Carbonyl oxides, formation ozonolysis

Ozonolysis in the presence of NaOH or NaOCH3 in methanol with CH2CI2 as a cosolvent leads to formation of esters. This transformation proceeds by trapping both the carbonyl oxide and aldehyde products of the fragmentation step.149... 

Schindler and coworkers verified the formation of hydroxyl radicals kinetically and further RRKM calculations by Cremer and coworkers placed the overall concept on a more quantitative basis by verifying the measured amount of OH radical. An extensive series of calculations on substituted alkenes placed this overall decomposition mechanism and the involvement of carbonyl oxides in the ozonolysis of alkenes on a firm theoretical basis. The prodnction of OH radicals in solution phase was also snggested on the basis of a series of DFT calculations . Interestingly, both experiment and theory support a concerted [4 4- 2] cycloaddition for the ozone-acetylene reaction rather than a nonconcerted reaction involving biradical intermediates . 

Gutbrod, R., R. N. Schindler, E. Kraka, and D. Cremer, "Formation of OH Radicals in the Gas Phase Ozonolysis of Alkenes The Unexpected Role of Carbonyl Oxides, Chem. Phys. Lett., 252, 221-229 (1996). 

Ah initio calculations suggest that in ozonolysis, as the two fragments formed by dissociation of the primary ozonide start to move apart, a strong electrostatic attraction builds up between newly formed dipoles.157 The torque created causes a flip of one relative to the other, with formation of a dipolar complex which converts to the secondary ozonide. Thus, the authors suggest that the carbonyl oxide and carbonyl are never actually separated to a van der Waals distance. This argument goes some way to explaining some observed experimental stereoselectivities. 

The study was continued with diastereoisomeric vinyl ethers. The ( )-isomer afforded on ozonolysis at —70 °C in ether a ry -carbonyl oxide, and with or without trifluoroacetophenone only the intramolecular ozonide 167, while the (Z)-isomer led to an anti-carbonyl oxide and to co-ozonolysis in the presence of trifluoroacetophenone, because the tf //-carbonyl oxide cannot adopt a suitable conformation for the intramolecular ozonide formation (Scheme 56) < 1996JOC5953, 1998JOC5617>. 

The carbonyl oxide, a valence-unsaturated species, is not the final product of an ozonolysis. Rather, it will react further in one of two ways. Carrying out the ozonol-ysis in methanol leads to the capture of the carbonyl oxide by methanol under formation of a hydroperoxide, which is structurally identical to the ether peroxide of isopropyl methyl ether. However, if the same carbonyl oxide is formed in the absence of methanol (e.g., if the ozonolysis is carried out in dichloromethane) the carbonyl oxide undergoes a cycloaddition. If the carbonyl oxide is formed along with a... 

Ozonolysis of 4,4-diphenyl-l-methyl-cyclohex-2-ene-l-ol in DCM at — 78 °C in the presence of AczO led to the oxygen-bridged 1,2-dioxocin 213 in 30% yield. Its formation is explained in terms of intramolecular trapping of the carbonyl oxide intermediate, followed by cyclic acetal formation. Since 294 is unstable and spontaneously decomposed to ketoaldehyde 215 (see Section 14.04.5.4), it was necessary to acetylate it in order to have the stable 1,2-dioxocin 213 (Scheme 60) <1995JA9927>. 

Using modern analytical methods, a number of transient intermediates and byproducts could be verified [19, 20]. The first step in the mechanism of ozonolysis is the 1,3-dipolar cycloaddition of the dipole ozone to the double bond of OA. A 1,2,3-trioxolane is formed, the unstable primary ozonide or molozonide. The primary ozonide collapses in a 1,3 dipolar cycloreversion to a carbonyl compound and a carbonyl oxide, the so-called Criegee zwitterion. Since OA is substituted with two diverse groups at the double bond, two different opportunities exist for the formation of carbonyl compound and carbonyl oxide. Again, a 1,3-dipolar cycloaddition of these intermediates leads to three different pairs of 1,2,4-trioxolane derivatives (cisltram), the secondary ozonides, which are more stable than the primary ones. Their oxidative cleavage results in AA and PA. 

However, drawbacks are the required high energy use as well as the toxicity and explosivity of ozone and ozonides. In past times, and also today, proposals for the improvement of safety and process control of ozonolysis have been claimed [28-31]. The carbonyl oxide is converted with water into a hydroperoxide, which reacts to an aldehyde and hydrogen peroxide [32]. The latter is used for the subsequent oxidation of the aldehyde group to the carboxylic acid group. It is claimed that an avoided formation of secondary ozonides makes the process safer and diminishes the amount of side products. 

All carbonyl oxides proved to be highly photolabile, and on photolysis yield dioxiranes 3 or split off oxygen atoms to produce ketones. Oxygen atoms are also formed thermally from vibrationally excited 1. Thus, if the large exothermicity of the ozonolysis reaction is taken into account, 1 might be a source of O atoms and OH radicals in the troposphere. The role of dioxiranes has not yet been discussed in context with atmospheric chemistry, although the formation of these species in contrast to the isomeric carbonyl oxides - in ozone/alkene reactions has been unequivocally demonstrated [13]. 

The main results of our investigations are (i) The first step in the alkene/ozone reaction is the formation of a 7i-complex with absorptions in the near UV or visible range, (ii) At 50-70 K this complex reacts to the primary ozonide (POZ) and, depending on substituents, traces of the secondary ozonide (SOZ). (iii) No carbonyl oxide was observed under any conditions used in our ozonolysis experiments, (iv) Some of the partially oxidized products formed are not in accordance with the Criegee mechanism and thus alternative mechanisms have to be considered. 

Mechanism of Ozonolysis (Criegee mechanism) The initial step of the reaction involves a 1,3-dipolar cycloaddition of ozone to the alkene leading to the formation of the primary ozonide (molozonide or 1,2,3-trioxolane), which decomposes to give a carbonyl oxide and a carbonyl compound. The carbonyl oxides are similar to ozone in being 1,3-dipolar compounds and undergo 1,3-dipolar cycloaddition to the carbonyl compound with the reverse regio-chemistry, leading to a relatively stable secondary ozonide (1,2,4-trioxolane) (Scheme 5.47). 

Criegee et al. have examinedthe intramolecular choices of the intermediate carbonyl oxides in the ozonolysis of substituted cyclopentenes (78). Formation of... 

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