The role of chirality in directing the formation of cup-shaped porphyrins and the coordination characteristics of such hosts.

Cup-shaped porphyrin 1a 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 (pTsOH 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 1a. We found that the oligomerization of racemic 7(rac) would give 1a-d in the ratio different from the statistical one, though a minuscule quantity of 1a (<5 %) formed. The oligomerization of enantioenriched 7 (80-85 % ee), however, led to the formation of greater amounts of 1a (31-47 %) along with other stereoisomers 1b-d. Importantly, pTsOH catalyzed the conversion of enantiopure 7 (99 % ee) into 1a (>95 % diastereomeric excess (de), 25 % overall yield) in CHCl(3) although prolonged reaction times or greater concentration of the catalytic acid gave rise to 1b-d at the expense of 1a. 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) -1a, 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.