The Avinoam Adam Department of Natural Sciences, The Open University of Israel , 1 University Road, Ra'anana 43537, Israel.
Skaggs School of Pharmacy and Pharmaceutical Sciences, University of California, San Diego , 9500 Gilman Drive, La Jolla, California 92093, United States.
J Am Chem Soc. 2017 Jun 21;139(24):8138-8145. doi: 10.1021/jacs.6b13005. Epub 2017 Jun 7.
In this paper, we report on the noteworthy 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 spectroscopy, IR spectroscopy, X-ray crystallography, and computational methods. Remarkably, while the distance between the portal plane and most atoms at the guest end groups increases progressively with the molecular size, the β-nitrogen atoms maintain a constant distance from the portal plane in all homologues, 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.
在本文中，我们报告了有机叠氮化物与瓜环的门户羰基之间引人注目的吸引力相互作用。我们制备了五个同系的双-α，ω-叠氮乙基铵烷烃，其中铵基之间的亚甲基数从 4 到 8 不等。通过 NMR 光谱、IR 光谱、X 射线晶体学和计算方法研究了它们与瓜环的相互作用。值得注意的是，虽然门户平面与客体末端基团的大多数原子之间的距离随着分子尺寸的增加而逐渐增加，但在所有同系物中，β-氮原子与门户平面保持恒定的距离，这表明叠氮基团与门户之间存在强烈的吸引力相互作用。晶体学和 NMR 都支持羰基和叠氮β-氮之间的特定静电相互作用，这种相互作用稳定了具有β-氮带正电荷和γ-氮带负电荷的典型共振形式。量子计算分析强烈支持静电相互作用，以正交偶极-偶极相互作用的形式，作为这种吸引力的主要驱动力。n → π*轨道离域的替代机制似乎在这种相互作用中不起重要作用。计算研究还表明，这种相互作用不仅限于叠氮化物，而且推广到其他等电子杂烯功能，如异氰酸酯和异硫氰酸酯。这种基本上未被利用的吸引力相互作用可以更广泛地用作工具，不仅与瓜环化学有关，而且与新型超分子架构的设计有关。
