O-heterocycles

A brief introduction to the oxidation of oxygenated compounds is appropriate as they are important intermediates in the total oxidation of an alkane through to CO2 + H2O. In the temperature region below 1000 K, the major oxygenated species are aldehydes (ketones from highly branched alkanes) and O-heterocyclic compounds formed via RO2 QOOH iso-merizations. An impression of the relative activity of oxiranes and oxetanes 

For oxiranes, isomerization occurs exothermically due mainly to the strain energy in the ring (ca 120kJmol ). It is probable that oxirane isomerization proceeds via chemically activated CH3CHO. 

The major products from methyloxirane (propene oxide) are C2H5CHO, CH3COCH3 and CH2=CHCH20H [90], Similarly, isobutyraldehyde is the dominant product from isobutene oxide. Oxetanes are known to undergo homolysis through ring splitting, so that isobutene and HCHO are major products from 3,3-dimethyloxetane. 

In a combustion environment, removal by free radical attack will also occur, although the C—H bond strength in oxirane is surprisingly high [113], 

The resulting oxiranyl radicals isomerize exothermically (AH 120 kJ mol ) to a variety of products. 

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Nitroazoles The C-Nitro Derivatives of Five-Membered N- and N, O-Heterocycles by Joseph H. Boyer... 

The l,3-oxazin-2-ones and 1,3-oxazine-2-thiones previously synthesized were used to prepare various N- and O-heterocyclic systems fused with 1,3-oxazine rings.172 For example, furan-l,3-oxazin-2-one or furan-l,3-oxazine-2-thiones (190a,b) and pyran-l,3-oxazin-2-ones or pyran-l,3-oxazine-2-thiones (190c,d) were prepared in very good yields, ranging from 83% to 90%, by montmorillonite K-10 clay-catalyzed cyclodehydration of 189a,b and 189c,d, respectively (Scheme 35). 

Allylic diacetates were converted into N,N, 0,0, and N, O-heterocycles in a palladium catalyzed two step nucleophilic displacement sequence. The reaction of o-aminophenol and cyclopentenyl-diacetate, for example, gave the cyclopentene condensed phenoxazine derivative in acceptable yield (4.21.), while o-phenylenediamine and catechol gave the corresponding phenazine and dioxine derivative in good yield.24... 

Small radicals such as tert-butylperoxy and ethylperoxy can, however, react via 1,4 H-transfer only the strain energy involved in O-heterocycle formation is 28 kcal. per mole. In this case, k.4(x — 106 sec."1 whereas krta = 10r> 4 sec. 1 and when [02] = 200 mm. of Hg, ko[02] = 105,3 sec. 1, so that k.4ct < < (tkr,a + k [02]). The result is that in the oxidation of small alkyl radicals, the route via alkylperoxy radicals will be blocked because reverse Reaction —4 competes successfully with Reaction 5. Reaction 2 will thus be a more effective mode of reaction of alkyl radicals with oxygen and the conjugate alkene will be a major product. 

Reaction 5. Several kinds of decomposition of C H2 OOH to a product (or products) + OH are possible. For simplicity, these calculations have been performed only for the case in which a single product P is formed, and that is an O-heterocycle. This mode of decomposition is a major one, particularly at temperatures characteristic of cool flames (3, 19, 20, 24, 45). 

Formation of O-Heterocycles as Major Products of the Gaseous Oxidation of -Alkanes... 

The gaseous oxidation of n-alkanes can, in suitable circumstances, yield substantial amounts of O-heterocycles of the same carbon number as the initial hydrocarbon. A comparative study has been carried out of the formation of O-heterocyclic products during the combustion of n-butane, n-pentane, and n-hexane. The way in which the yields of such compounds vary with reaction conditions has been investigated. As a result of the optimization of the amounts of O-heterocycles it has been possible to obtain maximum yields of these compounds of up to 30% from n-pentane but only about 10% from n-butane and n-hexane. An attempt is made to account for the observed differences in the amounts and nature of the O-heterocyclic products formed from the three n-alkanes. 

The successful isolation of substantial amounts of O-heterocycles will depend to a considerable extent on the design of the reaction vessel. Thus, for example, high yields of such compounds are produced in the falling cloud reactor under conditions corresponding to cool-flame combustion (10, 11). Comparatively little information is available, however, as to the variation in yield with molecular structure of the initial hydrocarbon. 

In the present work, therefore, a comparative study of the production of O-heterocycles during the cool-flame combustion of three consecutive n-alkanes—viz., n-butane, n-pentane, and n-hexane—was carried out under a wide range of reaction conditions in a static system. The importance of carbon chain length, mixture composition, pressure, temperature, and time of reaction was assessed. In addition, the optimum conditions for the formation of O-heterocycles and the maximum yields of these products were determined. The results are discussed in the light of currently accepted oxidation mechanisms. 

The production of O-heterocycles from each of the three hydrocarbons was examined according to a standard procedure. Yields of O-heterocycles were expressed as percentages of the hydrocarbon introduced... 

Figure 1. Optimum mixture compositions for production of O-heterocycles...

Table I. Yields of O-Heterocycles Obtained during Combustion of Different Alkanes under Optimum Conditions...

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