Sequence Isomerism in [3]Rotaxanes

摘要

We describe a strategy for assembling different macrocycles onto a nonsymmetrical rotaxanenthread in a precise sequence. If the macrocycles are small and rigid enough so that they cannot pass eachnother then the sequence is maintained mechanically, affording stereoisomerism in a manner reminiscentnof atropisomerism. The method is exemplified through the synthesis of a pair of [3]rotaxane diastereomersnthat are constitutionally identical other than for the sequence of the different macrocycles on the thread.nThe synthesis features the iterative binding of different palladium(II) pyridine-2,6-dicarboxamide complexesnto a pyridine ligand on the thread followed by their macrocyclization by ring-closing olefin metathesis.nRemoval of the palladium(II) from the first rotaxane formed frees the pyridine site to coordinate to a second,ndifferent, palladium(II) pyridine-2,6-dicarboxamide unit which, following macrocyclization, provides a multiringnrotaxane of predetermined macrocycle sequence.