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 C-76 in their asymmetrically distorted pi-electronic cavity and are capable of spectral discrimination of the enantiomers of C-76 by means of H-1 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 C-76 With 1(2H) is more than twice as large as that with 2(2H). The Delta H values for the association of C-76 with these hosts are hardly different from one another, but a rather big difference exists in their AS values (1(2H), -17.7 +/- 0.7 J mol(-1); 2(2H), -26.3 +/- 1.0 J mol(-1)). The difference in Delta S 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 H-1 NMR spectra of the hosts upon inclusion of C-76.