Radical mechanisms biradicals

The degradation process has a free radical mechanism. It is initiated by free radicals P that appear due to, for example, hydroperoxide decomposition induced thermally or by trace amounts of metal ions present in the polysaccharide. One cannot exclude even direct interaction of the polysaccharide with oxygen in its ground triplet state with biradical character. Hydroperoxidic and/or peracid moieties are easily formed by oxidation of semiacetal chain end groups. The sequence of reactions on carbon 6 of polysaccharide structural unit that ultimately may lead to chemiluminescence is shown in Scheme 11. 

The chemical details of the reactions of representative alkyl radicals, alkoxy radicals, and biradicals with oxygen should be established. Both the rate constants and the immediate products are needed to construct realistic mechanisms for the model. 

The hidden-radical mechanism (also dubbed the mechanism of biradical origin ) does not, however, take into account all the peculiarities of the reaction participants. The redox potential of OH/ OH couple is equal to 0.9 V with regard to the saturated calomel electrode in AN (Tsang et al. 1987). In conventional reference to the normal hydrogen electrode and in water, this potential is equal to 1.9 (Berdnikov and Bazhin 1970) or 1.3 V in AN (Eberson 1987, Chapter 4, p. 62). 

The highly strained double bond in methylenecyclopropane displays enhanced reactivity in cycloaddition reactions. In addition to normal [4+2] cycloaddition to 1,3-dienes (e.g. equation 13)32, methylenecyclopropane and its derivatives have a pronounced tendency to undergo thermal [2+2] cycloaddition reactions. For example, thermal dimerization of methylenecyclopropane in the gas phase results in formation of isomeric dispirooctanes 16 and 17 (equation 14)33. This unusual cyclization is considered to proceed via a stepwise radical mechanism involving the intermediacy of biradical 18 (equation 15)34. Equation 15 demonstrates that methylenecyclopropanes possessing substituents capable of stabilizing intermediate radicals undergo efficient [2+2] dimerization even... 

The products of the thermolysis of 3-phenyl-5-(arylamino)-l,2,4-oxadiazoles and thiazoles have been accounted for by a radical mechanism.266 Flash vacuum pyrolysis of 1,3-dithiolane-1-oxides has led to thiocarbonyl compounds, but the transformation is not general.267 hi an ongoing study of silacyclobutane pyrolysis, CASSF(4,4), MR-CI and CASSCF(4,4)+MP2 calculations using the 3-21G and 6-31G basis sets have modelled the reaction between silenes and ethylene, suggesting a cyclic transition state from which silacyclobutane or a trcins-biradical are formed.268 An AMI study of the thermolysis of 1,3,3-trinitroazacyclobutane and its derivatives has identified gem-dinitro C—N bond homolysis as the initial reaction.269 Similar AMI analysis has determined the activation energy of die formation of NCh from methyl nitrate.270 Thermal decomposition of nitromethane in a shock tube (1050-1400 K, 0.2-40 atm) was studied spectrophotometrically, allowing determination of rate constants.271... 

The P-cleavage of halogen atoms and thiyl radicals from biradicals appears to follow a simple enough mechanism, as shown in Sch. 2. After being formed, the 5-substituted 1,4-biradical can undergo radical cleavage in competition with its normal decay reactions. That leaves an... 

Support for a free-radical mechanism for dextrinization is afforded by comparison of the processes conducted in air with those carried out under a neutral atmosphere and under vacuum. The absence of oxygen accelerates dextrinization, and thus, the overall process yielding dextrins is not merely oxidation furthermore, free radicals are trapped by molecular oxygen, a biradical. Pyrolysis of a-D-glucose at 300° to 1,4 3,6-dianhydro-f -D-... 

Oxirane formation can also occur via free radical mechanisms, as in the reaction of certain fluoroalkenes with oxygen. Under pressure at elevated temperatures, oxygen alone can suffice, but activation is frequently provided in the form of radical initiators (e.g., tribromofluoromethane) and ultraviolet light. Thermolysis of dioxole 5, comonomer from which DuPont s Teflon-AF is made, offers an unusual route to an oxirane. Rearrangement of the heterocycle presumably takes place via a biradical intermediate. ... 

The mechanism of the unsensitized photoreaction with oxygen is similar to that of photoreaction without oxygen. Both of them involve free radical or biradical formation. Photooxidation without sensitizer generally gives a more complex result than sensitizer photooxidation, probably because of co-occurrence of the photoreaction of the substrate itself, oxidation of the initial photoproduct, or further photochemical transformation of the photooxdation product (Matsuura and Saito, 1976). 

Also, as shown in this example and similar to the Diels-Alder reaction, the normal or traditional mode of reaction is for the lowest unoccupied molecular orbital (LUMO) of the electron-deficient enophile to react with the highest occupied molecular orbital (HOMO) of the electron-rich ene. However, there are some carbon and hydrogen containing systems that have been proposed to proceed via radical or stepwise pathways. For example, allenyl alkynes 7 and 8 were thermally cyclized using PhMe under relatively mild conditions to provide 10 and 11 in good yield. The authors suggested a radical mechanism for this transformation via the fulvene biradical intermediate 9. ... 

When the sulfonium-based polymerization reaction of Figure 17.14 B is run at low temperatures, a short-lived intermediate with an absorption at 317 nm is observed. Propose a structure for this intermediate. From this intermediate, the mechanism could proceed along one of two different paths. One involves radical and biradical-type structures, and the other is... 

Homolysis, Oxidation, and Reduction.—Photolysis converts 2-nitrosoimino-3-phenyl-2,3-dihydrobenzothiazole (48 R = Ph), with loss of nitric oxide, into bis[o-(N-phenylcyanamino)phenyl] disulphide in 74% yield. Under varied conditions, 2-imino-3-phenyl-2,3-dihydrobenzothiazole is also formed (20%). The relative rates of the photolysis of (48) were measured in a number of different solvents. An interpretation of the results in terms of a free-radical mechanism involving the biradical (49) was proposed."... 

The two most widely accepted mechanisms for the spontaneous generation of radicals from S are the biradical mechanism (top half of Scheme 3.61) first proposed by Flory314 and the Mayo315 or MAH (molecule assisted homolysis) mechanism (lower pari of Scheme 3.61). 

As the mechanism, a radical and a cationic pathway are conceivable (Eq. 31). The stereochemical results with rac- or mcjo-1,2-diphenyl succinic acid, both yield only trans-stilbene [321], and the formation of a tricyclic lactone 51 in the decarboxylation of norbornene dicarboxylic acid 50 (Eq. 32) [309] support a cation (path b, Eq. 31) rather than a biradical as intermediate (path a). 

The ozonolysis of ethylene in the liquid phase (without a solvent) was shown to take place by the Criegee mechanism.This reaction has been used to study the structure of the intermediate 16 or 17. The compound dioxirane (21) was identified in the reaetion mixture at low temperatures and is probably in equilibrium with the biradical 17 (R = H). Dioxirane has been produced in solution but it oxidatively cleaves dialky] ethers (such as Et—O—Et) via a chain radical process, so the choice of solvent is important. 

Thermal decomposition of 1-methyl-A -phospholen in toluene at 356-444 °C yielded butadiene as the primary product. The activation parameters are in agreement with a mechanism involving ring opening to a biradical followed by fragmentation into butadiene, phosphorus, and methyl radicals. ... 

Although this mechanism could explain the inertness of di-t-butyl sulphide towards oxidation due to the absence of a-hydrogen atoms, it was later ruled out by Tezuka and coworkers They found that diphenyl sulphoxide was also formed when diphenyl sulphide was photolyzed in the presence of oxygen in methylene chloride or in benzene as a solvent. This implies that a-hydrogen is not necessary for the formation of the sulphoxide. It was proposed that a possible reactive intermediate arising from the excited complex 64 would be either a singlet oxygen, a pair of superoxide anion radical and the cation radical of sulphide 68 or zwitterionic and/or biradical species such as 69 or 70 (equation 35). 

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