Atom transfer mechanism

The kinematic theory can now be used to determine the appropriate values of Js, lQtotal, and It (Equations 15, 16, and 17, respectively) for various assumed values of k, the rate constant of Reaction O—a quantity about which nothing is known—and for the various mechanisms. Obviously from the chemistry of the system, Reactions U, V, and W cannot occur by a hydrogen atom transfer mechanism hence, only two cases need be considered—Reactions S, T, U, V, and W, occurring by... 

Suggestions that the sulphate catalysed paths may involve a mechanism with a sulphate-bridged activated complex, as opposed to a hydrogen-atom transfer mechanism, have been made. ... 

Ce(IV) + Fe(II) system, as calculated by Dulz and Sutin, on the grounds that the rate of the Fe(III)+Fe(II) exchange (/ci,i) and the corresponding oxidation potential relate to HCIO4 media, whereas the rate (A 2,2) and oxidation potentia of the Ce(IV)+Ce(III) system are for H2SO4 media. Adamson et arrive at a calculated value of 1.3 x 10 l.mole sec for the rate coefficient (A i,2) of the Ce(lV)+Fe(II) reaction in 0.5 M HCIO4 at 0 °C. Since this value is very much at variance with the observed value (700 l.mole sec" ), they conclude that this oxidation takes place by an atom-transfer mechanism, to which the theoretical treatment of Marcus is not appropriate. 

The reactions of several Co(ii) complexes have been examined (Halpern, 1974), namely, pentacyanocobaltate(n) (Chock and Halpern, 1969 Halpern and Maher, 1964, 1965 Kwiatek and Seyler, 1965,1968 Kwiatek, 1967), bis-(glyoximato)cobalt(il) (Schneider et al., 1969), cobalt(li) Schiff s base (Marzilli et al., 1970, 1971) and bis(dioximato)cobalt(ii) (Halpern and Phelan, 1972) complexes. A halogen-atom-transfer mechanism has been proposed for most halides (158, 159), with the exception of the reaction of cobalt(ii) Schiflf s... 

Similar investigations have been carried out and similar conclusions reached with the reaction of chromium(ii) complexes with alkyl halides (Castro, 1963 Kochi and Davis, 1964 Kochi and Mocadlo, 1966 Kray and Castro, 1964). The main argument in favour of the halogen-atom-transfer mechanism in this case was the order of reactivity of the halides tertiary > secondary > primary. 

Oxidation of Fe " aq in solution by ozone, at pH 0 2, showed evidence for an oxygen atom transfer mechanism with as a transient intermediate en route to Fe +. The rate constant at 298 K is... 

X 10 M at 1.0 M ionic strength. A simple outer-sphere mechanism can be ruled out since k is more than two orders of magnitude greater than predicted by the Marcus cross-relation, and an oxygen atom transfer mechanism has been proposed. ... 

For the one-electron reduction of nitrite to NO by nitrate reductase, an O atom transfer mechanism is unlikely, inasmuch as O atom transfer is inherently a two-electron mechanism. 

Organonickel(II) species are believed to be formed during the reaction between [Ni(TMC)] and primary alkyl halides, and subsequently undergo hydrolysis with cleavage of the Ni—C bond. Kinetic data measured in the presence of excess alkyl halide indicate a rate law -dlNi1 (TMC)+]/cft = MNi (TMCr][RX]. The rate constants increase for R and X in the order methyl < primary < secondary < allyl < benzyl halides and Cl < Br < I (133, 140). This suggests that the rate-determining step is electron transfer from the Ni(I) complex to R—X via an inner-sphere atom-transfer mechanism (143). 

Prior to these two FTIR studies, Nangia and Benson advanced thermo-chemical arguments in favor of an alternative H-atom transfer mechanism to account for then available experimental results [52] ... 

The last three entries in Table 13 reflect a marked preference for a-methylene versus methyl proton transfer for electron-withdrawing a substituents. These amines are also unusual in that they react with t in nonpolar solvents and do not display exciplex fluorescence. While this unusual behavior was initially attributed to a free radical hydrogen atom transfer mechanism leading to the formation of exceptionally stable "merostabilized" a-aminoallyl radicals (115), our current view is that the high kinetic acidity of the a-C-H bond of these amines when complexed with t is responsible for their behavior. 

Alkoxyl radicals can be generated by a variety of methods including peroxide reduction, nitrite ester photolysis, hypohalite thermolysis, and fragmentation of epoxyalkyl radicals (for additional examples of alkoxyl radical generation, see Section 4.2.S.2). Hypohalites are excellent halogen atom donors to carbon-centered radicals, and a recent example of this type of cyclization from the work of Kraus is illustrated in Scheme 43.182 Oxidation of the hemiketal (57) presumably forms an intermediate hypoiodite, which spontaneously cyclizes to (58) by an atom transfer mechanism. Unfortunately, the direct application of the Barton method for the generation of alkoxyl radicals fails because the intermediate pyridine-thione carbonates are sensitive to hydrolytic reactions. However, in a very important recent development, Beckwith and Hay have shown that alkoxyl radicals are formed from N-alkoxypyridinethiones.183 Al-... 

Photo-induced H-abstraction of anthraquinone from xanthene has been studied using nuclear polarization-detected EPR and the structure of the resulting short-lived radical pair determined.194 The retrodisproportionation reactions of a variety of styrenes with 9,10-diliydroanthracene (DHA), xanthene (XAN), and 9,10-dihydroacridine (DHAc) have been studied in order to determine if there was any evidence of the alternative hydride-transfer mechanism in competition with the proposed H-atom-transfer mechanism. No such evidence was found.195 The reaction between azulene and DHAC... 

Our previous results on hypericin indicate that excited-state H-atom transfer occurs even when one of the carbonyls is prohibited from accepting a hydrogen. The presence of such a transfer is apparent under very acidic conditions in AOT reverse micelles and cannot be excluded upon chelation of Tb3+ [76]. There is thus no evidence for a concerted H-atom-transfer mechanism in hypericin. In the present study, contrary to our initial expectations, we are not even able to demonstrate that hypomycin B executes an excited-state H-atom transfer hence our investigation sheds no light on the general question of how many H atoms are transferred in the perylene quinones and whether the transfer is concerted or stepwise. On the other hand, if further investigation reveals that H-atom transfer does not occur in hypomycin B, the result would have considerable implications for an understanding of the reaction coordinate for the H-atom transfer. 

Here a formal two-electron oxidation is initiated by an electron transfer step, and a second electron transfer step involving a neutral radical is interspersed between two chemical ones. As we shall see, such mechanisms are not always easily distinguished from atom-transfer mechanisms. 

Figure 1. Postulated oxygen atom transfer mechanism for the enzymic reduction of nitrate by Mo(H20)63+, assuming NADPH, H+ to be the substrate for reducing Mo02(H20) to Mo(H20)63+...

Subsequently, RR was used to successfully detect structural changes between the oxidized and reduced forms of both DMSOR and BSOR that are consistent with the proposed oxygen atom transfer mechanism of the catalytic reaction (95, 97). These experiments make use of the readily measurable isotopic shifts in vibration frequency between ieO=Mo and lsO=Mo to follow the fate of the oxygen atom removed from DMSO (or BSO) by the Mo. In this way, the clean transfer of 180 from DMSlsO to Mo(IV) to yield the oxidized form of the active site as Mo(VI)=180 was directly observed as well as the substrate-bound intermediate, (DMS180)Mo(IV). Further discussion of the technique of RR applied to metal dithiolenes and dithiolene-containing enzymes is included in Chapter 4 in this volume (98). 

Mohammad has also found that benzoyl and 4-nitrobenzoyl chlorides parallel the benzyl halides, the one reacting with 66 by an atom transfer mechanism and the other by electron transfer, in the rate-determining step.237 By contrast, 1,3-dinitrobenzene, although a more powerful electron acceptor than 4-nitrobenzyl chloride, reacts slower than the latter with 66 (by a factor > 105), albeit by a mechanism whose initial step is electron transfer.238 The proposed mechanism is as follows ... 

Only in the coupled oxidation of nitrite using glucose oxidase-glucose- 0 (as H 0 0H generating system) and beef liver catalase, which results in the formation of isotopically enriched nitrate (183a), are the results compatible with an oxygen atom transfer mechanism [Eqs.(28) and (29) ]. Even in this case, alternative interpretations are possible, such as oxidation of nitrite via outer or inner sphere electron transfers [Eqs. 

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