Four-membered peroxides

The homolyses of 104 to 105 would be expected on the basis of findings that diaryl peroxides are stable only at very low temperatures, and decompose upon warming to aryloxy radicals (Walling, 1957). The conversion of 105 to 106 finds analogy in the known conversion of triphenylmethoxy radical to the more stable diphenoxymethyl radical (Wieland, 1911). The four-membered peroxide 107 would be expected by precedent to cleave to the ketoaldehyde 108 (Wei et al., 1967 White et al., 1969 Fenical et al., 1969). 

On the basis of mechanistic studies, mainly on these isolable cychc four-membered peroxides (1 and 2), two main efficient chemiexcitation mechanisms can be defined in organic peroxide decomposition (i) the unimolecular decomposition or rearrangement of high-energy compounds leading to the formation of excited-state products, exemplified here in the case of the thermal decomposition of 1,2-dioxetane (equation i)". 5,i9. 

The experimentally observed substituent effect on the triplet and singlet quantum yields in the complete series of methyl-substituted dioxetanes, as well as the predicted C—C and 0—0 bond strength for the four-membered peroxidic rings , have led to the hypothesis that a more concerted, almost synchronized, decomposition mechanism should lead to high excitation quantum yields (as in the case of tetramethyl-l,2-dioxetane), whereas the biradical pathway presumably leads to low quantum yields (as in the case of the unsubstituted 1,2-dioxetane)" . However, it appears that this criterion of concertedness is difficult to apply generally to structurally dissimilar dioxetane derivatives. 

It can be shown by 13C Fourier transform NMR spectroscopy at — 78 °C that a six-membered peroxide is an intermediate which disappears completely on warming, with formation of the observed reaction products. The extrusion of nitrogen from the six-membered peroxide yielded the four-membered peroxide (equation 25) accompanied by light emission103. 

Four-membered peroxides are an interesting group of compounds, and will be discussed briefly. Such compounds have the ability to yield excited state carbonyl fragments upon thermolysis 154 in such cases, chemiluminesence is often observed. A classic illustration of this type of chemistry is the reaction of hydrogen peroxide with oxalyl derivatives, for example, when the 2,4-dinitro-... 

No three- or four-membered peroxide rings are known at present. Though stable peroxides were often formulated as three- or four-membered rings in the older literature,1 3 detailed examination always showed that the structure was actually different. 

Cyclic peroxides are obtained from diketones when a five- or six-membered peroxide ring can be formed, as is the case with acetyl-acetone, acetonylacetone, triacetylmethane, and benzalacetylace-tone. In the reaction of 1,2-diketones with hydrogen peroxide, the C—C bond is broken to give carboxylic acids, without intermediate formation of a four-membered peroxide ring.37... 

Later, fireflv oxyluciferin was successfully synthesi2ed (403. 408) and has been isolated and identified in firefly lanterns (luciola cruaciata) after the lanterns were treated with pyridine and acetic anhydride to prevent decomposition (409). In 1972, Suzuki and Goto firmly established that oxyluciferin is involved in the bioluminescence of firefly lanterns and in the chemiluminescence of firefly luciferin (403. 410).. A. mechanism involving a four-membered ring cyclic peroxide has been proposed for the reaction (406. 411). However, it was not confirmed by 0 -labelinE experiments (412). 

Based on the available knowledge on the chemiluminescence and bioluminescence reactions of various luciferins (firefly, Cypridina, Oplophorus and Renilla), the luminescence reaction of coelenterazine is considered to proceed as shown in Fig. 5.4 (p. 171). The reaction is initiated by the binding of O2 at the 2-position of the coelenterazine molecule, giving a peroxide. The peroxide then forms a four-membered ring dioxetanone, as in the case of the luminescence... 

If a four-membered ring peroxide (1.2-dioxetane) is involved in a reaction, its concerted bond cleavage into two carbonyl moieties should yield one of these in its excited electronic state on the basis of the orbital symmetry conservation rules of R. B. Woodward and R. Hoffmann ... 

As mentioned earlier (see p. 122) the previously postulated dioxetane intermediate in firefly bioluminescence has been challenged as no 180 is in-corporated in the carbon dioxide released during oxidation of firefly luciferin with 18C>2. In view of the crucial significance of the 180. experiments De Luca and Dempsey 202> rigorously examined the reliability of their tracer method. They conclude from their experiments that all available evidence is in favour of a linear, not a cyclic peroxide intermediate — in contrast to Cypridina bioluminescence where at least part of the reaction proceeds via a cyclic peroxide (dioxetane) as concluded from the incorporation of 180 into the carbon dioxide evolved 202,203). However, the dioxetane intermediate is not absolutely excluded as there is the possibility of a non-chemiluminescent hydrolytic cleavage of the four-membered ring 204>. 

Singlet oxygen reacts with electron rich or highly strained alkenes to form 1,2-dioxetanes. These four-membered ring peroxides decompose on warming to two carbonyl compounds (or moieties), usually with appearance of light emission (chemiluminescence). The macrocyclic bis-lactone in (6.17)608>, a musk fragrance, has been synthesized via such a sequence. 

Another reaction in which an oxygen cation is plausible as an intermediate is in the ozonization of olefins. Ozonides are now known to have many structures, but the molozonide precursor of the classical" or most common ozonide is believed to have a four-membered, cyclic structure. Criegee and the author have independently proposed a mechanism in which heterolytic fission of the cyclic peroxide bond leads to an intermediate that can rearrange either to the classical ozonide or to an "abnormal ozonide 816 328... 

Acyltetramic acids (39) are almost completely enolized in solution as well as in the crystalline state [80JCS(P1)1057], whereby the exo-enol form (a) is favored. Irradiation of 39a in peroxide-free cyclohexene leads to [2 + 2]-cycloaddition at the exocyclic double bond. Subsequent thermal reaction opens the four-membered ring of cycloadduct 59. 3-(2-Acetyl-cyclohexyl)-l,5,5-trimethyl-2,4-pyrrolidinedione 60 is obtained in 85% yield. In solution, 60 establishes an equilibrium with cyclic hemiacetal 61 (92M93). (See Fig. 28.)... 

Adam, W., in Four-membered Ring Peroxides 1,2-Dioxetanes and -Peroxylac-tones. The Chemistry of Peroxides, Patai, S. (Ed.), Wiley, New York, 1983, Chapter 24, pp. 829-920. 

Adam, W., Heil, M., Mosandl, T. and Saha-Moeller, C.R. (1992) Dioxetanes and a-peroxy lactones, four-membered ring cyclic peroxides, in Organic Peroxides (ed. W. Ando), John Wiley Sons, Chichester, UK, pp. 221-254. 

A four-membered ring structure (91) has also been proposed for the primary ozonide, but this seems unlikely in view of the appreciable strain that should be present in four-membered ring structures.84 90 decomposes readily into the peroxidic zwitterion64 (carbonyl oxide84) (92) and a carbonyl fragment (93).84a... 

Figure 9 describes a proposed mechanism for the dioxygenase reaction based on enzymatic and structural studies. It is proposed that the substrate binds as a dianion, which ligates the metal ion. Oxygen would then react to form an organic peroxide anion, which would then form a eyerie peroxide intermediate. Trie different reaction specificity is proposed to result from attack of the peroxide oxygen on alternative carbons. For example, the Ni-ARD would direct attack of the peroxide anion at the C3 carbonyl to form a five-membered peroxide intermediate, while the Fe-ARD would attack at carbon 2 to form a four-membered cyclic peroxide. 

The photooxygenation of three- and four-membered heterocyclic compounds affords five- and six-membered heterocyclic peroxides (Scheme 59) [164, 165]. In the case of the photooxygenation of aziridines, the cis and trans isomer ratio of the five-membered peroxides depends on the bulkiness of substituents on the nitrogen atom. 

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