Selective Amine Recognition Driven by Host鈥揋uest Proton Transfer and Salt Bridge Formation

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• 作者：Calogero Capici ; Giuseppe Gattuso ; Anna Notti ; Melchiorre F. Parisi ; Sebastiano Pappalardo ; Giovanna Brancatelli ; Silvano Geremia
• 刊名：The Journal of Organic Chemistry
• 出版年：2012
• 出版时间：November 2, 2012
• 年：2012
• 卷：77
• 期：21
• 页码：9668-9675
• 全文大小：487K
• 年卷期：v.77,no.21(November 2, 2012)
• ISSN：1520-6904

文摘

The stepwise synthesis of ionizable p-tert-butylcalix[5]arenes 1a路H and 1b路H, featuring a fixed cone cavity endowed with a carboxyl moiety at the narrow rim, is described. Single-crystal X-ray analyses have shown that in the solid state 1a路H and 1b路H adopt a cone-out conformation with the carboxylic OH group pointing in, toward the bottom of the aromatic cavity, as a result of a three- or two-center hydrogen-bonding pattern between the carboxyl group and the phenolic oxygen atom(s). The affinity of amines for calix[5]arene derivatives 1a路H and 1b路H was probed by ¹H NMR spectroscopy and single-crystal X-ray diffraction studies. These carboxylcalix[5]arenes are shown to selectively recognize linear primary amines鈥攐ver branched, secondary, and tertiary amines鈥攂y a two-step process involving a proton transfer from the carboxyl to the amino group to provide the corresponding alkylammonium ion, followed by binding of the latter inside the cavity of the ionized calixarene. Proton transfer occurs only with linear primary amines, that is, when the best size and shape fit between host and substrate is achieved, while the other amines remain in their noncompeting unprotonated form. The role of the solvent in the ionization/complexation process is discussed. Structural studies on the n-BuNH₂ complexes with 1a路H and 1b路H provide evidence that binding of the in situ formed n-BuNH₃⁺ substrate to the cavity of the ionized macrocycle is ultimately secured, in the case of 1a路H, by the formation of an unprecedented salt-bridge interaction.