Decomposition of cyclic oxides

The six-membered trioxides , all aldehyde trimers, undergo Hoffmann-Woodward allowed concerted, unimolecular reactions (Table 22). Single bond rupture, followed by a rapid unzipping to products, is excluded by the reaction exothermicity. Ring opening of trioxane, viz. 

5-dihydrofuran elimination of hydrogen is also Woodward-Hoffmann allowed. The reaction proceeds at relatively low temperatures ( 650 °K) with a low activation energy which is characteristic of such concerted processes. Conservation of orbital symmetry requires exclusive cw-2,5-hydrogen elimination accompanied by 7t-bond migration, viz. 

It is interesting to compare this reaction with that of 2,3-dihydrofuran . The latter must give furan by a 1,2-elimination of hydrogen, viz. 

Conservation of orbital symmetry requires that 1,2-elimination be trans. This, of course, is sterically prohibited. In agreement with the rules, decomposition of 2,3-dihydrofuran occurs only at high temperatures, and gives CO and propene via a complex free radical path. 

The four-membered ring oxide decompositions (oxetane and 3,3-dimethyl oxetane ) are well-behaved unimolecular reactions. Products are shown in Table 22. Reaction products and experimental Arrhenius parameters are consistent with the biradical mechanism below. Calculated parameters are those of O Neal and Benson . 

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