Electron transfer mechanism for

Fig. 10-1. Electron transfer mechanism for a chain process of iodo-de-diazoniation in the solid state (after Gougoutas, 1979).

The electron transfer mechanism for antioxidant activity corresponding to eq. 16.5 makes the standard reduction potentials of interest for evaluation of antioxidative activity. The standard reduction potential of the phenoxyl radical of several flavonoids has been determined and forms the basis for correlation of rate of electron transfer for various oxidants from the flavonoid (Jovanovic etal., 1997 Jorgensen and Skibsted, 1998). The standard reduction potentials have also been used to establish antioxidant hierarchies. 

It has been proposed that there may be a single electron transfer mechanism for the Mukaiyama reaction under certain conditions.72 For example, photolysis of benzaldehyde dimethylacetal and 1-trimethylsilyloxycyclohexene in the presence of a... 

Manganese in its various oxidation states, such as, Mn+7, Mn+4 and Mn+3, had been exploited in several studies. An electron transfer mechanism for the inter-molecular arylation of malonate esters in the presence of manganese (III) triacetate... 

Electron-transfer mechanism for nucleophilic addition. In accord with Mulliken theory, irradiation of the charge-transfer band of [Py+, BMeT] directly affords the radical pair via one-electron transfer (equation 46). 

The electron-transfer mechanism for electrophilic aromatic nitration as presented in Scheme 19 is consistent with the CIDNP observation in related systems, in which the life-time of the radical pair [cf. (87)] is of particular concern (Kaptein, 1975 Clemens et al., 1984, 1985 Keumi et al., 1988 Morkovnik, 1988 Olah et al., 1989 Johnston et al., 1991 Ridd, 1991 Rudakov and Lobachev, 1991). As such, other types of experimental evidence for aromatic cation radicals as intermediates in electrophilic aromatic nitration are to be found only when there is significant competition from rate processes on the timescale of r<10 los. For example, the characteristic C-C bond scission of labile cation radicals is observed only during the electrophilic nitration of aromatic donors such as the dianthracenes and bicumene analogues which produce ArH+- with fragmentation rates of kf> 1010s-1 (Kim et al., 1992a,b). 

The proposed electron transfer mechanism for this photoreduction parallels that given earlier for nitrobenzene and is supported by the observation of two first order transient absorptions (t 1 ms) in flashed acidified 2-propanol solutions of 7. These absorptions are assigned to the radical 4 and its conjugate acid 5. 

Fig. 9. Possible electron transfer mechanism for NOS utilizing a pterin radical. The oxy-complex in 2 is shown as the ferric (Fe +)-superoxide complex. The role of the pterin then is to donate an electron to the iron, thus giving the peroxy dianion in 3. The dianion is a potent base that abstracts a proton from the substrate, giving 4. The system is now set up for a peroxidase-like heterolytic cleavage of the 0-0 bond to give the active hydroxylating intermediate in 5 and, finally, the first product in 6.

The electron transfer mechanism for the CNT-MPc modified electrode may be represented as shown in Figure 3, where the immobilized MPc and CNT act as electrocatalyst and electron conducting species, respectively. 

Haber and Weiss postulated an electron transfer mechanism for the ferrous ion catalyzed decomposition of peroxides, the metal donating an electron... 

Intermediates 162 may be generated electrochemically,224,225 with zinc,219,221,223 sodium, potassium, magnesium,221 or organometallics.2,8,220 An electron-transfer mechanism for a reaction of perchlorate 158 with tert-butylmagnesium bromide was considered220 as follows (Eq. 7). 

The series of elementary steps which constitute the overall electron transfer mechanisms for outer-sphere and inner-sphere reactions are illustrated in Schemes 1 and 2. 

The discovery that lithium and its alkyls produce a highly cis-1,4 polyisoprene in hydrocarbon solvents (103) has led to a renewed interest in metal and metal alkyl initiated polymerization. About the same time Szwarc (109) postulated an electron transfer mechanism for the initiation of polymerization by sodium naphthalene in ether solvents. This was extended to lithium metal catalysis by Tobolsky (80) and Overberger (83) and subsequently generalized to cover all alkali metal initiation, e" + M M (1) ... 

The electron transfer (if there is an ion radical pair) intermediate is generally thought to participate in the polar mechanism for cycloaddition reactions28. Recently, the electron transfer mechanism for the zwitterionic cycloaddition of tetracyanoethylene and bis(4-methoxycinnamyl) ether has been discounted and there is now strong support for the theory that a polar mechanism is also operative for other systems29. 

Fig. 9. Proposed electron-transfer mechanism for the sensitization of YbIn luminescence by the excited state of tryptophan (Trp). Figures are energies of the states in eV. Redrawn from (deW. Florrocks et al., 1997).

The homolysis of a metal alkyl may be regarded as a reductive elimination however, it is convenient to consider such reactions separately (Section V,B). Electron-transfer mechanisms for organometallic intermediates in catalytic reactions have been reviewed (129) examples are in the formation of transient RCu(I) or RCr(III) in oxidation (by Cu(II)) or reduction (by Cr (II)) of R, and in the role of Fe in the Kharasch-Grignard... 

Doerge DR (1986) Oxygenation of organosulfur compounds by peroxidases evidence of an electron transfer mechanism for lactoperoxidase. Arch Biochem Biophys 244 678-685... 

Further work by Flowers examined the role of solvent polarity in the electron transfer process.30 Inner-sphere electron transfer kinetics show a weak dependence on solvent polarity due to the considerable orbital overlap of the donor-acceptor pair in the transition state. In an outer-sphere process, changes in solvent polarity alter the energetics of electron transfer. The addition of excess HMPA, beyond that required to saturate Sml2, resulted in a linear correlation to the rate of reduction for alkyl iodides, whereas no impact was observed on the rate of ketone reduction.30 Thus the experiments showed a striking difference in the electron transfer mechanism for the substrate classes, which is consistent with the operation of an outer-sphere-type process for the reduction of alkyl iodides and an inner-sphere-type mechanism for the reduction of ketones.30 These findings are consistent with the observations of Daasbjerg and Skrydstrup.28,29... 

It was suggested at one time that the chlorination of polymethyl-substituted aromatic hydrocarbons might proceed by a non-bonded electron-transfer mechanism (for arguments for and against, see Kochi, 1975 Baciocchi and Illuminati, 1975 Hart et al., 1977). The low value of E° for the C12/C12 couple, 0.6 V (Malone and Endicott, 1972), makes this suggestion rather unlikely (E° of, e.g. hexamethylbenzene is 1.85 V). 

Mechanistic speculations about the molybdoenzymes must be considered to be in their infancy with the possible exception of those for xanthine oxidase. Although the detailed structural nature of the molybdenum site is unknown, there is sufficient information from biochemical and coordination chemistry studies to allow informed arguments to be drawn. Here we first discuss evidence for the nuclearity of the molybdenum site and then discuss both oxo-transfer and proton-electron transfer mechanisms for molybdenum enzymes. A final discussion considers the unique aspects of nitrogenase and the possible reasons for the use of molybdenum in enzymes. 

The study of mercuration and thallation provides a shaip focus on the experimental delineation of stepwise and concerted mechanisms for arene activation. Thus the unequivoc demonstration of arene radical cations as key intermediates in thallation, particularly of durene and pentamethylbenzene, is consistent with a stepwise (electron-transfer) mechanism for arene activation (compare Scheme 6 and equation 39). 

Pyridinyl radicals react with haloorganic compotinds at ratra that are very sensitive to the strength of the halogen-carbon bond. Both atom-transfer and electron-transfer mechanisms for the reaction have been detected, with the latter exhibiting a large response to solvent polarity change and to variations in the electron affinity of the organic halide. 

Boranes are generally less electron rich than corresponding aluminium analogues and electron transfer mechanisms are usually not considered for reduction reactions. However, an increase of the hydridic character in complex hydrides such as.LiBEt3H ( super hydride ) [68] or LiAlH4 [66] allows for the formation of well-characterized radical products in reactions with unsaturated acceptor heterocycles such as (3). Electron transfer mechanisms for the reduction by complex hydrides should be quite intricate because the coordinatively saturated donor moiety (MHn ) and the a acceptor part (e.g. Li" ) can now well separately interact with the coordinating n acceptor substrate. 

Triorganostannates such as LiSnRs are sufficiently electron rich to be potential candidates for electron transfer to halides [52,126]. As for the silicon analogues MSiRa [115], the electron transfer mechanisms for the reduction by such at-complexes may be quite complicated because of the solvent-induced separation between the SiRJ donor anion and the a acceptor cation M. ... 

The nomenclature [348] is such that a mediator exchanges electrons with substrates by an outer-sphere electron-transfer mechanism (for instance ferrocene/... 

The dynamics of both static and dynamic quenching of the fluorescent singlet states of diazapyrenium salts by nucleotides has been investigated by Brun and Harriman using sub-nanosecond time-resolved transient absorption spectroscopy [88]. Observation of the reduced acceptor DAP+ (Table 5) supports an electron transfer mechanism for fluorescence quenching. Diffusion-controlled rate constants were observed for quenching of DAP + by all four deoxynucleotides. Excitation of... 

The potential stereoselectivity of this photocyclization process has recently been investigated for the two rigid 2-allylanilines 70 and 71169. Irradiation of compound 70 at room temperature gives a mixture of diastereomers trans-12 and cis-12 with a little stereoselectivity (equation 23). While a poor stereoselectivity is also observed for 71, the photocyclization is regioselective, where the products trans-12 and cis-12 are minor (equation 24). However, the diastereoselectivity of trans-12 vs cis-12 is increased in the case of 71 when the temperature is changed, indicating that the reaction is significantly entropy-controlled. In addition, the observation of fluorescent exciplex formation for 70 and 71 supports the electron-transfer mechanism for the photocyclization of 2-allylanilines. 

The photochemistry of 4-chloroanilines in methanol, dioxane-water and diox-ane-methanol solvents has been investigated for more than thirty years by Latowski185,186. Large quantum yields of HC1 formation (hci) have been observed for the photolysis of 91a in protic solvents (e.g. Hci = 0.78 in methanol at 254 nm). However, the values of 4>hx are relatively small for 4-bromoaniline (HBt = 0.19), 4-iodoaniline (cbm = 0.29), 2-chloroaniline (hci < 0.02) and 3-chloroaniline (hci = 0.02) under the same condition. N-Acetylation of 91a to 4-chloroacetanilide also inhibits the photolytic process. In conjunction with the solvent- and concentration-dependent photolysis rates of 91a, these results indicate an electron-transfer mechanism for the photochemical reaction electron transfer occurred from an excited 91a to an unexcited 91a molecule, followed by ionization reactions. However, recent analysis of photoproducts from 91a in water/methanol mixtures has shown that benzidine (92) is a major product along with aniline (equation 29)187. As a result, a carbene mechanism that leads to the formation of aniline radicals was put forward in analogy to the photochemistry of 4-halophenols188,189. For example, the photolysis of 91a in aqueous solution first results in the transient species carbene 93 followed by the formation of the aniline radical 94 that was observed as the primary product (Scheme 13)190. In addition to la and 92, other identified secondary products include 4-aminodiphenylamine, 2-aminodiphenylamine, hydrazobenzene, 4-chloronitrosobenzene and 4-chloronitrobenzene, but they are all in low yields191. 

For the case of CF , the rate constants have been measured at pH 1 for a series of -substituted phenols, the value for phenol being 2.5 x 10 M s . The rate constants increase with increasing electron-donating power of the substituent. A plot of the rate constants vi the Hammett a values yields p = — 1.5, indicating an electron transfer mechanism for the formation of the phenoxyl radicals . The weaker oxidant Br2 reacts with phenol more slowly, k = 6x 10 M s . However, upon increasing the reducing power by going from phenol to phenolate, the rate constant increases to ca 4 X 10 s . (SCN)2 and 12 are even weaker oxidants than Br2 and... 

---