Facile synthesis and characterization of urushiol analogues from tung oil via ultraviolet photocatalysis

Abstract In this work, the urushiol analogues (CAT-ME) carrying the functional groups of double bonds and phenolic hydroxyls were synthesized by Friedel-Crafts alkylation reaction of methyl eleostearate (the main component of tung oil) with catechol via ultraviolet photocatalysis. By comparison of catalytic results of different photoinitiators, it was found that cationic photoinitiators (triarylsulfonium salts) could be used to catalyze this alkylation reaction, which could not only dramatically boost the yield (71%) of the urushiol analogues, but also greatly shorten the reaction time (8 min) at a low UV light (100 W). And the chemical structures of the target compounds were also confirmed by Fourier transform infrared spectroscopy (FTIR), ultraviolet and visible spectroscopy (UV–vis), nuclear magnetic resonance spectroscopy (NMR) and liquid chromatography-mass spectroscopy (LC–MS). In addition, the photoactivity of the urushiol analogues was also discussed and the results showed that the UV-cured polyurethane films mixed with CAT-ME had excellent mechanical property and thermal stability, and the gel fraction of the film mixed with 15% CAT-ME reached 99.5%, and the tensile strength was up to 25.07 MPa while the breaking elongation increased by 4.57%. Thus, this approach of photocatalysis using cationic photoinitiators as catalysts showcases a promising method to direct other chemical reactions and provides a platform to guide the further modification of other unsaturated natural oil, expanding the application of vegetable oil.