The Role of Chirality in Directing the Formation of Cup-Shaped Porphyrins and the Coordination Characteristics of such Hosts

摘要

Cup-shaped porphyrin 1 a has four norbornane rings for encircling space and this type of host could be of interest in supramolecular and catalytic chemistry. We used 1H NMR spectroscopy to investigate the acid-catalyzed (pTsOH in CHCl₃ and TFA in CH₃CN) condensation of racemic, enantioenriched (80–85 % enantiomeric excess (ee)), and enantiopure (99 % ee) pyrromethanecarbinol 7 into 1 a. We found that the oligomerization of racemic 7_rac would give 1 a–d in the ratio different from the statistical one, though a minuscule quantity of 1 a (<5 %) formed. The oligomerization of enantioenriched 7 (80–85 % ee), however, led to the formation of greater amounts of 1 a (31–47 %) along with other stereoisomers 1 b–d. Importantly, pTsOH catalyzed the conversion of enantiopure 7 (99 % ee) into 1 a (>95 % diastereomeric excess (de), 25 % overall yield) in CHCl₃ although prolonged reaction times or greater concentration of the catalytic acid gave rise to 1 b–d at the expense of 1 a. The metallation of 1 a with Zn(OAc)₂ led to the formation of ZnII-1 a and we used computational (DFT: RI-BP86/SV(P),TZVP) and experimental (1H NMR spectroscopy) methods to study the partitioning of smaller N-methylimidazole 13 (94 Å3) and bigger 1,5-dicyclohexylimidazole 14 (268 Å3) between the inner and outer side of the host. We found that bigger 14 was mostly encapsulated (90 %) inside ZnII-1 a at 298.0 K, whereas smaller 13 would equally partition between the two sides of the host. Furthermore, the out/in equilibrium was, in the case of 14-ZnII-1 a, favored by entropy (TΔS°_out/in=3.5±0.1 kcal mol−1) indicating that perhaps differential solvation of the coordinated ligand assisted the encapsulation.