CS‑Symmetric Pyridine(diimine) Iron Methyl Complexes for Catalytic [2+2] Cycloaddition and Hydrovinylation: Metallacycle Geometry Determines Selectivity

A series of CS-symmetric (aryl,alkyl)-substituted pyridine(dimine) iron methyl (CyARPDI)FeCH3 complexes have been prepared, characterized, and evaluated as precatalysts for the [2+2]-cycloaddition of butadiene and ethylene. Mixtures of vinylcyclobutane and (Z)-hexa-1,4-diene were observed in each case. By comparison, C2v-symmetric, arylated (PDI) iron catalysts are exclusively selective for reversible [2+2]-cycloaddition to yield vinylcyclobutane. The alteration in the chemoselectivity of the catalytic reaction was investigated through a combination of precatalyst stability studies, identification of catalytic resting state(s), and 2H and 13C isotopic labeling experiments. While replacement of an aryl-imine substituent with an N-alkyl group decreases the stability of the formally iron(0) dinitrogen and butadiene complexes, two diamagnetic metallacycles were identified as catalyst resting states. Deuterium labeling and NOESY/EXSY NMR experiments support 1,4-hexadiene arising from catalytic hydrovinylation involving reversible oxidative cyclization leading to accessible cis-metallacycle. Cyclobutane formation proceeds by irreversible C(sp3)–C(sp3) bond-forming reductive elimination from a trans-metallacycle. These studies provide key mechanistic understanding into the high selectivity of bis(arylated) pyridine(diimine) iron catalysts for [2+2]-cycloaddition, unique, thus far, to this class of iron catalysts.