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Thinking Outside the “Blue Box”: Induced Fit within a Unique Self-Assembled Polycationic Cyclophane

Blanco-Gómez, Arturo, Fernández-Blanco, Ángel, Blanco, Víctor, Rodríguez, Jaime, Peinador, Carlos, García, Marcos D.
Journal of the American Chemical Society 2019 v.141 no.9 pp. 3959-3964
acidity, hydrazones, hydrophobicity, isomerization, pH, protons, trapping
We present herein the development of a new polycationic molecular receptor, inspired by the ubiquitous cyclobis(paraquat-p-phenylene)cyclophane (“blue box”). Our analogue, the “white box”, has been easily self-assembled on a preparative scale in water, using a template-assisted process by acyl hydrazone bonding of complementary bis(pyridinium)xylylene tweezers, followed by kinetic trapping of the empty receptor. The obtained macrocycle was found to display a marked pH responsiveness in water, because of an abnormal acidity of the amide protons within its structure. Consequently, and because of the concurrence of rotational isomerism under acidic conditions (fixed at higher pH values), the compound was found to display a dual behavior as a conformationally locked/flexible molecular host, being able to recognize appropriate aromatic substrates, in a lock and key or induced fit fashion, by a conjunction of π–π, C–H···π, and, crucially, the hydrophobic effect.