Chromium-manganese redox couple

Scheme 7.18. Chromium-manganese redox couple-mediated domino process for the synthesis of benzoxazoles.

Copper(II) and cerium(IV) have been studied as oxidants in acetonitrile. The copper(II)-copper(I) couple has an estimated electrode potential of 0.68 V relative to the silver reference electrode. It has been studied as an oxidant for substances such as iodide, hydroquinone, thiourea, potassium ethyl xanthate, diphenylbenzidine, and ferrocene. Cerium(IV) reactions are catalyzed by acetate ion. Copper(I) is a suitable reductant for chromium(VI), vanadium(V), cerium(IV), and manganese(VII) in the presence of iron(III). For details on many studies of redox reactions in nonaqueous solvents, the reader is referred to the summary by Kratochvil. ... 

The last method, F, which uses NiCl2-2,2 -bipyridine complex, works well only for the most reactive aryl iodides [31]. It is mechanistically more complicated because here lead(II) bromide acts as the source of lead(0) which actually reduces Ni"(bpy)X2 back to the catalytically active Ni°(bpy). Since the 2,2 -bip)ridme 75 is somewhat more electron-rich ligand than simple 2,2 -bipyridine (5), the resulting nickel(II) complex employed in the related Chen s method is sUghtly more reactive with electron-poor aryl halides, e.g. bromides, but generally the method is appUcable only with aryl iodides [30]. For instance, both iodo- and bromobenzene gave biphenyl (8) in 98 and 88% yields, respectively, but 2-bromothiophene was coupled in only 22% yield [30], Scheme 8. Chromium(II) chloride in a catalytic amount was used as an intermediate reductant as it provides an effective redox system, Cr(II) / Cr(III) between Ni(0) / Ni(II) and metallic manganese as the ultimate reductant. 

In most natural waters at near neutral pH, Cr is the dominant form due to the very high redox potential for the couple Cr /Cr (Rai etal., 92>9). Chromium(III) forms strong complexes with hydroxides. Rai et al. (1987) report that the dominant hydroxo species are CrOH at pH values 4-6, Cr(OH)3 at pH values from 6 to 11.5, and Cr(OH)4 at pH values above 11.5. The OH ligand was the only signifrcant complexer of Cr in natural aqueous solutions that contain environmental concentrations of carbonate, sulfate, nitrate, and phosphate ions. The only oxidant in natural aquatic systems that has the potential to oxidize Cr ° to is manganese dioxide. This compound is common on Earth s surface and thus... 

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