Expanding Cavitand Chemistry: The Preparation and Characterization of [n]Cavitands withn≥4

Abstract

The preparation of cavitands composed of 4, 5, 6, and 7 aromatic subunits ([n]cavitands, n=4-7) is described. The simple, two-step synthetic procedure utilized readily available starting materials (2-methylresorcinol and diethoxymethane). The two cavitand products having 4 and 5 aromatic subunits exhibited highly symmetric cone conformations, while the larger cavitands (n = 6 and 7) adopt conformations of lower symmetry. 1H NMR spectroscopic studies of [6]cavitand and [7]cavitand revealed that these hosts undergo exchange between equivalent conformations at room temperature. The departure of these two cavitands from cone conformations is related to steric crowding on their Ar-O-CH2-OAr bridges and is predicted by simple molecular mechanics calculations (MM2 force field). X-ray diffraction studies on single crystals of the [4]cavitand, [5]cavitand, and [6]cavitand hosts afforded additional experimental support for these conclusions.

Many Simple Molecular Cavitands Are Intrinsically Porous (Zero-Dimensional Pore) Materials

“…(hemi)carcerands 79-81 ) highlights this design feature and provides the crystal structures of several bowl-occupied solvates. [55][56][57][58][59][60][61][62][63][64][65][66][67][68][69][70][71][72][73][74] It was noted early on that many simple cavitands essentially always crystallize from solution with included solvent, some forming highly stable solvates. For example, Cram and coworkers found that the aryl-footed cavitand H,4-CH 3 C 6 H 4 ,CH 2 strongly holds ethyl acetate and apparently could only be emptied by sublimation at 450 °C under vacuum.…”

“…3 where necessary. New calix [4]resorcinarenes and cavitands were made by methods analogous to the methods in the literature 55,[60][61][62] and their protocols are described below. Except where otherwise noted, all compounds are stereopure (>95%) rccc isomers.…”

Metallo-supramolecular capsules

“…Following this study, Sherman and co-workers synthesised a series of expanded [n]cavitands where n ≥ 4 (Scheme 2B). [92] In addition, C-ethylpyrogallol [4 and 6]arene were also synthesised in a mixture, separated and structurally characterised by Nissinen and co-workers (Scheme 2C) [93]. Although this is the case, the pyrogallolarene pentamer is yet to be isolated.…”

“…Larger resorcin[4]arenes, cavitands and pyrogallolarenes that have recently been synthesised under various reaction conditions[91][92][93].…”

Exploring the Scope of Macrocyclic “Shoe-last” Templates in the Mechanochemical Synthesis of RHO Topology Zeolitic Imidazolate Frameworks (ZIFs)

“…Clearly, also, MeHCH 2 -6 cannot fit within a completely formed ZnIm 2 d8r motif. However, MeHCH 2 -6 exhibits a "double-bowl" type C 2v symmetric conformation in the solid state and in solution [55]. The conformation ( Figure 6) is such that two narrow ends of the elongated, pinched double-bowl structure, mimic, almost perfectly, three of the four rings of the four-fold symmetric MeHCH 2 -4 cavitand.…”