Ozone reaction with alkynes

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. 

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. 

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,... 

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,... 

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). ... 

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. ... 

Oxygenated alkynes also can be ozonated. The reaction of ozone with alkynyl ethers followed by reductive workup provides a convenient method for the production of a-keto esters in moderate yields (eq 19). ... 

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. 

Scheme 6.64. A representation of the ozonolysis of a nonsymmetrically substituted internal alkyne to yield an a-diketone and, with sufficient ozone (O3), carboxylic acids. The quantity of ozone (O3) added to a reaction mixture in, for example,dichloromethane (H2CCI2) solvent is difficult to measure as the air or oxygen stream is passed through the silent electric discharge and then into the solution. As a consequence, mixtures of carboxylic acids and a-diketones are the usual products.

SCHEME 8.7 Schematic illustration of the process of ozone prelrealment, graft copolymerization PVDF with PMA, preparation of PVDF-g-PPMA membrane with clickable surface by phase inversion, preparation of functional PVDF-g-P[PMA-cZ/ck-MPS] membranes via surface thiol-yne click reaction of thiols on the PVDF-g-PPMA membrane, and preparation of PVDF-g-P[PMA-c//ck-P-CD] membranes via surface alkyne-azide click reaction on the PVDF-g-PPMA membranes. PVDF, poly(vinylidene fluoride) PMA, propargyl methacrylate MPS, 3-mercapto-l-propanesulfonic acid sodium salt azido-fl-CD, mono(6-azido-6-desoxy)-P-cyclodexIrin. Reprinted with permission from Reference 168. Copyright 2011 American Chemical Society. 

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