Cyclic peroxide, formation

Due to the formal analogy to the classical Diels-Alder reaction, the mechanism of cyclic peroxide formation through cycloadditions of 1,3-dienes with O2 was considered for a long time to involve a concerted suprafacial [4 4- 2]-cycloaddition of a super-dienophile, namely a singlet oxygen to 1,3-dienic system In such a case, the concerted or almost concerted cycloaddition must be c -stereospecific and the stereochemical properties of the diene must be reflected in the three-dimensional structure of cyclic peroxide according to well-defined rules. Indeed, it was found in early stereochemical... 

Depending on the reaction conditions— primarily pH, concentration of reagents— the cyclic dimer, trimer form from their linear analogs. Cyclic dimer and trimer crystallize from the reaction mixture when catalyzed by an acid. The reaction mechanisms of cyclic peroxide formation have been investigated by several authors and are schematically summarized below [1-7]. 

The mechanism for cyclic peroxide formation presumably involves formation of 3-peroxy alkyl radicals like 3 which can react with oxygen to yield ultimately cyclic peroxide products. 

The lucigenin radical 78 is not involved, according to Janzen and coworkers 136), in the direct formation of this cyclic peroxide one would, however, expect a reaction of 78 with oxygen radical anion to be a possible way of forming the cyclic peroxide, although lucigenin radicals were not detected in the presence of hydrogen peroxide. 

The oxidation of naphthalic acid (1,8-naphthalenedicarboxylic acid) by peroxide, rather surprisingly, does not proceed by formation of a cyclic peroxide but rather via a dioxirane [53] (a three-membered ring containing a carbon atom and a peroxide group). CL is observed from this reaction. 

Hydrocarbons with conjugated double bonds are oxidized with the formation of cyclic peroxides [46,80,82], for example ... 

Polynuclear aromatic hydrocarbons can be oxidized photolytically with the formation of cyclic peroxide. For example, anthracene is photooxidized to peroxide with the quantum yield 0 = 1.0 [205], The introduction of quenchers lowers the peroxide yield. 

Crosslinking of alkyds containing conjugated double bonds results in more carbon-carbon bonds in the crosslinks than in the alkyds containing unconjugated double bonds. The crosslinking mechanism involves the formation and decomposition of cyclic peroxides, to yield radicals that initiate crosslinking by 1,4-polymerization of the polymer molecules. 

TABLE 4. Enthalpies of formation of cyclic peroxides and multiply-linked peroxides (kJ mol )... 

The dialkyl denomination also includes cyclic peroxides (endoperoxides). The most significant route for peroxide formation is probably that of autoxidation of organic materials, leading to their gradual degradation. Although hydroperoxides are the main products of this process, also peroxyesters are formed, as is the case, for example, of isoprostane bicyclic endoperoxides (25) mentioned in Section II.A.2.C. 

Condensation of diethyl difluorosilane with 100% hydrogen peroxide in the presence of ammonia results in the formation of ethoxysilane, whereas an analogous condensation with diethyl dichlorosilane produces polymeric peroxides containing ethoxy and siloxane units. The initially formed straight chain or cyclic peroxides probably rearrange to products 3 or 4 containing alkoxy and siloxane moieties (equation 5) . 

The unimolecular decomposition of 1,2-dioxetanes and 1,2-dioxetanones (a-peroxylac-tones) is the simplest and most exhaustively studied example of a thermal reaction that leads to the formation, in this case in a single elementary step, of the electronically excited state of one of the product molecules. The mechanism of this transformation was studied intensively in the 1970s and early 1980s and several hundreds of 1,2-dioxetane derivatives and some 1,2-dioxetanones were synthesized and their activation parameters and CL quantum yields determined. Thermal decomposition of these cyclic peroxides leads mainly to the formation of triplet-excited carbonyl products in up to 30% yields. However, formation of singlet excited products occurs in significantly lower yields (below... 

Experimental evidence of the involvement of a biradical intermediate in the decomposition of 3,3-dimethyl-l,2-dioxetane (10) has been obtained by radical trapping with 1,4-cyclohexadiene (CHD). Decomposition of 10 in neat CHD was shown to result in the formation of the expected 1,4-dioxy biradical trapping product, 2-methyl-1,2-propanediol (11) ° . However, more recently, it has been shown that the previously observed trapping product 11 was formed by induced decomposition of the dioxetane, initiated by the attack of the C—C double bond of the diene on the strained 0—0 bond of the cyclic peroxide (Scheme 9)"°. 

A CIEEL approach can also be used to explain chemiexcitation in luminol chemilumi-nescence . Two possibilities arise (i) an electron transfer from the amino group to the peroxidic moiety in the antiaromatic peroxide 33, resulting in bond cleavage followed by intramolecular back-electron transfer and formation of excited 3-aminophthalate (Scheme 24) (ii) the equilibrium between the peroxycarboxylic aldehyde 34, formed after elimination of nitrogen, and the cyclic peroxy semiacetal 35 is shifted in the direction of 35, as the result of an electron transfer from the amino group to the cyclic peroxide moiety, followed by 0—0 bond cleavage . Back-electron transfer would result in chemiexcitation (Scheme 25). 

Cyclic hydrocarbons, diastereoselective allylic hydroperoxide formation, 861-3, 864 Cyclic olefins, final ozonides, 718 Cyclic peroxides... 

Formation of cyclic peroxides by conjugated dienes is a general reaction and although ultraviolet light often initiates the reaction, better results are achieved by carrying out the... 

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