Self-thickening materials derived from phenylpropanoid ene reactions

Authors: Biswas, Atanu; Cheng, Huai; Chisholm, Bret; BENI, RYAN - Tennessee State University; Liu, Zengshe; Vermillion, Karl; Appell, Michael; Forson, Kelton; EL SEOUD, OMAR - University Of São Paulo; ALVES, CARLUCIO - Universidade Estadual Do Ceara; FURTADO, ROSELAYNE - Embrapa

Submitted to: Molecules
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/17/2025
Publication Date: 2/20/2025
Citation: Biswas, A., Cheng, H.N., Chisholm, B., Beni, R., Liu, Z., Vermillion, K., Appell, M., Forson, K., El Seoud, O., Alves, C.R., Furtado, R.F. 2025. Self-thickening materials derived from phenylpropanoid ene reactions. Molecules. https://doi.org/10.3390/molecules30050977.
DOI: https://doi.org/10.3390/molecules30050977

Interpretive Summary: Phenylpropanoids represent a diverse group of natural compounds found throughout the plant kingdom, playing key roles in plant defense, structural support, and overall survival. They are also critical components of several structural polymers, such as lignin and condensed tannins. One group of phenylpropanoids of interest is related to allylbenzene, including allylbenzene, eugenol, and methyl eugenol. As part of our ongoing efforts to transform agro-based materials for industrial use, we have explored the reactions of these three natural products. A particularly green and eco-friendly process is Alder’s ene reaction, which can occur under mild conditions without the need for a catalyst or the formation of byproducts. All three compounds reacted with an ene reagent (diethyl azodicarboxylate), producing highly viscous or solid products. These products dissolved readily in toluene, forming thick solutions, and their viscosity dropped significantly upon dilution. This self-thickening property suggests potential applications as thickening agents in organic solvent formulations, including paints, sealants, coatings, and adhesives.

Technical Abstract: In this work, we report the observation of uncatalyzed ene reactions between several phenylpropanoid compounds and diethyl azodicarboxylate (DEAD). For allylbenzene, the reaction produces the ene product at molar ratios of up to 1:2 of allylbenzene to DEAD. At higher levels of DEAD, more complex reactions are observed. For the reaction between methyl eugenol and DEAD, similar ene reaction products have been found. However, the reaction of eugenol with DEAD is more complex; in addition to the ene reaction, other reactions happen at the same time. Most of the structures of the resulting products have been elucidated using NMR spectroscopy (¹H, ¹³C, and 2D), and the findings have been further corroborated by FTIR analysis. Interestingly, these products appear to undergo molecular aggregation, which results in self-thickening in their neat form. However, the viscosity significantly decreases upon dilution with a solvent. This self-thickening property suggests their potential use as thickening agents in organic solvent formulations.