Iron Catalysts for the Head-to-Head Dimerization of α-Olefins and Mechanistic Implications for the Production of Linear α-Olefins

A series of pyridine bis-imine iron catalysts previously reported for the oligomerization of ethylene and propylene are now reported as active catalysts for the dimerization of α-olefins to make linear internal olefins. The dimers possess up to 80% linearity, depending on the catalyst structure and the reaction conditions. The dimer byproducts consist almost exclusively of methyl-branched species and only traces of vinylidene (2-alkylalkene), trisubstituted, or α-olefin products. The linear dimers are formed by a unique mechanism that precludes the formation of multiple dimer products. The dimer content is verified by hydrogenation, which produces only a linear paraffin and a methyl-branched paraffin upon GC analysis. The dimer composition is also verified by 1H NMR, which shows >95% disubstituted internal olefin and less than 5% total vinylidene and trisubstituted olefin. Observations regarding the dimer structures are used to elucidate the structures of the branched α-olefin (BAO) byproducts that are made when ethylene is oligomerized by the same catalysts.