Ozonation of alkynes

Compared to ozonation of alkenes, much less is known about the ozonation of alkynes,710 which yields 1,2-dicarbonyl compounds, carboxylic acids, and anhydrides. 1,2,3-Trioxolene (91), analogous to 74 in alkene ozonation mechanism (Scheme 9.14), and zwitterionic intermediates (92) were formulated on the basis of IR studies and trapping experiments ... 

Olefins with hindered double bonds may be transformed stereospecifically to oxiranes by treatment with ozone. The epoxidation of propylene has been achieved with alkoxyalkyl-hydroperoxides obtained by the ozonization of olefins in the presence of alcohol.The yield depends on whether the alcohol is a primary, secondary, or tertiary one. The low-temperature (—70°) epoxidation of olefins with a yield of about 30% has been performed with electrophilic intermediates produced in the course of the ozonization of alkynes these intermediates are probably five-membered cyclic trioxides. This epoxidation is almost totally stereospecific. 

Ozonation of Alkynes. Reactions of alkynes with ozone afford either carboxylic acids or, if reductive procedures are used, a-dicarbony 1 compounds. For the production of carboxylic acids, MeOH has been shown to be superior to CH2CI2 as reaction solvent. As with alkenes, a number of reducing agents can be used to produce a-dicarbonyl compounds. An easy option which results in high yields of a-dicarbonyl compounds involves the addition of Tetracyanoethylene directly to an ozonation reaction mixture as an in situ reducing agent (eq 16). ... 

The two major characteristic oxidation processes of alkynes are their transformation to 1,2-dicarbonyl compounds and their cleavage reaction to carboxylic acids.710 The structure of the starting compounds has a decisive effect on the selectivity of oxidation. Since 1,2-dicarbonyl compounds proved to be intermediates in further oxidations, carefully controlled reaction conditions are often necessary to achieve selective synthesis. Certain oxidizing agents such as peroxyacids and ozone are nonselective oxidants. 

Further chemistry of alkenes and alkynes is described in this chapter, with emphasis on addition reactions that lead to reduction and oxidation of carbon-carbon multiple bonds. First we explain what is meant by the terms reduction and oxidation as applied to carbon compounds. Then we emphasize hydrogenation, which is reduction through addition of hydrogen, and oxidative addition reactions with reagents such as ozone, peroxides, permanganate, and osmium tetroxide. We conclude with a section on the special nature of 1-alkynes— their acidic behavior and how the conjugate bases of alkynes can be used in synthesis to form carbon-carbon bonds. 

A number of useful reactions for the preparation of aldehydes and ketones, such as ozonization of alkenes and hydration of alkynes, have been considered in previous chapters. These and other methods of preparation are summarized in Tables 16-7 and 16-8 at the end of the chapter. Only a few rather general methods that we have not discussed will be taken up here. 

The mechanism and kinetics of the atmospheric oxidation of alkynes, initiated by OH radicals, have been studied particularly to determine the role of alkyne oxidation in tropospheric ozone formation. A general mechanism for OH-initiated oxidation of alkynes has been developed with the aid of thermodynamic calculations. In general, the significance of atmospheric alkynes to the formation of tropospheric ozone was found to be smaller than for alkanes and alkenes, due to the absence of the hydroxy peroxy-forming product channel in the OH-initiated atmospheric oxidation of alkynes.227... 

Dry ozonization of alkraes and alkynes has also been explraed. Sometimes these results resemble those of the homogeneous ozonization in an i rotic solvent ... 

The reactions of ozone with alkynes are essentially slower than those with alkenes. This is shown by the values of rate constants, which are lower in the case of alkynes due to the higher activation energy. Alkanes react very slowly with O3 and thus, they are unimportant in atmospheric processes. 

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,... 

Alkynes react slower with ozone than do alkenes and selective reaction of alkenes can be achieved in the presence of alkynes (eq 17). Conversely, the example of eq 18 shows that alkynes are more reactive toward ozone than aromatic rings. ... 

Treatment of alkynes with ozone (O3) under the same conditions as those used... 

R is hydrogen, alkenyl, or alkyne. In remote tropospheric air where NO concentrations ate sometimes quite low, HO2 radicals can react with ozone (HO2 + O3 — HO + 2 O2) and result in net ozone destmction rather than formation. The ambient ozone concentration depends on cloud cover, time of day and year, and geographical location. 

Alkynes, like alkenes, can be cleaved by reaction with powerful oxidizing agents such as ozone or KMnC, although the reaction is of little value and we mention it only for completeness. A triple bond is generally less reactive than a double bond and yields of cleavage products are sometimes low. The products obtained from cleavage of an internal alkyne are carboxylic acids from a terminal alkyne, CO2 is formed as one product. 

Ozonium Ion (H03+). Ozone is a resonance hybrid of canonical structures 50a-50d.135 Ozone does in fact act as a 1,3-dipole—that is, either as an electrophile or a nucleophile. The electrophilic nature of ozone has been recognized for a long time in its reactions with alkenes, alkynes, arenes, amines, phosphines,... 

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