Radicals nature

Step (5) is rapid due to the radical nature of Br02, so the overall stoichiometric process given by (4) + 2 x (5) has the fomi... 

At elevated temperatures (250-400°C) bromine reacts with thiazole in the vapor phase on pumice to afford 2-bromothiazole when equimolecu-lar quantities of reactants are mixed, and a low yield of a dibromothiazole (the 2,5-isomer) when 2 moles of bromine are used (388-390). This preferential orientation to the 2-position has been interpreted as an indication of the free-radical nature of the reaction (343), a conclusion that is in agreement with the free-valence distribution calculated in the early application of the HMO method to thiazole (Scheme 67) (6,117). 

The addition of phosphines to fiuoroolefins is of free radical nature [6, 7, (equations 2-5)... 

Due to the radical nature of the Hofmann-Lbffler-Freytag reaction, a deviation was observed when there was a pendant terminal olefin on the substrate. When the aminyl radical from N-chloroamine 31 had a choice between addition to the double bond... 

This is the starting chain and is identical to the IPi species in Equation (13.28), The notation explicitly shows the free-radical nature of the polymerization, and the moiety denoted by I represents a fragment from the chemical initiator e.g. a butyl group when f-butyl peroxide is the initiator. The propagation reaction is... 

These are oxidised by both Fe(III) and Cu(II) octanoates (denoted Oct) in nonpolar solvents at moderate temperatures . 80-90 % yields of the corresponding disulphide are obtained with Fe(III) and this oxidant was selected for kinetic study, the pattern of products with Cu(II) being more complex. The radical nature of the reaction was confirmed by trapping of the thiyi radicals with added olefins. Simple second-order kinetics were observed, for example, with l-dodecane thiol oxidation by Fe(Oct)3 in xylene at 55 °C (fcj = 0.24 l.mole . sec ). Reaction proceeds much more rapidly in more polar solvents such as dimethylformamide. The course of the oxidation is almost certainly... 

Acyclic ketones containing radical-stabilizing groups similarly undergo decarbonylation. In this case photolysis of mixtures of ketones results in products arising from mixed radical combinations/67 This result is direct evidence for the free radical nature of these decarbonylations ... 

Not surprisingly, the majority of examples presented in this chapter are of pure radical nature, since, as mentioned above, radical reactions are well-suited to sequencing and can be regarded as the prototype of a chain reaction. Nevertheless,... 

Previously unknown deoxygenation was reported with o-, m-, and p-nitro-benzonitrile oxides on reactions with NO (112) this was interpreted as being due to the radical nature of the latter (Scheme 1.11). 

Most of the kinetic models predict that the sulfite ion radical is easily oxidized by 02 and/or the oxidized form of the catalyst, but this species was rarely considered as a potential oxidant. In a recent pulse radiolysis study, the oxidation of Ni(II and I) and Cu(II and I) macrocyclic complexes by SO was studied under anaerobic conditions (117). In the reactions with Ni(I) and Cu(I) complexes intermediates could not be detected, and the electron transfer was interpreted in terms of a simple outer-sphere mechanism. In contrast, time resolved spectra confirmed the formation of intermediates with a ligand-radical nature in the reactions of the M(II) ions. The formation of a product with a sulfonated macrocycle and another with an additional double bond in the macrocycle were isolated in the reaction with [NiCR]2+. These results may require the refinement of the kinetic model proposed by Lepentsiotis for the [NiCR]2+ SO/ 02 system (116). 

Tt-radical nature of these complexes, the one-electron oxidized species 30 was formed (33). 

The free-radical nature of the endgroup is, however, powerful enough to recombine with the radical of another growing chain and terminate the growth of the later ... 

Nozaki reported the reaction of trialkylboranes with styryl sulfoxides and sulfones. Alkyl radicals generated from trialkylboranes add at the -position of /3-styryl sulfoxides and sulfones (a- to the sulfur atom). The resulting radicals fragment and deliver the -styryl adducts [108]. Interestingly, the sulfoxides eliminate very rapidly leading to partially stereospecific substitution (Scheme 44). The radical nature of the process is demonstrated by the presence of a side product derived from the solvent (THF) by hydrogen atom abstraction. 

Interestingly, this allylation process seems to be very general. For instance, introduction of a dienyl moiety using penta-2,4-dienyl phenyl sulfone has been achieved (Scheme 47, Eq. 47a). The modest yield (50%) for the conversion is due to the instability of the dienyl sulfone which readily polymerizes. Finally, the radical nature of the process has been demonstrated by running... 

We made a similar observation when we reported a mild and efficient radical mediated reduction of organoboranes (Scheme 58, Eq. 58a) [128]. An in situ generated B-methoxycatecholborane-methanol complex acts as a reducing agent. The radical nature of the process was demonstrated by using (+)-2-carene as a radical probe (Eq. 58b). Water, ethanol and trifluoroethanol can be used instead of MeOH with very similar efficiency. 

Recently reported additions of dibutyl phosphinite20 and tetra-alkoxydiphos-phines21 to alkenes are probably of free-radical nature, although the reactions only take place with alkenes possessing electron-withdrawing groups. 

As the AO with a direct nonspecific mechanism of action we have chosen Hypoxene - sodium poly(2,5-dihydroxiphenyl)-4-thiosulfonate. Besides a direct AO effect as a scavenger of free radicals it exerts an anti-hypoxic effect shunting I and II complexes of mitochondrial respiratory chain, which are inhibited as a consequence of hypoxia (Eropkin et al., 2007). Hypoxene was introduced into cell incubation media before illumination and left during cells further incubation. Hypoxene in the concentration of 40pg/ml, comparable to doses applied in vivo, completely blocked C60-induced phototoxicity (Table 7.3). Cellular viability has completely recovered to control level, which is a convincing evidence of free radical nature of cellular damage in photodynamic effect of fullerene. 

Kruus also conducted experiments in the presence of the radical scavenger diphe-nylpicryhydracyl (DPPH) and observed induction periods which were roughly proportional to concentration of DPPH employed. This clearly demonstrates the free radical nature of the polymerisation. By assuming that each of the monomer radicals produced by the cavitation process (Eq. 5.30) reacted with one DPPH molecule, he was able to deduce the following kinetic relationship ... 

The pronounced tendency of radicals to engage in one-electron transfer reactions is well documented [3]. This reaction channel is favored because it provides the simplest way for radicals to lose their radical nature, i.e. to b ome species with an even number of electrons (closed-shell molecules). The direction of the electron flow between the radical X and the molecule Y depends on the oxidizing or reducing power of X and on the ability of Y to either donate or accept an electron the final result of the interaction between X and Y is then the either one-electron-reduced or -oxidized former radical (X" or X ) or the open-shell molecle (Y or Y"" ), (cf. Eq. 1) ... 

The supposition that ozone is mutagenic or carcinogenic in man is based primarily on information on the biochemical mechanism of ozone toxicity and to a lesser extent on in vitro and animal studies. The biochemical evidence is for the most part indirect and depends on an analogy between the free-radical nature of ozone toxicity and of radiation and other carcinogenic agents. 

Schmittel and Burghart (1997) had published a well-structured review on cation-radical chemistry. The review gave a general picture of the cation-radical nature compiling data of those days. This section scrutinizes data relevant to acid-base reactivity of organic ion-radicals. 

To get support for the postulation of ion-radical nature of the rate-determining step, one can plot the reaction rate constant values against the oxidation and reduction potentials of the reaction partners. When both plots occur to be linear, it will support the postulation of the ion-radical route. 

To discern the ion-radical nature of reactions, the so-called intramolecular and intermolecular proton/deuterium isotope effects may be of use. Baciocchi et al. (2005) revealed ion-radical mechanism for A-demethylation of A,A-dimethylanilines, (CH3)2NAr, by phthalimide-A-oxyl radical (Scheme 4.14). In this reaction, ( e/ D)intra values were derived for reactivity of (CD3)(CH3)NAr, whereas ( H/ D)inter was referred for the reactivity of (CD3)2NAr. The values of (A e/ D)intra were found to be always different and higher than These results, although are incompat-... 

While investigating the mechanism, Pobedimskii and Buchachenko (1968a, 1968b) concluded that these reactions have an ion-radical nature and consist of electron transfer from phosphites or sulfides (denoted further as D) to hydroperoxides as follows ... 

In a moderately alkaline medium, the ter Meer reaction proceeds through a considerable induction period the kinetic curves are S-shaped. Peroxide compounds and UV irradiation accelerate the process (Bazanov et al. 1978). Radical traps inhibit the reaction (as discussed earlier). This indicates the radical nature of the process. The rate of formation of active radical centers obeys the second-order equation in the total concentration of chloronitroethane introduced into the reaction. In nonionized substrate and anion conjugated with it, the reaction is a first-order one. The rate of the whole reaction is independent of the nitrite concentration. 

The ion-radical mechanism is characteristic in cases of substrates, which are ready for one-electron oxidation and capable to give stable cation-radicals in appropriate solvents. As the cited examples show, such a mechanism can really be revealed. However, very rapid transformations of aromatic cation-radicals can mask the ion-radical nature of many other reactions and create an illusion of their nonradical character. At the same time, the ion-radical mechanism demands its own approaches for farther optimization of commercially important cases of nitration. This mechanism deserves onr continned attention. 

The rate of an electron transfer from the reduced catalyst to the substrate is also important. If the rate is excessively high, the electron exchange will occur within the preelectrode space and the catalytic effect will not be achieved. If the rate is excessively low, a very high concentration of the catalyst will be needed. However, at high concentration, the anion-radicals of the catalyst will reduce the phenyl radicals. Naturally, this will be unfavorable for the chain process of the substitution. As catalysts, substances that can be reduced at potentials by 50 mV less negative than those of the substrates should be chosen. The optimal concentration of the catalyst must be an order lower than that of a substrate (Swartz and Stenzel 1984). 

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