Pyridine(diimine) Iron Diene Complexes Relevant to Catalytic [2+2]-Cycloaddition Reactions

The synthesis, characterization, and catalytic activity of pyridine(diimine) iron piperylene and isoprene complexes are described. These diene complexes are competent precatalysts for (i) the selective cross-[2+2]-cycloaddition of butadiene or (E)-piperylene with ethylene and α-olefins and (ii) the 1,4-hydrovinylation of isoprene with ethylene. In the former case, kinetic analysis implicated the diamagnetic η⁴-piperylene complex as the resting state prior to rate-determining oxidative cyclization. Variable temperature ¹H NMR and EXSY experiments established that diene exchange from the diamagnetic, 18e^– complexes occurred rapidly in solution at ambient temperature through a dissociative mechanism. The solid-state structure of (^Me(Et)PDI) Fe(η⁴-piperylene) (^Me(Et)PDI=2,6-(2,6-Me₂-C₆H₃N=CEt)₂C₅H₃N) was determined by single-crystal X-ray diffraction and confirmed the s-trans coordination of the monosubstituted 1,3-diene. Possible relationships between ligand-controlled diene coordination geometry, metallacycle denticity, and chemoselectivity of iron-mediated cycloaddition reactions are discussed. (Figure presented.).