Self-folding cavitands

A novel class of resorcinarene-based cavitands 2a-e that fold,into a deep (8 x 10 Angstrom dimensions) open-ended cavity by means of intramolecular hydrogen bonds has been synthesized. As follows from the FTIR and H-1 NMR spectral data in apolar solvent, a seam of eight intramolecular hydrogen bonds is stitched along the upper rim of the structure 2a-e; the amide C=O ... H-N interactions bridge adjacent rings-interannular binding-and are held in place by the seven-membered intraannular hydrogen bonds. The self-folding in 2a-e is reversibly controlled by solvent and temperature. Complexation of self-folding cavitands 2a-e with organic molecules such as (1-substituted) adamantanes, lactams, and cyclohexane derivatives was demonstrated by H-1 NMR spectroscopy in CDCl3, benzene-d(6) and p-xylene-d(10); the binding energy -Delta G degrees values of 2-4 kcal mol(-1) in p-xylene-d(10) at 295 K were calculated. The exchange between complexed and free guest species is slow on the NMR time-scale, and it is proposed that hydrogen bonds are responsible for these unique features. Employing the pronounced upfield H-1 NMR shifts of the complexed guest molecules, attempts were made to study the structure of the caviplexes "from inside", and the orientation of the encapsulated adamantanes 12,13, as well as noncovalent interactions of complexed epsilon-caprolactam 9b with the host walls were deduced. Even though the guest-exchange process in 2a-e is slow on the NMR time scale (k = 2 +/- 1 s(-1)), it is still faster than that observed for the completely closed hydrogen-bonded calixarene-based capsules or for covalently sealed hemicarceplexes. This places them in an unusual position in the scale of cavity-containing receptors and opens new perspectives to use 2a-e in catalysis and as H-1 NMR supramolecular shift reagents.