Pliego Josefredo R, Piló-Veloso Dorila
Departamento de Química, Universidade Federal de Minas Gerais 31270-901, Belo Horizonte, MG, Brazil.
J Phys Chem B. 2007 Feb 22;111(7):1752-8. doi: 10.1021/jp066580p. Epub 2007 Feb 1.
Reaction of the fluoride ion with secondary alkyl halides leads to 90% of elimination reaction and only 10% of nucleophilic substitution in dipolar aprotic solvents. Adding water to the organic phase, the SN2 yield increases in the cost of decreased reactivity. Using ab initio calculations, we have shown that it is possible to increase the reaction rate and the selectivity toward the SN2 process through supramolecular organocatalysis. The catalytic concept is based on selective solvation of the transition state through two hydrogen bonds provided by the 1,4-benzenedimethanol. The two hydrogen bonds between the catalyst and the SN2 transition state favor this pathway while just one strong hydrogen bond between the catalyst and the fluoride ion leads to a lower stabilization of the nucleophile, resulting in a higher reaction rate. Our calculations predict that the substitution product increases to 40% yield because of the selective catalysis provided by the 1,4-benzenedimethanol.
在偶极非质子溶剂中，氟离子与仲卤代烷的反应会导致90%的消除反应，而亲核取代反应仅占10%。向有机相中加水，SN2产率会增加，但反应活性会降低。通过从头算计算，我们表明通过超分子有机催化可以提高反应速率和对SN2过程的选择性。催化概念基于1,4-苯二甲醇提供的两个氢键对过渡态的选择性溶剂化作用。催化剂与SN2过渡态之间的两个氢键有利于该反应途径，而催化剂与氟离子之间只有一个强氢键会导致亲核试剂的稳定性降低，从而提高反应速率。我们的计算预测，由于1,4-苯二甲醇提供的选择性催化作用，取代产物的产率会提高到40%。
