The influence of macrocyclic polyether constitution upon ammonium ion/crown ether recognition processes

Secondary dialkylammonium (R2NH2+) ions are bound readily by dibenzo[24]cro wn-8 (DB24C8) to form threaded complexes, namely [2]pseudorotaxanes. The ef fect of replacing one or both of the catechol rings in DB24C8 with resorcin ol rings upon the crown ether's ability to bind R2NH2+ ions has now been in vestigated. When only one aromatic ring is changed from catechol to resorci nol, a crown ether with a [25]crown-8 constitution is created-namely benzom etaphenylene[25]crown-8 (BMP25C8). A [2]pseudorotaxane is formed in the sol id state when BMP25C8 is co-crystallized with dibenzylammonium hexafluoroph osphate. as evidenced by its X-ray crystal structure. Furthermore. this cro wn ether has been shown to bind R2NH2+ ions in solution, an observation whi ch has been exploited in the synthesis of the first BMP25C8-containing [2]r otaxane. The methodology employed to generate this [2]rotaxane-the reaction of an amine with an isocyanate: to form a urea-was tested initially on a s ystem incorporating DB24C8 and was shown to work efficiently. Both [2]rotax anes have been fully characterized by H-1 and C-13 NMR spectroscopies, FAB mass spectrometry and X-ray crystallography. Interestingly, the unsymmetric al nature of the dumbbell-shaped component in each of the two [2]rotaxanes renders each face of the encircling macrocyclic polyether diastereotopic, a feature that is apparent upon inspection of their H-1 NMR spectra. The res onances associated with the diastereotopic protons on each face of the macr orings are well enough resolved to enable the faces of the crown ethers to be readily identified with respect to their protons by H-1 NMR spectroscopy . Unambiguous assignments can he made as a result of the fact that the prot ons on each face of the macrocyclic polyether experience a unique set of th rough-space interactions, as evidenced by T-ROESY experiments. Additionally , the two-dimensional NMR analyses are in agreement with the X-ray crystall ographic studies performed on these [2]rotaxanes. indicating that the crown ethers are located intimately around the NH2- centers as expected. Replace ment of both catechol rings in the DB24C8 constitution with resorcinol ring s results in a crown ether with a [26]crown-8 constitution-namely bismetaph enylene[26]crown-8 (BMP26C8). All the evidence to date points to the fact t hat this further change in constitution results in a crown ether that lives not bind R2NH2+ ions in either the solution or solid states.