Iron-Catalyzed Intermolecular [2π + 2π] Cycloaddition

The bis(imino)pyridine iron dinitrogen compounds, (^iPrPDI)Fe(N₂)₂ and [(^MePDI)Fe(N₂)]₂(μ₂-N₂) (^RPDI = 2,6-(2,6-R₂-C₆H₃NCMe)₂C₅H₃N; R = ⁱPr, Me), promote the catalytic intermolecular [2π + 2π] cycloaddition of ethylene and butadiene to form vinylcyclobutane. Stoichiometric experiments resulted in isolation of a catalytically competent iron metallocycle intermediate, which was shown to undergo diene-induced C–C reductive elimination. Deuterium labeling experiments establish competitive cyclometalation of the bis(imino)pyridine aryl substituents during catalytic turnover.