(506f) Bio-Terephthalic Acid Synthesis from Cross Metathesis of Unsaturated Carboxylic Acids and Consecutive One-Pot Cycloaddition and Aromatization Reactions

Terephthalic acid in its pure form (PTA) is essential for the preparation of a variety of polyester fibers and is largely produced via the catalytic oxidation of petroleum-derived p-xylene [1]. The ongoing exchange of naphtha crackers with shale gas crackers is causing a shortage in some aromatic species. As a consequence, alternative and sustainable routes to produce terephthalic acid from biomass derivatives, are highly desirable [2]. PTA precursors can be prepared via Diels-Alder cycloaddition with ethylene from bio-derived chemicals, like furnaic compounds or carboxylic dienes. These synthetic pathways are, however, limited by the low yields of either the starting chemicals or the final product [3-5]. In the last decade, Ru-catalyzed olefin metathesis, and in particular cross-metathesis, has become an important tool for the formation of carbon–carbon bonds and the formation of valuable organic molecules [6]. Di-ester muconates, in particular, can be synthesized from cross-metathesis of bio-sourced olefins. This route has never been considered before for the synthesis of bio-sourced terephthalates.

Results and discussion

We have investigated the ruthenium-catalyzed cross metathesis of sorbates with acrylates to form di-ester muconates. The effect of alkyl substituents and solvents was investigated over various organometallic ruthenium-based catalysts to optimize the conversion and selectivity towards the muconates. Conversions of up to 60% and 40% yields of the muconates were achieved within the first hour of reaction with low amounts (3 mol.%) of ruthenium-based catalyst containing a N-heterocyclic carbene (NHC) ligand. With the aim to develop a greener process, the acrylate reactant was used in excess with no other solvent added. In a later step, a one pot synthesis of diester terephthalates from the as-prepared di-ester muconates was achieved. First, the Diels-Alder cycloaddition between muconate esters and ethylene forms a cyclohexene dicarboxylic compound in >95% yield, and a consecutive aromatization on the same reaction batch in the presence of Pd/C catalyst affords the formation of terephthalic acid diesters (~ 40% yield). Upon further development, this route can provide a sustainable alternative for the industrial production of biomass derived polyethylene terephthalate (PET).

References

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