Synthesis and Structure of Feet-to-Feet Connected Bisresorcinarenes

Bisresorcinarenes 1a–d were obtained
in excellent yields, and 1e was finally obtained in 50%
yield. X-ray diffraction analysis showed that 1a and 1b adopted helical conformations, whereas the two resorcinarenes
of 1c–e were in parallel orientations
in which the clefts of the aliphatic chains entrapped one or two solvent
molecules. The conformational study revealed that the helix interconversion
between the (P)- and (M)-helical
conformers depended on the length of the aliphatic chains. 1a had the largest energetic barrier to helix interconversion, while
in 1b, its more flexible aliphatic chains lowered its
energetic barriers. The P/M interconversion
of 1a was coupled with the clockwise/anticlockwise interconversion
of the interannular hydrogen bonding of the two resorcinarenes. The
large negative entropic contributions indicate that the transition
state is most likely more ordered than the ground states, suggesting
that the transition state is most likely symmetric and is solvated
by water molecules. Calculations at the M06-2X/6-31G­(d,p)
level revealed that the more stable (P)-conformation
has clockwise interannular hydrogen bonding between the two resorcinarenes.