Molecular-level insight into the multiple mechanistic pathways in iron-catalysed alkene dimerisation.

As the least expensive, least toxic and most abundant of the first-row transition metals, iron catalysis underpins the future of sustainable synthesis. Yet the mechanistic understanding remains limited, particularly for pathways involving low oxidation-state intermediates. The reductive dimerisation of alkenes is a prime example, with very few iron-catalysed examples reported and with no in-depth mechanistic analyses. As simple, 1,2-disubstituted alkenes, methyl crotonates can be selectively dimerised to 2-ethylidene-3-methylpentanedioates with two stereogenic units. This rare non-arene example of a C(sp2)-H functionalisation offers a platform for molecular-level understanding of broad scope iron-catalysed C-H functionalisation. In-depth mechanistic studies of this dimerisation, including the speciation of [(dmpe)2FeH2] through a combination of X-ray diffraction, kinetic analysis and in situ NMR monitoring, has uncovered hidden pathways that show this "simple" dimerisation is in fact a mechanistically complex system.