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 alpha-olefins catalyzed by alpha-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 ((DI)-D-Mes)Fe(COD) ((DI)-D-Mes = [2,4,6-Me-3-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, ((DI)-D-iPr)Fe(eta(4)-C5H8), was isolated and characterized. A combination of single crystal X-ray diffraction, Mossbauer spectroscopy, magnetic measurements, and DFT calculations established that the complex is best described as a high-spin Fe(I) center (S-Fe = 3/2) engaged in antiferromagnetic coupling to an alpha-diimine radical anion (S-DI = 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 beta-hydrogen elimination.