Sciences Chimiques de Rennes, Equipe MaCSE, Université de Rennes 1, Campus de Beaulieu, 5042 Rennes cedex, France.
Phys Chem Chem Phys. 2010 Jul 21;12(27):7506-12. doi: 10.1039/b920552g. Epub 2010 Jun 2.
The irreversible dimerization of the acetophenone radical anion, chosen as an example of a carbon-carbon coupling reaction between two charged species, was investigated in a series of 1,3-dialkylimidazolium and 1,2,3-trialkylimidazolium ionic liquids. Indeed, such ion dimerizations which display slow kinetics despite small activation energies, are controlled by a subtle competition between bond formation, Coulombic repulsion and solvation. The effects of viscosity, "polarity" and ionic solvation on the reactivity of the radical anions were examined. The dimerization rate constants were demonstrated to be only weakly affected by the high viscosity of the medium or its apparent polarity. When the acetophenone radical anion is "solvated" in imidazolium-based ionic liquids, a strong interaction between the negatively-charged intermediates and the imidazolium cation occurs. The ensuing charge stabilization allows a fast dimerization step in all the ionic liquids used. The kinetic effect is even enhanced in the 1,3-dialkylimidazolium salts as compared to the 1,2,3-trialkylimidazolium ones because the interaction between the radical anions and the 1,3-dialkylimidazolium cations are stronger, probably due to the formation of H-bond. The reactivity of the ion radical is demonstrated not only to be mainly dominated by electrostatic interactions, but also that the nature of the ionic liquid cations with respect to that of the ion radical is a major factor that affects the reaction kinetics.
选择苯乙酮自由基阴离子作为两个带电物种之间碳-碳偶联反应的示例，研究了一系列 1,3-二烷基咪唑鎓和 1,2,3-三烷基咪唑鎓离子液体中的不可逆二聚化。实际上，尽管这些离子二聚化的活化能很小，但动力学却很慢，这是由于键形成、库仑排斥和溶剂化之间的微妙竞争所控制的。考察了粘度、“极性”和离子溶剂化对自由基阴离子反应性的影响。二聚化速率常数仅受介质高粘度或表观极性的微弱影响。当苯乙酮自由基阴离子在基于咪唑鎓的离子液体中“溶剂化”时，带负电荷的中间体与咪唑鎓阳离子之间会发生强烈相互作用。由此产生的电荷稳定化允许在所有使用的离子液体中快速进行二聚化步骤。与 1,2,3-三烷基咪唑鎓相比，在 1,3-二烷基咪唑鎓盐中，动力学效应甚至更强，因为自由基阴离子与 1,3-二烷基咪唑鎓阳离子之间的相互作用更强，可能是由于氢键的形成。证明离子自由基的反应性不仅主要由静电相互作用决定，而且离子液体阳离子相对于离子自由基的性质也是影响反应动力学的主要因素。
