Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Stable radical anions

Reductive dunenzation to form fluorinated benzopinacols proceeds m the partly fluormated case either with zinc or by photolysis but is not observed with perfluorobenzophenone [651 (equation 53). Trifluoroacetophenone is reduced electrochemically in dimethylformamide to a stable radical anion, which, m the presence ot lithium ion, rapidly dunerizes to pinacol in higher yield than that available by photoreduction [66] (equation 54)... [Pg.309]

The redox chemistry of stannyl metalcarbonyl clusters has been studied in one case177. CV of the cluster PhSnCo3(CO)i2 shows an irreversible reduction peak —0.74 V vs Ag wire, at 100 mV s 1. Faster scans were not reported. It is concluded that the radical anion [PhSnCo3(CO)i2r is unstable, in contrast to the silicon cluster PhSiCo3(CO)n which gives a stable radical anion (reported lifetime of 2.3 s) at —0.26 V (200 mV s 1). [Pg.699]

The CVA method shows that electrochemical reduction of 3,3 -bis(2-R-5,5-dimethyl -4-oxopyrrolidinylidene)-1,1 -dioxides (237) (Fig. 2.20), which per se are dinitrones conjugated with a C=C double bond, is an EE process that produces stable radical anions and dianions. Oxidation is an EEC- or EE-process that gives stable RC and dications (431, 432). [Pg.201]

In general, it may be expected that thiocar-bonyl compounds are easier to reduce ca-thodically than the corresponding carbonyl structures. There are a few examples dealing with the cathodic reduction of thiones. Let us quote the case of thiobenzophenone, which affords readily a rather stable radical anion at a potential that is -1-0.5 V more positive than that of benzophenone, when... [Pg.256]

The photo-reduction of 2,l,3-benzothiadiazole-4,7-dicarbonitrile (13) by EDTA in the presence of micelles gave a stable radical anion which could be observed by ESR <84CC1324>. The observed 17 line ESR signal was attributed to an overlapping quintet of quintets from a radical with (NA) = 0.255 mT and fl(NB) = 0.075 mT. The radical appears to be protected within the micelle when electron transfer is inhibited. [Pg.360]

One of the key issues in understanding these stepwise dissociative reductions is to define the thermodynamics (or at least relative thermodynamics) of the fragmentation itself. It is equally important to understand on which side of the scissile bond the charge actually resides, as this has important mechanistic implications. This is most easily inferred by comparison of the measured standard (or peak) potential with standard potentials of similarly substituted aromatic molecules that make stable radical anions. [Pg.107]

The ESR spectra of the stable radical anions of the dibenzometalle-pins (XI M = Si, R2 = Ph2, PhMe, and Me2 and M = Ge, R2 = Ph), show little sensitivity to the metal atom or metal atom substituent that lies in a node of the occupied molecular orbital (117a). In these radical anions, only d orbitals on the metal have the appropriate symmetry to interact with the orbital occupied by the unpaired electron. [Pg.305]

Processes for two-electron reductions, two sequential one-electron reductions with a radical anion intermediate, and reactions of dianions with unreduced parents to give radical anions were observed. Structural reorganization is occasionally observed, particularly in the case of Fe(CO)2 and Fe(CO)3 complexes (26). There appears to be little correlation between structure and electrochemical behavior. In general, the presence of metal-metal bonds in the substrate appears to correlate well with the ability to yield a stable radical anion on reduction. The lack of a metal-metal bond correlates, although poorly, with the ability to form radical cations (25). At present, the predictability of results from reduction in metal-carbonyl complexes is very low. The area remains one in which a great deal more work is needed. [Pg.311]

Radical cations of bisarene-transition metal complexes are produced relatively easily by oxidation (39). However, since the coordination of a neutral 7r-system to a metal is accompanied by a net metal - ligand charge transfer, the energy of the LUMO is raised upon complex formation, and the electron affinity of the complex decreases (9). Thus, bisbenzenechromium could not be converted to a stable radical anion... [Pg.312]

The reactions most commonly involved in flavin redox chemistry are shown in Equations 1.15-1.17. One-electron reduction of the flavin (Eq. 1.15) produces a relatively stable radical anion. Protonation of the radical anion produces an unstable neutral radical (Eq. 1.16), which will be rapidly reduced by another electron (Eq. 1.17) to give the flavohydroquinone anion. [Pg.19]

A reasonable model has been proposed to accommodate these results (2/y 23). The presence of quinoid functions in lignin would give rise to electron donor-acceptor complexes with existing phenolic groups. These complexes, like quinhydrone, would form stable radical anions (semiquinone anions) on basification, according to the scheme shown below. Both biological and chemical oxidation would create more quinone moieties, which in turn would increase the contribution of Reactions 1 and 2. Alternately, enzymatic (< ) and/or alkaline demethylation 16) would produce... [Pg.66]

The plot of the pH-dependence (Fig. 18) indicates qualitatively a participation of an intermediate acid-base equilibrium. Evaluation of rate constants kr and kg is made difficult by the inaccessibility of the dissociation constant of reaction (24 b) which corresponds to protonation of a radical anion. ESR would be a suitable method for the determination of the dissociation constants of at least the more stable radical anions. Another possibility for obtaining at least an approximate value of the equilibrium constant is the measurement of the shifts of the half-wave potentials of the more negative wave at potential 3 with pH. Because the half-wave potential of this wave is known to be sensitive to the... [Pg.33]

Of the array of aromatic diimides available, PMIs (1), NDIs (2), and PDIs (3) (Figure 11.1) have been the most widely used, owing to the commercial availability of the starting dianhydride precursors. Although the PDIs are the most attractive in terms of their optical properties, they suffer tremendously from solubility problems-that is, aggregation. In particular, substituted NDIs form stable radical anion and dianion species [1-7], so from this aspect NDIs have become more attractive as supramolecular components. In recent years, NDIs have been shown to be a powerful tool in organic supramolecular chemistry to separate charges from photoexcited donor molecules. In this respect, NDIs are utilized as the electron acceptors [2-5]. [Pg.265]

The ratio of 171a and 171b depends on the method used to neutralize the reaction, though both isomers were always formed. Only arylation at y was observed. Coupling of the radical in the y-position gave the most stable radical anion in which the double bond and the nitrile group are conjugated. [Pg.1438]

In the photostimulated reaction of 2-naphthoxide ion (198) with an o-dihalobenzene, an aromatic a radical may be formed very close to the oxygen functionality along the chain propagation cycle of the S l mechanism. This spatial proximity and the fact that the intramolecular coupling between the two moieties will form a relatively stable radical anion will favour the reaction between both reactive centres. Thus in the photostimulated reaction of o-dihalobenzenes with 198 in liquid ammonia, the formation of the monosubstitution 351 and of the cyclization product 352 were reported in yields that depend on the substrate and on the reaction conditions (equation 205)346. [Pg.1474]

Reduction of halonitrobenzenes in liquid NH3 to form stable radical anions [136]... [Pg.49]


See other pages where Stable radical anions is mentioned: [Pg.27]    [Pg.166]    [Pg.699]    [Pg.258]    [Pg.838]    [Pg.847]    [Pg.1099]    [Pg.79]    [Pg.74]    [Pg.96]    [Pg.181]    [Pg.309]    [Pg.201]    [Pg.1035]    [Pg.94]    [Pg.137]    [Pg.489]    [Pg.96]    [Pg.658]    [Pg.341]    [Pg.365]    [Pg.63]    [Pg.172]    [Pg.181]    [Pg.429]    [Pg.266]    [Pg.1400]    [Pg.1401]    [Pg.84]    [Pg.194]    [Pg.606]    [Pg.655]    [Pg.655]   
See also in sourсe #XX -- [ Pg.88 ]




SEARCH



Radicals stable

© 2024 chempedia.info