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A direct route to adipic acid esters

By Gerald Ondrey |

Adipate diesters are building blocks of polyamides and polyesters, and are used in plasticizers, perfumes, lubricants, solvents, various pharmaceutical active ingredients and, in terms of quantity, mainly for the production of nylon. Currently, adipate diesters are produced industrially by oxidizing a mixture of cyclohexanol and cyclohexanone with an excess of nitric acid, followed by esterification with the corresponding alcohols. This process requires special equipment due to the corrosive effect of the acid. In addition, stoichiometric amounts of the powerful greenhouse gas nitrous oxide are released.

Now, a “greener,” simpler route to adipic acid is possible thanks to a new catalyst developed by a team of German scientists, led by professor Matthias Beller, director of the Leibniz Institute for Catalysis (Rostock; www.catalysis.de) and Robert Franke, head of hydroformylation research at Evonik Performance Materials GmbH (Essen Germany; www.evonik.com) and associate professor of chemistry at the Ruhr University Bochum (www.ruhr-uni-bochum.de). The scientists have succeeded in producing adipates (salts of adipic acid) by the direct dicarbonylation of 1,3-butadiene. Normally, this reaction forms many byproducts caused by secondary reactions, including telomerization, hydroalkoxylation and (co)polymerization.

The scientists have developed a palladium catalyst with a specially designed pyridyl-functionalized bisphosphine ligand (HeMaRaphos) that enables the highly selective and efficient double alkoxy carbonylation of 1,3-butadiene to adipic acid esters in one step. The key to success was the ligand design. The combination of a bidentate phosphine ligand with a basic pyridyl substituent on the phosphorus and a palladium precursor provides dialkyl adipates in the dicarbonylation of 1,3-butadiene in higher than 95% yield and with selectivity of 97% or more, as reported in a recent issue of Science.

The catalyst system can also be transferred to other dienes, paving the way for a revolutionary synthesis process for producing many fine chemicals, says Evonik.

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