Die nucleofuge Aktivität von Substituenten am Brückenkopf von Bicyclo[2.2.2]octan und Chinuclidin Das kOTs/kBr-Verhältnis als mechanistisches Kriterium. Fragmentierungsreaktionen, 24. Mitteilung |
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Authors: | C. A. Grob K. Kostka F. Kuhnen |
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Abstract: | The rate ratios kOTs/kBr of the bridgehead tosylate and bromide of bicyclo [2.2.2]-octane and quinuclidine, respectively, have been determined in 80 vol.-% ethanol. The high ratio for the bicyclo [2.2.2] octane derivatives (3,3 · 103 at 25°) indicates a highly ionic transition state. Since nucleophilic assistance to ionisation by solvent is precluded in this system a ratio of 103 or greater is typical for the «limiting» SN1 mechanism. Lower kOTs/kBr ratios therefore reflect increasing nucleophilic participation in the transition state of substitution and elimination reactions. The high kOTs/kBr ratio for 4-substituted quinuclidines, namely 1,6 · 103 at 25°, indicates the absence of anchimeric nitrogen participation in the transition state of this synchronous fragmentation. The rates of the 4-haloquinuclidines follow the normal order of nucleofugal activity for halides in solvolysis reactions. |
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