Bridgehead Carbocations: Solvolysis of a Series of 5-Substituted Bicycle [3.1.1]heptyl Bromides. Nucleophilic Solvent Assistance to Ionization of 1-Bromobicyclo[3.1.1]heptane
Bridgehead Carbocations: Solvolysis of a Series of 5-Substituted Bicycle [3.1.1]heptyl Bromides. Nucleophilic Solvent Assistance to Ionization of 1-Bromobicyclo[3.1.1]heptane
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桥头碳阳离子：一系列 5-取代双环[3.1.1]庚基溴的溶剂分解。
DOI：
10.1071/ch9950967
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发表时间：
1995
影响因子：
1.1
通讯作者：
G. Elsey
中科院分区：
文献类型：
--
作者：
E. Della;G. Elsey
合成了一系列用于溶剂分解研究的5-取代双环[3.1.1]庚基溴。发现取代基对体系的溶剂分解速率有很大的影响，其作用主要与其诱导/场常数σI的大小一致。取代基效应的最引人注目的例子是COOMe基团，它导致甲醇分解速率延迟6.5°105。虽然log k和σI曲线中的线性关系通常符合预期，但对于双环[3.1.1]庚基桥头阳离子介导的机制，两种溴化物，1-溴双环[3.1.1]庚烷及其5-甲氧基衍生物，显示出异常行为，并且反应速度比基于Hammett曲线预测的更快。证明了溶剂的亲核辅助作用是母体反应速率提高的主要原因。在5-甲氧基双环[3.1.1]庚基溴的溶剂分解中的嵌合协助归因于甲氧基取代基的强大p-供体性质，其在独特的协同开环和电离步骤中稳定过渡态。
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.