Iron‐Catalyzed Olefin Metathesis with Low‐Valent Iron Alkylidenes

Abstract

Inspired by recent reports of low-valent iron-complex-catalyzed formal [2+2] cycloaddition of olefins, we demonstrate computationally that with such low-valent iron complexes and with "strong" ligands, the olefin metathesis is also preferred over the undesired cyclopropanation side-reaction, competition already studied by Hoffmann and co-workers almost 40 years ago (J. Am. Chem. Soc. 1981, 103, 5582). The [2+2] cycloaddition step in metathesis propagation, which gives a Chauvin-type metallacyclobutane intermediate, is proposed to proceed either via a planar four-electron Craig-Möbius aromatic [π2 +π2 ] transition-state structure with a low barrier of 4.7 kcal mol or, alternatively, via a twisted Zimmerman-Möbius aromatic [π2 +π2 ] transition state with a 5.5 kcal mol activation-energy barrier, with respect to an "encounter" π-complex minimum obtained from an Fe alkylidene and the entering olefin, while the corresponding triplet pathways are all disfavored.

Olefin Metathesis by First‐Row Transition Metals

“…[64] However, some theoretical studies showed the possibility of low valent Fe-catalyzed OM. [73,75] Finally, apply rigid chelating ligands to control the geometry of the alkylidene and metallacycle to favor singlet states for both. [70,73,76] Scheme 16.…”

“…Third, use Fe(IV) complexes because, according to Hückel calculations, cyclopropanation is forbidden for d 4 metal centers [64] . However, some theoretical studies showed the possibility of low valent Fe‐catalyzed OM [73,75] . Finally, apply rigid chelating ligands to control the geometry of the alkylidene and metallacycle to favor singlet states for both [70,73,76] …”

NNNO‐Heteroscorpionate nickel (II) and cobalt (II) complexes for ethylene oligomerization: the unprecedented formation of odd carbon number olefins

“…A mechanism explaining these experimental facts suggests three simultaneous processes: oligomerization, isomerization and metathesis catalyzed by iron bisiminopyridine complex. Later several theoretical and computational studies [62,63] established that iron catalysts have a high potential in the reaction of olefin metathesis.…”

Cyclopropanations via Heme Carbenes: Basic Mechanism and Effects of Carbene Substituent, Protein Axial Ligand, and Porphyrin Substitution

“…47, 51 On the other hand, investigations of cyclopropanations with formally ferric iron-porphyrins suggest in situ reduction of the metalloporphyrin and thus the active role of Fe II -porphyrin carbene in these reactions. 29, 32, 36 Computationally, several different mechanisms have been proposed for metal carbene cyclopropanations, including elimination from metallocyclobutane 60–62 and addition via both concerted 63–65 and step-wise 66, 67 pathways. Since both the metal center and ligand environment greatly influence the operating mechanism, it is critical to study the mechanism in newly developed systems such as the hemoprotein and Fe II -porphyrin based catalysts investigated here.…”