Redox dimerization

Such reactions are used to prepare bridging ligands having one main symmetry element (mirror plane or Cj axis) passing between two (carbon-) bonded atoms. Except for some metal-catalyzed reactions, redox dimerizations are generally interpreted as the recombination of two radical species—followed, if needed, by a dehydrogenation step—hence generating a simple bond. 

A number of reductive procedures have found general applicability. a-Azidoketones may be reduced catalytically to the dihydropyrazines (80OPP265) and a direct conversion of a-azidoketones to pyrazines by treatment with triphenylphosphine in benzene (Scheme 55) has been reported to proceed in moderate to good yields (69LA(727)23l). Similarly, a-nitroketones may be reduced to the a-aminoketones which dimerize spontaneously (69USP3453279). The products from this reaction are pyrazines and piperazines and an intermolecular redox reaction between the initially formed dihydropyrazines may explain their formation. Normally, if the reaction is carried out in aqueous acetic acid the pyrazine predominates, but in less polar solvents over-reduction results in extensive piperazine formation. 

In view of the facile oxidation of 10.13a-c it is not surprising that some metathetical reactions with metal halides result in redox behaviour. Interestingly, lithium halides disrupt the dimeric structures of 10.13a or 10.13c to give distorted cubes of the type 10.14, in which a molecule of the lithium halide is entrapped by a Ei2[E(N Bu)3] monomer. Similar structures are found for the MeEi, EiN3 and EiOCH=CH2 adducts of 10.13a. In the EiN3 adduct, the terminal... 

Metal-induced reductive dimerization of carbonyl compounds is a useful synthetic method for the formation of vicinally functionalized carbon-carbon bonds. For stoichiometric reductive dimerizations, low-valent metals such as aluminum amalgam, titanium, vanadium, zinc, and samarium have been employed. Alternatively, ternary systems consisting of catalytic amounts of a metal salt or metal complex, a chlorosilane, and a stoichiometric co-reductant provide a catalytic method for the formation of pinacols based on reversible redox couples.2 The homocoupling of aldehydes is effected by vanadium or titanium catalysts in the presence of Me3SiCl and Zn or A1 to give the 1,2-diol derivatives high selectivity for the /-isomer is observed in the case of secondary aliphatic or aromatic aldehydes. 

Detailed kinetic studies in connection with digital simulations do confirm the RR coupling mechanism postulated in older publications as well as the oxidation of the resulting dimer D to the dication D. But the surprising drop in the height of the reduction wave for the redox pair as the concentration... 

Two ferredoxins were isolated and purified from D. vulgaris Miyazaki. Fdl, the major form, contains two redox centers with distinct behavior and a high sequence homology to D. africanus Fdlll (73). The protein is a dimer of a polypeptide chain of 61 amino acids with 7 cysteines. D. vulgaris Miyazaki Fdll is a dimer of 63 amino acids, containing 7 cysteines but only one [4Fe-4S] cluster (73-75). 

The Na/K ATPase has been extensively purified and characterized, and consists of a catalytic a subunit of around 95 kDa and a glycoprotein 0 subunit of approximately 45 kDa (Skou, 1992). The functional transporter exists as a dimer with each monomer consisting of an a and /3 subunit. Hiatt aal. (1984) have su ested that the non-catalytic jS subunit may be involved in the cottect insertion of the a subunit into the lipid bilayer and, therefore, it is conceivable that a modification of the 0 subunit structure may be reflected by changes in the catalytic activity of the a subunit. Therefore, in studies involving the manipulation of tissue glutathione levels, alterations of intracellular redox state may have an effect on substrate binding at an extracellular site on this ion-translocating protein. 

Table IV lists the redox potentials of conjugated ferrocene oligomers (mainly dimers with a single bridge). Potential values are denoted against different reference electrodes as given in the references. The values can be primarily compared using the relationship mentioned in the footnote of the table, although care should be taken with some errors derived from junction potentials which depend on experimental conditions. There have been several reports on the quantitative estimation of structural factors affecting internuclear electron delocalization.

Azo-bridged ferrocene oligomers also show a marked dependence on the redox potentials and IT-band characteristics of the solvent, as is usual for class II mixed valence complexes 21,22). As for the conjugated ferrocene dimers, 2 and 241 the effects of solvents on the electron-exchange rates were analyzed on the basis of the Marcus-Hush theory, in which the t/max of the IT band depends on (l/Dop — 1 /Ds), where Dop and Ds are the solvent s optical and static dielectric constants, respectively (155-157). However, a detailed analysis of the solvent effect on z/max of the IT band of the azo-bridged ferrocene oligomers, 252,64+, and 642+, indicates that the i/max shift is dependent not only on the parameters in the Marcus-Hush theory but also on the nature of the solvent as donor or acceptor (92). 

The third fact that seemed to argue in favor of the occurrence of radicals 10 was the observation that reactions of a-tocopherol under typical radical conditions, that is, at the presence of radical initiators in inert solvents or under irradiation, provided also large amounts of two-electron oxidation products such as o-QM 3 and its spiro dimerization product 9 (Fig. 6.8).16,25,26 This was taken as support of a disproportionation reaction involving a-tocopheroxyl radical 2 and its hypothetical tautomeric chromanol methide radical 10, affording one molecule of o-QM 3 (oxidation) and regenerating one molecule of 1 (reduction). The term disproportionation was used here to describe a one-electron redox process with concomitant transfer of a proton, that is, basically a H-atom transfer from hypothetical 10 to radical 2. 

Formation of the ethano-dimer of a-tocopherol (12) by reduction of spiro dimer (9) proceeds readily almost independently of the reductant used. This reduction step can also be performed by tocopheroxyl radicals as occurring upon treatment of tocopherol with high concentrations of radical initiators (see Fig. 6.10). The ready reduction can be explained by the energy gain upon rearomatization of the cyclohexadienone system. Since the reverse process, oxidation from 12 to 9 by various oxidants, proceeds also quantitatively, spiro dimer 9 and ethano-dimer 12 can be regarded as a reversible redox system (Fig. 6.22). 

The methano-dimer of a-tocopherol (28)50 was formed by the reaction of o-QM 3 as an alkylating agent toward excess y-tocopherol. It is also the reduction product of the furano-spiro dimer 29, which by analogy to spiro dimer 9 occurred as two interconvertible diastereomers,28 see Fig. 6.23. However, the interconversion rate was found to be slower than in the case of spiro dimer 9. While the reduction of furano-spiro dimer 29 to methano-dimer 28 proceeded largely quantitatively and independently of the reductant, the products of the reverse reaction, oxidation of 28 to 29, depended on oxidant and reaction conditions, so that those two compounds do not constitute a reversible redox pair in contrast to 9 and 12. 

FIGURE 6.22 Spiro dimer of a-tocopherol (9) formation, redox reactions, and fluxional nature. 

FIGURE 6.23 Methano-dimer of a-tocopherol (28) formation and redox reactions, including oxidation to the two fluxationally interconvertible diastereomers of furano-spiro dimer 29. 

FIGURE 6.24 Redox behavior of the methano-dimer of a-tocopherol (bis(5-tocopheryl) methane, 28) temperature dependence of the oxidation with bromine. 

Reductive activation of the quinone shown in Scheme 7.9 and incubation in methanol afforded a complex mixture of products consisting mainly of head-to-tail coupling at C-5 or C-7 (Scheme 7.10). Minor reactions involve transfer of H2 from the hydroquinone to the ene-imine (internal redox reaction) and methanol trapping. The structures of the dimers and trimers in Scheme 7.10 were derived from H-NMR,... 

A vanadium catalyst is essential although the combination of Zn and MejSiCl is capable of reductive dimerization of aldehydes [20]. A reversible redox cycle for the in situ generated low-valent vanadium species mediating the electron transfer is achieved in the presence of Zn as the stoichiometric co-reductant (Scheme 4). 

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