Selective Amine Recognition Driven by Host\u2013Guest Proton Transfer and Salt Bridge Formation

The stepwise synthesis of ionizable p-tert-butylcalix[5]arenes 1a\ub7H and 1b\ub7H, 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\ub7H and 1b\ub7H 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\ub7H and 1b\ub7H 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 - over branched, secondary, and tertiary amines - by 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\ub7H and 1b\ub7H 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\ub7H, by the formation of an unprecedented salt-bridge interaction

Selective Amine Recognition Driven by Host–Guest Proton Transfer and Salt Bridge Formation

The stepwise synthesis of ionizable <i>p</i>-<i>tert</i>-butylcalix­[5]­arenes <b>1a</b>·H and <b>1b</b>·H, featuring a fixed <i>cone</i> cavity endowed with a carboxyl moiety at the narrow rim, is described. Single-crystal X-ray analyses have shown that in the solid state <b>1a</b>·H and <b>1b</b>·H adopt a <i>cone-out</i> 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 <b>1a</b>·H and <b>1b</b>·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over branched, secondary, and tertiary aminesby 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 <i>n</i>-BuNH₂ complexes with <b>1a</b>·H and <b>1b</b>·H provide evidence that binding of the in situ formed <i>n</i>-BuNH₃⁺ substrate to the cavity of the ionized macrocycle is ultimately secured, in the case of <b>1a</b>·H, by the formation of an unprecedented salt-bridge interaction

Selective Amine Recognition Driven by Host–Guest Proton Transfer and Salt Bridge Formation

The stepwise synthesis of ionizable <i>p</i>-<i>tert</i>-butylcalix­[5]­arenes <b>1a</b>·H and <b>1b</b>·H, featuring a fixed <i>cone</i> cavity endowed with a carboxyl moiety at the narrow rim, is described. Single-crystal X-ray analyses have shown that in the solid state <b>1a</b>·H and <b>1b</b>·H adopt a <i>cone-out</i> 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 <b>1a</b>·H and <b>1b</b>·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over branched, secondary, and tertiary aminesby 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 <i>n</i>-BuNH₂ complexes with <b>1a</b>·H and <b>1b</b>·H provide evidence that binding of the in situ formed <i>n</i>-BuNH₃⁺ substrate to the cavity of the ionized macrocycle is ultimately secured, in the case of <b>1a</b>·H, by the formation of an unprecedented salt-bridge interaction

Species Plantarum

Additional notes on 'Fucus', numbers (16 and 18) indicating intended positions relative to material on p. 590