Konformative Beweglichkeit Flexibler Ringsysteme—X. Untersuchungen mit Hilfe der Protonenresonanz-Spektroskopie. Substituentenabhängigkeit der Ringinversion bei 1.3-Dioxanen, 1.3-Dithianen und 1.3-Oxathianen |
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Authors: | Horst Friebolin Helmut G Schmid Siegfried Kabuß Wilhelm Faißt |
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Abstract: | Activation energies Eα and free enthalpies of activation ΔG? were determined by NMR for the ring inversions of nineteen 1.3-dioxanes, 1.3-dithianes and 1.3-oxathianes and their methyl-substituted derivatives. The rate constant k of the chair to chair-inversion of these rings depend on the number and positions of the substituents. In substituted 1.3-dioxanes k is lower than in 1.3-dioxane itself if two geminal CH3-groups are situated in position 5. However, k becomes higher when the geminal CH3-groups are in positions 2 or 4. The rate constants are particularly high for 4.4.6.6-tetramethyl-1.3-dioxane. A similar dependence of k on substituents has been observed in the oxathianes, while with 1.3-dithianes only a small influence is noticed. The effect of the methyl substituents in positions 2 or 4 in 1.3-dioxanes can be explained by assuming that the chair conformation is deformed by 1.3-interactions. In 1.3-dithianes such 1.3-interactions are expected to be smaller because of the larger C? S bond length. For 1.3-oxathianes the dependence of k on the substituents is more difficult to understand since these molecules are not symmetrical. |
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