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 -piperylene with ethylene and α-olefins and (ii) the 1,4-hydrovinylation of isoprene with ethylene. In the former case, kinetic analysis implicates the diamagnetic η-piperylene complex as the resting state prior to rate-determining oxidative cyclization. Variable temperature H NMR and EXSY experiments established that diene exchange from the diamagnetic, 18e complexes occurs rapidly in solution at ambient temperature through a dissociative mechanism. The solid-state structure of ((Et)PDI)Fe(η-piperylene) ((Et)PDI = 2,6-(2,6-Me-CHN═CEt)CHN), was determined by single-crystal X-ray diffraction and confirmed the s- 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.