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

Abstract

Mechanistic studies are reported on the inter- and intramolecular [2 + 2] alkene cycloadditions to form cyclobutanes promoted by (tricPDI)­Fe­(N2) (tricPDI = 2,6-(2,4,6-tricyclopentyl)­C6H2N = CMe)2C5H3N). A combination of kinetic measurements, freeze-quench 57Fe Mössbauer 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 N2 pressure. Both 57Fe Mössbauer and infrared spectroscopic measurements identified (tricPDI)­Fe­(N2)­(η2-1-octene) as the catalyst resting state. Rate-determining association of 1-octene to (tricPDI)­Fe­(η2-1-octene) accounts for the first order dependence of alkene and the inverse dependence on N2. Heavy atom 13C/12C 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 N2 pressure were measured. A pyridine­(diimine) iron trans-bimetallacycle was identified as the catalyst resting state and was isolated and characterized by X-ray diffraction and 1H NMR and 57Fe Mössbauer spectroscopies. Dissolution of the iron trans-bimetallacycle in benzene-d 6 produced predominantly the cis-cyclobutane product, establishing interconversion between the trans and cis metallacycles during the catalytic reaction and consistent with a Curtin-Hammett kinetic regime. A primary 13C/12C 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.

Catalyst Design Principles Enabling Intermolecular Alkene-Diene [2+2] Cycloaddition and Depolymerization Reactions

“…18 Instead, a paramagnetic iron metallocycle was implicated where the olefin was dissociated from the metal, giving rise to an Fe(III) dialkyl-type intermediate analogous to the S = 1 complexes observed and crystallographically characterized in alkene−alkene [2 + 2] cycloaddition. 20,27,28 This phenomenon is likely a result of the decreased coordination affinity of the methyl-substituted propenyl olefin of the metallocycle to iron, leading to increased dissociation of the propenyl group. With ruthenium, the resulting Ru(II) metallocycle is low-spin and diamagnetic due to the stronger coordination of internal olefins to the metal compared to iron.…”

“…As discussed in the previous section, this dissociation likely results in a spin state change from S = 0 to S = 1 where the triplet intermediate is best described as intermediate spin iron(III) (S Fe = 3/2) engaged in antiferromagnetic coupling to a chelate radical anion (Scheme 6B). 20,27,28 The ferric oxidation state is particularly significant, as it has been previously implicated in iron dialkyl complexes to promote C−C bond formation. 50 Spin crossover during reductive elimination from an 18electron, S = 0 metallocycle has been explored computationally by Chen and co-workers, 29 although this study did not explicitly address the role of alkene dissociation on a potential change in spin surface that may ultimately lead to cyclobutane formation.…”

Alkene oligomerization via metallacycles: Recent advances and mechanistic insights

“…In 2020, Chirik and co-workers carried out a comparative study between [2+2] cycloadditions of different unactivated alkene: 1-octene and 1,7-octadiene. [101] 1-octene is reported to be dimerized to trans-substituted-cyclobutane by the iron complex whereas the diene formed cis-cyclobutane. These results are supported by kinetic and DFT studies and analysis of the different resting states.…”

Ligand-Metal Cooperation Enables C–C Activation Cross-Coupling Reactivity of Cyclopropyl Ketones

“…[7][8][9][10][11] On the other hand, while it is known that bi-and tridentate N-donor ligands promote cross-coupling reactions with alkyl halides to form C(sp 2 )-C(sp 3 ) and C(sp 3 )-C(sp 3 ) bonds, 24-28 these ligands have not been reported for C-C bond activation reactions. Building upon studies showing redox-active ligands can enable new reactivity at metal centers, [29][30][31][32] our solution to this long-standing challenge relies upon forcing a well-known cross-coupling catalyst, nickel terpyridine ((tpy)Ni), into a reactive, reduced state, that turns on C-C activation activity while retaining cross-coupling activity. This approach relies on metal-ligand cooperativity between a nickel metal center and a multidentate π-acceptor ligand capable of storing significant spin-density.…”