Enzyme-like Acyl Transfer Catalysis in a Bifunctional Organic Cage

Amide-based organic cage cavities are, in principle,
ideal enzyme
active site mimics. Yet, cage-promoted organocatalysis has remained
elusive, in large part due to synthetic accessibility of robust and
functional scaffolds. Herein, we report the acyl transfer catalysis
properties of robust, hexaamide cages in organic solvent. Cage structural
variation reveals that esterification catalysis with an acyl anhydride
acyl carrier occurs only in bifunctional cages featuring internal
pyridine motifs and two crucial antipodal carboxylic acid groups. 1H NMR data and X-ray crystallography show that the acyl carrier
is rapidly activated inside the cavity as a covalent mixed-anhydride
intermediate with an internal hydrogen bond. Michaelis–Menten
(saturation) kinetics suggest weak binding (KM = 0.16 M) of the alcohol pronucleophile close to the internal
anhydride. Finally, activation and delivery of the alcohol to the
internal anhydride by the second carboxylic acid group forms ester
product and releases the cage catalyst. Eyring analysis indicates
a strong enthalpic stabilization of the transition state (5.5 kcal/mol)
corresponding to a rate acceleration of 104 over background
acylation, and an ordered, associative rate-determining attack by
the alcohol, supported by DFT calculations. We conclude that internal
bifunctional organocatalysis specific to the cage structural design
is responsible for the enhancement over the background reaction. These
results pave the way for organic-phase enzyme mimicry in self-assembled
cavities with the potential for cavity elaboration to enact selective
acylations.