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 Cu(2+) ion. It was found that, when Cu(2+) was 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 Cu(2+) ion 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 Cu(2+)-containing nanostructures can function as an asymmetric catalyst for Diels-Alder cycloaddition between cyclopentadiene and aza-chalcone. In comparison with the other Cu(2+)-containing nanostructures, the Cu(2+)-mediated nanotube structure showed not only accelerated reaction rate, but enhanced enantiomeric selectivity. It was suggested that, through the Cu(2+) mediated 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 Cu(2+)-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.