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Multicomponent, Hydrogen-Bonded Cylindrical Capsules

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journal contribution
posted on 04.09.2009, 00:00 by Dariush Ajami, Julius Rebek
Self-assembled, hydrogen-bonded capsules emerge from synthetic resorcinarene-derived cavitands and soluble glycolurils when appropriate guest molecules are present. The assembly consists of 2 cavitands, 4 glycolurils and guest(s), and the arrangement of glycolurils leads to a chiral structure. The capsule features a space of ∼620 Å3 and accommodates narrow guests such as n-alkanes from C14 to C19, or other molecules (e.g., capsaicin) and combinations of molecules of up to ∼22 Å in length (e.g., two p-methylstyrene molecules). Positions of encapsulated nuclei can be predicted from NMR chemical shifts, with intense shielding of δΔ = −5 ppm near the resorcinarene ends and mild deshielding of +0.5 to 1 ppm near the glycolurils at the capsule’s center. Computational methods using nucleus independent chemical shifts (NICS) were used to map the induced magnetic shielding/deshielding for the inner space of the cavity. The asymmetric arrangement of the spacers creates a chiral steric and magnetic environment in the capsule and the geminal hydrogen atoms of encapsulated alkanes show diastereotopic proton signals. The two enantiomers interconvert (racemize) through an achiral intermediate involving a slight rotation of the spacers and lengthening of the cavity. Accordingly, longer, compressed alkanes accelerate the racemization by applying pressure from the inside on the capsule’s ends. Guests that place hydrogen bond donors and acceptors near the glycolurils in the middle (e.g., p-isopropylbenzyl alcohol) also accelerate the racemization by facilitating the rotation of the glycolurils. Slow tumbling of guest on the NMR time scale inside the capsule leads to social isomerism of para-disubstituted benzenes such as p-methylstyrene. Flexible guests such as hexane tumble inside the cavity with an activation barrier of ΔG =16.2 kcal/mol. The middle of the extended capsule is narrow, but still accommodates phenyl groups such as those presented by p-quaterphenyl and alkylated biphenylcarbonitriles. The aromatic units in these guests report their positions by imparting magnetic anisotropy to the capsule components. Gases such as propane, butane, isobutane, propylene, 2-methylpropene, and 1,3-butadiene even xenon are coencapsulated with other guests and their motions inside are examined.