Single-electron transfer processes

DP) and leads (Equation 4.5) to the formation of an anthracene cation radical as a result of the single-electron transfer process. The resulting ion-radical pair [AN, DP is the critical intermediate that subsequently evolves to cycloadduct (AD). 

In the reaction of Mes2Ge = NS02PhMe 137 with 3,5-di-tert-butyl-o-benzoquinone, a single electron transfer process was evidenced by an ESR study, as well as by the characterization of isobutene. The initially formed o-semiquinonic species could afford the dimesitylgermadioxolane 148 according to two different pathways590 (Scheme 27). 

The reactions of sodium dimethyl and diisopropyl phosphite with 4-nitrobenzyl chloride, 9-chlorofluorene, and diphenylchloromethane provided information that supported the proposed reaction mechanism. The RaPO anion acts towards an arylmethyl chloride as a base and abstracts a proton to form a carbanion, which can then participate in single-electron transfer processes to produce carbon-centred radicals. ... 

Steric constraints dictate that reactions of organohalides catalysed by square planar nickel complexes cannot involve a cw-dialkyl or diaryl Ni(iii) intermediate. The mechanistic aspects of these reactions have been studied using a macrocyclic tetraaza-ligand [209] while quantitative studies on primary alkyl halides used Ni(n)(salen) as catalyst source [210]. One-electron reduction affords Ni(l)(salen) which is involved in the catalytic cycle. Nickel(l) interacts with alkyl halides by an outer sphere single electron transfer process to give alkyl radicals and Ni(ii). The radicals take part in bimolecular reactions of dimerization and disproportionation, react with added species or react with Ni(t) to form the alkylnickel(n)(salen). Alkanes are also fonned by protolysis of the alkylNi(ii). 

This technique was used to synthesize some cyclohexane derivatives by substitution with electrophiles specifically in the axial or equatorial position. Whereas alkylation with dimethyl sulfate occurred smoothly, alkylation with haloalkanes failed completely. Rather nonstereoselective protonation of the lithium compound, as well as dehydrogenation to enol ethers, occurred. Operation of single electron transfer processes are believed to account for these results27. 

The difference in reactivity between saturated and unsaturated organomagnesium compounds towards a thiocarbonyl group has been interpreted by Fukui [338], and the thiophilic addition mechanism has been discussed in terms of single-electron transfer processes [329, 339, 340],... 

The deoxygenation reaction by similar single electron transfer process is also useful for the reduction of mesylates, while tosylates regenerate by hydrolysis the alcohol [86, 87] in the presence of base without epimerization. High yields are obtained in general, while in alcoholic solution, vV,/V-dimethyl thiocarbamates lead to reduction products in low yields with a predominant deprotection process [88] (Scheme 45). 

It should be noted that the sum of a and a2 is not necessarily equal to unity [37], However, under the assumption that the reaction is a one-step, single-electron transfer process, the following relations apply ... 

Recently it was found that iodine(v) compounds like 2-iodoxybenzoic acid (IBX) 7 can be used to affect selective oxidations at carbon atoms adjacent to aromatic systems. The mechanism of this transformation is believed to proceed via a SET (Single-Electron-Transfer) process. A postulated mechanism for the oxidation of benzylic positions is outlined in Scheme 32. This oxidation is quite general and proceeds efficiently in fluorobenzene/DMSO mixtures or in DMSO at 80 °C [ 135]. Starting from compounds 70, the corresponding aldehydes 71 can be obtained easily in good yields. 

Recently a number of groups, most significantly those of Crabtree and Richmond, have devised methods of carrying out reductive defluorination/ functionalization processes of saturated fluorocarbons. It is assumed that these processes involve successive single electron transfer processes [82]. Richmond and... 

The first step of the reaction involves the dimerization of MPG via single electron transfer process by Ti(III) and the sequential Ti(IV)-catalyzed intramolecular heterolytic cleavage of the dimer, regenerating M PG and the nucleophilic radical (Equations 14.25 and 14.26). 

The possibility of a single electron transfer process in substitution reactions at carbon and metal was at first gradually and now widely accepted. 

Gassman, P.G. and Bottorff, K.J. (1987) Photoinduced lactonization. a useful but mechanistically complex single electron transfer process. Journal of the American Chemical Society, 109, 7547-7548. 

N-Phthalimide-linked peptides that contain C-terminal a-amidotrimethylsilyl groups undergo macrocyclization reactions via sequential single-electron transfer processes [11, 12]. Thus, electron transfer from the neighboring amide to the excited phthalimide chromophore leads to an amide radical cation. Hole migration to the ot-amidotrimethylsilane center is followed by desilylation to form an 1, co-biradical intermediate, which finally cydizes to the macrocydic peptide (Scheme 9.9). 

Alkyl bromides, on the other hand, undergo halogen-metal exchange via single electron transfer processes, and cyclisations of alkyl bromide-derived organolithiums may proceed with significant contribution from radical pathways.117... 

Blechert et al. have opened a very elegant alternative to the classical hetero Diels-Alder methodology by generating the cationic radical 3-31 - which might be conceived as 1-aza-1,3-butadiene - from the 2-vinyl indole 3-30 by means of a single electron transfer process. In the presence of the tetrahydropyridine derivative 3-32 the radicalic intermediate was transformed into the tetracyclic compound 3-33 which contains the complete skeleton of the alkaloid gonio-mitine [250, 251]. The authors have postulated a stepwise mechanism for this [4+2] process, therefore it should be understood as a formal hetero Diels-Alder reaction (Fig. 3-11). 

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... 

Single-electron transfer processes have been implicated." When combined with asymmetric hydrosilylation using a chiral catalyst, optically active alcohols can be generated. ... 

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