O-heterocyclics

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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