posted on 2013-10-30, 00:00authored byQi Zhang, Konrad Tiefenbacher
Molecular capsules have attracted
interest as simple enzyme mimetics
and several examples of catalytic transformations in water-soluble
metal–ligand based systems have been reported. This is not
the case for hydrogen-bond based molecular capsules, which in contrast
can be employed in organic solvents. We describe herein our investigations
of such a system: The resorcin[4]arene hexamer is one of the largest
hydrogen bond-based self-assembled capsules and has been studied intensively
due to its ready availability. We present evidence that the capsule
acts as a reasonably strong Brønsted acid (pKa approximately 5.5–6). This finding explains the
capsule’s high affinity toward tertiary amines that are protonated
and therefore encounter cation-π interactions inside the cavity.
We were able to translate this finding into a first synthetic application:
A highly substrate-selective Wittig reaction. We also report that
this property renders the capsule an efficient enzyme-like catalyst
for substrate selective diethyl acetal hydrolysis.