Pillar[5]arene as a Co-Factor in Templating Rotaxane Formation (original) (raw)

2013, Journal of the American Chemical Society

rate, often ca. 10 5-fold compared to the uncatalyzed reaction. Since its discovery by Mock, the CB-catalyzed 1,3-dipolar cycloaddition (CB-AAC) has found application in the construction of polymers, 14 mechanically interlocked molecules (MIMs) 15 and pH responsive controlled-release systems. Despite the fact that both the CB-AAC and the Cu(I)-catalyzed cycloaddition (CuAAC) exhibit favorable kinetics and regiospecificities, the former has found comparatively few applications compared to the now ubiquitous CuAAC. Perhaps the fact that CB is not as freely available as copper salts and is less convenient to handle because of its poor solubility 17 in water 18 and other common laboratory solvents, 19 add up to explanations of a sort. The greatest drawback of the CB-AAC, however, is its substrate scope − to date, all reports are restricted to describing reactions between propargylammonium and azidoethyl-ammonium derivatives. This limited scope can be rationalized by considering the cyclization mechanism and noncovalent bonding interactions which underpin the reaction. CB-AAC proceeds by means of the initial formation of a hetero-ternary complex, which renders the cyclization a pseudo-unimolecular process as a result of bringing the triple bond in the propargylammonium ion and the azide function in the azidoethylammonium ion into close proximity as well as aligning them so that they are poised to undergo trizaole ring formation. 11a, 20 The entropic cost of bringing the CB and AAC precursors together is compensated 21 for by the favorable binding enthalpy and the release of high energy water molecules from the cavity of the CB. Charge-dipole and hydrogen bonding interactions between the secondary dialkylammonium ions and the carbonyl groups around the rim of the CB are maximized when the NH 2 + centers are close to the planes of carbonyl oxygens, a requirement which dictates the geometry of the ternary complex. Consequently, altering the

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Thermodynamic, kinetic, and structural study of the ligational properties of the macrobicyclic aza-ligand 4,7,10,17,23-pentamethyl-1,4,7,10,13,17,23-heptaazabicyclo[11.7.5]pentacosane (L1) and of its macrocyclic precursor 1,4,7,13-tetramethyl-1,4,7,10,13,16-hexaazacyclooctadecane (L2). Crystal st...

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