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 alpha -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 alpha -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 H-1 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 alpha -olefin (BAO) byproducts that are made when ethylene is oligomerized by the same catalysts.