The synthesis of a range of 5-substituted bicyclo [3.1.1] heptyl bromides for solvolytic studies is described. It is found that the substituent has a profound effect on the rate of solvolysis of the system and acts principally in accordance with the magnitude of its inductive/field constant σI. The most spectacular example of the effect of the substituent is provided by the COOMe group which leads to a retardation in the rate of methanolysis by a factor of 6.5°105. While a linear relationship in the plot of log k and σI is generally obeyed, as expected for a mechanism mediated by the bicyclo [3.1.1] heptyl bridgehead cation, two of the bromides, 1-bromobicyclo[3.1.1] heptane and its 5-methoxy derivative, show deviant behaviour and react more rapidly than predicted on the basis of the Hammett plot. Evidence is presented to show that the enhanced rate of the parent is the result of nucleophilic assistance by the solvent. Anchimeric assistance in the solvolysis of 5-methoxybicyclo[3.1.1] heptyl bromide is attributed to the powerful p-donor property of the methoxy substituent which stabilizes the transition state in a unique concerted ring-opening and ionization step.
本文介绍了一系列 5 取代双环 [3.1.1] 庚基溴化物的合成方法,用于溶解研究。研究发现,取代基对体系的溶解速率有着深远的影响,其作用主要取决于其感应/场常数 σI 的大小。取代基影响最明显的例子是 COOMe 基团,它导致甲烷分解速率减慢 6.5°105 倍。虽然 log k 与 σI 的关系图一般符合由双环[3.1.1]庚桥头阳离子介导的机制的预期,但其中两种溴化物、1-溴双环[3.1.1]庚烷及其 5-甲氧基衍生物表现出偏离行为,其反应速度比根据 Hammett 图预测的要快。有证据表明,母体反应速率的提高是溶剂亲核作用的结果。 在 5-甲氧基双环[3.1.1]庚基溴的溶解过程中出现的嵌合体辅助作用归因于甲氧基取代基强大的对供体特性,它在独特的协同开环和电离步骤中稳定了过渡态。