Self-Assembly of Copper(II) Ion-Mediated Nanotube and Its Supramolecular Chiral Catalytic Behavior.

Self-assembly of several low-molecular-weight l-glutamic acid-based gelators, which individually formed helical nanotube or nanofiber structures, was investigated in the presence of Cu2誉. It was found that, when Cu2솫 added into the system, the self-assembly manner changed significantly. Only in the case of bolaamphiphilic glutamic acid, N,N′-hexadecanedioyl-di-l-glutamic acid (L-HDGA), were the hydrogel formation as well as the nanotube structures maintained. The addition of Cu2誉 caused a transition from monolayer nanotube of L-HDGA to a multilayer nanotube with the thickness of the tubular wall about 10 nm. For the other amphiphiles, the gel was destroyed and nanofiber structures were mainly formed. The formed Cu2+containing nanostructures can function as an asymmetric catalyst for Diels–Alder cycloaddition between cyclopentadiene and aza-chalcone. In comparison with the other Cu2+containing nanostructures, the Cu2+mediated nanotube structure showed not only accelerated reaction rate, but enhanced enantiomeric selectivity. It was suggested that, through the Cu2駧扪힝 nanotube formation, the substrate molecules could be anchored on the nanotube surfaces and produced a stereochemically favored alignment. When adducts reacted with the substrate, both the enantiomeric selectivity and the reaction rate were increased. Since the Cu2+mediated nanotube can be fabricated easily and in large amount, the work opened a new way to perform efficient chiral catalysis through the supramolecular gel. [ABSTRACT FROM AUTHOR]