Single-electron reductant

The anomeric configuration is set in the reductive lithiation step, which proceeds via a radical intermediate. Hyperconjugative stabilization favors axial disposition of the intermediate radical, which after another single electron reduction leads to a configurationally stable a-alkoxylithium intermediate. Protonation thus provides the j9-anomer. The authors were unable to determine the stereoselectivity of the alkylation step, due to difficulty with isolation. However, deuterium labeling studies pointed to the intervention of an equatorially disposed a-alkoxylithium 7 (thermodynamically favored due to the reverse anomeric effect) which undergoes alkylation with retention of configuration (Eq. 2). 

SCHEME 2.13 Activation of QMPs (quinone methide precursors) by base catalysis and single-electron reduction (reproduced from Ref. [47] with permission from American Chemical Society). 

Single Electron Reduction of Nucleobases in Biology and in Model Systems... 

Although the reduction potentials of DNA bases and UV induced DNA lesions inside a DNA double strand or inside the active site of a DNA photolyase, together with the reduction potential of the photoexcited FADH- in the photolyases, are not known, currently available redox potentials indicate that the single electron reduction of a nucleobase or a UV induced dimer lesion by a reduced and deprotonated flavin coenzyme is a weakly exothermic process. The reduced and deprotonated FADH- in its photoexcited state is... 

Although the reduction potentials argue for thymine, as the most easily reducable base in protic solvents like water, subsequent protonation reactions need to be considered as well. The coupling of single electron reduction with a subsequent protonation step will strongly affect the ease of single electron reduction. Table 2 contains the pKa-values of some nucleobases in their reduced and neutral states [37]. It is clear that the thymine radical anion, due to its rather neutral pKa-value of about 7 is unlikely to become pro-tonated either by water or by the adenine counter base in the DNA strand. 

Scheme 3 Depiction of an A T and of a G C base pair after single electron reduction of the corresponding pyrimidine base...

The single-electron reduction and oxidation of Co(salen) is solvent dependent as a result of the available coordination sites perpendicular to the CoN202 plane.1220 Furthermore, substituents on the phenyl rings modulate the observed redox potentials and subsequently the 02 binding constants. Hammett correlations are obtained.1221 Potentiometric titrations were performed to determine the 02 binding constants and species distribution as a function of pH for a variety of Schiff base Co complexes.1222... 

Reduction of the dimer appears to take place in two stages. In the first cathodic wave, a single-electron reduction generates a radical which decomposes in a chemical step in an ECE sequence ... 

A single-electron reduction creating one anion and one radical ... 

Qrunones can accept one or two electrons to form the semiquinone anion (Q ") and the hydroquinone dianion (Q ). Single-electron reduction of a quinone is catalyzed by flavoenzymes with relatively low substrate selectivity (Kappus, 1986), for instance NADPH cytochrome P-450 reductase (E.C. 1.6.2.3), NADPH cytochrome b5 reductase (E.C. 1.6.2.2), and NADPH ubiquinone oxidoreductase (E.C. 1.6.5.3). The rate of reduction depends on several interrelated chemical properties of a quinone, including the single-electron reduction potential, as well as the number, position, and chemical characteristics of the substituent(s). The flavoenzyme DT-diphorase (NAD(P)H quinone acceptor oxidoreductase E.C. 1.6.99.2) catalyzes the two-electron reduction of a quinone to a hydroquinone. 

The previous chapter covered radical cation cyclization reactions that were a consequence of single-electron oxidation. In the following section, radical anion cyclization reactions arising from single-electron reduction will be discussed. In contrast to the well documented cyclization reactions via carbon-centered free radicals [3, 4], the use of radical anions has received limited attention. There are only a few examples in the literature of intramolecular reductive cyclization reactions via radical anions other than ketyl. Photochemi-cally, electrochemically or chemically generated ketyl radical anions tethered to a multiple bond at a suitable distance, have been recognized as a promising entry for the formation of carbon-carbon bonds. 

GICs, via the one-electron process in both cases. According to Shu et al., these intermediate species underwent further single-electron reduction and produce Li2C03 and propylene gas, while alkyl carbonates are generated via radical termination as shown in Schemes 1 and yss zn become the major ingredients in the surface film. 

The reductive dehalogenation of polyfluoroarenes by zinc in aqueous ammonia gave products derived from the removal of one or two halogen atoms. A radical anion is suggested to form initially by direct electron transfer from the zinc to substrate which then fragments. Ceo undergoes single-electron reduction by the electron-rich. 

The electrochemical properties of 4,8-bis(dicyanomethylene)-4,8-dihydrobenzo[l,2-A4,5- ]difuran 72 have been investigated by cyclic voltammetry in the search for new materials with enhanced conducting properties <2000JOC2577>. Upon single-electron reduction the central quinonoid ring gains the aromatic stability of benzene, whereas the fused heterocyclic part loses the aromaticity of the furan nucleus. 

Flavin redox states in a dual flavin enzyme. (Left) Single-electron reduction of the isoalloxazine ring generates the semiquinone radical, while reduction by two electrons generates the fully reduced species. (Right) Five possible oxidation levels of a dual flavin enzyme, where the FMN reduction potential is held at a more positive value relative U) FAD. The flavins can theoretically accept a maximum of four electrons obtained from two NADPH. However, in NADPH-cytochrome P450, reductase, full reduction of the flavins is not normally reached when NADPH serves as the reductant. 

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