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

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 1 H NMR spectroscopy to investigate the acid‐catalyzed ( p TsOH in CHCl 3 and TFA in CH 3 CN) 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, p TsOH catalyzed the conversion of enantiopure 7 (99 %  ee ) into 1 a (>95 % diastereomeric excess ( de ), 25 % overall yield) in CHCl 3 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 Zn II ‐ 1 a and we used computational (DFT: RI‐BP86/SV(P),TZVP) and experimental ( 1 H NMR spectroscopy) methods to study the partitioning of smaller N ‐methylimidazole 13 (94 Å 3 ) and bigger 1,5‐dicyclohexylimidazole 14 (268 Å 3 ) between the inner and outer side of the host. We found that bigger 14 was mostly encapsulated (90 %) inside Zn II ‐ 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 ‐Zn II ‐ 1 a , favored by entropy ( T Δ S ° out/in =3.5±0.1 kcal mol −1 ) indicating that perhaps differential solvation of the coordinated ligand assisted the encapsulation.