Versatile concave receptors with binding properties that can be controlled by external stimuli are rare. Herein, we report on a calixcrypturea (1) that features two different binding sites in close proximity, that is, a tris(2-aminoethyl)amine (tren)-based tris-ureido cap that provides convergent hydrogen-bond-donor sites and a hydrophobic cavity suitable for the inclusion of organic guests. The binding properties of this heteroditopic receptor have been evaluated by NMR spectroscopic studies. Compound 1 behaves as a remarkably versatile host that strongly binds neutral molecules, anions, or contact ion pairs. Within each family of guests, compound 1 is able to discriminate between different guests with a high degree of selectivity. Indeed, neutral molecules that possess hydrogen-bond donor and acceptor groups, chloride anions, and linear ammonium ions associated to F(-) or Cl(-) are particularly well recognized. In comparison with all the related receptors, compound 1 displays several unique features: 1) charged or neutral species are also recognized in polar or protic solvents, 2) thanks to the flexibility of the calixarene structure, induced-fit processes allow the binding of large, biologically relevant ammonium salts such as neurotransmitters, and 3) the protonation of the basic cap leads to a positively charged receptor, 1.H(+), which is reluctant to host anions and in which host properties are now governed by strong charge-dipole interactions with the guests. In other words, compound 1 presents an acid-base controllable tris-ureido recognition site protected by a hydrophobic corridor that can select guests through induced-fit processes. Thus, its versatile host properties can be allosterically controlled by protonation and selective guest-switching processes are possible. To illustrate all these remarkable features, a sophisticated three-pole supramolecular switch, based on the interconversion of host-guest systems displaying either charged or neutral guests, is described.
[Show abstract][Hide abstract] ABSTRACT: The synthesis of benzyl chlorides, cyclic derivatives cyclotriveratrylene and cyclotripiperotrylene were carried out in using the HCl/dioxane system as a catalyst. The reaction proceeded with high selectivity and is sensitive to the number of alkyl and methoxy substituent on the aromatic ring.
[Show abstract][Hide abstract] ABSTRACT: (1,3)-Calixarene-crown-3 derivatives 1a and 1b, bearing either three n-butylureido or 1-naphthylureido moieties at the lower rim, respectively, have been synthesised. Their cationic and anionic binding domains have been investigated by a combination of 1D- and 2D-NMR, UV/vis absorption and fluorescence and ESIMS techniques. Complexation data show that the attachment of 1-naphthyl groups at the lower rim of calixcrown-3 1b dramatically increases its anion-binding ability over the n-butyl-bearing derivative 1a. Overall, both 1a and 1b act as highly efficient heteroditopic receptors for n-butylammonium halides and in so doing bind these salt species as spatially-separated ions much more effectively than the single ionic components. The X-ray crystal structure of the pivotal tris-[2-(2-chloroethoxy)ethoxy]-calixcrown precursor 2 is also reported. (C) 2010 Elsevier Ltd. All rights reserved.
[Show abstract][Hide abstract] ABSTRACT: Azatripyrrolic 1 and azatetrapyrrolic 2 macrocycles were synthesized in a single step by the Mannich reaction of pyrrole in the presence of primary amine hydrochloride and were structurally characterized among several other higher analogue azapyrrolic macrocycles. Binding constants for the halide anion complexes are determined by (1)H NMR titrations and they show different binding stoichiometries.
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