Lower oxidation potentials

This ether has properties similar to the p-methoxybenzyl (MPM) ether except that it can be repioved from an alcohol with DDQ in the presence of an MPM group with 98% selectivity The selectivity is attributed to the lower oxidation potential of the DMPM group 1.45 V for the DMPM versus 1.78 V for the MPM. 

Eisch, Behrooz and Galle196 give compelling evidence for the intervention of radical species in the desulphonylation of certain acetylenic or aryl sulphones with metal alkyls having a lower oxidation potential at the anionic carbon. The primary evidence presented by these workers is that the reaction of 5-hexenylmagnesium chloride outlined in equation (85) gives a mixture of desulphonylation products, in accord with the known behaviour of the 5-hexenyl radical, in which the cyclopentylmethyl radical is also formed. 

The nucleophilic reaction with the solvent is of crucial importance. Monomers with lower oxidation potentials (aniline and pyrrole) can easily be polymerized even in aqueous electrolytes. For monomers with higher oxidation potentials, aprotic solvents must be used, such as acetonitrile... 

In section 2.2, we reported that the rate constants of the hole transfer from Py + to oxG were 3xl04 s 1 for Py-TTT-oxG and smaller than lxlO3 s 1 for Py-TTTTT-oxG [45]. Compared with the rate constants for Py-oxG-ODNs, the hole transfer rates for PtzPy-1 and PtzPy-3 were much faster. This result is probably explained by the difference of AG. The energy gap (AE) between the oxidation of Py and oxG is 0.31 eV, whereas AE between the oxidation of Py and Ptz is 0.64 eV. Namely, the lower oxidation potential of Ptz is considered to be responsible for the increase in the hole transfer rate. 

I luorophorcs having lower oxidation potentials also enhance the PO-CL reaction. The addition of these compounds to the postcolumn reagent is very effective so as to increase the sensitivity. The attempt to enhance CL intensity using micelles of surfactants has also been reported [33], but it has yet not been applied to HPLC. 

Thus, the oxidation potential of the former type of diene (limonene) is substantially the same as that of the corresponding monoolefin (1-Me-cyclohexene), whereas norbor-nadiene and bicyclo[2.2.2]octadiene show much lower oxidation potentials than those of norbornene and cyclohexene. 

Moreover, one should mention that in spite of similar electronic structures, PBN and the isoquinoline nitrone (278) react in a different way. Under no circumstances does PBN give an oxidative methoxylation product, whereas nitrone (278) reacts readily to form a,a-dialkoxy-substituted nitroxyl radical (280) (517). Perhaps this difference might be due to the ability to form a complex with methanol in aldo-nitrones with -configuration. This seems favorable for a fast nucleophilic addition of methanol to the radical cation (RC), formed in the oxidation step. The a-methoxy nitrone (279), obtained in the initial methoxylation, has a lower oxidation potential than the initial aldo-nitrone (see Section 2.4). Its oxidation to the radical cation and subsequent reaction with methanol results in the formation of the a,a-dimethoxy-substituted nitroxyl radical (280) (Scheme 2.105). 

It was Breslow (Breslow, 1982 Breslow et ai., 1982) who first paid attention to this theory. Knowing that the pentachlorocyclopentadienyl cation (Breslow et ai, 1964 Saunders et ai, 1973), the hexachlorobenzene dication (Wasserman et al., 1974) and the 2,3,6,7,10,11-hexamethoxy-triphenylene (HMT, [36]) dication are all ground-state triplets, in good agreement with theory, Breslow and coworkers set out on the synthesis of analogues that should have lower oxidation potentials, be chemically more stable and therefore form CT complexes more readily (Fig. 21c Breslow et al., 1982, 1984 Breslow, 1985, 1989 LePage and Breslow, 1987). 

The photoreaction of polysilanes with Ceo has also been investigated [35]. Reaction (8.16) shows an example in which the irradiation in benzene with a low-pressure mercury-arc lamp afforded a product that contains 14wt% of Ceo into the polysilane chain. The incorporation of Ceo into the polysilane backbone has not been observed upon irradiation with X > 300 nm, when the cleavage of Si—Si bond does not take place. The adduct obtained from Reaction (8.16) has a lower oxidation potential than C6o( + 0-77 vs + 1.21 V) and a lower reduction potential than polysilane (—1.24 vs — 2 V). 

The second competing fragmentation reaction can be avoided by using a vinyl sulphide substrate 34. Vinyl sulphides have a lower oxidation potential compared... 

The hydroxyl radical can also abstract a single electron from dG to generate the base radical cation (G ). In duplex DNA, the G " " will be stabilized by its delocalization into adjacent bases. Both calculations and kinetic measurements " indicate that GG sequences have a lower oxidation potential than an isolated G. Nucleo-bases on the 3 -side of G determine the extent of G formation, and here purines are more effective than pyrimidines at lowering the oxidation potential of G, which accounts for the GG effect and that GA sites are also reactive. ... 

A similar pattern of reactivity has been observed by Burrows and coworkers for the reaction between A -acetyllysine methyl ester (Lys) and dG. This reaction was studied in order to gain an understanding of structural aspects of DNA-protein cross-links (DPCs). These cross-links are regarded as a common lesion of oxidative damage to cells, but remain, from a chemical point, a poorly understood DNA lesion. As pointed out by Burrows, oxidation of protein-DNA complexes should occur preferentially at the primary amines since these sites have a lower oxidation potential (1.1 V vs. NHE, pH 10) than G. While protonation of the primary amine inhibits the oxidative process, transient deprotonation of a lysine residue would give rise to a lysine aminyl radical (or aminium radical cation). Using... 

Disilenes have much lower oxidation potentials than olefins , and consequently they are much more reactive toward 02. Typically, disilenes 93 react in solution with triplet oxygen to give 1,2-disiladioxetanes 94 as the major product, accompanied at room temperature by a smaller amount of disilaoxirane 95 (equation 93) °. ... 

According to calculations by Schweig and coworkers X -phosphorins should be oxidized to the radical cation at a lower oxidation potential than the X -phosphorins. Careful experiments on l.l-dimethoxy-2.4.6-triphenyl-X -phos-phorin by Weber confirmed these predictions. Accordingly, the 6jt system of X -phosphorin loses an electron more readily than that of X-phosphorin it is thus not the lone-electron pair at phosphorus, but rather the 6jr-electron system which is responsible for the easy oxidation of the phosphorins to radical cations. 

Antioxidants are very effective in stabilizing products undergoing a free-radical mediated chain reaction. These products possess lower oxidation potentials than the active drug. Ideally, antioxidants are stable over a wide pH range and remain soluble in the oxidized form, colorless, and nontoxic. A listing of commonly used antioxidants can be found in Table 3. 

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