Bioderived Muconates by Cross‐Metathesis and Their Conversion into Terephthalates

• Published in: ChemSusChem Vol. 11; no. 4; pp. 773 - 780
• Main Authors: Saraçi, Erisa, Wang, Lan, Theopold, Klaus H., Lobo, Raul F.
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 22-02-2018; Wiley Blackwell (John Wiley & Sons)
• Subjects: Acids; Acrylates; aromatization; bioderived chemicals; Catalysis; Catalysts; Chemical industry; Chemical synthesis; Cycloaddition; Cyclohexene; Decomposition reactions; Dehydrogenation; Economic conditions; Esterification; Fermentation; Genetic modification; Metathesis; Microorganisms; NMR spectroscopy; Polyethylene terephthalate; Solvents; Sorbates; terephthalates; Terephthalic acid; Tetrahydrofuran; bioderived chemicals; aromatization; cycloaddition; metathesis; terephthalates
• ISSN: 1864-5631; 1864-564X
• DOI: 10.1002/cssc.201701874

Polyethylene terephthalate that is 100 % bioderived is in high demand in the market guided by the ever‐more exigent sustainability regulations with the challenge of producing renewable terephthalic acid remaining. Renewable terephthalic acid or its precursors can be obtained by Diels–Alder cycloaddition and further dehydrogenation of biomass‐derived muconic acid. The cis,cis isomer of the dicarboxylic acid is typically synthesized by fermentation with genetically modified microorganisms, a process that requires complex separations to obtain a high yield of the pure product. Furthermore, the cis isomer has to be transformed into the trans,trans form and has to be esterified before it is suitable for terephthalate synthesis. To overcome these challenges, we investigated the synthesis of dialkyl muconates by cross‐metathesis. The Ru‐catalyzed cross‐coupling of sorbates with acrylates, which can be bioderived, proceeded selectively to yield diester muconates in up to 41 % yield by using very low catalyst amounts (0.5–3.0 mol %) and no solvent. In the optimized procedure, the muconate precipitated as a solid and was easily recovered from the reaction medium. Analysis by GC–MS and NMR spectroscopy showed that this method delivered exclusively the trans,trans isomer of dimethyl muconate. The Diels–Alder reaction of dimethyl muconate with ethylene was studied in various solvents to obtain 1,4‐bis(carbomethoxy)cyclohexene. The cycloaddition proceeded with very high conversions (77–100 %) and yields (70–98 %) in all of the solvents investigated, and methanol and tetrahydrofuran were the best choices. Next, the aromatization of 1,4‐bis(carbomethoxy)cyclohexene to dimethyl terephthalate over a Pd/C catalyst resulted in up to 70 % yield in tetrahydrofuran under an air atmosphere. Owing to the high yield of the reaction of dimethyl muconate to 1,4‐bis(carbomethoxy)cyclohexene, no separation step was needed before the aromatization. This is the first time that cross‐metathesis is used to produce bioderived trans,trans‐muconates as precursors to renewable terephthalates, important building blocks in the polymer industry.
Cross‐road to green terephthalate rings metathesized: Cross‐metathesis is used for the first time to produce renewable polyethylene terephthalate precursors. Moderate amounts of a Ru catalyst and no solvents give the pure trans isomers of the muconates, which can then be readily converted into terephthalates by combining consecutive Diels–Alder and aromatization reactions in the same solvent with very high efficiency.