Acetoxy radical

For the acetoxy radical, the for decarboxylation is about 6.5 kcal/mol and the rate is about 10 s at 60°C and 10 s at —80°C. Thus, only very rapid reactions can compete with decarboxylation. As would be expected because of the lower stability of aryl radicals, the rates of decarboxylation of aroyloxy radicals are slower. The rate for p-methoxybenzoyloxy radical has been determined to be 3 x 10 s near room temperature. Hydrogen donation by very reactive hydrogen-atom donors such as triethylsilane can compete with decarboxylation at moderate temperatures. 

When the decomposition is carried out in an inert solvent, methyl acetate and ethane are formed, whereas in the gas-phase decomposition methyl acetate is completely absent and ethane is produced in much smaller quantity, It was suggested that the dimers in solution represent the recombination of methyl, and the combination of methyl and acetoxy radicals, within the solvent cage. ... 

Thermal decomposition of lead tetraacetate gives rise to methyl radicals, again through the initial formation of acetoxy radicals. " An ionic mechanism for the decomposition has also been postulated, and it is possible that both mechanisms may occur, depending on the conditions. 

For benzoyl and acetyl peroxides, loss of carbon dioxide occurs in a stepwise process. Estimates of the rate constants for step c in Scheme 1 are 7 x 10 sec (benzene, 60°). The corresponding process for acetyl peroxide has k = 2x 10 sec (n-hexane, 60°), so that the lifetime of radical pairs containing acetoxy radicals is comparable to the time necessary for nuclear polarization to take place (Kaptein, 1971b Kaptein and den Hollander, 1972 Kaptein et al., 1972). Propionoxy radicals are claimed to decarboxylate 15-20 times faster than acetoxy radicals (Dombchik, 1969). 

Pb(IV) is most usually employed as the tetraacetate and the action of this compound is complex in that it can function either as a two-equivalent oxidant giving Pb(II) or as a source of acetoxy radicals, viz. 

The hydroxyl radicals may also initiate homopolymerization by addition to monomer. In the redox system (20) the hydroxyl radicals are reduced to hydroxyl ions leaving the acetoxy radicals which are more specific for grafting (21)., ... 

The anode acetoxylation of aromatic compounds in solutions of acetic acid carrying alkali or tetraalkylammonium acetates takes the same route. As shown (Eberson 1967, Eberson and Jonsson 1981), the process starts with one-electron oxidation at the anode and then passes through the same stages as in oxidation with cobalt trifluoroacetate. The reaction takes place at potentials sufficient to oxidize the substrate but not sufficient to convert acetate ion into acetoxy radical. 

Acetoxy radicals from acetyl peroxide undergo partial decarboxylation and radical combination within the solvent cage leads to stable products incapable of producing radicals ... 

Mechanism A clearly predicts the formation of more double-labeled oxygen, in this case a 36/34 ratio of 0.0149, calculated from statistical considerations. Mechanism B, involving cage recombination of acetoxy radicals, predicts a more random distribution and a 36/34 ratio of 0.00828, closer to the experimentally determined ratio of 0.00835. 

It is clear (23) that three products result from reactions of acetoxy radical pairs within the solvent cage acetyl peroxide (estimated at 38% ), methyl acetate (12.4% ), and ethane (2.9% ). The implied near equality of rates for decarboxylation of the acetoxy radicals and diffusion from the cage has been given quantitative expression in work of Braun, Rajben-bach, and Eirich (2). These workers studied the variation in the amounts of ethane and methyl acetate formed from acetyl peroxide as a function of solvent viscosity, and they derived a rate constant for the decarboxylation of acetoxy radical at 60°C. of 1.6 X 109 sec."1. 

They further noted (2) that the over-all rate constant for disappearance of acetyl peroxide decreases monotonically with increasing solvent viscosity. The attractive hypothesis (19, 22, 23) that the observed rate decrease with increased viscosity in this homologous series of hydrocarbon solvents reflects the increased importance of cage recombination of acetoxy radicals in the more viscous solvents is subject to further test from the data of this paper. 

Let us assume that fci is equal to k9, the rate constant for the gas phase decomposition (15), where no cage effect is expected. This assumption does not always hold (15, 18). For example, it is known (18) that di-f erf-butyl peroxide (DPB) decomposes about 30% slower in the gas phase than in solution. We can calculate from our value of k8 and the known value of kg, from the work of Szwarc (7, 21), a value for the fraction of acetoxy radical pairs recombining, fR, where... 

Since fR = l — where fn is the fraction of acetoxy radical pairs reacting to give other products, and... 

Figure 4. The prohahility that a radical generated at time zero from acetyl peroxide will be an acetoxy radical (A), a methyl radical (M) or will have diffused from the solvent cage, a quantity proportional to (f). These curves were used to duplicate the observed product yields...

In Reaction 21a there is no net electron transfer to the metal, and the only product is acetic acid. In Reaction 22a, a one-electron transfer to the metal ion occurs, and the peracetic acid moiety of the complex is transformed into an acetoxy radical which will decompose rapidly to CH3 and CO2. 

The polyazophenylene units are formed from the polyrecombination of the decomposition products from bis(nitrosoacetyl)benzidine. Chain termination can occur by disproportionation of the polymer radicals and by recombination with acetoxy radicals. Despite the rate constant for the recombination of the phenyl and azophenyl radicals being much larger than that of the initiation reaction for isoprene, it is possible to synthesise copolymers from these materials by a careful choice of the various reaction parameters. However, block copolymers could only be obtained using emulsion techniques (see Table 4.11) and not in bulk or in solution. 

Anodic acetoxylation is an illustrative example of these principles. Anodic oxidation of sodium acetate in acetic acid at a platinum anode under constant current conditions yields ethane in almost quantitative yield. The mechanism was supposed to be discharge of acetate ion at the anode with formation of an acetoxy radical, which subsequently would undergo decarboxylation with formation of methyl radicals as shown in Eqs. (14) and (15). 

To demonstrate the intervention of acetoxy radicals, an aromatic substrate (anisole or naphthalene) was added to the electrolyte and the reaction run under constant current conditions. The isolation of aryl acetates from this reaction was considered as evidence for the intermediacy of acetoxy radicals, the aryl acetate being formed via a homolytic attack of the acetoxy radical on the aromatic compound 4S"47). 

The effect of concentration gradients in electrode reactions is really not a problem of mechanism but rather a troublesome source of possible systematic ambiguity in the interpretation of the product distributions observed, one of the tasks that lies close to the heart of the organic chemist. To see how this comes about, it is instructive to make the mental experiment that we generate acetoxy radicals by the Kolbe reaction of acetate ion in acetic acid [eqn (52)] at an electrode of 1 cm2 surface area, passing a current of 1 A during... 

In this reaction, the radical pair consisting of the methyl and acetoxy radicals, [CHs- OOCCH3], is important for the CIDNP signals of its product. The phase of CIDNP signals can be explained by Eq. (4-17), which needs the g-values of the methyl and acetoxy radicals and the HFC constant of the methyl radical. 

Attempts to attack the i4a-methyl group of lanosterol from a 9a-alkoxy-radical gave only a 9,io-seco-9-ketone [53], although the 7a-alkoxy radicai readily formed a cyclic ether with the I4a-methyl group [54]. Finally, a selection of secondary alkoxy-radicals derived from alcohols adjacent to quaternary centres e.g. la- or ij8-0H [55], 3j5-OH-4,4-dimethyl [56], 17/S-OH [36J, and 2i-OH-20-cyclic ketal [56]) gave fragmentation products which were either unsaturated carbonyl compounds, or acetoxy derivatives resulting from combination of alkyl and acetoxy radicals, e.g. ... 

The acyl peroxide bonds (RCO2-OCR) are very weak (see Table 63), and the decomposition of the acetoxy radical is rather exothermic, viz. 

The reported Arrhenius parameters for the diacyl peroxides appear quite reasonable. For example, the heat of formation of the acetoxy radical deduced from the observed activation energies is A/f (CH3COO-) = —49.7 kcal.mole . This gives a bond dissociation energy of the acidic hydrogen in acetic acid of >(CH3C02 -H) = 106 kcal.mole , very comparable to (O-H) bond dissociation energies in alcohols, -factors are also very reasonable. Since two internal rotations become partially restricted in the transition state as shown below... 

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