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Molecular Meccano, 48Probing Co‐Conformational Changes in Chiral [2]Rotaxanes by 1 H‐NMR Spectroscopy
Author(s) -
Ashton Peter R.,
Bravo José A.,
Raymo Françisco M.,
Stoddart J. Fraser,
White Andrew J. P.,
Williams David J.
Publication year - 1999
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/(sici)1099-0690(199904)1999:4<899::aid-ejoc899>3.0.co;2-c
Subject(s) - chemistry , chirality (physics) , planar chirality , rotaxane , cyclophane , crystallography , enantiomer , stereochemistry , nuclear magnetic resonance spectroscopy , yield (engineering) , proton nmr , axial chirality , circular dichroism , supramolecular chemistry , crystal structure , enantioselective synthesis , organic chemistry , symmetry breaking , catalysis , chiral symmetry breaking , physics , quantum mechanics , materials science , nambu–jona lasinio model , metallurgy
The template‐directed syntheses of three [2]rotaxanes possessing either chiral centers or elements of planar chirality, in one of their two mechanically interlocked components, have been realized and their solid‐state structures have been analyzed by X‐ray crystallography. In one instance, an enantiomerically pure dumbbell‐shaped component incorporating a 1,5‐dioxynaphthalene recognition site and two (1 R ,2 S ,5 R )‐menthyl stoppers was employed to template the formation of the achiral tetracationic cyclophane, cyclobis(paraquat‐ p ‐phenylene). The resulting enantiomerically pure [2]rotaxane was isolated in a yield of 55%. In the other two instances, an achiral 1,5‐dioxynaphthalene‐based dumbbell‐shaped component was employed to template the formation of bipyridinium‐based cyclophanes possessing either one or two elements of planar chirality. The resulting [2]rotaxane, possessing one element of planar chirality, was isolated as a racemate in a yield of 24%. The related [2]rotaxane, possessing two elements of planar chirality, was isolated as a mixture of a meso form and an enantiomeric pair in an overall yield of 28%. The 1 H‐NMR‐spectroscopic analysis of this mixture revealed a diastereoisomeric ratio of 4:1. A degenerate co‐conformational change was identified by variable‐temperature 1 H‐NMR spectroscopy in all [2]rotaxanes. The symmetry loss arising from the introduction of one or two elements of planar chirality enabled the elucidation of the mechanism of this dynamic process in two instances.