Cross Metathesis of Unsaturated Carboxylic Acids to Bioterephthalic Acid Intermediates

Terephthalic acid 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 aromatics species, such as p-xylene [2]. As a consequence, alternative and sustainable routes to produce terephthalic acid from biomass derivatives, e.g. bio-sourced sorbates [3], are highly desirable. In the last decade, Ru-catalyzed olefin metathesis, and in particularly cross-metathesis, has become an important tool for the formation of carbon–carbon bonds and the formation of valuable organic molecules [4].

We have investigated the ruthenium-catalyzed cross metathesis of sorbates with acrylates to form di-ester muconates. In a later step, the reaction between alkyl muconates and ethylene affords the aromatization to dialkyl terephthalate. The effect of alkyl substituents and solvents was investigated over various organometallic ruthenium-based catalysts to increase conversion and selectivity towards the alkyl muconates. Conversions of up to 20% were achieved within the first hour of reaction with very low amounts (0.1 mol.%) of ruthenium-based catalyst containing a N-heterocyclic carbene (NHC) ligand. If a solvent was used, the cross metathesis products were formed in higher yields with the use of more polar solvents, i.e. CH₂Cl₂. With the aim to develop a greener process, the acrylate reactant was used in excess with no other solvent added. In this case the highest selectivity to the desired product was observed (~ 40%). By using different alkyl substitutents the steric effect of the substitutents was investigated. Using reactants (sorbates and acrylates) with smaller alkyl substitutents the yield of cross metathesis products was favored, possibly due to easier accessibility of the reactants to the catalyst metal site. Upon further development, this route for obtaining di-ester muconates, may provide a sustainable alternative for producing biomass derived precursors to polyethylene terephthalate (PET).

References

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[2] C. Luke Williams, Chun-Chih Chang, Phuong Do, Nima Nikbin, Stavros Caratzoulas, Dionisios G. Vlachos, Raul F. Lobo, Wei Fan, and Paul J. Dauenhauer. ACS Catal. 2 (2012) 935

[3] Sandrine Bérard, Christophe Vallée, and Damien Delcroix. Ind. Eng. Chem. Res. 54 (2015) 7164

[4] Georgios C. Vougioukalakis, and Robert H. Grubbs. Chem. Rev. 110 (2010) 1746