Oxidation, of alkyl radicals by copper

The redox reactions of carbon free radicals and copper(II) compounds have been portrayed as ligand-transfer and electron-transfer processes 178). The electron-transfer oxidation of alkyl radicals by copper(II) complexes, which are efficient radical interceptors, is considered to proceed via a metastable alkylcopper species which is consumed primarily by oxidative elimination [Eq. (129)] and oxidative solvolysis [Eq. (130)] 143b). The anionic counterion exerts a dominant effect in the selection... 

When copper(i) oxide is treated in benzene with trifluoromethanesulphonic anhydride, the complex (Cu0 S02 CF3)2(C H,), which releases benzene only above 120 °C, is obtained. The trifluoromethanesulphonate ion does not compete with olefins for co-ordination sites on the copper cf. ref. 28, p. 2%) and cationic Cu complexes are formed. Oxidation of alkyl radicals by copper(ii) trifluoromethanesulphonate in acetic acid gives more of the alkyl acetate, by oxidative solvolysis, and less olefin, than oxidation by copper(ii) acetate, the difference in product ratio being due to the greater dissociation of the copper trifluoromethanesulphonate. Silver(ii) complexes Ag (bipy)2-(O3S CF,) (bipy = 2,2 -bipyridyl) may be prepared from the silver(i) complex either by electrolysis or by treatment with silver(ii) oxide and trifluoro-methanesulphonic acid. ... 

The oxidation of alkyl radicals by copper(ir) involves the transient formation of alkyl-copper complexes,and a general scheme has been proposed for the electron-transfer processes (R = radical, X = ligand) in which the formation of the complex is followed by either oxidative elimination, (e), oxidative displacement, (d), or oxidative solvolysis, (s) ... 

Facile ligand-transfer oxidation of alkyl radicals is accomplished by copper(II) halides or pseudohalides 143a). Two processes occur simultaneously (1) oxidative substitution via cationic intermediates and an alkylcopper species, as in electron-transfer oxidation processes and (2) homolytic atom transfer. The former is akin to the oxidative displacement... 

Sexton BA (1979) Observation of formate species on a copper(lOO) surface by high resolution electron energy loss spectroscopy. Surface Sci 88 319-330 Sexton BA, Madix RJ (1981) A vibrational study of formic acid interaction with clean and oxygen-covered silver(llO) surfaces. Surface Sci 105 177-195 Sheldon RA, Kochi JK (1968) Photochemical and thermal reduction of cerium(IV) carboxylates. Formation and oxidation of alkyl radicals. J Am Chem Soc 90 6687-6698... 

In the presence of a catalytic amount of copper(II) acetate, unsaturated aldehydes were formed331 by electron transfer oxidation of the intermediate alkyl radical by Cu(II), e.g.,... 

A unified concept of these reactions is provided by the proposal that alkyl copper intermediates are involved in each of these reactions at the stage of the oxidation of the radical ... 

Closely related routes to alkoxyamines have also been described by Studer and Grubbs research groups. They both used nitroso compounds as radical traps. Studer et used the reaction between hydrazine and lead oxide to promote the alkyl radical formation, whereas Grubbs et generated the alkyl radical by reaction of an alkyl bromide with copper(I) complex. The advantage of this technique relies on the in situ production of the nitroxide. However, yields for alkoxyamines are often moderate and the method is limited to alkoxyamines beating two identical alkyl groups in the stracture. 

One advantage of a CRP, such as ATRP, is the ability to control the MW and MWD of copolymers containing functional monomers. Control over MW requires efficient initiation and preservation of activity in the majority of the polymeric chains. However, termination and other side reaaions are also present in ATRP, and they become more prominent as higher MW polymers are targeted. For example, in the copper-mediated ATRP of styrene, a slow termination process was observed, mainly arising from the interaction of the Cu species with both the growing radical and the maaomolecular alkyl halide. The catalytic species can participate in the OSET reactions by either oxidation of polystyryl radicals to carbocations or reduction of radicals to carbanions via reactions with Cu(II) and Cu(l) species. Studies with model compounds and maao-molecular PS species demonstrated that the elimination reaction was accelerated in the presence of the Cu(II) complex. This process was faster for bromine-mediated ATRP than for chlorine-based systems and was more noticeable in polar... 

Iron(II) salts, usually in conjunction with catalytic amounts of copper(II) compounds, have also been used to mediate radical additions to dienes91,92. Radicals are initially generated in these cases by reductive cleavage of peroxyesters of hydroperoxides to yield, after rearrangement, alkyl radicals. Addition to dienes is then followed by oxidation of the allyl radical and trapping by solvent. Hydroperoxide 67, for example, is reduced by ferrous sulfate to acyclic radical 68, which adds to butadiene to form adduct radical 69. Oxidation of 69 by copper(H) and reaction of the resulting allyl cation 70 with methanol yield product 71 in 61% yield (equation 29). 

Oxidation of cyclohexene by peroxydisulfate in the presence of copper(II) salts results in the formation of cyclopentanecarboxaldehyde as the main product in an aqueous acetonitrile solution (equation 261), and 2-cyclohexenyl acetate in an acetic acid solution (equation 262).588,589 Reaction (261) has been interpreted as the formation of a radical cation (186) by oxidation of cyclohexene with S2Og, followed by hydrolysis of (186) to the /3-hydroxy alkyl radical (187), which is oxidized by copper(II) salts to the rearranged aldehydic product (188 equation 263).589... 

Copper(I) can also be regenerated by electron transfer oxidation of radicals produced by hydrogen transfer from solvent by alkoxy radicals.101 Thus, reactions carried out in hydrocarbon solvents will produce alkyl radicals that are oxidized by Cu(II) at rates approaching diffusion control.95-100... 

Copper(II) compounds also enhance the rate of decarboxylation of alkanoic acids with lead(IV) acetate by more efficient electron-transfer oxidation of the alkyl radicals produced alkylcopper species may be intermediates 179). 

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