Chirality sensing of fullerenes using cyclic hosts having a chiral N-substituted porphyrin: a remote substituent effect.

Cyclic host molecules 1 2H and 2 2H, having a chiral N-methylporphyrin unit, include C76 in their asymmetrically distorted pi-electronic cavity and are capable of spectral discrimination of the enantiomers of C76 by means of 1H NMR. Although the only structural difference between 1 2H and 2 2H is in the para-substituents R on their meso-phenyl groups, the association constant of C76 with 1 2H is more than twice as large as that with 2 2H. The Delta H values for the association of C76 with these hosts are hardly different from one another, but a rather big difference exists in their DeltaS values (1 2H, -17.7 +/- 0.7 J mol(-1); 2 2H, -26.3 +/- 1.0 J mol(-1)). The difference in DeltaS is most likely due to a steric effect of the substituents on the conformational freedom of the hexamethylene linkers. Although the R groups are topologically remote from the binding site, they also affect the resolution of the diastereoisomerically split N-Me signals in the 1H NMR spectra of the hosts upon inclusion of C76.