Attractive Interactions between Heteroallenes and the Cucurbituril Portal

Abstract

This study reports on the remarkable attractive interaction between organic azides and the portal carbonyls of cucurbiturils. Five homologous bis-α,ω-azidoethylammonium alkanes were prepared, where the number of methylene groups between the ammonium groups ranges from 4 to 8. Their interactions with cucurbit[6]uril were studied by NMR, IR and X-ray crystallography, and by computational methods. Remarkably, while the distance between the portal plane and most atoms at the guest end groups increase progressively with the molecular size, the β-nitrogen atoms maintain a constant distance from the portal plane in all homologs, pointing at a strong attractive interaction between the azide group and the portal. Both crystallography and NMR support a specific electrostatic interaction between the carbonyl and the azide β-nitrogen, which stabilizes the canonical resonance form with positive charge on the β-nitrogen and negative charge on the γ-nitrogen. Quantum computational analyses strongly support electrostatics, in the form of orthogonal dipole-dipole interaction, as the main driver for this attraction. The alternative mechanism of n→π* orbital delocalization does not seem to play a significant role in this interaction. The computational studies also indicate that the interaction is not limited to azides, but generalizes to other isoelectronic heteroallene functions, such as isocyanate and isothiocyanate. This essentially unexploited attractive interaction could be more broadly utilized as a tool not only in relation to cucurbituril chemistry, but also for the design of novel supramolecular architectures.

Hetero‐Bambusurils

“…[36] We used this chemistry to achieve the corresponding nitrogen analogs. Remarkably, 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 this one-pot transformation converted semithio-BU [6], a single anion receptor, into various semiaza-BU [6]s, which were found to be multiple ion receptors (vide infra). With no need to work up the reaction mixture and isolate the latter triflate salts, addition of a primary amine to the reaction mixture afforded semiaza-BU [4]s, 5 a-d, and semiaza-BU[6]s, 6 a-d, in satisfactory overall yields (Scheme 5).…”

“…The semithio-glycoluril precursor, 2 f, was sufficiently stable to react with formaldehyde under various acidic conditions (typically, p-toluene sulfonic acid, PTSA), leading to successful one-step syntheses of either semithio-BU [4], 3, or semithio-BU [6], 4, in satisfactory yields (35 % and 82 %, respectively) adopting Heck's optimized conditions (Scheme 4). [4,42] Interestingly, the unsubstituted semithio-glycoluril, 2 g, decomposed under these conditions.…”

“…[24] Similar attempts were reported by Heaven. We show here that by converting a semithio-bambus [6]uril into the corresponding semiaza-bambusuril we can convert a single anion receptor into a potential anion channel, which simultaneously accommodates three anions linearly positioned within the cavity along the main symmetry axis. Our studies started with calculations of the molecular electrostatic potential of various glycoluril and bambusuril analogs, which predicted that heteroatom replacements would significantly alter their electrostatic surface, leading to a general trend of anion affinity: X = S > O > NH.…”

“…This major difference explains why the CBs bind neutral molecules and gases within their hydrophobic cavity, [5,6] whereas the BUs are anion-binders. In contrast, in the BUs the glycoluril units present their convex face towards the interior with the methine hydrogens pointing inward, thus forming a bamboo-like object.…”

“…[19] Thiacalixarenes have proven useful ion sensors [20] and those bearing thiophenol groups have exhibited unique conformational preferences and specific binding to metal ions. Interestingly, Mock has reported on the preparation of semithio-glycoluril shortly after the discovery of CB [6] structure, probably reflecting his efforts to prepare thio-CBs. Interestingly, Mock has reported on the preparation of semithio-glycoluril shortly after the discovery of CB [6] structure, probably reflecting his efforts to prepare thio-CBs.…”