Investigations into the Mechanism of Inter- and Intramolecular Iron-Catalyzed [2+2] Cycloaddition of Alkenes

Mechanistic studies are reported on the inter-and intramolecular [2 + 2] alkene cycloadditions to form cyclobutanes promoted by ((PDI)-P-tric)Fe(N-2) (tricPDI = 2,6-(2,4,6-tricyclopentyl)-C6H2N = CMe)(2)C5H3N). A combination of kinetic measurements, freeze-quench Fe-57 Mossbauer and infrared spectroscopic measurements, deuterium labeling studies, natural abundance 13C KIE studies, and isolation and characterization of catalytically relevant intermediates were used to gain insight into the mechanism of both inter-and intramolecular [2 + 2] cycloaddition reactions. For the stereo-and regioselective [2 + 2] cycloaddition of 1-octene to form trans-1,2-dihexylcyclobutane, a first-order dependence on both iron complex and alkene was measured as well as an inverse dependence on N-2 pressure. Both Fe-57 Mossbauer and infrared spectroscopic measurements identified ((PDI)-P-tric)Fe(N-2)(eta(2)-1-octene) as the catalyst resting state. Rate-determining association of 1-octene to ((PDI)-P-tric)Fe(eta(2)-1-octene) accounts for the first order dependence of alkene and the inverse dependence on N2. Heavy atom C-13/C-12 kinetic isotope effects near unity also support post rate-determining C-C bond formation. By contrast, the intramolecular iron-catalyzed [2 + 2] cycloaddition of 1,7-octadiene yielded cis-bicyclo[4.2.0]octane in 92:8 d.r. and a first order dependence on the iron precursor and zeroth order behavior in both diene and N-2 pressure were measured. A pyridine(diimine) iron transbimetallacycle was identified as the catalyst resting state and was isolated and characterized by X-ray diffraction and H-1 NMR and Fe-57 Mossbauer spectroscopies. Dissolution of the iron trans-bimetallacycle in benzene-d(6) produced predominantly the ciscyclobutane product, establishing interconversion between the trans and cis metallacycles during the catalytic reaction and consistent with a Curtin-Hammett kinetic regime. A primary C-13/C-12 kinetic isotope effect of 1.022(4) was measured at 23 degrees C, consistent with irreversible unimolecular reductive elimination to form the cyclobutane product. Despite complications from competing cyclometalation of chelate aryl substituents, deuterium labeling experiments were consistent with unimolecular C-C reductive elimination that occurred either by a concerted pathway or a radical rebound sequence that is faster than C-C bond rotation.