Dicarbonyl compounds Ozone

Carbon—nitrogen double bonds in imines, hydrazones, oximes, nitrones, azines, and substituted diazomethanes can be cleaved, yielding mainly ketones, aldehydes and/or carboxyHc acids. Ozonation of acetylene gives primarily glyoxal. With substituted compounds, carboxyHc acids and dicarbonyl compounds are obtained for instance, stearoHc acid yields mainly azelaic acid, and a smaH amount of 9,10-diketostearic acid. 

Alkenes with at least one vinjdic hydrogen undergo oxidative cleavage when treated with ozone, yielding aldehydes (Section 7.9). If the ozonolysis reaction is carried out on a cyclic alkene, a dicarbonyl compound results. 

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. 

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 ... 

Regarding ozonation processes, the treatment with ozone leads to halogen-free oxygenated compounds (except when bromide is present), mostly aldehydes, carboxylic acids, ketoacids, ketones, etc. [189]. The evolution of analytical techniques and their combined use have allowed some researchers to identify new ozone by-products. This is the case of the work of Richardson et al. [189,190] who combined mass spectrometry and infrared spectroscopy together with derivatization methods. These authors found numerous aldehydes, ketones, dicarbonyl compounds, carboxylic acids, aldo and keto acids, and nitriles from the ozonation of Mississippi River water with 2.7-3 mg L 1 of TOC and pH about 7.5. They also identified by-products from ozonated-chlorinated (with chlorine and chloramine) water. In these cases, they found haloalkanes, haloalkenes, halo aldehydes, haloketones, haloacids, brominated compounds due to the presence of bromide ion, etc. They observed a lower formation of halocompounds formed after ozone-chlorine or chloramine oxidations than after single chlorination or chlorami-nation, showing the beneficial effect of preozonation. 

Spectra from the chemiluminescent gas phase reactions at 0,5 torr, of ozone with ethylene, tetramethylethylene, trans-2-hutene, and methyl mercaptan at room temperature are presented, and a summary of the general features of the emissions obtained from reaction in the gas phase of ozone with fourteen different olefins is given. The emitting species in the ozone-olefin reactions have been tentatively identified as electronically excited aldehydes, ketones, and a-dicarbonyl compounds. The reaction of ozone with hydrogen sulfide, methyl mercaptan, and dimethylsulfide produces sulfur dioxide in its singlet excited state. 

Although a-dicarbonyl compounds are not known to be products of the ozonolysis of olefins, biacetyl has been isolated in photochemically initiated reactions 14, 15) which result in the net oxidation of olefins in the gas phase. For example, when a mixture of ci5-2-butene, nitric oxide, and air is irradiated, small amounts of biacetyl are isolated. One of the pathways suggested to explain the production of biacetyl involves the reaction of ozone with ci5-2-butene (14) ... 

Bailey et al. (1965) found that in a-diazocarbonyl compounds with a terminal diazo group (-CH=N2) the diazo function is replaced by an O-atom, i.e., a glyoxal derivative is formed primarily, but the final products result from CH bond cleavage. In other cases, however, the a,yff-dicarbonyl compound is stable. An example, described by Ursini et al. (1992), is the oxo-de-diazoniation (9-35) of various 6-diazopenicillanates (9.70, R = 4-NO2C6H4CH2 and other alkyl groups). The a-diketone is obtained at -15 to -10°C in dichloromethane with ozone in 86-97% yield. 

Ozonolysis of cyclic alkenes gives dicarbonyl compounds. For example, treatment of cyclohexene in methanol with ozone, followed by addition of hydrogen peroxide gave adipic acid (5.101). Particularly useful are variants of this process that lead to differentiated functional groups, thereby making subsequent selective reactions feasible. Ozonolysis at low temperature followed by addition of... 

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). ... 

Figure IX-L-8. Cross sections for some aromatic dicarbonyl compounds as measured in hexane solution (Wang et al., 2006). The cross sections for benzaldehyde in hexane solution were measured by Ling (1961-1971). Phthalide, a product of the photolysis of phthaldialdehyde, is not photochemically active at the wavelengths of sunhght available in the troposphere. Actinic flux is given for an overhead Sun in the lower troposphere with an overhead ozone column of 350 DU.

These compounds, exemplified by acrolein, crotonaldehyde, and methyl vinyl ketone, are known to react with ozone and with OH radicals. Photolysis and N03 radical reaction are of minor importance. Under atmospheric conditions the 03 reactions are also of minor significance (Atkinson and Carter, 1984), leaving the OH radical reaction as the major loss process. For the aldehydes, OH radical reaction can proceed via two reaction pathways OH radical addition to the double bond and H-atom abstraction from the -CHO group (Atkinson, 1989). For crotonaldehyde, for example, the OH reaction mechanism is given in Fig. 3. As can be noted from Fig. 3, these a,/3-unsaturated aldehydes are expected to ultimately give rise to a-dicarbonyls such as glyoxal and methylglyoxal. For the a,/3-unsaturated ketones such as methyl vinyl ketone, the major... 

In the original formulation [81a], DBN and a slight molar excess of AgN03 in MeCN solution were used. Depletion of the starting penicillanic ester was followed by tic and, when complete, obtained silver azetidinyl mercaptides 126 were in situ treated with acyl chlorides to afford thioesters 127 these in turn were oxidized at the butenoate double bond to give lH-4-thioesters 128, key intermediates of Woodward s original phosphorane-thioester cyclization route to penems [1, 48], or ozonized to the iV-oxalo compounds 129, immediate penem precursors by a newly discovered reductive dicarbonyl coupling [82]. 

Cyclic a,/3-unsaturated carbonyl compounds lacking a y-hydrogen can be ozonized to 1,5-dicarbonyl intermediates, but cycUzation to enol lactones is not possible. Reversal... 

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