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

The rate ratios k_OTs/k_Br 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» S_N1 mechanism. Lower k_OTs/k_Br ratios therefore reflect increasing nucleophilic participation in the transition state of substitution and elimination reactions. The high k_OTs/k_Br 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.