Solvent cage radical recombination

The high rate of decarboxylation of aliphatic acyloxy radicals is also the prime reason behind low initiator efficiencies (see 3.3.2.1.3). Decarboxylation occurs within the solvent cage and recombination gives alkane or ester byproducts. Cage return for LPO is 18-35% at 80 °C in -octane as compared to only 4% for BPO under similar conditions.144... 

In reaction (a), represents the total eflBciency of the initiator defined [48] as the fraction of primary radicals that come out of the solvent cage escaping recombination therein and take part in initiation and primary radical termination of growing chains. Evidently / should be greater than the conventional / used in ideal polymerization equation (6.26). 

Triplet n,7t -state cleavage reactions (typically for aromatic ketones, ISc of which is usually 1.0 Section 2.1.6) are more efficient due to the longer triplet lifetimes and relatively large cleavage rate constants. Furthermore, recombination of the primary triplet radical pair formed is spin forbidden, which allows the radicals to escape the solvent cage. The photochemical racemization of the chiral phenylpropiophenone 264, for example, was found to depend on a partitioning between in-cage radical recombination and diffusion rate constants (Scheme 6.110).921... 

Whether or not a fragmentation according to reactions (15) and (16) takes place depends on the reactivity of the primary formed oxygen-centered radicals toward the monomer. In the case of BPO, there is a fragmentation with phenyl radical formation [reaction (15)] only in the absence of the monomer. In the presence of the monomer, the benzoyl oxy radicals react with monomer before decarboxylation. Aliphatic acyloxy radicals, on the other hand, undergo fragmentation already in the solvent cage whereby recombination products are produced that are not susceptible to further radical formation. As a result, the radical yield Ur for these initiators is smaller than 1 ... 

In discussing mechanism (5.F) in the last chapter we noted that the entrapment of two reactive species in the same solvent cage may be considered a transition state in the reaction of these species. Reactions such as the thermal homolysis of peroxides and azo compounds result in the formation of two radicals already trapped together in a cage that promotes direct recombination, as with the 2-cyanopropyl radicals from 2,2 -azobisisobutyronitrile (AIBN),... 

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. ... 

The radicals do not drift apart because they are held together by the solvent cage. According to this mechanism, the radicals must recombine rapidly in order to account for the fact that does not racemize. Other evidence in favor of mechanism a is that in some cases small amounts of coupling products (R R have been isolated,which would be expected if some R leaked from the solvent cage. However, not all the evidence is easily compatible with mechanism a. It is possible that another mechanism (b) similar to mechanism a, but involving ion... 

Further evidence consistent with the polar radical pair mechanism was provided by a crossover experiment (Scheme 6.26). A 1 1 mixture of labeled 8Z /8 and unlabeled 8Z/8E was heated in xylene at 125 °C for 2h and at 135 °C for 4h to afford hydroxypyrimidinones 3 and 3. Analysis of the products by high resolution mass spectrometry showed no crossover between the labeled and unlabeled fragments. This result reinforces the computational results discussed previously wherein PRP recombines to give product within the solvent cage (Scheme 6.24). 

The hypothesis of Kellogg 38> described above, that autoxidation reactions display low quantum yields in spite of high yields of excited products, due to oxygen quenching in the solvent cage, is criticized by J. Beutel 13) who very thoroughly investigated the chemiluminescent autoxidation of dimedone (1.1. dimethyl 3.5 cyclohexandione). Here the recombination of dimedone peroxy radicals should be the excitation step ... 

The chain termination is a result of tertiary alkylperoxyl radical recombination in the solvent cage. The values of the rate constants for chain termination through the disproportionation of tertiary peroxyl radicals are collected in Table 2.15. They vary in the range 103 to 105 L mol 1 s 1 at room temperature. The probability of a pair of alkoxyl radicals to escape cage recombination is sufficiently higher than that of cage recombination. The values of rate constants of the reaction 2 R02 > 2 RO + 02 measured by the EPR technique are presented in Table 2.16. 

The cage effect was also analyzed for the model of diffusion of two particles (radical pair) in viscous continuum using the diffusion equation [106], Due to initiator decomposition, two radicals R formed are separated by the distance r( at / = 0. The acceptor of free radicals Q is introduced into the solvent it reacts with radicals with the rate constant k i. Two radicals recombine with the rate constant kc when they come into contact at a distance 2rR, where rR is the radius of the radical R Solvent is treated as continuum with viscosity 17. The distribution of radical pairs (n) as a function of the distance x between them obeys the equation of diffusion ... 

Single Electron Transfer A single electron transfer (SET) mechanism is often difficult to distinguish from an SN2 reaction because the principal product of these two pathways is the same, apart from the stereochemistry at carbon (race-mization instead of inversion). The radicals formed can recombine rapidly in a solvent cage (inner-sphere ET) [2, 193, 194]. The [HFe(CO)5] -catalyzed deiodina-tion of iodobenzene may serve as an example [179] (Eq. (13)). 

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