Activation of the S(N)2 Reaction by Adjacent pi Systems: The Critical Role of Electrostatic Interactions and of Dissociative Character

作者：Robiette Raphael^*; Tran Trieu Van; Aggarwal Varinder K^*; Harvey Jeremy N^*

来源：Journal of the American Chemical Society, 2016, 138(3): 734-737.

DOI：10.1021/jacs.5b11402

The activation of the S-N(2) reaction by systems is well documented in textbooks. It has been shown previously that this is not primarily due to classical (hyper)conjugative effects. Instead, pi-conjugated substituents enhance favorable substrate-nucleophile electrostatic interactions, with electron-withdrawing groups (EWG) on the sp(2) system leading to even stronger activation. Herein we report computational and experimental results which show that this activation by sp2 EWG-substitution only occurs in a fairly limited number of cases, when the nucleophile involves strong electrostatic interactions (usually strongly basic negatively charged nucleophiles). In other cases, where bond breaking is more advanced than bond making at the transition state, electrophile-nucleophile electrostatic interactions are less important. In such cases, (hyper)conjugative electronic effects determine the reactivity, and EWG-substitution leads to decreased reactivity. The basicity of the nucleophile as well as solvent effects can help to determine which of these two regimes occurs for a given electrophile.

• 出版日期2016-1-27