Binding of Acetylcholine and Tetramethylammonium to Flexible Cyclophane Receptors: Improving on Binding Ability by Optimizing Host's Geometry

The structure of a cyclophanic tetraester (1), previously employed for investigations on the cation−π
interaction, has been optimized to better accommodate acetylcholine (ACh) and tetramethylammonium (TMA) guests. Following indications from molecular modeling calculations, a flexible
cyclophane receptor of significantly improved binding properties has been obtained by removing
the four carbonyl groups of the parent host. 2,11,20,29-Tetraoxa[3.3.3.3]paracyclophane (2) was
prepared by an improved procedure, which was conveniently devised to avoid the formation of
contiguous cyclooligomers that caused serious separation issues. Association of 2 with TMA picrate
was measured in CDCl3 at T = 296 K by 1H NMR titrations and compared to binding data obtained
for a set of reference hosts, including the parent tetraester 1, the corresponding cyclophanic
tetraamine, the open-chain counterpart of 2, and its cyclooligomers from pentamer to octamer.
Binding enhancements ranging from 15-fold (with respect to the tetraester and the tetraamine) to
over 80-fold (with respect to the open-chain tetraether) were achieved by geometry optimization of
the host. Binding of 2 to ACh and TMA was investigated for a variety of counterions. A constant
binding free energy increment of nearly 8 kJ mol-1 with respect to 1 was observed, independent
from the anion and irrespective of the different structure of the cationic guests. Results showed
that the electrostatic inhibiting contribution of the counterion to the cation's binding is a
characteristic constant of each anion. The value of −ΔG° = 44.9 kJ mol-1 extrapolated for TMA in
the absence of a counterion indicates that 28−34 kJ mol-1 of binding free energy are lost in ion
pairing.