Journal
CHEMISTRY-A EUROPEAN JOURNAL
Volume 13, Issue 25, Pages 7091-7098Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.200601657
Keywords
cyclodextrins; kinetics; rotaxanes; supramolecular chemistry; unidirectional
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New [2]- and [3]pseudorotaxanes containing a-cyclodextrin (alpha-CDs) molecules as rotors and alkyl pyridinium derivatives as axles were prepared by a slipping process. The inclusion behavior of these rotaxanes was investigated by using one- and two-dimensional NMR spectroscopy. The methyl group at the 2-position of the pyridinium moiety at the end of each axle molecule was found to control the rates of threading of the alpha-CD onto the axle molecules. alpha-CD can approach axle molecules from a particular direction to form inclusion complexes. Axle molecules that contain a 2-methylpyridinium moiety at one end and a bulky stopper at the other end can regulate the direction of approach to give a [2]pseudorotaxane such as 2b-alpha-CD. A [3]pseudorotaxane in which two alpha-CD molecules are arranged facing in the same direction at two stations of the tetracationic axle molecule was also obtained. These face-selective behaviors are dominated by kinetic processes rather than thermodynamic processes.
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