Highly efficient synthesis of sustainable bisphenols from hydroxycinnamic acids

Abstract

Three stilbene based bisphenols: (E)-4,4′-(ethene-1,2-diyl)diphenol (1), (E)-4,4′-(ethene-1,2-diyl)bis(2-methoxyphenol) (2), and (E)-4,4′-(ethene-1,2-diyl)bis(2,6-dimethoxyphenol) (3) were synthesized from p-coumaric, ferulic, and sinapic acid, respectively. This was accomplished with a high-yielding two-step approach, which combined catalyst-free thermal decarboxylation of the cinnamic acids followed by olefin metathesis. The thermal decarboxylation proceeded in quantitative yield merely by refluxing the cinnamic acids in DMF under a nitrogen atmosphere. The resulting vinyl phenols were briefly isolated and then immediately subjected to olefin metathesis with a Grubbs–Hoveyda 2nd generation catalyst, which resulted in direct precipitation of the bisphenols from the reaction mixture. Overall yields of 1, 2, and 3 were 92, 88, and 85%, respectively, starting from the cinnamic acids. The compounds were isolated as pure solids without the need for crystallization or column chromatography. Bisphenol 1 was isomerized to the cis-isomer by irradiation with UV light. The resulting 80 : 20 cis : trans mixture exhibited a glass transition at low temperatures and a decreased melting temperature. To further expand the suite of bisphenols that can be produced from cinnamic acids, p-coumaric acid was directly converted into a 4-vinylphenol dimer by thermal decarboxylation/dimerization in a biphasic aqueous base/toluene solution to yield (E)-4,4′-(but-1-ene-1,3-diyl)diphenol (4). The ability to efficiently produce bisphenols from abundant, low-cost, and easily sourced bio-based substrates holds great promise for the development of more sustainable polymers.