Radical pathways oxidative addition

Alkyl halides that do not readily undergo nucleophilic attack may oxidatively add to a metal by radical mechanisms. Oxidative addition reactions that occur by radical mechanisms show loss of stereochemistry, nomeproducible rates, inhibition by radical inhibitors, and acceleration by O2 or light. Reactions of lr(Cl)(CO)(PMe3)2 with methyl and benzyl halides showed no indication of radical behavior, but other saturated alkyl halides, vinyl, and aryl halides showed characteristics consistent with a radical-chain pathway. 

Photolysis Abiotic oxidation occurring in surface water is often light mediated. Both direct oxidative photolysis and indirect light-induced oxidation via a photolytic mechanism may introduce reactive species able to enhance the redox process in the system. These species include singlet molecular O, hydroxyl-free radicals, super oxide radical anions, and hydrogen peroxide. In addition to the photolytic pathway, induced oxidation may include direct oxidation by ozone (Spencer et al. 1980) autooxidation enhanced by metals (Stone and Morgan 1987) and peroxides (Mill et al. 1980). 

Consequently, the benzene oxidation mechanism was further developed by considering additional decomposition and oxidation steps. Sethuraman et al. proposed that phenyl radical decomposition can occur by either of two key pathways (3-scission of phenyl radical or by breakdown of the phenylperoxy radical formed by the oxidation of phenyl radical (Fig. 9). Using PM3 calculations,which were ultimately verified by DFT studies,Carpenter predicted that another species, 2-oxepinoxy radical (3 in Fig. 9b), is an important intermediate due to its relative stability, formed via a spirodioxiranyl intermediate (2 in Fig. 9b) from phenylperoxy radical. Pathway A in Fig. 9b is the thermodynamically preferred pathway at temperatures increasing up to 432 K, while pathway B has an entropic benefit at higher temperatures. While pathway B essentially matched the traditional view of benzene combustion, pathway A introduced a new route for phenylperoxy radical, which could resolve discrepancies observed using previous models. 

Lead(IV) in acidic media has been found to promote oxidative addition of Cl-, CF3CO2-, AcO-, MeS03- and CIO4- to cyclohexene, 1-hexene and styrene433. Sonochemical switching from ionic to radical pathway in the reactions of styrene and fraws-/l-methylstyrene with (AcO Pb has been observed434. 

In addition to their use for the calibration of rates for radical reactions, radical clocks can be employed to distinguish between ionic and radical pathways. In the simplest embodiment of this idea, a suitable clock reaction that undergoes a known fast rearrangement with easily identifiable products is incorporated into the reaction system to be studied. This approach has been exploited in the pioneering work of Newcomb and co-workers in studies of the mechanism of cytochrome P450 oxidation reactions [13]. Newcomb has developed a range of ultrafast radical clocks able to detect radical species with lifetimes of 80-200 fs. 

Br)]2, exclusively. Low concentrations of 1,2-bromochloroethane, however, yield the mixed halide metal dimers Pt2(pop)4(Br)(Cl) " and Ir2(p-pz)2(C0D)2(Br)(Cl). This result is predicted by the proposed mechanism (Figure 5). Photolysis results in formation of Pt2(pop)4 (Br)4 or Xr2(p-pz)2(C0D)2(Br) as intermeditaes. The intermediate can react with another bromochloroethane molecule, as it does when the latter species is in high concentration, to yield the dibromide dimer or it can react with the chloroethane radical to yield the mixed halide metal species. The latter pathway becomes competitive at low halocarbon concentrations. In general, the oxidative addition of halocarbons is typical of the photochemistry arising from electron transfer from d -d metal dimers with the final product being the stable d -d metal-metal bonded dimers (24-25). 

We are here concerned with various organometallic reactions for which there is evidence that organic free radicals are implicated in the reaction pathway. Many of these arc formally two-electron oxidative additions or their retrogressions, the reductive eliminations (Section V,A). We shall focus attention on systems in which transition metal Group VIII complexes are involved. 

Especially for alkyl halides 6 the transfer of a single electron from the metal center is facile and occurs at the halide via transition state 6C, which stabilizes either by direct abstraction of the halide to a carbon-metal complex radical pair 6D or via a distinct radical anion-metal complex pair 6E. This process was noted early but not exploited until recently (review [45]). Alkyl tosylates or triflates are not easily reduced by SET, and thus Sn2 and/or oxidative addition pathways are common. The generation of cr-radicals from aryl and vinyl halides has been observed, but is rarer due to the energy requirement for their generation. Normally, two-electron oxidative addition prevails. 

When aryl halides were applied in catalytic coupling reactions, the mechanistic evidence points to initial SET reduction by low-valent nickel phosphine species (selected investigations in [23, 24]). The competition of cage collapse to ArNi(PR3)2X vs. dissociation of the aryl halide radical anion to a free radical and Ni(I) complexes determines the cross-coupling manifolds. Thus, Ni(0)-Ni(II) and Ni(I)-Ni(III) catalytic cycles can occur interwoven with each other and a distinction may be difficult. Common to both is that the coupling process with aryl halides is likely to occur by a two-electron oxidative addition/reductive elimination pathway. 

S-Allyloxy tellurides also underwent similar radical 5-exo cyclizations catalyzed by 7 mol% of Ni(acac)2 and 2 equiv. of Et2Zn [113]. The reaction proceeded with high r/.v-selectivity in 56% yield. In contrast to the reactions of 5-hexenyl iodides shown above, 5-exo cyclization reactions of 5-hexynyl iodides were proposed to proceed by a two-electron pathway consisting of alkyne coordination/oxidative addition/intramolecular carbonickelation and reductive elimination, resulting in alkylidenecyclopentanes [114]. 

---