Chemistry Reference
In-Depth Information
NMe
2
NMe
2
R'
3
Si
R
R
SiR'
3
N
N
CO
2
Tg
CO
2
Tg
n
n
CO
2
Tg
CO
2
Tg
Figure 2.29 General structure of oligo(
m
-phenylene ethynylene) foldamers.
interactions and favorable side chain-solvent interactions while minimizing destabi-
lizing backbone-solvent contact. Unlike their hydrogen-bonded counterparts, the sol-
vents used for these foldamers are more polar in nature (e.g., aqueous solutions).
Several examples of this class of foldamers were reported by Moore and coworkers
[81], who elaborated their oligo(
m
-phenylene ethynylene) foldamer to incorporate a
4-dimethylaminopyridine (DMAP) ring in the center (Figure 2.29). These foldamers
are capable of selecting, preferentially binding and reacting with substrates that pres-
ent a better match to their size and shape. Thus, these foldamers can be likened to
enzymes, such as
t
RNA synthetases, that use multiple sieving protocols to ensure
specificity and fidelity. Hecht and coworkers modified the oligo(
m
-phenylene ethyny-
lene) backbone pioneered by Moore and coworkers by introducing an azobenzene
ring to take advantage of its photoswitching properties [82].
2.4 Organization Induced by External Agents
Several authors noticed that the organization adopted by aromatic foldamers is strongly
influenced by external agents such as solvents or ions. As the organization of foldamers
driven by metal cations will be extensively discussed in the following chapters of this
book, we report here a short overview of some examples of the effect of solvents and
anions on foldamer organization.
2.4.1 Organization Induced by Solvents
We present here only one very interesting example, which was reported by Huc and cow-
orkers; the authors demonstrated that pyridinedicarboxamide helices can be capped with
two quinoline-2-carboxylates on either end to form isolated capsules capable of binding
small guests like methanol and water in chloroform [83]. The host molecules give rise to
distinct NMR signals depending on whether they are empty, half-full or full, thereby facil-
itating the analysis. The binding event is slow and can be captured on the NMR timescale.
Capsules with larger cavities have been generated by replacing the pyridine ring with a
1,8-diaza-anthracene motif; these are capable of binding alkanediol guests [84].
Progressively increasing the size of the helical capsules by including additional mono-
mers in the center of their sequence allows their binding properties to be tuned in a modu-
lar fashion. More guests or larger guests can be included when the capsule size is
rationally increased (Figure 2.30). Capsules with larger cavities prefer to accommodate
larger guests rather than a large number of water molecules for obvious entropic reasons.
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