Electron-transfer-mediated benzylic

The key step involves an electron transfer-mediated benzylic umpolung of the (central-chiral) complex 25 by means of the one electron reducing agent LiDBB (lithium di-tert-butyl diphenyl). Obviously, both the formation of the (planar-chiral) reactive intermediate 26 and the subsequent alkylation proceed in a stereo-controlled manner with overall retention of configuration. 

Topics that have formed the subjects of reviews this year include contemporary issues in electron transport research, dynamics of bimolecular photoelectron transfer reactions, photophysical properties of functionalised fullerene derivatives, carbon-carbon bond formation via radical ions, photoinduced electron transfer processes in ketone, aldehyde, and ester synthesis, photochemical reactions between arenenitriles and benzylic donors, photo-oxidation of conjugated dienes, photoredox reactions of aromatic nitro compounds, electron transfer-mediated photochemistry of some unsaturated nitrogen-containing compounds, reactions of 02( Ag), carbon dioxide activation by aza-macrocyclic complexes, and photochromism of chalcone derivatives. ... 

Lund and coworkers [131] pioneered the use of aromatic anion radicals as mediators in a study of the catalytic reduction of bromobenzene by the electrogenerated anion radical of chrysene. Other early investigations involved the catalytic reduction of 1-bromo- and 1-chlorobutane by the anion radicals of trans-stilhene and anthracene [132], of 1-chlorohexane and 6-chloro-l-hexene by the naphthalene anion radical [133], and of 1-chlorooctane by the phenanthrene anion radical [134]. Simonet and coworkers [135] pointed out that a catalytically formed alkyl radical can react with an aromatic anion radical to form an alkylated aromatic hydrocarbon. Additional, comparatively recent work has centered on electron transfer between aromatic anion radicals and l,2-dichloro-l,2-diphenylethane [136], on reductive coupling of tert-butyl bromide with azobenzene, quinoxaline, and anthracene [137], and on the reactions of aromatic anion radicals with substituted benzyl chlorides [138], with... 

Palmore et al. applied the biocatalytic approach, utilizing the enzyme diaphorase (EC 1.6.4.3) to catalyze the reoxidation of NADH homogeneously, transferring electrons to a mediator, benzyl violo-gen. The mediator was then reoxidized at an electrode surface, with the overall scheme... 

The ozone-mediated reaction of bicumene and some derivatives (11) with nitrogen dioxide in dichloromethane (kyodai nitration) at low temperatures results in the cleavage of the central C—C bond to yield the benzyl nitrate and products therefrom, in contrast to the behaviour of bibenzyl.36 This result is interpreted in terms of electron transfer from the substrate to NO3- to give a radical cation species which fragments to form tertiary benzylic species in the former cases. 

Benzyl chloride and its substituted derivatives are electrochemically reduced indirectly through the mediator, l,4-dihydro-4-methoxycarbonyl-l-methylpyridine anion 35, (equation 27). The rates of the electron transfer between the mediators and benzyl halides, measured by cyclic voltammetry, were found to be about 961 80 M 1 s1. The ratedetermining step was proposed to involve a single electron transfer from the mediator to... 

Recently, electron-transfer catalysis by viologen compounds has attracted much attention. The compounds function as mediators of electron transfer and have been applied in the reduction of aldehydes, ketones, quinines, azobenzene, acrylonitrile, nitroalkenes, etc., with zinc or sodium dithionite in a monophase or a two-liquid phase system [13]. Noguchi et al. [13] found that a redox-active macrocyclic ionene oligomer, cyclobis(paraquat-/ -phenylene), acted as an electron phase-transfer catalyst for the reduction of quinines, as compared with acyclic benzyl viologen. The enhanced activity of this compound is due to the inclusion of the substrate into the catalyst cavity. 

The magnesium-mediated reductive trifluoromethylation also works for other structurally diverse chlorosilanes. Chlorotriethyl-silane, f-hutyldimethylsilyl chloride, and tris(trimethylsilyl)silyl chloride have been applied to prepare corresponding trifluoro-methyl-containing silanes. However, the reductive trifluoromethylation did not take place with other electrophiles such as aldehydes, ketones, allyl bromide, benzyl chloride, or tributyltin chloride. Even tributyltin hydride and allyltrimethylsilane showed no reactivity The reason for such behavior is not clear. Probably, chlorosilanes play an important role during the reductive trifluoromethylation both as a silylating agent and a single-electron transfer promoter. 

The authors proposed a mechanism based on a cage-mediated guest-to-host electron transfer (Fig. 9.30) in which the cage acted as a photosensitizing molecular flask. Excitement of the coordination cage, followed by electron transfer from alkyne to an electron-deficient cage and the reaction of a molecule of water (solvent) with the obtained phenyl alkyne radical cation, results in benzylic radicals and subsequently the anti-Markovnikov product. 

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