Electron aqueous chemistry scheme

Fe(III) salts are known to oxidise electron-rich centres to foster the formation of radical species. They are particularly efficient in the oxidation of aromatic systems or a carbanion to the corresponding carbon-centred radical which undergoes C-C bond formation to yield the coupled products. For a successful synthesis, it is important to work in the absence of reactive synthetic molecules other than those which form the combination of radicals. Barton et al. used a simple water-soluble diselenide derivative that shows radical scavenger properties towards alkyl and hydroxyl radicals in Fenton-type chemistry (Fe2+-H202)4 The reaction rate between the produced alkyl radical and the diselenide overwhelms self-termination and halogen transfer reactions. The ability of diselenide to scavenge alkyl and hydroxyl radicals [ 3(0 °C) = 6.1 x 108 M-1 s-1] could be exploited as a new tool in both synthetic and mechanistic work conducted in aqueous media (Scheme 8.5).4... 

Chromium(n). The applications of chromium(n) salts in preparative organic chemistry have been reviewed.69 The kinetics of oxidation of Cr11 to Crm by halogen radical anions, a particularly simple one-electron oxidation scheme, have been determined.70 Hydrated electrons are formed71 during the photochemical oxidation of aqueous chromium(n). 

DT, which might indicate electronic participation. The recent radical chemistry studies on 1,3,5-TT performed by radiation chemical techniques, have provided further insight into the one-electron oxidation of this compound by OH radicals in aqueous solutions. Of particular interest are consecutive reactions of 1,3,5-TT. The most characteristic feature in the oxidation of 1,3,5-TT is the formation of the C-centered radical with a dithioester function CH2SCH2SCH=S (1,3,5-TE) (reactions in Scheme 3). 

Both our photochemical and radiation studies have focused on the chemistry of very reactive species in aqueous solution. Indeed, it is because the photochemical work involved aqueous media that radiation chemistry techniques could be so useful to us. Our pulse radiolysis work has led to a number of highly unusual mechanistic conclusions. In the area of low-oxidation-state chemistry, several of the systems violate standard organometaUic dogma. We investigated the rate of hydride formation in another cobalt(I) system, that derived from the high-spin d polypyridyl-cobalt(I) complexes (28). Remarkably, electron transfer was found to be the rate-determining step for formation of the hydride complex, and contributions from Bronsted acid pathways contribute neghgibly to the rate. Rather, the hydride formation appears to involve H-atom transfer from the protonated bpy radical. The H-atom receptor may be either Co(bpy)2 or Co(bpy) as shown in Scheme II. 

Mikheev (1988,1989,1992) has obtained extensive evidence through cocrystallization that almost all the tripositive lanthanide (except for Sm, Eu, Tm and Yb) and actinide ions (U, Np, Pu, Cm, Bk) can be reduced and have (M /M ) in the neighborhood of — 2.5 to — 2.9 V. The lanthanide potentials are not consistent with the experimentally confirmed generalized f electron energetics scheme developed by Nugent (1975). The potentials are not consistent with potentials inferred from pulse-radiolysis studies (Sullivan et al. 1976,1983,1988). If the potentials (M /M ) proposed by Mikheev for uranium, —2.54 V, and plutonium, —2.59 V, at macroscopic concentrations (Mikheev etal. 1991) were correct, phase diagram studies and electrochemistry ih molten salts should have revealed the ions and Pu ", but no evidence other than cocrystallization has been presented. In fact, the crystal chemistry of the reduced uranium halide NaUjCl (Schleid and Meyer 1989) is consistent with ions and metallic electrons. The cocrystallization model (Mikheev and Merts 1990) may not be transferable to aqueous solution and thus Mikheev s (M /M ) potentials are not cited in table 5. 

In all of the photochemistry of UO2 , the nature of the bimolecular product formed by association of the reactant with the excited state U02 is an important facet of the chemistry. In aqueous solution the uranyl ion exists as the hydrate, U02(H20)5, where the five water molecules are complexed in the equatorial plane. Replacement of one or more of these complexed water molecules by an incoming substrate allows for bimolecular photoreaction to occur in the vicinity of the metal center. The reaction types can be summarized by the series of reactions shown in Scheme 8.5, where the radical pair can undergo dissociation, or either forward or back electron transfer reactions. Since the potentials for the (U02 /U02) and the (UOj/U ) couples are 0.06 V and 0.55 V, respectively, the UO2 ion can either be readily oxidized back to UOi, or it can act as a one-electron oxidant to a substrate that reacts as a reducing agent. By observing the reactions... 

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