Ozone, complexes with alkenes

Ozone reacts with alkenes (olefins) through addition to the double bond. The initially formed ozonides decompose further. Schbnbein appears to have performed the first such identified ozonolysis (on ethylene). Turpentine is a complex mixture of olefinic terpenes (see P.S. Bailey, Ozonation in Organic Chemistry, Academic Press, New York, 1978, pp. 1—4, for a brief historical perspective). 

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). 

In summary, the reaction of ozone with alkenes is important in the atmospheric degradation of alkenes. In all cases the reaction leads to rupture of the > C = C < double bond. The double bond is replaced by a carbonyl group on one side and a Criegee biradical on the other. The Criegee biradical is formed energetically excited and decomposes by a variety of different routes to give a complex mixture of oxygenated products (mainly carbonyls). 

Of the range of reations of hydrocarbons in the atmosphere particularly important are those with ozone, which may be generated in the NO2 photolysis process. It reacts readily with alkenes with producing different products, including lower hydrocarbons, aldehydes, organic acids, CO, CO2 and HjO. It is assumed that the initial reaction of alkenes with O3 in the gaseous phase is accomplished in two steps as follows the formation of an initial complex, which is further decomposed to an aldehyde or ketone and a peroxybiradical [8] ... 

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. 

Highly enantioselective epoxidation of unfunctionalized alkenes was developed by using chiral metalloporphyrin catalysts.1214-1218 Remarkable anion axial ligand effects were observed with [Fe(TPFPP)X] complexes (X = triflate, perchlorate, nitrate).1219 Hexafluoroacetone was found to be an efficient cocatalyst with H202,1220 and alkenes could be epoxidized by ozone at ambient temperature.1221... 

The rate constants of the reaction of 2,6-dimethyloct-7-en-2-ol separately with ozone and hydroxyl radical, in the gas phase, have been determined. The OH radical can either abstract hydrogen or add to the double bond. Ozone adds to the double bond. The formation of acetone, 2-methylpropanal, 2-methylbutanal, ethanedial, and 2-oxopropanal was discussed.191 The rate laws and activation parameters for the ozone oxidation of alcohols in aqueous solution have been determined and explained on the basis of formation of an ozone-alcohol complex.192 The reactivity of alkenes towards ozone, in aqueous solution, correlates well with Taft s equation.193... 

A historically important use of the ozonolysis reaction was in the area of structure determination. In the days before the advent of spectroscopic techniques (Chapters 13-15), the structure of an unknown organic compound was determined by submitting it to a host of reactions. Often, a complex molecule was broken into several fragments to simplify the structural problem. After the individual fragments were identified, the original molecule could be mentally reconstructed from them. Alkenes were often cleaved to aldehydes and ketones by reaction with ozone. 

A combination of rtiodium(III) chloride with silver acetate, and treatment of rhodium(II) acetate in acetic acid solution with ozone, are two methods for generation of the (is-oxotrimetal-acetato complex of rhodium [Rhs0(0Ac)6 2O)3]0Ac. This RhsO complex was found to effect catalytic allylic oxidation of alkenes efficiently to give the corresponding a -unsaturated carbonyl compounds in the presence of a reoxidant such as r-butyl hydroperoxide, although in disappointing yield (equation 44). 

The mechanism of the reaction of ozone with double bonds (equation 68) is very complex and still subject to arguments. An alkene and ozone may first form a it complex (a), which forms a a complex (b), a molo-zonide (c), or both. The molozonide may change to a dipolar ion (d), which breaks down with the fission of the carbon-carbon bond to a carbonyl compound and another dipolar ion (e). The two species recombine to give the ultimate product, ozonide (f) (1,3,4-trioxolane, also known as 1,3,4-trioxacyclopentane) [76], The temporary presence of the carbonyl com-... 

Applications of tetracyanoethylene have been reviewed recent information on molecular complexes, ozonization of alkenes and alkynes (the Criegee reaction), and reactions with ketones is included. Dicyanoketene has been prepared in situ from 2,5-diazido-3,6-dicyano- and 2,6-diazido-3,5-dicyano-l,... 

New Applications of Tetracyanoethylene (TCNE) in Organic Chemistry, A. J. Fatiadi (1986). This review with 501 references deals with reactions of tetracyanoethylene used in organic synthesis. Information on molecular complexes, ozonization of alkenes and acetylenes, dehydrogenation and tricyanovinylation, reactions of TCNE oxide, reactions with ketones and diketones, synthesis of heterocycles and cationic polymerizations are included in this survey. Some industrial and analytical applications are also discussed. 

Although ozone-alkene reactions were not specified in the original objectives of LACTOZ they constitute a loss process for ozone and are important for degradation of unsaturated hydrocarbons, in particular alkenes with multiple double bonds and complex structures, such as are found in the biogenic hydrocarbons. Emphasis in LACTOZ has been on the rates and mechanisms under atmospheric conditions. These studies led to downward revision of the rate constants of the O3 alkene reactions, due to complications arising in many earlier investigations from secondary reactions of radicals produced in the primary step. Some important hitherto unknown aspects of ozone reactions, such as the formation of peroxides and their dependence on the water vapour concentrations, have been discovered in LACTOZ. 

---