The Rhodium catalyzed maleinisation of monounsaturated fatty acids leads to cyclic 3,6-disubstituted-1,2,3,6-tetrahydro-phthalic acid derivatives and acyclic maleinated byproducts, which are similar to the products formed by Ene-reaction. Two different catalysts, RhCl3(H2O)3 and Rh(OAc)2, were investigated. It was found that Rh(OAc)2 shows a higher selectivity toward the cyclic products as well as higher conversions compared to RhCl3(H2O)3. Furthermore it was observed that the double bond configuration of the fatty acid influences the yields and the product composition of the maleinated products. The Rh(OAc)2-catalysis leads mainly to the cyclic THPAs. If the cis-configurated oleic acid is used, one cyclic main product is formed, whereas the trans-configurated elaidic acid leads to three different isomers. Furthermore a decrease of the overall yield of the maleinated products was observed for elaidic acid, compared to oleic acid. In contrast to that, the RhCl3(H2O)3-catalyst tends to result favorably in the formation of the acyclic products. If oleic acid is used, the formation of two different constitutional syn-isomers was observed. In addition to that the maleinisation of elaidic acid gives also the anti-form, so that four different products could be detected by NMR-experiments.
Practical applications: Fatty acid based 3,6-disubstituted-1,2,3,6-tetrahydro-phthalic acid (THPA) derivatives are promising monomers for polycondensation processes. The substructure of the six-membered carbon ring is of great interest for the hardness of the resulting polyesters. The mentioned THPA derivatives are commonly synthesized via Diels-Alder reaction of conjugated fatty acids and maleic anhydride. The main disadvantage of this synthesis route is caused by monounsaturation of many natural fatty acids, e.g., oleic acid, or by the non-conjugation of the most natural fatty acids, e.g., linoleic acid which requires a further step for conjugation. The Rh-catalyzed maleinisation enables the synthesis of such cyclic structures using monounsaturated fatty acids, e.g., oleic acid. The resulting THPAs are considered to be biobased substitutes with good prospects, e.g., for phthalic acid anhydride, trimellitic acid and their di-, tetra-, or hexahydro-derivatives, which are commonly used in polycondensation processes. These fatty acid based THPAs may open the door towards new advanced biobased products, e.g., for lacquers and coatings.
The Rh-catalyzed addition of maleic anhydride to oleic and elaidic acid was investigated. Two different catalysts were used. RhCl3(H2O)3 leads preferred to the acyclic products, whereas Rh(OAc)2 favors the formation of the cyclic products. The influence of the double bond configuration of the fatty acid component on the product composition is also discussed.