Abstract

Abstract

Abstract

Metal ions are superb at templating the synthesis of small macrocycles that are decorated with Lewis basic sites. However, for the synthesis of larger macrocycles, or the synthesis of macrocycles devoid of an array of Lewis basic sites, metal ions are less useful. Here we demonstrate that resorcinarenes can be used as templates to engender the efficient formation of large crown ethers. A three step process of 1) tethering moieties to the template, 2) linking those moieties, and 3) then cleaving off the template leads to the efficient formation of a family of aromatic crown ethers. If adaptable, this approach will prove useful for the construction of macrocycles that are hard to obtain from a step-wise synthesis.

The synthesis of three different nanoscale molecular hosts is reported. These cavitands each possess a highly preorganized cavity with an open portal (nearly 1 nm wide), by which guests can enter and egress the cavity. Additionally, these hosts are deep-functionalized with a crown of weakly acidic benzal CH groups which can form a variety of noncovalent interactions with guest molecules residing within the cavity. Thirty-one guests were examined for their propensity to form complexes with the hosts. Guests that possess halogen atoms were the strongest binders, suggesting the formation of polydentate CH⋅⋅⋅XR hydrogen bonds with the deep crown of benzal hydrogens. Exchange rates between the free and bound states were noted to be dependent on the size of the guest and the solvent used to study complexation. In general, stronger binding and slower exchange were noted for complexations carried out in DMSO with highly complementary guests. The orientation of each guest within the cavity was determined using either EXSY NMR spectroscopy or ¹H NMR shift data. Cumulatively these results showed that the principal factors directing orientation were interactions with the benzal groups and the type of solvent. Van't Hoff analyses of selected complexations were also carried out. As well as revealing that all complexations were entropically unfavorable, these experiments provided support for guest orientation determinations, and gave an estimation that the formation of a CH⋅⋅⋅IR hydrogen bond releases between 1 and 1.5 kcal mol⁻¹.

Zauber des Kristalls: Ein kürzlich beschriebenes Calix[4]aren, das eine annähernd hexagonal dichteste Packung aus quasi-kugelförmigen Trimeren sowohl in Gegenwart als auch in Abwesenheit von Gastmolekülen bildet (siehe Struktur im Kristall), schlägt eine Brücke zwischen Kristallen, die sich beim Entfernen des Gastes zersetzen, und solchen, deren Gäste nicht entfernbar sind.

The beauty of crystals: A bridge between crystals that degrade when their guests are removed and crystalline solids that refuse to release their guests has been provided by Atwood et al. with the report of a calix[4]arene that forms an approximate hexagonal close-packed array of quasi-spherical trimers in the presence and absence of guest molecules (see X-ray crystal structure).