Enhanced
Helical Folding of <i>ortho</i>-Phenylenes through the Control
of Aromatic Stacking Interactions
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Abstract
The <i>ortho</i>-phenylenes are a simple class of foldamers,
with the formation of helices driven by offset aromatic stacking interactions
parallel to the helical axis. For the majority of reported <i>o</i>-phenylene oligomers, the perfectly folded conformer comprises
perhaps 50–75% of the total population. Given the hundreds
or thousands of possible conformers for even short oligomers, this
distribution represents a substantial bias toward the folded state.
However, “next-generation” <i>o</i>-phenylenes
with better folding properties are needed if these structures are
to be exploited as functional units within more complex architectures.
Here, we report several new series of <i>o</i>-phenylene
oligomers, varying both the nature and orientation of the substituents
on every repeat unit. The conformational behavior was probed using
a combination of NMR spectroscopy, DFT calculations, and X-ray crystallography.
We find that increasing the electron-withdrawing character of the
substituents gives oligomers with substantially improved folding properties.
With moderately electron-withdrawing groups (acetoxy), we observe
>90% of the perfectly folded conformer, and stronger electron withdrawing
groups (triflate, cyano) give oligomers for which misfolded states
are undetectable by NMR. The folding of these oligomers is only weakly
solvent-dependent. General guidelines for the assessment of <i>o</i>-phenylene folding by NMR and UV–vis spectroscopy
are also discussed