A family of single-component iron precatalysts for the [4+4]-cyclodimerization and intermolecular cross-[4+4]-cycloaddition of monosubstituted 1,3-dienes is described. Cyclooctadiene products were obtained with high regioselectivity, and catalyst-controlled access to either cisor transdiastereomers was achieved using 4-substituted diene substrates. Reactions conducted either with single-component precatalysts or with iron dihalide complexes activated in situ proved compatible with common organic functional groups and were applied on multi-gram scale (up to >100 g). Catalytically relevant, S = 1 iron complexes bearing 2-iminopyridine ligands, (R PI)FeL 2 (R PI = [2-(2,6-R 2-C 6 H 3-N=CMe)-C 5 H 4 N] where R = i PR or Me, L 2 = bis-olefin) were characterized by single-crystal X-ray diffraction, mößbauer spectroscopy, magnetic measurements, and dft calculations. The structural and spectroscopic parameters are consistent with an electronic structure description comprised of a high spin iron(I) center (S fe = 3/2) engaged in antiferromagnetically coupling with a ligand radical anion (S PI = −1/2). Mechanistic studies conducted with these single-component precatalysts-including kinetic analyses, 12 C/ 13 C isotope effect measurements, and in situ mößbauer spectroscopy-support a mechanism involving oxidative cyclization of two dienes that determines regio-and diastereoselectivity. Topographic steric maps derived from crystallographic data provided insights into the basis for the catalystcontrol through stereoselective oxidative cyclization and subsequent, stereospecific allylisomerization and CC bond-forming reductive elimination.