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RESEARCH PRODUCT

A bibracchial lariat aza-crown ether as an abiotic catalyst of malonic acid enolization

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A bibracchial lariat aza-crown ether (L) consisting of 2-aminoethylnaphthyl moieties appended to a 2 : 2 azapyridinophane structure displays significant activation of H–D exchange in malonic acid. The compound forms very stable adducts with malonate anions (MA) in the 2–10 pH range. Molecular dynamics studies performed for the species resulting from the interaction of the hexaprotonated macrocycle and the dianion show that malonate is encapsulated by L with distances between the CH2 group of malonate and the pyridine nitrogens of ca. 3.5 A. The pendant arms of L cap above and below the anion, defining a pseudo-cage structure. Quantum chemical calculations for α-proton abstraction from malonate by the protonated ligand L, at the B3LYP/6-31G** computational level using reduced models, have allowed transition states to be obtained that denote a concerted mechanism in which α-proton abstraction is accompanied by proton transfer from protonated amino groups of the receptor to the carboxylate groups. Kinetic measurements for ligand L, performed at pD = 6 following the variation of the 1H-NMR MA-CH2 and MA-CHD signals, allow the following rate constants to be calculated: kH1 = (60.2 ± 0.5) × 10−3 min−1 and kH2 = (75.3 ± 0.6) × 10−3 min−1, with values that equal the experimental values of the free substrate at pD = 0. The rate enhancement produced by the open-chain reference ligand (L1), containing the polyamine tren (tris(2-aminoethyl)amine) functionalized at one of its terminal amino groups with a naphthyl moiety, is much more moderate, with kH1 = (9.2 ± 0.2) × 10−3 min−1 and kH2 = (11.56 ± 0.03) × 10−3 min−1.