(Cyclopentadienone)iron Complexes: Synthesis, Mechanism and Applications in Organic Synthesis

Abstract

(Cyclopentadienone)iron tricarbonyl complexes are catalytically active, inexpensive, easily accessible and air‐stable that are extensively studied as an active pre‐catalyst in homogeneous catalysis. Its versatile catalytic activity arises exclusively due to the presence of a non‐innocent ligand, which can trigger its unique redox properties effectively. These complexes have been employed widely in (transfer)hydrogenation (e. g., reduction of polar multiple bonds, Oppenauer‐type oxidation of alcohols), C−C and C−N bond formation (e. g., reductive aminations, α‐alkylation of ketones) and other synthetic transformations. In this review, we discuss the remarkable advancement of its various synthetic applications along with synthesis and mechanistic studies, until February 2021.

Cyclopentadienone Iron Complex-Catalyzed Hydrogenation of Ketones: The Influence of the Charge-Tag on Catalytic Performance

“…The full reaction is shown in Scheme . Scheme shows the simplified, generally accepted reaction mechanism for the hydrogenation of a ketone with a cyclopentadienone iron complex as a catalyst and has been reported in several review articles. − In the present work, the catalytic cycle is entered either by removal of a CO ligand from the tricarbonyl precatalyst by Me 3 NO or by thermally cleaving the Fe–N bond in acetonitrile dicarbonyl precatalysts. The 16 valence electron dicarbonyl complex (oxidized form), which corresponds to the resting state in the catalytic cycle, is able to split molecular hydrogen into a hydride on iron and a proton on oxygen (reduced form).…”

Mechanistic Studies on Iron-Catalyzed Dehydrogenation of Amines Involving Cyclopentadienone Iron Complexes─Evidence for Stepwise Hydride and Proton Transfer

“…40 Recently, we have disclosed the iron-catalyzed biomimetic oxidation of alcohols, amines, and N-heterocycles to the corresponding ketones (aldehydes), imines, and saturated heterocycles using (cyclopentadienone)iron tricarbonyl complexes. 29,30,39 Despite the promise of iron complexes II as pre-catalysts for dehydrogenation reactions, 41 important details concerning their catalytic mechanisms have remained unclear. It is known that in situ activation of complex II (18e) results in the formation of the active intermediate II′ (16e complex), which, in the presence of a hydrogen source, can be reduced to iron hydride VI (Scheme 2).…”

“…Despite the promise of iron complexes II as pre-catalysts for dehydrogenation reactions, important details concerning their catalytic mechanisms have remained unclear. It is known that in situ activation of complex II (18e) results in the formation of the active intermediate II′ (16e complex), which, in the presence of a hydrogen source, can be reduced to iron hydride VI (Scheme ).…”

Substituent Effects and Mechanistic Insights on the Catalytic Activities of (Tetraarylcyclopentadienone)iron Carbonyl Compounds in Transfer Hydrogenations and Dehydrogenations

“…During the past decade, a variety of oxidative and reductive catalytic processes using (cyclopentadienone)iron carbonyl compounds have been developed, including reductions of carbonyls and imines, oxidations of alcohols, and borrowing hydrogen reactions. − Additionally, it has become clear that cyclopentadienone substitution affects the catalyst activity. For example, changing the substitution on the ring fused to the cyclopentadienone affects its activity in alcohol oxidations, − aldehyde and ketone reductions, ,− imine reductions/reductive aminations, − and borrowing hydrogen processes. − Unfortunately, there have been few studies systematically exploring the underlying effects of these modifications in large part because structural modifications to the cyclopentadienone often affect both its steric and electronic properties, making it difficult to develop structure–activity relationships.…”