Using computational methods to explore improvements to Knölker's iron catalyst

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

(Cyclopentadienone)iron carbonyl compounds are catalytically active in carbonyl/imine reductions, alcohol oxidations, and borrowing hydrogen reactions, but the effect of cyclopentadienone electronics on their activity is not well established. A series of (tetraarylcyclopentadienone)iron tricarbonyl compounds with varied electron densities on the cyclopentadienone were prepared, and their activities in transfer hydrogenations and dehydrogenations were explored. Additionally, mechanistic studies, including kinetic isotope effect experiments and modifications to substrate electronics, were undertaken to gain insights into catalyst resting states and turnover-limiting steps of these reactions. As the cyclopentadienone electron density increased, both the transfer hydrogenation and dehydrogenation rates increased. A catalytically relevant, trimethylamine-ligated iron compound was isolated and characterized and was observed in solution under both transfer hydrogenation and dehydrogenation conditions. Importantly, it was catalytically active in both reactions. Kinetic isotope effect data and initial rates in transfer hydrogenation reactions with 4'-substituted acetophenones provided evidence that hydrogen transfer from the catalyst to the carbonyl substrate occurred during the turnover-limiting step, and NMR spectroscopy supports the trimethylamine adduct as an off-cycle resting state and the (hydroxycyclopentadienyl)iron hydride as an on-cycle resting state. In transfer dehydrogenations of alcohols, the use of electronically modified benzylic alcohols provided evidence that the turnover-limiting step involves the transfer of hydrogen from the alcohol substrate to the catalyst. The trimethylamine-ligated compound was proposed as the primary catalyst resting state in dehydrogenations.

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

(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.

Organic synthesis with the most abundant transition metal–iron: from rust to multitasking catalysts

The promising aspects of iron in synthetic chemistry are being explored for three-four decades as a green and eco-friendly alternative to late transition metals. This present review unveils these rich iron-chemistry towards different transformations.

(Cyclopentadienone)iron-Catalyzed Transfer Dehydrogenation of Symmetrical and Unsymmetrical Diols to Lactones

Air-stable iron carbonyl compounds bearing cyclopentadienone ligands with varying substitution were explored as catalysts in dehydrogenative diol lactonization reactions using acetone as both the solvent and hydrogen acceptor. Two catalysts with trimethylsilyl groups in the 2- and 5-positions, [2,5-(SiMe₃)₂-3,4-(CH₂)₄(η⁴-C₄C═O)]Fe(CO)₃ (1) and [2,5-(SiMe₃)₂-3,4-(CH₂)₃(η⁴-C₄C═O)]Fe(CO)₃ (2), were found to be the most active, with 2 being the most selective in the lactonization of diols containing both primary and secondary alcohols. Lactones containing five-, six-, and seven-membered rings were successfully synthesized, and no over-oxidations to carboxylic acids were detected. The lactonization of unsymmetrical diols containing two primary alcohols occurred with catalyst 1, but selectivity was low based on alcohol electronics and modest based on alcohol sterics. Evidence for a transfer dehydrogenation mechanism was found, and insight into the origin of selectivity in the lactonization of 1°/2° diols was obtained. Additionally, spectroscopic evidence for a trimethylamine-ligated iron species formed in solution during the reaction was discovered.

Homogeneously catalyzed hydrogenation and dehydrogenation reactions – From a mechanistic point of view

Homogeneously catalyzed hydrogenation/dehydrogenation reactions represent not only one of the most synthetically important chemical transformations, but also a promising way to renewably utilize the hydrogen energy. In order to rationally design efficient homogeneous catalysts for hydrogenations/dehydrogenations, it is of fundamental importance to understand their reaction mechanisms in detail. With this aim in mind, we herein provide a brief overview of the mechanistic understanding and related catalyst design strategies. Hydrogenations and dehydrogenations represent the reverse process of each other, and involve the activation/release of H2 and the insertion/elimination of hydride as major steps. The mechanisms discussed in this chapter include the cooperation (bifunctional) mechanism and the non-cooperation mechanisms. Non-cooperation mechanisms usually involve single-site transition metal (TM) catalysts or transition metal hydride (TM-H) catalysts. Cooperation mechanisms usually operate in the state-of-the-art bifunctional catalysts, including Lewis-base/transition-metal (LB-TM) catalysts, Lewis-acid/transition-metal (LA-TM) catalysts, Lewis-acid/Lewis-base (LA-LB; the so-called frustrated Lewis pairs - FLPs) catalysts, newly developed ambiphilic catalysts, and bimetallic transition-metal/transition-metal (TM-TM) catalysts. The influence of the ligands, the electronic structure of the metal, and proton shuttle on the reaction mechanism are also discussed to improve the understanding of the factors that can govern mechanistic preferences. The content presented in this chapter should both inspire experimental and theoretical chemists concerned with homogeneously catalyzed hydrogenation and dehydrogenation reactions, and provide valuable information for future catalyst design.

A mechanistic study and computational prediction of iron, cobalt and manganese cyclopentadienone complexes for hydrogenation of carbon dioxide

A series of cobalt and manganese cyclopentadienone complexes are proposed and examined computationally as promising catalysts for hydrogenation of CO₂ to formic acid with total free energies as low as 20.0 kcal mol^-1 in aqueous solution. Density functional theory study of the newly designed cobalt and manganese complexes and experimentally reported iron cyclopentadienone complexes reveals a stepwise hydride transfer mechanism with a water or a methanol molecule assisted proton transfer for the cleavage of H₂ as the rate-determining step.

Synthesis and applications to catalysis of novel cyclopentadienone iron tricarbonyl complexes

New approaches to enantiomerically-pure cyclopentadienone iron complexes, and their applications to ketone reductions reactions, are described.

DFT investigation of the ring contraction reaction of (η4-1,2-disilacyclohexadiene)iron tricarbonyls: a crucial intramolecular Si–Si bond activation

The ring contraction reaction mechanism of (η⁴-1,2-disilacyclohexadiene)iron tricarbonyls: crucial intramolecular Si–Si bond activation.

Iron cyclopentadienone complexes derived from C2-symmetric bis-propargylic alcohols; preparation and applications to catalysis

The following complexes were prepared through cyclisation of a bis-propargylic alcohol and were tested as redox catalysts for hydrogen transfer reactions of alcohols and ketones.