Reversing a rotaxane recognition motif: Threading oligoethylene glycol derivatives through a dicationic cyclophane

An already well-established recognition motif-namely one in which the NH₂ ⁺ centers in the rod sections of the dumbbell components of rotaxanes are encircled by macrocyclic polyether components-has been turned simultaneously outside-in and inside-out, a fact that has been proved beyond any doubt by the stoppering of both ends of a [2]pseudorotaxane to give a stable [2]rotaxane. The [2]pseudorotaxane is formed in nitromethane when a benzylic dibromide, obtained after reacting an excess of 1,4-bis(bromomethyl)benzene with hexaethylene glycol, is added to an equimolar amount of a dicationic cyclophane in which two -CH₂OCH₂- chains link a pair of dibenzylammonium ions through the para positions on their phenyl rings. When the [2]pseudorotaxane is reacted in nitromethane with triphenylphosphine, a [2]rotaxane and the corresponding free dumbbell compound are isolated in 58 and 31% yields, respectively. The structure of the [2]rotaxane is established by using mass spectrometry (FABMS and ESMS) and NMR (¹H and ¹³C) spectroscopy in nitromethane-d₃. The [2]rotaxane exhibits quite dramatic changes in the ¹H chemical shifts of the signals for its CH₂N⁺ and CH₂O protons compared with those in the free dumbbell compound. The ¹H NMR spectrum of the [2]pseudorotaxane shows many similar features. Titration experiments with three of the six different CH₂O probes give an average K_a value of 2900 ± 750 M^-1 in nitromethane-d₃. The new recognition motif for the template-directed synthesis of rotaxanes can now be exploited at both the molecular and macromolecular levels of structure with numerous potential applications in sight.