Synthesis of Dimer Fatty Acid Methyl Esters by Catalytic Oxidation and Reductive Amination: An Efficient Route to Branched Polyamides

A novel and versatile route toward dimer fatty acid methyl esters (dimer FAMEs) via catalytic oxidation and reductive amination is described. The oxyfunctionalization of mono‐unsaturated FAMEs bearing different chain lengths (C11, C18, C22) is accomplished by a co‐catalyst‐free Wacker Oxidation process in a high pressure reactor. The applied catalytic system of palladium(II) chloride in a dimethylacetamide/water mixture enabled the formation of keto‐FAMEs in the presence of molecular oxygen as sole re‐oxidant. In a first attempt, partially renewable dimer FAMEs are synthesized by reductive amination of keto‐FAME (C18) in the presence of various aliphatic and aromatic diamines and sodium triacetoxyborohydride as selective reducing agent. In another approach, the keto‐FAMEs directly underwent reductive amination using Raney‐Nickel in order to obtain the corresponding amino‐FAMEs. Subsequently, the keto‐ and amino‐FAMEs are used for the synthesis of fully renewable dimer FAMEs via reductive amination with sodium triacetoxyborohydride as reducing agent. In order to demonstrate a possible application for these new dimer FAMEs, three out of the thirteen synthesized dimer FAMEs are selected and studied in a polycondensation with renewable 1,10‐diaminodecane using TBD as catalyst. The polyamides are obtained in molecular weights (M_n) of up to 33 kDa and are carefully characterized by ¹H‐NMR spectroscopy, FTIR, SEC, and DSC analysis.

Practical Applications: The described catalytic route to defined dimer fatty acids has potential applications in lubricants, detergents, polymeric materials, and others. Compared to commercially available dimer fatty acids, a molecularly highly defined mixture of regioisomers is obtained, without contamination of monofunctional or trifunctional fatty acid derivatives, which is advantegous for most of the mentioned applications. Especially for polycondensations, the known stoichiometry of exactly two is a considerable benefit.

The synthesis of keto‐fatty acid methyl esters via a straightforward co‐catalyst‐free Wacker oxidation process allows access to partially and fully renewable dimer fatty acid methyl esters, by using the obtained keto‐fatty acid methyl esters as renewable raw materials in a reductive amination process. Some of the obtained dimer fatty acids are exemplarily studied as monomers for polyamide synthesis.