Ozone, cycloaddition with

Dipolar cycloadditions are another important family, with the impressive sequence of reactions involved when ozone reacts with an alkene as an example here. At -78°, ozone adds 1.3 (arrows) to give the molozonide 1.4. On warming, this undergoes a 1,3-dipolar cycloreversion (1.4 arrows),... 

A coozonolysis (two compounds in presence of ozone) is possible if one precursor generates the carbonyl oxide in situ that then reacts with the second compound - the carbonyl. JV-Methyl oximes have been found to be ideal precursors, because they readily react as dipolarophiles in a 1,3-dipolar cycloaddition with ozone. A retro- 1,3-dipolar cycloaddition then leads to the formation of the carbonyl oxide and methyl nitrite ... 

Figure 3 Direct pathways of ozone reaction with organics. (A) Criegge mechanism. (B) Electrophilic aromatic substitution and 1,3-dipolar cycloaddition. (C) Nucleophilic substitution.

Ozone is a symmetrical bent molecule with a central positively charged oxygen atom and two terminal oxygen atoms that share a negative charge. It is a 1,3-dipole and does typical 1,3-dipolar cycloadditions with alkenes. 

Ozonolysis is widely used both in degradation work to locate the position of double bonds and in synthesis for the preparation of aldehydes, ketones, and carboxylic acids. Ozone is a 1,3-dipole and undergoes 1,3-dipolar cycloadditions with alkenes. 

Many combinations of atoms are conceivable, among them azides, nitrones, nitrile oxides, and ozone. As these systems have four tt electrons, they are analogous to dienes, and cycloadditions with alkenes and alkynes are allowed [4 + 2] reactions. These are discussed in Section 10.3. 

The reaction of ozone with alkenes is one of the most useful 1,3-dipolar cycloadditions. Ozone undergoes [3 -I- 2] cycloaddition to the alkene to give a 1,2,3-trioxolane, which immediately decomposes by a [3 -I- 2] retro-cycloaddition to give a carbonyl oxide and an aldehyde. When the ozonolysis is carried out in the presence of an alcohol, the alcohol adds to the carbonyl oxide to give a hydroperoxide acetal. In the absence of alcohol, though, the carbonyl oxide undergoes another [3 -b 2] cycloaddition with the aldehyde to give a 1,2,4-trioxolane. 

Carbonyl oxides can be also used for the synthesis of 3-vinyl-1,2,4-trioxalanes (a-vinylozonides) by a [3+2] cycloaddition with a,P-unsatu-rated aldehydes. Howeve, a,P-unsaturated ketones are practically inactive in this reaction. The reaction of 3-vinyl-l,2,4-trioxalanes with ozone leads to the formation of the corresponding dizonides [79],... 

Ozone transforms the double bond to a mixed anhydride at one end and a ketone oxide at the other. Reaction of the amide group with the mixed anhydride results in a succinimide wherein the ketone oxide undergoes cycloaddition with one of the lactam carbonyls to yield the observed false ozonide. 

It was not their reactivity but their chemical inertness that was the true surprise when diazirines were discovered in 1960. Thus they are in marked contrast to the known linear diazo compounds which are characterized by the multiplicity of their reactions. For example, cycloadditions were never observed with the diazirines. Especially surprising is the inertness of diazirines towards electrophiles. Strong oxidants used in their synthesis like dichromate, bromine, chlorine or hypochlorite are without action on diazirines. Diazirine formation may even proceed by oxidative dealkylation of a diaziridine nitrogen in (186) without destruction of the diazirine ring (75ZOR2221). The diazirine ring is inert towards ozone simple diazirines are decomposed only by more than 80% sulfuric acid (B-67MI50800). 

A well-known example for a 1,3-dipolar compound is ozone. The reaction of ozone with an olefin is a 1,3-dipolar cycloaddition (see ozonolysis). 

The reaction of alkenes with ozone constitutes an important method of cleaving carbon-carbon double bonds.138 Application of low-temperature spectroscopic techniques has provided information about the rather unstable species that are intermediates in the ozonolysis process. These studies, along with isotope labeling results, have provided an understanding of the reaction mechanism.139 The two key intermediates in ozonolysis are the 1,2,3-trioxolane, or initial ozonide, and the 1,2,4-trioxolane, or ozonide. The first step of the reaction is a cycloaddition to give the 1,2,3-trioxolane. This is followed by a fragmentation and recombination to give the isomeric 1,2,4-trioxolane. The first step is a... 

The reaction of singlet oxygen with conjugated double bonds usually is a 1,4-cycloaddition leading to formation of derivatives of the 1,2-diox -ene ring system. This can be achieved either by photooxidation or by reaction in the presence of triphenyl phosphite-ozone adduct (Section Vin.D.2), shown in equations 85 and 86 . ... 

Several oxidizing reagents react with alkenes under mild conditions to give, as the overall result, addition of hydrogen peroxide as HO—OH. Of particular importance are alkaline permanganate (MnO40) and osmium tetroxide (0s04), both of which react in an initial step by a suprafacial cycloaddition mechanism like that postulated for ozone. 

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