Dramatic Acceleration of the Menschutkin Reaction and Distortion of Halide Leaving-Group Order

摘要

Compared to structurally related linear trialkylamines,a simple macrocyclic amine with an anion-binding cavity exhibits very large rate enhancements(>10~5)for stoichiometric N-alkylation with primary alkyl,allyl,and benzyl halides in the weakly polar solvent CDCl3.There is also a major distortion of the halide leaving-group order.For example,with benzyl halides the relative leaving-group order with a control amine is Cl(1)4-t-BuBnBr(2200 M)>4-t-BuBnI(35 M);thus,the free energy of activation is selectively decreased for organohalides having smaller and more charge dense leaving groups.Likely reasons for this selective enhancement effect are:(a)increased transition-state stabilization due to hydrogen bonding in the macrocyclic pocket and(b)reduced entropic penalty in the transition state due to an increased fraction of prereaction complexes that are oriented in a near attack conformation.The study suggests that it should be possible to develop highly reactive macrocyclic amines that selectively sense or scavenge carcinogenic haloalkanes from the atmosphere.