The stability of imine-containing dynamic [2]rotaxanes to hydrolysis

Abstract

Large amounts (>100 mol equivalents) of water are required to effect by hydrolysis the partial disassembly of the rings from the dumbbell components of two dynamic [2]rotaxanes. The two dynamic [2]rotaxanes are comprised of [24]crown-8 rings—each of which incorporate two imine bonds—encircling a dumbbell component composed of a dibenzylammonium ion in which each of the two benzyl substituents carries two methoxyl groups attached to their 3- and 5-positions. A mechanism for the partial disassembly of the two dynamic [2]rotaxanes, involving the cleavage of the kinetically labile imine bonds by water molecules, is proposed. The most important experimental observation to be noted is the fact that the hydrolysis of the macrocyclic diimines, associated with the templating –CH₂NH₂⁺CH₂–centres in the middle of their dumbbells, turns out to be an uphill task to perform in the face of the molecular recognition provided by strong [N⁺–H⋯O] hydrogen bonds and weaker, yet not insignificant, [C–H⋯O] interactions. The dynamic nature of the imine bond formation and hydrolysis is such that the acyclic components produced during hydrolysis of the imine bonds can be enticed to cyclise once again around the –CH₂NH₂⁺CH₂–template, affording the [2]rotaxanes. The reluctance of imine bonds, present in substantial numbers in larger molecular and extended structures, is significant when it comes to exercising dynamic chemistry in compounds where multiple imine bonds are present.