Peroxides fragmentation

Let us briefly overview the results of relevant studies. The mechanism of the photolysis of hydrocarbon peroxide radicals of different types was studied [119], and the decomposition of the electron-excited peroxide fragment with an 0-0 bond cleavage was found to be the primary act of the process. When the reaction is not complicated by the adsorption of molecules on the solid surface, its kinetic parameters can be determined. The rate constants of the reactions of radicals =Si, =Si-0-0, =Si-CH, =Si-0-CH, =Si-CH2-CH, and =Si-0- C = O with H2(D2), CH4, C2H6, and C3H8 were measured in Ref. [16]. 

Scheme 2.99 Other commonly used methods for generation of perfluoroalkyl radicals [14-16], The activation energy of perfluorodiacyl peroxide fragmentation is approximately 24 kcal mol resulting in a half-life at room temperature of ca. 5 h.

Lastly a peroxide fragmentation has been used to prepare the... 

Radical destruction by combination with peroxide fragments ... 

Only under combined action of anion and cation the most essential changes of the peroxide fragment (C-O-O-C) conformation in the ROOR molecule are observed. It is the dominant factor of peroxide bond activation. Considering models V and VI it is impossible to prefer finally only one structure. But comparative analysis of all considered models allows conclude about selective association of peroxide with tetraethylammonium bromide ions i.e. catalyst attack should be stereospecific. 

Ozonation ofAlkenes. The most common ozone reaction involves the cleavage of olefinic carbon—carbon double bonds. Electrophilic attack by ozone on carbon—carbon double bonds is concerted and stereospecific (54). The modified three-step Criegee mechanism involves a 1,3-dipolar cycloaddition of ozone to an olefinic double bond via a transitory TT-complex (3) to form an initial unstable ozonide, a 1,2,3-trioxolane or molozonide (4), where R is hydrogen or alkyl. The molozonide rearranges via a 1,3-cycloreversion to a carbonyl fragment (5) and a peroxidic dipolar ion or zwitterion (6). 

The thermal fragmentation of unsaturated bicyclic 1,4-peroxides, often readily made from 1,4-dienes (Scheme 84), has become an important route to novel bis(oxiranes) (80T833, 81CRV91). 

Content of prime - tertiary peroxide groups was measured by the quantity of products of complete decay, which were measured by chromatography. It is known that the main contents in products of the complete decay of Oct-MA-TBPMM samples are acetone and 2,2-dimethylpropanol, which arise in reactions of chain fragmentation of tert-butylperoxy radical or in reaction of chain transfer of this radical. In this case the sum of acetone and 2,2-dimethylpropanol molecules is equal to the quantity of peroxide groups in polymer. As an internal standard we used chloroform. 

Acyl radicals can fragment with toss of carbon monoxide. Decarbonylation is slower than decarboxylation, but the rate also depends on the stability of the radical that is formed. For example, when reaction of isobutyraldehyde with carbon tetrachloride is initiated by t-butyl peroxide, both isopropyl chloride and isobutyroyl chloride are formed. Decarbonylation is competitive with the chlorine-atom abstraction. 

Chemistry and biological activity of artemisinin (sesquiterpene y-lactone with oxepane fragment and transannular peroxide bridge) and related antimalari-als 99H(51)1681. 

Natural peroxides and pharmacological preparations possessing oxirane fragment 99MI3. 

Aliphatic acyloxy radicals undergo facile fragmentation with loss of carbon dioxide (Scheme 3,69) and, with few exceptions,428 do not have sufficient lifetime to enable direct reaction with monomers or other substrates. The rate constants for decarboxylation of aliphatic acyloxy radicals are in the range l 10xl09 M 1 s at 20 °C.429 lister end groups in polymers produced with aliphatic diacyl peroxides as initiators most likely arise by transfer to initiator (see 3.3.2.1,4). The chemistry of the carbon-centered radicals formed by (3-scission of acyloxy radicals is discussed above (see 3.4.1). 

In the case of allyl peroxides (12 X= CH2, A=CH2, BO),1 1 1 intramolecular homolytic substitution on the 0-0 bond gives an epoxy end group as shown in Scheme 6.18 (1,3-Sn/ mechanism). The peroxides 52-59 are thermally stable under the conditions used to determine their chain transfer activity (Table 6.10). The transfer constants are more than two orders of magnitude higher than those for dialkyi peroxides such as di-f-butyl peroxide (Q=0.00023-0.0013) or di-isopropyl peroxide (C =0.0003) which are believed to give chain transfer by direct attack on the 0-0 bond.49 This is circumstantial evidence in favor of the addition-fragmentation mechanism. 

Depending on the choice of transfer agent, mono- or di-cnd-functional polymers may be produced. Addition-fragmentation transfer agents such as functional allyl sulfides (Scheme 7.16), benzyl ethers and macromonomers have application in this context (Section 6.2.3).212 216 The synthesis of PEG-block copolymers by making use of PEO functional allyl peroxides (and other transfer agents has been described by Businelli et al. Boutevin et al. have described the telomerization of unsaturated alcohols with mercaptoethanol or dithiols to produce telechelic diols in high yield. 

However, in most cases photolysis of XS(0)Y alone produced much weaker signals from the radical XSO than when mixtures of the compounds with peroxides were irradiated. A mechanism has been proposed which involves hydrogen abstraction to form species 4, the fragmentation of which gives the sulfinyl radical, namely13,14,16... 

The probable reaction mechanism involves formation of the peroxide 55, which then fragments to give the observed products. 

---