Self-Folding Cavitands

Abstract

A novel class of resorcinarene-based cavitands 2a − e that fold into a deep (8 × 10 Å dimensions) open-ended cavity by means of intramolecular hydrogen bonds has been synthesized. As follows from the FTIR and 1H 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 1H NMR spectroscopy in CDCl3, benzene-d 6 and p-xylene-d 10; the binding energy -ΔG° 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 1H 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 ε-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 1H NMR supramolecular shift reagents.

Alkali metal mediated resorcarene capsules: An ESI-FTICRMS study on gas-phase structure and cation binding of tetraethyl resorcarene and its per-methylated derivative

“…Therefore, the skeleton may oscillate between two conformations. This kind of behavior has been reported earlier for resorcarene derivatives [35] and resorcarenes: NMR studies have shown that the average C 4v structure symmetry is the result of two interconverting C 2v boat conformers [36]. However, the rate of this change is so fast that severe conformational changes to the macrocyclic ring do not occur and the crown conformation is maintained.…”

Quinone‐Based, Redox‐Active Resorcin[4]arene Cavitands

“…[20] We attributed this to the open-top structure of the cavi- Table 1: Kinetic parameters (DH°, DS°, and DG°2 98 K ) for the kite1-kite2 interconversion, thermodynamic quantities (DH, DS, and DG 298 K ) for the vase!kite equilibrium, and chemical shifts d 298 K (*) of the methine protons of cavitands ox-1a-c, red-1 b, ox-2 b, and red-2 tands [6e, 10] and to their structural flexibility. [21] To address the first issue, we prepared the triptycene-derived diquinone cavitand ox-3 and its reduced form, red-3 ( Figure 5). 1 H NMR studies showed that the conformational properties of cavitands ox-3 and red-3 are very similar to those of cavitands ox-2 a-c and red-2 a-c.…”

Self-fulfilling cavitands: Packing alkyl chains into small spaces

“…The nine-membered diether rings impart flexibility on the cavitand, allowing the walls to draw closer or move farther from the center of the cavity in response to a guest (19). The amide groups of the upper rim stabilize this vase conformation by forming a seam of hydrogen bonds in either a clockwise or counterclockwise orientation (20,21) determined by the handedness of the Nacylbenzimidazole. The plasticity of hydrogen bonds permits extensive ''breathing'' at the upper rim.…”