subsequent fate of the resulting radical ion pair depends on the ability of the nucleophile to form a stabilized radical, and also on the choice of solvent [10]. For phenoxides [4a, 5, 11] and thiophenoxides [4c, Ila] a reaction pathway via difluorocarbene is usually preferred whereas enamines and ynamines are halodifluoromethylated by a radical chain mechanism (see also Section 2.2.1) [12] (Scheme 2.169). 

R. Bacaloglu, C. A. Bunton, and F. Ortega,/. Am. Chem. Soc., 110, 3512 (1988). Single-Electron Transfer in Aromatic Nucleophilic Substitution in Reaction of 1-Substituted 2,4-Dinitronaphtlanenes with Hydroxide Ion. 

The mechanism of UGM is of great interest and is still not completely resolved. Both a single-electron transfer mechanism and a nucleophilic mechanism have been proposed (Scheme 28). Both mechanisms involve the formation of a substrate—flavin N5 adduct, which has been trapped during turnover. This could arise due to direct nucleophilic attack of the reduced flavin on the sugar substrate. Alternatively, this adduct could arise from one-electron transfer from the reduced flavin to an oxocarbenium ion generated by elimination of UDP, followed by radical recombination of the flavin semiquinone and hexose radical. " An oxocarbenium ion is a proposed intermediate based on positional isotope exchange experiments and studies with substrate analogues, possible. 

In general, all reactions between closed-shell electrophiles and nucleophiles are describable by the same diagram type [11] with R and P states, which are vertical charge transfer states that involve an electron transfer from the nucleophile to the electrophile, while coupling the single electron on the oxidized nucleophile to that on the reduced electrophile to form a bond-pair. One of the many examples is the nucleophihc assisted cleavage of an ester where the rate-determining step [62,63] is the formation of a tetrahedral intermediate, as depicted in Fig. 23.8. 

We have noted that both 5 2 and SnAr reactions may occur through SET processes. There is good evidence that the SnAc reaction may involve such a pathway also. Figure 8.55 shows species identified by Bacaloglu and coworkers in a fast kinetic spectroscopy study of the reaction of hydroxide ion with l-chloro-2,4,6-trinitrobenzene (picryl chloride, 71). D ending on reaction conditions, these workers could see transients ascribed to the n complex (72), an intermediate produced by single electron transfer (73), and one or more cr complexes (74, 75). In addition, evidence was obtained for the reversible formation of a phenyl carbanion (76) and a dianion (77) that probably do not lead directly to the substitution product (78). Further support for the role of SET processes in SNAr reactions comes from the detection of radical intermediates by EPR spectrometry and by correlations of reactivity with the oxidation potentials of the nucleophiles in some studies. ... 

Mayer, M.U. and Breslow, R., Antihydrophobic evidence for the single electron transfer mechanism of nucleophilic substitution, /. Am. Chem. Soc., 1998, 120, 9098-9099. 

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). 

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. 

Reaction of 2-chloromethyl-4//-pyrido[l,2-u]pyrimidine-4-one 162 with various nitronate anions (4 equiv) under phase-transfer conditions with BU4NOH in H2O and CH2CI2 under photo-stimulation gave 2-ethylenic derivatives 164 (01H(55)535). These alkenes 164 were formed by single electron transfer C-alkylation and base-promoted HNO2 elimination from 163. When the ethylenic derivative 164 (R = R ) was unsymmetrical, only the E isomer was isolated. Compound 162 was treated with S-nucleophiles (sodium salt of benzyl mercaptan and benzenesulfinic acid) and the lithium salt of 4-hydroxycoumarin to give compounds 165-167, respectively. 

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