Cage disproportionation

The photochemical interaction of EtjGeCFFCH CI I2 (13) with CC Br is also accompanied by the formation of trace amounts of polarized chloroform CHCI3 (Table 6 and Figure 13). The sign of chloroform polarization (emission) allows one to suggest that CHCI3 is a product of the in-cage disproportionation of the initial... 

The disproportionation reaction is depicted above although other modes are possible. j8-Hydrogens are abstracted by the radical, either from the methyl group or the ring, to yield the hydrocarbon 49 with largely retained configuration and also the two olefins 90a, b. A cage disproportionation reaction has also been observed in the thermal decomposition of trans-2-phenylcyclopropanoyl peroxide in carbon tetrachloride. 

As seen in Scheme 14, molecular elimination processes (including cage disproportionation reactions) account for about half of the primary decomposition. This is in contrast to the behavior of the Isosteric ethers such as diethyl (177) and methyl n-propyl ethers (88), where these processes are rather less important (Schemes 6 and 7). It is not known whether the formaldehyde formed in reaction 4 (Scheme 14) incorporates the carbon atom from methylene or methyl. 

Figure 4 indicates that the yields of olefins produced by radiolysis are independent of the absorbed dose. These unsaturated products are pentene, light hydrocarbons, and eventually decenes produced by Reaction 15. Cage disproportionation of alkyl radicals produced by C—C bond scission and molecular fragmentation account for the formation of light olefins. Pentene would be formed either by elimination of molecular hydrogen (Reaction 5) or by disproportionation of radicals when the temperature of the sample is raised (Reactions 9 and 16). The latter process is thus a postirradiation production of pentene. 

A possible explanation is that the unsaturated by-product from cage-disproportionation (e.g. MAN from AIBN, Section 3.3.1.1.3)" may copolymcrizc. This may result in an apparent functionality of >2. 

It is not likely that all A-Sty trimer arises from an ene reaction. (I.e., from reaction in Fig. lA.) As I have pointed out (J), phenyltetralin probably arises from the disproportionation reaction of caged radicals, shown as eq in Fig. lA. (Any A that diffuses into free solution would be expected to add to styrene and not to abstract hydrogen to give phenyltetralin.) If cage disproportionation occurs, then cage combination also must occur. (See eq Ji in Fig. lA.) However, the conclusion reached here is not dependent on whether trimer arises from the combination of radicals within a cage or an ene process, since neither reaction produces free radicals that can initiate the polymerization of styrene. 

Random scissions below 300 °C probably involve initial homolysis (Scheme 13). Since no volatile products result, the radicals A and B produced are believed to undergo a cage disproportionation ... 

The assumption that k values are constant over the entire duration of the reaction breaks down for termination reactions in bulk polymerizations. Here, as in Sec. 5.2, we can consider the termination process—whether by combination or disproportionation to depend on the rates at which polymer molecules can diffuse into (characterized by kj) or out of (characterized by k ) the same solvent cage and the rate at which chemical reaction between them (characterized by kj.) occurs in that cage. In Chap. 5 we saw that two limiting cases of Eq. (5.8) could be readily identified ... 

For /-butyl peresters there is also a variation in efficiency in the series where R is primary secondary>tertiary. The efficiency of /-butyl peroxypentanoate in initiating high pressure ethylene polymerization is >90%, that of /-butyl peroxy-2-ethylhexanoate ca 60% and that of/-butyl peroxypivalate ca 40%.196 Inefficiency is due to cage reaction and the main cage process in the case where R is secondary or tertiary is disproportionation with /-butoxy radicals to form /-butanol and an olefin.196... 

Recombination of the ion radicals within the cage is thought of as forming the path to rearrangement whilst escape of the radicals and subsequent reaction with the hydrazo compound leads to the formation of disproportionation products often observed. The theory is mainly directed at the two-proton mechanism and does not accommodate well the one-proton mechanism, since this requires the formation of a cation and a neutral radical, viz. 

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 CL that accompanied the disproportionation of secondary and primary peroxyl radicals (see later 2.5.4). The R1 R2C 000 biradical formed in the cage is a predecessor of exited carbonyl compound and singlet dioxygen. 

It can be seen that primary and secondary R02 radicals disproportionate in the cage involving the a-C—H bond, which explains why the substitution of D in the a-position for H retards the disproportionation of R02 . Because of this, R02 radicals of unsaturated compounds with a double bond in the a-position to the peroxyl free valence disproportionate more rapidly than structurally analogous aliphatic peroxyl radicals [195],... 

Dialkyl peroxides decompose with splitting of the weakest O—O bond [3,4]. The pair of forming alkoxyl radicals recombine or disproportionate in the cage or go out the cage ... 

Another mechanism of nitroxyl radical regeneration was proposed and discussed in the literature [67-71]. The alkoxyamine AmOR is thermally unstable. At elevated temperatures it dissociates with cleavage of the R—O bond, which leads to the appearance of an [AmO + R ] radical pair in the cage of polymer. The disproportionation of this radical pair gives hydroxylamine and alkene. The peroxyl radical reacts rapidly with hydroxylamine thus... 

Azobisisobutyronitrile, 182, reacts thermally or photochemically to give the intermediate 183, which leads, in inert solvents, to combination products 184 and 185, and disproportionation products 186 and 187. The parent compound is dimorphic, and both crystal forms behave similarly on photolysis, yielding 95% disproportionation and 5% 184. In contrast, in both fluid and rigid solution the disproportionation products form only 5% of the total. The cage effect in the solid is almost quantitative. 

The thermal decomposition of azoalkanes bearing geminal a-cyano and a-trimethylsiloxy groups has been the subject of a report. The symmetrical compound (107) decomposes near room temperature to afford entirely C—C dimers, whereas the unsymmetrical azoalkane (108) requires heating to 75 °C. A NMR product study of photolysed (107) in the presence of TEMPO showed that the fate of caged t-butyl-l-trimethylsiloxy-l-cyanoethyl radical pairs is disproportionation (17%), cage recombination (20%), and cage escape (63%). 

As noted, the products of electron transfer retained in the solvent cage. Benzene was used as a solvent in these experiments. The cage complex [R -O, OH, D+ ] decays either on disproportionation in the cage or dissociation. Disproportionaton leads to phosphinoxides or sulfoxides mentioned earlier. Dissociation results in the passage of radicals out of the cage into the solvent pool. 

This inference is checked stereochemically. If hydroperoxide produces alcohol at the expense of disproportionation of radicals not leaving the cage, the enantiomeric hydroperoxide should give alcohol that retains its optical activity. And this is what actually takes place (Davies and Feld 1958). 

The reactivity of acyl radicals inside and outside the solvent cage has been a matter of discussion. It has been postulated that aryloxy and acyl radicals could disproportionate within the cage to give phenol (or naphthol) and ketene (37) but the results are not conclusive (Scheme 14). On the one hand, photolysis of 1-naphthyl acetate in a solvent without abstractable H-like Freon 113 (1,1,2-trichl-oro-l,2,2-trifluoroethane) yields low amounts of 1-naphthol (< 0.1%) in comparison with the same reaction in acetonitrile (7%) [50]. This reveals that dispropor-... 

Recently, we reported that an Fe supported zeolite (FeHY-1) shows high activity for acidic reactions such as toluene disproportionation and resid hydrocracking in the presence of H2S [1,2]. Investigations using electron spin resonance (ESR), Fourier transform infrared spectroscopy (FT-IR), MiJssbauer and transmission electron microscopy (TEM) revealed that superfine ferric oxide cluster interacts with the zeolite framework in the super-cage of Y-type zeolites [3,4]. Furthermore, we reported change in physicochemical properties and catalytic activities for toluene disproportionation during the sample preparation period[5]. It was revealed that the activation of the catalyst was closely related with interaction between the iron cluster and the zeolite framework. In this work, we will report the effect of preparation conditions on the physicochemical properties and activity for toluene disproportionation in the presence of 82. ... 

Studies on the photochemical reactions of dihydropyridines have proven to be interesting. There are a number of 1,4-dihydropyridines that are known to disproportionate when irradiated (equation 19) (B-76PH240). Analogous intramolecular reductions have also been observed by other workers (55JA447). In contrast to these results, the 1,4-dihydropyridine (59) rearranged to its 1,2-dihydro isomer (60). Further irradiation resulted in dimerization. Interestingly, the photodimer (61) cyclized to the cage compound (62). 

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