Reactivity electron transfer mechanisms

The equation does not take into account such pertubation factors as steric effects, solvent effects, and ion-pair formation. These factors, however, may be neglected when experiments are carried out in the same solvent at the same temperature and concentration for an homogeneous set of substrates. So, for a given ambident nucleophile the rate ratio kj/kj will depend on A and B, which vary with (a) the attacked electrophilic center, (b) the solvent, and (c) the counterpart cationic species of the anion. The important point in this kind of study is to change only one parameter at a time. This simple rule has not always been followed, and little systematic work has been done in this field (12) stiH widely open after the discovery of the role played by single electron transfer mechanism in ambident reactivity (1689). 

In SiCl4-mediated Mukaiyama-Michael reactions, an electron-transfer mechanism is proposed for the case in which ketene silyl acetals bearing less hindered silyl substituent are used as substrates.342-344 As shown in Scheme 82, ketene silyl acetals having more substituents at the /3-position are much more reactive. 

The stereoselective 1,4-addition of lithium diorganocuprates (R2CuLi) to unsaturated carbonyl acceptors is a valuable synthetic tool for creating a new C—C bond.181 As early as in 1972, House and Umen noted that the reactivity of diorganocuprates directly correlates with the reduction potentials of a series of a,/ -unsaturated carbonyl compounds.182 Moreover, the ESR detection of 9-fluorenone anion radical in the reaction with Me2CuLi, coupled with the observation of pinacols as byproducts in equation (40) provides the experimental evidence for an electron-transfer mechanism of the reaction between carbonyl acceptors and organocuprates.183... 

However, the existence of an extremely reactive bound hydroxyl radical is questionable because it is difficult to understand why it does not immediately react with adjacent molecules (most of the reactions of hydroxyl radicals proceed with the rates close to a diffusion limit). Therefore, the mechanism proposed by Zhang et al. [7,8] seems to be much more convincing. They suggested that the genuine oxidizing free radical formed during SOD inactivation is the bicarbonate radical anion CO/, which is formed as a result of the oxidation of bicarbonate. It has also been suggested that DMPO OH is formed by the addition of water to an intermediate of the reaction of DMPO with CO/ via a nucleophilic or electron transfer mechanism. 

Orbital interactions and long-range electron transfer, 38, 1 Organic materials for second-order non-linear optics, 32, 121 Organic reactivity, electron-transfer paradigm for, 35, 193 Organic reactivity, structure determination of, 35, 67 Orotidine monophosphate decarboxylase, the mechanism of, 38, 183... 

A review on the nitration of aromatics (using a range of species including N02 and NO3) has appeared. Evidence for electron-transfer mechanisms via radical cations has been reviewed. In addition, another review comparing the reactivity of a range of radicals and radical cations has appeared. While radicals prefer to add to the carbon of CN triple bonds, radical cations were found to prefer addition at the N atom. Ab initio calculations were performed to rationalize this behaviour. 

The COs radical anion is a strong one-electron oxidant ( 7-1.7 V vs NHE [15]) that oxidizes appropriate electron donors via electron transfer mechanisms [103]. Detailed pulse radiolysis studies have shown that carbonate radicals can rapidly abstract electrons from aromatic amino acids (tyrosine and tryptophan). However, reactions of CO3 with S-containing methionine and cysteine are less efficient [104-106]. Hydrogen atom abstraction by carbonate radicals is generally very slow [103] and their reactivities with other amino acids are negligible [104-106]. 

In contrast to autoxidation, tertiary C—H bonds are less reactive under these conditions. A reversible electron-transfer mechanism to form a radical was suggested 115... 

A further variant is the oxidation of olefins by Mn(III) acetate in the presence of halide ions. Thus, oxidation of cyclohexene by Mn(III) acetate in acetic acid at 70°C is slow, but addition of potassium bromide leads to a rapid reaction. Cyclohexenyl acetate was formed in 83% yield.223 In contrast to what would be expected for an electron transfer mechanism, norbomene (ionization potential 9.0 eV) was unreactive at 70°C, whereas cyclohexene (ionization potential 9.1 eV) and bicyclo[3,2,l] oct-2-ene reacted rapidly. The low reactivity of norbomene can be explained, if oxidation involves attack at the allylic position... 

Catalysis of these reactions by potassium bromide has been observed.232,236 Aralkyl bromides are formed as intermediates that undergo acetolysis to the corresponding acetates under the reaction conditions. Results236 were consistent with a free radical mechanism, even with reactive toluenes, in contrast to the electron transfer mechanism observed in the absence of potassium bromide. Relative reactivities236 corresponded closely to those observed in photochemical brominations, suggesting that bromine atoms, formed by electron transfer oxida-... 

The rates of cobalt(III) oxidations are also enhanced by chloride ions. Thus, the oxidation of toluene by Co(III) acetate in acetic acid required more than a week for reaction at 65°C, but reacted in less than 2 hr at room temperature in the presence of a tenfold excess of lithium chloride. The products and relative reactivities of various alkylbenzenes were consistent with an electron transfer mechanism. The dramatic enhancement in rate was attributed to the formation... 

For a series of methylbenzenes, the rates decreased in the order toluene > xylenes > mesitylene > durene > hexamethylbenzene. This order of reactivity is the reverse of that expected for a mechanism involving electrophilic substitution or electron transfer. However, Bushweller598 found that electron-releasing groups facilitate the benzylic oxidation of substituted toluenes by Pd(OAc)2 in acetic acid. p-Methoxy toluene gave a 96% yield of p-methoxybenzyl acetate, and p-nitrotoluene gave only 2% p-nitrobenzyl acetate, in agreement with either an electrophilic substitution or electron transfer mechanism. More mechanistic studies are necessary to clear up these anomalies. Steric effects may play an important role in these reactions. 

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