Rearrangements ozonides

Addition of thermally-generated ethoxycarbonylnitrene to 1,4-naphthoquinone in the presence of 02 gives a small amount of a 1,3,4-dioxazolidine derivative, thought to arise from an initially-formed 1,2,3-dioxazolidine by a process analogous to the molozonide-ozonide rearrangement (77CJC2363). 

Besides these scission products formic acid (0.6 mole) is found. This may be formed via an abnormal ozonide rearrangement from the nonpolar structure. The total yield of fission products is about 65% of the original trimethylpyrone. 

The photooxygenation of furans was investigated initially by Schenck in the mid-forties. A crystalline and highly explosive material 32 was isolated from the parent furan by Schenck and Koch. This secondary ozonide rearranges thermally to an epoxybutanolide and a fcis-epoxide reduction with triphenylphos-phine leads to maleic dialdehyde, methanolysis to a methoxy butenolide. 

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). 

When uradiated, fluonnated isomers of Dewar benzene yield pnsmane derivatives that rearrange thermally to benzene Photolysis of hexakis(mfluororaethyl)benzvalene ozonide gives tetrakis(tnfluoromethyl)cyclobutadiene and its dimer [J47]... 

Ozone reacts vigorously with alkenes to form unstable initial ozonides (molozonides) which rearrange spontaneously to form ozonides. 

The rearrangement is thought to go through dissociation of the initial ozonide into reactive fragments that recombine to give the ozonide. 

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... 

The reaction gave only the rearrangement products 333 and 334, and the side product 335, as expected from the reactivity of alkylidenecyclopropane derivatives (Scheme 49). Compound 333 might arise from the 0-0 bond cleavage followed by the rearrangement of a cyclopropyloxy cation to an oxoethyl cation (Scheme 49, path a). Spiro-hexanone 334 could arise from a different fragmentation of ozonide C-O bond and further cyclopropyloxy-cyclobutanone rearrangement (Scheme 49, path b). Oxirane 335 can eventually derive from the same path b or from other side processes [13b]. 

In general, open structures II are energetically preferred over the closed forms I [8, 9], In the ring closed isomers I two unfavorable double bonds within five-membered rings would be required. No monoadduct with such a structure has been observed. Fulleroids such as 1-3 are usually formed via rearrangement of their pyrazoline-, triazoline- or ozonide [6,6]-precursor adducts accompanied by extrusion of N2 or O2 (see Chapter 4) [7,10-12]. 

The most commonly employed transformations for the construction of five-membered rings containing three sulfur or oxygen atoms in the 1,2,4-positions are shown in Table 11. These have attracted more interest than the syntheses from acyclic components. The rearrangement of a 1,2,3-trioxolane (primary ozonide) to a 1,2,4-trioxolane (secondary ozonide) is the most generally applicable method for preparation of this ring system and will be discussed further in Sections... 

Trioxolane (1) has only been prepared by the ozonolysis of ethylene. The rearrangement of the primary ozonide occurs above — 100°C to give 1,2,4-trioxolane as a colorless, explosive liquid <42LA(553)187>. 1,2,4-Trithiolane (2) is still best prepared by a classical reaction of Na2S2.5 with excess dichloromethane. Some 1,2,4,5-tetrathiolane is also produced, but (2) can be isolated as a pale-yellow distillable liquid. It is best kept stored under inert atmosphere below 0°C to avoid polymerization <67CPB988>. Parent compounds (3)-(6) are not known and the 1- and 4-5-oxides for 1,2,4-trithiolane have been mentioned previously (see Section 4.16.5.2.3). 

Direct calorimetric measurements of two ozonides have been reported. Both the enthalpy of combustion of the ozonides and the direct enthalpies of ozonation of the precursor olefin were measured. The first species to be studied was the purported ozonide of A °octalin (l,2,3,4,5,6,7,8-octahydronaphthalene) . It is doubtful that the product of the octalin ozonation reaction would be the molozonide formed by direct addition with no subsequent rearrangement (i.e. ll,12,13-trioxabicyclo[4.4.3]tridecane) but perhaps even less likely is the rearranged and hence normal ozonide, the ll,12,13-trioxa[4.4.2.1]paddlane. From the published enthalpy of combustion of —5628 kJmoU, we derive an enthalpy of formation of this species, whatever it is, of —593.7 kJmoU. ... 

Oxidative hydrolysis transforms the intermediate ozonide into ketone(s) and/or carboxylic acid(s) in good yields. H202 in water, in sodium hydroxide solution, or in formic acid is the best proven oxidant.582,584,592 Peroxy acids and silver oxide are also employed. Rearrangement and overoxidation may be undesirable side-reactions. A simple two-step ozonation in MeOH yields methyl esters without added oxidizing agent.627... 

It has been shown in ozonizations of fluoroalkenes (vide supra) that fluoro substituents stabilize the ozonides formed from the rearrangement of the initial molozonides. Rather stable ozonide isomers 3 are formed from the trifluoromethylated alkenyl ether 2 as two separable dia-stereomers with isolated yields 27 and 26%. respectively.180 The stability was demonstrated... 

Takenaka and Lemal subsequently undertook an extensive study of the perfluorobenzene oxide (3)/perfluorooxepin (4) system.13,14 After considerable effort, benzene oxide 3 was successfully prepared by ozonization of tricyclic precursor 5 and photolytic decomposition of the resultant ozonides. Oxepin 4 was not observed directly, but was estimated by NMR computer simulations to be present in small proportion (3 %) at 55"C. The principal reaction of 3 is its rearrangement to cyclohexadienone 2, which occurs at room temperature in polar solvents, by heating in nonpolar solvents, or in the presence of Lewis acids. Photolysis of 3 with benzophenone as triplet sensitizer also produces 2. Furthermore, attempts to trap the oxepin, for example by treatment with bromine in the dark, result in cyclohexadienone 2. 

Ozonolysis of a double bond leads first to a so-called primary ozonide 40 through 1,3-dipolar cycloaddition. Rearrangement of primary ozonide 40 with ring cleavage produces a carbonyl oxide 42 and a carbonyl compound 41, which then recyclize to secondary ozonide 43. The reaction terminates with a redox process involving... 

Despite the complexities of alkene ozonolysis47, the reaction between alkenes and ozone may be summarized by Scheme 7. The reaction involves several steps48 with the formation of a variety of intermediates, such as a primary ozonide (1,2,3-trioxolane) (12), its isomer of rearrangement 13 and a carbonyl oxide (14). 

Hydrogen atoms in allylic position are favorite sites for hydroperoxidation of chains. So, this mechanism proceeds in the formation of lateral hydroperoxides, and not like for other polymers, in intramolecular peroxides. Rearrangement of chemical structures coming from ozonides are rapidly observed (Scheme 33). 

There are six possible five-membered monocycles 1-6 containing three oxygen or sulfur atoms in the 1,2,3-positions <1996CHEC-II(4)545>. 1,2,3-Trioxolane 1 is the parent compound of the so-called primary ozonides, the primary reaction products in the reaction of alkenes with ozone. They are extremely unstable and rearrange to the more stable ozonides (1,2,4-trioxolanes). This rearrangement represents a key step in the reaction of ozonolysis. However, the parent compound 1 and a few derivatives have been characterized at low temperatures (see Section 6.05.10.1). 1,2,3-Trithiolanes have been synthesized (Section 6.05.10.3) some of them undergo slow decomposition at room temperature. Derivatives of 1,2,3-dioxathiolane 3 are unknown, and the other heterocycles of the mixed types 4-6 are known only in the oxidized forms, mostly as -oxides and J -dioxides, and also A-imino and A-thiono derivatives <1996CHEC-II(4)545>. The A-oxides and AA -dioxides of... 

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