Recent developments in enantioselective iron-catalyzed transformations

Iron-catalyzed 1,3-dipolar cycloaddition of alkynes and aryl nitrones for the synthesis of chiral Δ4-isoxazolines

The synthesis of chiral Δ4-isoxazolines is a crucial area of research due to their biological activity and versatility in synthetic chemistry. Driven by the abundant reserves, low cost and high biocompatibility of iron catalysts, we herein investigated the catalytic potential of iron complexes in asymmetric 1,3-dipolar cycloaddition reactions between N-Bn,α-aryl nitrones and 3-propioloyloxazolidin-2-ones. By utilizing Fe(OTf)2 and (S,S)-DBFOX-Ph, a series of (R)-Δ4-isoxazolines were obtained in high yields (up to 99%) with good to excellent enantioselectivities (up to 98 : 2 er). Additionally, using Fe(OTf)3 and (±)-BINOL as the catalyst, good to excellent yields (up to 99%) and diastereoselectivities (up to >20 : 1 dr) were achieved for the (S,S)-Δ4-isoxazoline derivatives from 3-propioloyloxazolidin-2-ones bearing a chiral Evans auxiliary. Gram-scale synthesis and synthetic utility were also demonstrated.

Iron-Catalyzed Reductive Ring-Rearrangement Reaction of Bridged Benzo[b]oxocin-4-ones with Grignard Reagents to Afford Bridged Benzo[b]oxocin-2-ols

An iron-catalyzed reductive ring-rearrangement reaction of bridged benzo[b]oxocin-4-ones with Grignard reagents to produce bridged benzo[b]oxocin-2-ols is reported. Mechanistic studies indicate that an iron redox catalysis cycle involving oxidative addition to the C-O bond by low-valence iron and β-methoxyl elimination as key steps operates in this reaction.

Modelling an Fe-III High-Valent Pincer-type Transition Metal Complex for Dehydrogenation of Ammonia-Borane

Development of efficient and cost-effective catalysts for the dehydrogenation of Ammonia-Borane (AB) has been a challenge which affects the advancement of the hydrogen economy. Over the last decades, pincer-type transition metal complexes have been known to show promising results in catalyzing many chemical reactions ranging from CO2 reduction to C-H bond activation. In this work we investigate the ability of a high-valent Ni-III-Cl complex (complex 1) for the dehydrogenating AB. Our results show that complex 1 can dehydrogenate two equiv. of AB under reaction conditions slightly higher than room temperature. Although the abstraction of H2 from AB can occur at room temperature, higher temperature is required due to relatively higher free-energy barriers for the formation of molecular H2. However, when the Ni-III center is substituted by a Fe-III center (complex 2), AB dehydrogenation can occur at room temperature for one equiv. of AB with a free-energetic span of 21.07 kcal/mol, but this does not remain the same for the second catalytic cycle for complex 2 and the free-energy energetic span increases to 36.1 kcal/mol. Therefore, for the initial cycle of AB dehydrogenation, the Fe-III complex has better functionality and this work exhibits the impact of metal mono-substitution, specifically Fe in activating AB dehydrogenation at room temperature and further paves the way for simple modelling of transition metal-based complexes as catalysts for such reactions.

Hydrogenation of CO2 to formic acid over a non-precious metal-based polymeric catalyst

Catalytic hydrogenation of CO2 using non-precious metal based heterogeneous catalysts presents a significant challenge and promising opportunity for exploration. We report catalytic hydrogenation of CO2 using an iron-loaded (<1%) nitrogen rich polymer. This catalyst showed excellent catalytic activity and produced 32.5 mmol of formic acid with a high turnover number of 8419.

Green Synthesis of Biorelevant Scaffolds through Organocatalytic/ Enzymatic Dynamic Kinetic Resolution

This review highlights major developments in the application of green organocatalytic and enzymatic dynamic kinetic resolutions (DKRs) in the total synthesis of biorelevant scaffolds. It illustrates the diversity of useful bioactive products and intermediates that can be synthesized under greener and more economic conditions through the combination of the powerful concept of DKR, which allows the resolution of racemic compounds with up to 100% yield, with either asymmetric organocatalysis or enzymatic catalysis, avoiding the use of toxic and expensive metals. With the need for more ecologic synthetic technologies, this field will undoubtedly expand its scope in the future with the employment of other organocatalysts/enzymes to even more types of transformations, thus allowing powerful greener and more economic strategies to reach other biologically important molecules.

β3-Tryptophans by Iron-Catalyzed Enantioselective Amination of 3-Indolepropionic Acids

A straightforward and general strategy for the catalytic asymmetric synthesis of β3-tryptophans by carboxylic-acid-directed intermolecular C-H amination has been developed. The iron-catalyzed C-H amination of 3-indolepropionic acids with BocNHOMs (Boc, tert-butyloxycarbonyl; OMs, methylsulfonate) in the presence of the base piperidine provides N-Boc-protected β3-tryptophans in a single step with high enantiomeric excess (ee) of up to >99%. Mechanistic experiments and density functional theory calculations support a mechanism through carboxylate-directed iron-mediated C(sp3)-H nitrene insertion. The method incorporates two key sustainability criteria: the use of iron as an abundant, non-toxic, and environmentally benign metal, along with the achievement of streamlined enantioselective C-H functionalization.

Stereogenic-at-iron mesoionic carbene complex for enantioselective C-H amidation

Electronically tuned C 2-symmetric stereogenic-at-iron complexes, featuring strongly σ-donating 1,2,3-triazolin-5-ylidene mesoionic carbene (MIC) ligands, exhibit enhanced catalytic efficiency compared to conventional imidazol-2-ylidene analogs, as demonstrated in nitrene-mediated ring-closing C(sp3)-H amidation reactions. Furthermore, a chiral pinene-derived pyridyl triazole ligand enables a highly diastereoselective synthesis of a non-racemic chiral iron catalyst, thereby controlling the absolute configuration at the metal center, as confirmed by NMR and X-ray crystallography. This pinene-modified stereogenic-at-iron MIC complex demonstrates high catalytic activity and a respectable asymmetric induction in the ring-closing C(sp3)-H amination of N-benzoyloxyurea, yielding 2-imidazolidinones with enantiomeric ratios of up to 92 : 8. These findings reflect the profound potential of this new class of mesoionic carbene iron complexes in further understanding and tuning the reactivity of iron-based catalysts.

Heterogeneous Iron-Based Catalysts for Organic Transformation Reactions: A Brief Overview

Iron (Fe) is considered to be one of the most significant elements due to its wide applications. Recent years have witnessed a burgeoning interest in Fe catalysis as a sustainable and cost-effective alternative to noble metal catalysis in organic synthesis. The abundance and low toxicity of Fe, coupled with its competitive reactivity and selectivity, underscore its appeal for sustainable synthesis. A lot of catalytic reactions have been performed using heterogeneous catalysts of Fe oxide hybridized with support systems like aluminosilicates, clays, carbonized materials, metal oxides or polymeric matrices. This review provides a comprehensive overview of the latest advancements in Fe-catalyzed organic transformation reactions. Highlighted areas include cross-coupling reactions, C-H activation, asymmetric catalysis, and cascade processes, showcasing the versatility of Fe across a spectrum of synthetic methodologies. Emphasis is placed on mechanistic insights, elucidating the underlying principles governing iron-catalyzed reactions. Challenges and opportunities in the field are discussed, providing a roadmap for future research endeavors. Overall, this review illuminates the transformative potential of Fe catalysis in driving innovation and sustainability in organic chemistry, with implications for drug discovery, materials science, and beyond.

Iron-catalyzed stereoselective C-H alkylation for simultaneous construction of C-N axial and C-central chirality

The assembly of chiral molecules with multiple stereogenic elements is challenging, and, despite of indisputable advances, largely limited to toxic, cost-intensive and precious metal catalysts. In sharp contrast, we herein disclose a versatile C-H alkylation using a non-toxic, low-cost iron catalyst for the synthesis of substituted indoles with two chiral elements. The key for achieving excellent diastereo- and enantioselectivity was substitution on a chiral N-heterocyclic carbene ligand providing steric hindrance and extra represented by noncovalent interaction for the concomitant generation of C-N axial chirality and C-stereogenic center. Experimental and computational mechanistic studies have unraveled the origin of the catalytic efficacy and stereoselectivity.

Enantioselective radical cascade cyclization via Ti-catalyzed redox relay

Radical cascade cyclization reactions provide an efficient method for the construction of polycyclic architectures with multiple stereogenic centers. However, achieving enantioselectivity control of this type of reaction is a challenging task. Here, we report an enantioselective cyclization of polyfunctional aryl cyclopropyl ketone and alkyne units, wherein the stereochemical outcome is directed by a chiral Ti(salen) catalyst. This transformation was proposed to proceed via a radical cascade process involving the reductive ring-opening of the cyclopropyl ketone followed by two annulation events entailing cyclization of the ensuing alkyl radical onto the alkyne and subsequent addition of the incipient vinyl radical to the Ti(IV)-enolate.

Trading Symmetry for Stereoinduction in Tetradentate, non-C2 -Symmetric Fe(II)-Complexes for Asymmetric Catalysis

A series of stereogenic-at-metal iron complexes comprising a non-C₂ -symmetric chiral topology is introduced and applied to asymmetric 3d-transition metal catalysis. The chiral iron(II) complexes are built from chiral tetradentate N4-ligands containing a proline-derived amino pyrrolidinyl backbone which controls the relative (cis-α coordination) and absolute metal-centered configuration (Λ vs. Δ). Two chloride ligands complement the octahedral coordination sphere. The modular composition of the tetradentate ligands facilitates the straightforward incorporation of different terminal coordinating heteroaromatic groups into the scaffold. The influence of various combinations was evaluated in an asymmetric ring contraction of isoxazoles to 2H-azirines revealing that a decrease of symmetry is beneficial for the stereoinduction to obtain chiral products in up to 99 % yield and with up to 92 % ee. Conveniently, iron catalysis is feasible under open flask conditions with the bench-stable dichloro complexes exhibiting high robustness towards oxidative or hydrolytic decomposition. The versatility of non-racemic 2H-azirines was subsequently showcased with the conversion into a variety of quaternary α-amino acid derivatives.

Ferrocenophanium Stability and Catalysis

Ferrocenium catalysis is a vibrant research area, and an increasing number of ferrocenium-catalyzed processes have been reported in the recent years. However, the ferrocenium cation is not very stable in solution, which may potentially hamper catalytic applications. In an effort to stabilize ferrocenium-type architectures by inserting a bridge between the cyclopentadienyl rings, we investigated two ferrocenophanium (or ansa-ferrocenium) cations with respect to their stability and catalytic activity in propargylic substitution reactions. One of the ferrocenophanium complexes was characterized by single crystal X-ray diffraction. Cyclic voltammetry experiments of the ferrocenophane parent compounds were performed in the absence and presence of alcohol nucleophiles, and the stability of the cations in solution was judged based on the reversibility of the electron transfer. The experiments revealed a moderate stabilizing effect of the bridge, albeit the effect is not very pronounced or straightforward. Catalytic propargylic substitution test reactions revealed decreased activity of the ferrocenophanium cations compared to the ferrocenium cation. It appears that the somewhat stabilized ferrocenophanium cations show decreased catalytic activity.

Improving the Configurational Stability of Chiral-at-Iron Catalysts Containing Two N-(2-Pyridyl)-Substituted N-Heterocyclic Carbene Ligands

Recently, we introduced the first example of chiral-at-iron catalysts in which two achiral N-(2-pyridyl)-substituted N-heterocyclic carbene (NHC) ligands in addition to two labile acetonitriles are coordinated around a central iron, to generate a stereogenic metal center [Hong Y.Chiral-at-Iron Catalyst: Expanding the Chemical Space for Asymmetric Earth-Abundant Metal Catalysis. J. Am. Chem. Soc.2019, 141, 4569-4572]. A more facile synthesis of such chiral-at-iron catalysts was developed, which omits the use of expensive silver salts and an elaborate electrochemical setup. Configurational robustness was improved by replacing the imidazol-2-ylidene carbene moieties with benzimidazol-2-ylidenes. The π-acceptor properties of the altered NHCs were investigated by Ganter's ⁷⁷Se NMR method. The obtained benzimidazol-2-ylidene chiral-at-iron complex is an excellent catalyst for an asymmetric hetero-Diels-Alder reaction under open-flask conditions.

Fe-BPsalan Complex-Catalyzed Asymmetric [4 + 2] Cycloaddition of Cyclopentadiene with α,β-Unsaturated Heterocycles

An efficient iron-catalyzed asymmetric [4 + 2] cycloaddition of cyclopentadiene with α,β-unsaturated acyl imidazoles or 2-cinnamoylisoindoline-1,3-dione derivatives was developed to afford the addition products in high yield and selectivity. Interestingly, the absolute structures of the addition products were controlled by the auxiliaries via different coordination modes with the same type of catalyst.

Recent developments in enantioselective nickel(ii)-catalyzed conjugate additions

This review updates the field of enantioselective nickel-catalyzed conjugate additions since 2016.

Exploiting the Chiral Ligands of Bis(imidazolinyl)- and Bis(oxazolinyl)thiophenes-Synthesis and Application in Cu-Catalyzed Friedel-Crafts Asymmetric Alkylation

Five new C2-symmetric chiral ligands of 2,5-bis(imidazolinyl)thiophene (L1-L3) and 2,5-bis(oxazolinyl)thiophene (L4 and L5) were synthesized from thiophene-2,5-dicarboxylic acid (1) with enantiopure amino alcohols (4a-c) in excellent optical purity and chemical yield. The utility of these new chiral ligands for Friedel-Crafts asymmetric alkylation was explored. Subsequently, the optimized tridentate ligand L5 and Cu(OTf)2 catalyst (15 mol%) in toluene for 48 h promoted Friedel-Crafts asymmetric alkylation in moderate to good yields (up to 76%) and with good enantioselectivity (up to 81% ee). The bis(oxazolinyl)thiophene ligands were more potent than bis(imidazolinyl)thiophene analogues for the asymmetric induction of the Friedel-Crafts asymmetric alkylation.

Chiral-at-Iron Catalyst for Highly Enantioselective and Diastereoselective Hetero-Diels-Alder Reaction

This study demonstrates that chiral-at-iron complexes, in which all coordinated ligands are achiral and the overall chirality the consequence of a stereogenic iron center, are capable of catalyzing asymmetric transformations with very high enantioselectivities. The catalyst is based on a previously reported design (J. Am. Chem. Soc. 2017, 139, 4322), in which iron(II) is surrounded by two configurationally inert achiral bidentate N-(2-pyridyl)-substituted N-heterocyclic carbenes in a C2 -symmetric fashion and complemented by two labile acetonitriles. By replacing mesityl with more bulky 2,6-diisopropylphenyl substituents at the NHC ligands, the steric hindrance at the catalytic site was increased, thereby providing a markedly improved asymmetric induction. The new chiral-at-iron catalyst was applied to the inverse electron demand hetero-Diels-Alder reaction between β,γ-unsaturated α-ketoester and enol ethers provide 3,4-dihydro-2H-pyrans in high yields with excellent diastereoselectivities (up to 99 : 1 dr) and excellent enantioselectivities (up to 98 % ee). Other electron rich dienophiles are also suitable as demonstrated for a reaction with a vinyl azide.

Iron-Catalyzed Regiodivergent Hydrostannation of Alkynes: Intermediacy of Fe(IV)-H versus Fe(II)-Vinylidene

We report an iron system, Cp*Fe(1,2-R₂PC₆H₄X), which controls the Markovnikov and anti-Markovnikov hydrostannation of alkynes by tuning the ionic metal-heteroatom bonds (Fe-X) reactivity. The sequential addition of nBu₃SnH to the iron-amido catalyst (1, X = HN^-, R = Ph) affords a distannyl Fe(IV)-H species responsible for syn-addition of the Sn-H bond across the C≡C bond to produce branched α-vinylstannanes. Activation of the C(sp)-H bond of alkynes by an iron-aryloxide catalyst (2, X = O^-, R = Cy) affords an iron(II) vinylidene intermediate, allowing for gem-addition of the Sn-H to the terminal-carbon producing β-vinylstannanes. These catalytic reactions exhibit excellent regioselectivity and broad functional group compatibility and enable the large-scale synthesis of diverse vinylstannanes. Many new reactions have been established based on such a synthetic Fe-X platform to demonstrate that the initial step of the catalysis is conveniently controlled by the activation of either the tin hydride or the alkyne substrate.

Tackling N-Alkyl Imines with 3d Metal Catalysis: Highly Enantioselective Iron-Catalyzed Synthesis of α-Chiral Amines

A readily activated iron alkyl precatalyst effectively catalyzes the highly enantioselective hydroboration of N-alkyl imines. Employing a chiral bis(oxazolinylmethylidene)isoindoline pincer ligand, the asymmetric reduction of various acyclic N-alkyl imines provided the corresponding α-chiral amines in excellent yields and with up to >99 % ee. The applicability of this base metal catalytic system was further demonstrated with the synthesis of the pharmaceuticals Fendiline and Tecalcet.

Recent Advances in Asymmetric Iron Catalysis

Asymmetric transition-metal catalysis represents a fascinating challenge in the field of organic chemistry research. Since seminal advances in the late 60s, which were finally recognized by the Nobel Prize to Noyori, Sharpless and Knowles in 2001, the scientific community explored several approaches to emulate nature in producing chiral organic molecules. In a scenario that has been for a long time dominated by the use of late-transition metals (TM) catalysts, the use of 3d-TMs and particularly iron has found, recently, a widespread application. Indeed, the low toxicity and the earth-abundancy of iron, along with its chemical versatility, allowed for the development of unprecedented and more sustainable catalytic transformations. While several competent reviews tried to provide a complete picture of the astounding advances achieved in this area, within this review we aimed to survey the latest achievements and new concepts brought in the field of enantioselective iron-catalyzed transformations.

Intra- and intermolecular Fe-catalyzed dicarbofunctionalization of vinyl cyclopropanes

Design and implementation of the first (asymmetric) Fe-catalyzed intra- and intermolecular difunctionalization of vinyl cyclopropanes (VCPs) with alkyl halides and aryl Grignard reagents has been realized via a mechanistically driven approach. Mechanistic studies support the diffusion of alkyl radical intermediates out of the solvent cage to participate in an intra- or intermolecular radical cascade with a range of VCPs followed by re-entering the Fe radical cross-coupling cycle to undergo (stereo)selective C(sp2)-C(sp3) bond formation. This work provides a proof-of-concept of the use of vinyl cyclopropanes as synthetically useful 1,5-synthons in Fe-catalyzed conjunctive cross-couplings with alkyl halides and aryl/vinyl Grignard reagents. Overall, we provide new design principles for Fe-mediated radical processes and underscore the potential of using combined computations and experiments to accelerate the development of challenging transformations.

Recent Advances in First-Row Transition Metal/Chiral Phosphoric Acid Combined Catalysis

Since the pioneering independent reports of Akiyama and Terada, the use of chiral phosphoric acids (CPAs) and derivatives as a versatile tool for asymmetric synthesis with good reactivity, regioselectivity, diastereoselectivity and enantioselectivity has emerged, forming an important part of the implementation of asymmetric counteranion-directed catalysis reported to date. In these achievements, the combination of metals with CPAs has enabled various catalytic modes beyond the scope of typical acid catalysis, such as relay catalysis, ion-pairing catalysis, and binary acid catalysis. The first-row transition metals (Sc-Zn) are considered to be sustainable transition metals and have received a great deal of attention. These naturally abundant metals display excellent Lewis acidity and function as powerful redox catalysts in synthesis involving both one and two-electron transfers. Hence, in this chapter, we summarize recent advances in the development of asymmetric reactions using a combination of first-row transition metals and CPAs. Furthermore, we provide a detailed discussion of the mechanisms involved in order to understand the interaction of the metal/phosphate and the origins of the asymmetric control of the transformations.