Gallic acid-based dendrimers with a thiacalix[4]arene core: synthesis, aggregation and use for stabilization of Pd nanoparticles

Abstract

First-generation gallic acid-based “click” dendrimers with a thiacalix[4]arene core, including a symmetric dendrimer with four dendrons and amphiphilic dendrimers with alkyl lipoiphilic groups, were synthesized using convergent approach. For their synthesis, new CuAAC precursors of thiacalix[4]arene with propargyl ethylene glycol fragments in 1,3-alternate stereoisomeric form were synthesized and characterized using physical methods (including X-ray crystal analysis). Use of a bromomethylene propargyl derivative of gallic acid as a dendron was complicated by side reactions with tertiary amines during CuAAC modification due to the extreme mobility of the benzyl-type bromine atom. A synthetic strategy for the preparation of dendrons based on gallic acid with an ethylene glycol linker, azide focal point and terminal hydroxyl groups was developed. Dendrimers formed submicron aggregates in water containing 5% THF. The solubilizing ability of dendrimers with respect to the hydrophobic dye orange OT was established. Alkyl-containing amphiphilic dendrimers had a higher solubilization compared with their symmetric counterparts. Use of dendrimers in the stabilization of palladium nanoparticles obtained by chemical reduction produced palladium nanodendrites of size 4–8 nm, which showed excellent catalytic activity in the Suzuki coupling reaction and in nitroaromatic reduction reaction.