Reaction free radical pathway

Homolytic and free radical pathways in reactions of organochromium complexes. J. H. Espenson, Prog. Inorg. Chem., 1983, 30,189-212 (85). 

N2, and bromine trifluoride at 25-35°C " are also highly regioselective for tertiary positions. These reactions probably have electrophilic, not free-radical mechanisms. In fact, the success of the F2 reactions depends on the suppression of free-radical pathways, by dilution with an inert gas, by working at low temperatures, and/or by the use of radical scavengers. 

For example, photolysis of a suspension of an arylthallium ditrifluoro-acetate in benzene results in the formation of unsymmetrical biphenyls in high yield (80-90%) and in a high state of purity 152). The results are in full agreement with a free radical pathway which, as suggested above, is initiated by a photochemically induced homolysis of the aryl carbon-thallium bond. Capture of the resulting aryl radical by benzene would lead to the observed unsymmetrical biphenyl, while spontaneous disproportionation of the initially formed Tl(II) species to thallium(I) trifluoroacetate and trifluoroacetoxy radicals, followed by reaction of the latter with aryl radicals, accounts for the very small amounts of aryl trifluoroacetates formed as by-products. This route to unsymmetrical biphenyls thus complements the well-known Wolf and Kharasch procedure involving photolysis of aromatic iodides 171). Since the most versatile route to the latter compounds involves again the intermediacy of arylthallium ditrifluoroacetates (treatment with aqueous potassium iodide) 91), these latter compounds now occupy a central role in controlled biphenyl synthesis. 

There have also been several papers [61-63] on the importance of carefully establishing the reaction mechanism when attempting the copolymerization of olefins with polar monomers since many transition metal complexes can spawn active free radical species, especially in the presence of traces of moisture. The minimum controls that need to be carried out are to run the copolymerization in the presence of various radical traps (but this is not always sufficient) to attempt to exclude free radical pathways, and secondly to apply solvent extraction techniques to the polymer formed to determine if it is truly a copolymer or a blend of different polymers and copolymers. Indeed, even in the Drent paper [48], buried in the supplementary material, is described how the true transition metal-catalyzed random copolymer had to be freed of acrylate homopolymer (free radical-derived) by solvent extraction prior to analysis. 

Evidence for a free radical pathway in the foregoing cycloalkyl bromide reactions was secured by San Filippo and coworkers, who found that cis and iraws-4-r-butylcydohexyl bromide afford nearly identical product mixtures with MesSnLi, MesSnNa and MesSnK under a given set of conditions (Table 7)45. Of the various combinations examined, that of di-bromide and Me3SnLi appeared to be most favorable to S/v2 displacement. 

Espenson, James H., Homolytic and Free Radical Pathways in the Reactions of... 

The addition of hydrogen halides to simple olefins, in the absence of peroxides, takes place by an electrophilic mechanism, and the orientation is in accord with Markovnikov s rule.116 When peroxides are added, the addition of HBr occurs by a free-radical mechanism and the orientation is anti-Markovnikov (p. 751).137 It must be emphasized that this is true only for HBr. Free-radical addition of HF and HI has never been observed, even in the presence of peroxides, and of HCI only rarely. In the rare cases where free-radical addition of HCI was noted, the orientation was still Markovnikov, presumably because the more stable product was formed.,3B Free-radical addition of HF, HI, and HCI is energetically unfavorable (see the discussions on pp. 683, 693). It has often been found that anti-Markovnikov addition of HBr takes place even when peroxides have not been added. This happens because the substrate alkenes absorb oxygen from the air, forming small amounts of peroxides (4-9). Markovnikov addition can be ensured by rigorous purification of the substrate, but in practice this is not easy to achieve, and it is more common to add inhibitors, e.g., phenols or quinones, which suppress the free-radical pathway. The presence of free-radical precursors such as peroxides does not inhibit the ionic mechanism, but the radical reaction, being a chain process, is much more rapid than the electrophilic reaction. In most cases it is possible to control the mechanism (and hence the orientation) by adding peroxides... 

In studies of reaction pathways, nitrosyl radicals are frequently used as spin traps to provide evidence for free radical pathways. A caution in interpretation of these results is that the probe or products will interact with the transition metal complex in the reaction and affect the reactivity of the probe with the organic substrate or free radicals produced. A number of reactions of the stable free radicals RNO and R2NO with platinum(II) complexes have been carried out which show that such reactions must indeed be considered (equations 473-... 

Depending on whether the electron pair of the broken bond is shared or not by the two new entities, the reaction sequence will involve either an ionic or a free-radical pathway, as shown in Equation (4.49) and Equation (4.50). 

This energy-rich 02 has in water (H20) only a very short lifetime (k = 2.5 x 10s s 1 in D20 it lives much longer, k = 1.6 x 104 s 1). Singlet 02 shows often only a low reactivity, but with a series of compounds, including dGuo and its derivatives, it reacts quite rapidly (for a compilation of rate constants, see Wilkinson et al. 1995). Because of its prolonged lifetime in D20, the yields of 02(1Ag)-reac-tions are much higher in this solvent than in water. In photosensitized reactions considered to occur by a free-radical pathway, these 02(1Ag)-reactions may yield... 

In germanium chemistry the importance of free radical pathways in substitution reactions of secondary bromides with R3GeLi (R = CH3, CgHs) reagents is strongly indicated by product stereochemistry in cyclohexyl systems and by cyclization of the cA-heptene-2-yl moiety to yield [(2-methylcyclopentenyl)methyl]germanes, with the appropriate cis/trans ratio, as shown in equation 180, Table 9 and equations 181 and 182189. 

The autoxidation of organic substrates catalyzed by transition-metal salts has been used widely in the petrochemical industry for many years (32), but the oxidations are frequently nonselective since they operate by free-radical pathways that are sometimes initiated by the transition-metal ion. An example is the chain reaction of Reactions 2, 3, 4, and 5 (4). Propagation is maintained via Reactions 3 and 4, and any interaction between the metal ion and 02 would be incidental. 

Tabushi and Koga reported the use of manganese porphyrins to catalyze the 02-oxidation of cyclohexene to cyclohexanol and cyclohexene-ol in the presence of borohydride these workers suggest that an equilibrium such as depicted in Reaction 32 is involved in non free-radical pathways (112). 

Other Systems. Alkyl Thiols. The reduction of hemin with ethane-thiol has been suggested to occur by a free radical mechanism on the basis of product analysis (II). The reaction of n-hexanethiol with TPPFeCl in DMSO carried out in the ESR cavity gives rise to the signal illustrated in Figure 7. These are preliminary results, and the spectrum is of poor quality and probably reflects some saturation from the low steady-state concentration of the radical. Nevertheless the signal only appears during the autoreduction of the iron porphyrin and again indicates that the autoreduction occurs by a free radical pathway. 

One way of circumventing this activation energy barrier involves a free radical pathway in which a singlet molecule reacts with 302 to form two doublets (free radicals) in a spin-allowed process (Fig. 4.1, Reaction (1)). This process is, however, highly endothermic (up to 50 kcal mol-1) and is observed at moderate temperatures only with very reactive molecules that afford resonance stabilized radicals, e.g. reduced flavins (Fig. 4.1, Reaction (2)). It is no coincidence, therefore,... 

As noted above, dioxygen reacts with organic molecules, e.g. hydrocarbons, via a free radical pathway. The corresponding hydroperoxide is formed in a free radical chain process (Fig. 4.3). The reaction is autocatalytic, i.e. the alkyl hydroperoxide accelerates the reaction by undergoing homolysis to chain initiating radicals, and such processes are referred to as autoxidations [1]. 

Ethers (32) and peroxides (33) are seen as by-pn ucts in the catalytic selenium dioxide oxidation of cycloalkanes, and these materials can predominate in the case of small rings. Addition of hydioquinone to the reaction mixtures suppresses their formation and consequently a free radical pathway has been proposed (Scheme 8). 

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