Molecular shuttles by the protecting group approach

Two new [2]rotaxane-based molecular shuttles, in which a mechanically bound dibenzo[24]crown-8 (DB24C8) macroring shunts back and forth between two di alkylammonium recognition sites situated on a chemical dumbbell, have been constructed by a novel synthetic strategy that relies upon the use of the t ert-butoxycarbonyl (Boc) protecting group. During the syntheses of both mol ecular shuttles, this protecting group masks a dialkylammonium recognition center which is liberated only after the [2]rotaxane constitution is establ ished. In both cases, the molecular shuttles' other dialkylammonium center is essential for the rotaxane-forming reactions and it ensures that DB24C8 is interpenetrated by threadlike precursors, as a result of noncovalent bon ding interactions, to produce [2]pseudorotaxanes which are stoppered subseq uently through 1,3-dipolar cycloadditions between azides and bulky acetylen edicarboxylates. The new molecular shuttles have been examined by means of dynamic H-1 NMR spectroscopy, which reveals that the movements of the DB24C 8 macroring are very highly dependent both on solvent properties and on the nature of the spacer unit linking the two dialkylammonium centers. Thus, D B24C8 shunts facilely between the dialkylammonium centers when the shuttles are dissolved in solvents that readily donate their nonbonding electrons i nto noncovalent bonds, e.g., DMF, and when spacer units that do not offer m uch steric resistance to shuttling, e.g., hexamethylene, are used. On the o ther hand, shuttling is difficult in solvents that are less inclined to don ate their electrons into noncovalent bonds, e.g., (CDCl2)(2), and when rela tively bulky benzenoid spacer units, e.g., p-xylylene, link the two dialkyl ammonium centers. It has been proposed that the DB24C8 might act as a "ferr y" which carries a proton between dialkylammonium and dialkylamine moieties in a singly protonated [2]rotaxane by means of ion-dipole interactions.