Homochiral C3-symmetric tris-ureas 1–3 and heterochiral tris-urea 4 were synthesized. The homochiral tris-ureas 1–3 acted as low-molecular-weight gelators for various organic solvents. The optical purities of the tris-ureas strongly influenced their gelation. External chiral molecules influenced chiral supramolecular gel formation of chiral tris-ureas.
A multicomponent supramolecular gel constructed from the self-assembly of pyridyl-substituted tris-urea 1 and Pd(OAc) 2 is reported. A mixture of 1 and Pd(OAc) 2 in dimethyl sulfoxide provided a supramolecular gel after brief ultrasound irradiation. The supramolecular gel changed into a sol/solution upon addition of chelating agents such as diamines and bidentate phosphine ligands. Dissociation of the coordination bonds between 1 and the palladium ion via addition of a chelating agent caused the phase transition. The Wacker oxidation of styrene was performed in the supramolecular gel. The reaction in the gel phase required a longer induction period to produce acetophenone than did the reaction in the homogeneous solution.INTRODUCTION Supramolecular gels have potential applications in a wide range of fields in materials science. 1,2 The development of supramolecular gels through the self-assembly of a small molecule, called a low-molecularweight gelator (LMWG), has attracted significant attention in recent decades. 3-6 LMWGs self-assemble by noncovalent interactions, such as hydrogen bonding, π-π interactions, dipole-dipole interactions and solvophobic interactions, to form three-dimensional networks that immobilize fluids. Intermolecular hydrogen bonds of ureide groups are the common driving force for the formation of supramolecular gels. 7 A variety of LMWGs with mono-urea, 8,9 bis-urea, 10-15 trisurea 16,17 and tetrakis-urea 18 structures have been reported. We have also developed C 3 -symmetric tris-urea LMWGs. [19][20][21][22] Supramolecular gels are generally constructed from a single component of LMWGs; however, some are formed from multiple types of small molecules. 23 A metal-ligand coordination bond is a common aspect of the multicomponent supramolecular gels; the advantage in this case is that gel formation can be easily controlled by controlling the formation and dissociation of the metal-ligand coordination bond. Moreover, the metal center of the resulting gel can potentially function as a catalyst for organic reactions. [24][25][26][27][28][29] We have previously reported multicomponent supramolecular gels of trisurea, in which the gelation ability of phenyl-substituted tris-urea was increased by the addition of a small amount of pyridyl-substituted trisurea and metal ion or bis-carboxylic acid. 30 In this paper, we report supramolecular gel formation from pyridyl-substituted tris-urea in the presence of palladium ions in dimethyl sulfoxide (DMSO). The supramolecular gel underwent a gel to sol/solution phase transition upon the addition of chelating agents for palladium ions. The palladium-catalyzed Wacker oxidation of styrene was performed in the gel phase.
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