Selective [1,4]-Hydrovinylation of 1,3-Dienes with Unactivated Olefins Enabled by Iron Diimine Catalysts

The
selective, intermolecular [1,4]-hydrovinylation of conjugated dienes
with unactivated α-olefins catalyzed by α-diimine iron
complexes is described. Value-added “skipped” diene
products were obtained with exclusive [1,4]-selectivity, and the formation
of branched, (Z)-olefin products was observed with
no evidence for alkene isomerization. Mechanistic studies conducted
with the well-defined, single-component iron precatalyst (MesDI)­Fe­(COD) (MesDI = [2,4,6-Me3-C6H2-NCMe]2); COD = 1,5-cyclooctadiene)
provided insights into the origin of the high selectivity. An iron
diene complex was identified as the catalyst resting state, and one
such isoprene complex, (iPrDI)­Fe­(η4-C5H8), was isolated and characterized. A combination
of single crystal X-ray diffraction, Mößbauer spectroscopy,
magnetic measurements, and DFT calculations established that the complex
is best described as a high-spin Fe­(I) center (SFe = 3/2) engaged in antiferromagnetic coupling to an α-diimine
radical anion (SDI = −1/2), giving
rise to the observed S = 1 ground state. Deuterium-labeling
experiments and kinetic analyses of the catalytic reaction provided
support for a pathway involving oxidative cyclization of an alkene
with the diene complex to generate an iron metallacycle. The observed
selectivity can be understood in terms of competing steric interactions
in the transition states for oxidative cyclization and subsequent
β-hydrogen elimination.