Angewandte
Chemie
DOI: 10.1002/anie.201304636
Template Synthesis
Sodium Ions Template the Formation of Rotaxanes from BPX26C6 and
Nonconjugated Amide and Urea Functionalities**
You-Han Lin, Chien-Chen Lai, Yi-Hung Liu, Shie-Ming Peng, and Sheng-Hsien Chiu*
Pseudorotaxanes and rotaxanes are becoming increasingly
important materials for gelation,[1] drug delivery,[2] and
molecular electronics;[3] therefore, efforts continue toward
developing new threading systems and new methods to
synthesize these intertwined and interlocked molecules.[4]
Although many elegant interlocked molecular compounds
and threaded supramolecular complexes have been prepared
in the past two decades,[5] the number of recognition motifs
that can be exploited for the preparation of these systems
remains limited. This shortcoming arises mainly from our
limited ability to incorporate suitable recognition units in an
appropriate arrangement in the molecular structures of the
host and guest components so that weak noncovalent
interactions can collaborate to stabilize the resulting pseu-
dorotaxane complexes. In addition, the lack of structural
flexibility of the recognition units that can form pseudorotax-
ane complexes hinders the application of their unique
functions or structures into already used materials and/or
biologically important (macro)molecules, many of which do
not contain the necessary, suitably arranged recognition units
in their native molecular structures. One possible solution to
resolve this problem would be to develop a new molecular
recognition system in which a simple and abundant function-
ality—one that is found commonly in many materials—is
recognized by a host macrocycle with no need for structural
preprogramming. Urea and amide units are ubiquitous in
artificial and biological polymers (e.g. polyureas, nylons,
peptides). We thus suspected that if it were possible to
recognize macrocycles at single units of these functionalities,
then the barrier hindering the use of such host/guest
structures in practically useful materials would be decreased
significantly. This would potentially allow many new and
interesting properties to be introduced into molecules and
materials that we encounter widely in our daily lives. Noting
that diamide-based macrocycles have been applied previously
in the recognition of threadlike species containing amides[6]
and conjugated ureas,[7] herein we demonstrate that the
macrocycle bis-p-xylyl[26]crown-6 (BPX26C6) is capable of
forming pseudorotaxane-like structures with single, noncon-
jugated urea or amide moieties when assisted by templating
Na+ ions, thereby allowing the successful syntheses of the
corresponding rotaxanes.[8] By using this metal-templating
approach, we prepared rotaxanes featuring glycine residues
and the repeating units of nylon-6,6 as key components in
their threadlike units. This opens the possibility of employing
this recognition system for the formation of (pseudo)rotaxane
structures from biorelated (macro)molecules (e.g. peptides)
and practically useful materials (e.g. nylon).
We exploited primarily ion–dipole interactions between
=
the C O group(s) of the guest species and spherical alkali-
metal ions to achieve recognition of a single urea or amide
functionality by a macrocycle without requiring the guest
component to feature specific structural characteristics. Our
previous study had shown that an interlocked BPX26C6
moiety in a rotaxane could collaborate with a 2,2’-bipyridine
unit in the threadlike component to allow the mutual
complexation of various metal ions.[9] We thus suspected
that a suitable metal ion would template the threading of
a urea or amide guest through the cavity of BPX26C6 by
coordinating to the binding pocket formed from one dieth-
=
ylene glycol chain of the macrocycle and the C O group of
the guest (Scheme 1). In addition, N-H···O hydrogen bonds
Scheme 1. Structural representation of the concept of threading a non-
conjugated urea or amide moiety through the cavity of BPX26C6 with
the assistance of a templating metal ion.
[*] Y.-H. Lin, Y.-H. Liu, Prof. S.-M. Peng, Prof. S.-H. Chiu
Department of Chemistry and Center for Emerging Material and
Advanced Devices, National Taiwan University
formed between the NH proton of the urea or amide group
and the other diethylene glycol chain of the macrocycle would
presumably also assist in stabilizing the pseudorotaxane. We
suspected that physiologically important and abundant alkali-
metal ions, such as Na+ and K+ ions, might be appropriate
templates because of their relatively strong interactions with
the oxygen atoms of the oligo(ethylene glycol) chains of
crown ethers.[10]
No. 1, Sec. 4, Roosevelt Road, Taipei, Taiwan, 10617 (R.O.C.)
E-mail: shchiu@ntu.edu.tw
Prof. C.-C. Lai
Institute of Molecular Biology, National Chung Hsing University
and Graduate Institute of Chinese Medical Science
China Medical University, Taichung, Taiwan (R.O.C.)
[**] We thank the National Science Council (Taiwan) (NSC-101-2628-M-
002-010 and NSC-102-2119-M-002-007) and National Taiwan Uni-
versity (NTU-102-R890913) for financial support.
To test this hypothesis we synthesized the threadlike urea
1, in which the urea unit is conjugated to two aromatic rings.
As less-polar solvents would promote hydrogen bonding
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2013, 52, 1 – 7
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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