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)Fe(N) (PDI = 2,6-(2,4,6-tricyclopentyl)CHN = CMe)CHN). A combination of kinetic measurements, freeze-quench Fe Mössbauer and infrared spectroscopic measurements, deuterium labeling studies, natural abundance C 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 -1,2-dihexylcyclobutane, a first-order dependence on both iron complex and alkene was measured as well as an inverse dependence on N pressure. Both Fe Mössbauer and infrared spectroscopic measurements identified (PDI)Fe(N)(η-1-octene) as the catalyst resting state. Rate-determining association of 1-octene to (PDI)Fe(η-1-octene) accounts for the first order dependence of alkene and the inverse dependence on N. Heavy atom C/C 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 -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 pressure were measured. A pyridine(diimine) iron -bimetallacycle was identified as the catalyst resting state and was isolated and characterized by X-ray diffraction and H NMR and Fe Mössbauer spectroscopies. Dissolution of the iron -bimetallacycle in benzene- produced predominantly the -cyclobutane product, establishing interconversion between the and metallacycles during the catalytic reaction and consistent with a Curtin-Hammett kinetic regime. A primary C/C kinetic isotope effect of 1.022(4) was measured at 23 °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.