Total synthesis of insect sex pheromones: recent improvements based on iron-mediated cross-coupling chemistry

In the current ecological context, use of insect sex pheromones as an alternative to conventional pesticides is in constant growth. In this report, we discuss the recent contributions brought by our groups in the field of iron-catalyzed cross-couplings applied to the synthesis of insect pheromones. The pivotal question of the development of sustainable synthetic procedures involving cheap, non-toxic and efficient additives is also discussed, as well as the mechanistic features guiding the reactivity of such catalytic systems.

Mechanistic Facets of the Competition between Cross-Coupling and Homocoupling in Supporting Ligand-Free Iron-Mediated Aryl–Aryl Bond Formations

In the context of cross-coupling chemistry, the competition between the cross-coupling path itself and the oxidative homocoupling of the nucleophile is a classic issue. In that case, the electrophilic partner acts as a sacrificial oxidant. We investigate in this report the factors governing the cross- versus homocoupling distribution using aryl nucleophiles ArMgBr and (hetero)aryl electrophiles Ar′Cl in the presence of an iron catalyst. When electron-deficient electrophiles are used, a key transient heteroleptic [Ar₂Ar′Fe^II]⁻ complex is formed. DFT calculations show that an asynchronous two-electron reductive elimination follows, which governs the selective evolution of the system toward either a cross- or homocoupling product. Proficiency of the cross-coupling reductive elimination strongly depends on both π-accepting and σ-donating effects of the Fe^II-ligated Ar′ ring. The reactivity trends discussed in this article rely on two-electron elementary steps, which are in contrast with the usually described tendencies in iron-mediated oxidative homocouplings which involve single-electron transfers. The results are probed by paramagnetic ¹H NMR spectroscopy, experimental kinetics data, and DFT calculations.

Synthesis and structure of a new bulky bis-(alkoxide) ligand on a terphenyl platform

A new sterically bulky chelating bis-(alkoxide) ligand 3,3'-([1,1':4',1''-terphen-yl]-2,2''-di-yl)bis-(2,2,4,4-tetra-methyl-pentan-3-ol), (H2[OO]tBu), was prepared in a two-step process as the di-chloro-methane monosolvate, C36H50O2·CH2Cl2. The first step is a Suzuki-Miyaura coupling reaction between 2-bromo-phenyl-boronic acid and 1,4-di-iodo-benzene. The resulting 2,2''-di-bromo-1,1':4',1''-terphenyl was reacted with t BuLi and hexa-methyl-acetone to obtain the desired product. The crystal structure of H2[OO]tBu revealed an anti conformation of the [CPh2(OH)] fragments relative to the central phenyl. Furthermore, the hydroxyl groups point away from each other. Likely because of this anti-anti conformation, the attempts to synthesize first-row transition-metal complexes with H2[OO]tBu were not successful.

Additive and Counterion Effects in Iron-Catalyzed Reactions Relevant to C-C Bond Formation

The use of iron catalysts in carbon-carbon bond forming reactions is of interest as an alternative to precious metal catalysts, offering reduced cost, lower toxicity, and different reactivity. While well-defined ligands such as N-heterocyclic carbenes (NHCs) and phosphines can be highly effective in these reactions, additional additives such as N-methylpyrrolidone (NMP), N,N,N',N'-tetramethylethylenediamine (TMEDA), and iron salts that alter speciation can also be employed to achieve high product yields. However, in contrast to well-defined iron ligands, the roles of these additives are often ambiguous, and molecular-level insights into how they achieve effective catalysis are not well-defined. Using a unique physical-inorganic in situ spectroscopic approach, detailed insights into the effect of additives on iron speciation, mechanism, and catalysis can inform further reaction development. In this Perspective, recent advances will be discussed as well as ongoing challenges and potential opportunities in iron-catalyzed reactions.

Well-Defined Aryl-FeII Complexes in Cross-Coupling and C–H Activation Processes

Herein we explore the intrinsic organometallic reactivity of iron embedded in a tetradentate N₃C macrocyclic ligand scaffold that allows the stabilization of aryl-Fe species, which are key intermediates in Fe-catalyzed cross-coupling and C–H functionalization processes. This study covers C–H activation reactions using ^MeL_H and FeCl₂, biaryl C–C coupling product formation through reaction with Grignard reagents, and cross-coupling reactions using ^MeL_Br or ^HL_Br in combination with Fe⁰(CO)₅. Synthesis under light irradiation and moderate heating (50 °C) affords the aryl-Fe^II complexes [Fe^II(Br)(^MeL)(CO)] (1^Me) and [Fe^II(^HL)(CO)₂]Br (1^H). Exhaustive spectroscopic characterization of these rare low-spin diamagnetic species, including their crystal structures, allowed the investigation of their intrinsic reactivity.

Relevance of Single-Transmetalated Resting States in Iron-Mediated Cross-Couplings: Unexpected Role of σ-Donating Additives

Control of the transmetalation degree of organoiron(II) species is a critical parameter in numerous Fe-catalyzed cross-couplings to ensure the success of the process. In this report, we however demonstrate that the selective formation of a monotransmetalated Fe^II species during the catalytic regime counterintuitively does not alone ensure an efficient suppression of the nucleophile homocoupling side reaction. It is conversely shown that a fine control of the transmetalation degree of the transient Fe^III intermediates obtained after the activation of alkyl electrophiles by a single-electron transfer (SET), achievable using σ-donating additives, accounts for the selectivity of the cross-coupling pathway. This report shows for the first time that both coordination spheres of Fe^II resting states and Fe^III short-lived intermediates must be efficiently tuned during the catalytic regime to ensure high coupling selectivities.

Oxidative Addition of Aryl and Alkyl Halides to a Reduced Iron Pincer Complex

The two-electron oxidative addition of aryl and alkyl halides to a reduced iron dinitrogen complex with a strong-field tridentate pincer ligand has been demonstrated. Addition of iodobenzene or bromobenzene to (3,5-Me2MesCNC)Fe(N2)2 (3,5-Me2MesCNC = 2,6-(2,4,6-Me-C6H2-imidazol-2-ylidene)2-3,5-Me2-pyridine) resulted in rapid oxidative addition and formation of the diamagnetic, octahedral Fe(II) products (3,5-Me2MesCNC)Fe(Ph)(N2)(X), where X = I or Br. Competition experiments established the relative rate of oxidative addition of aryl halides as I > Br > Cl. A linear free energy of relative reaction rates of electronically differentiated aryl bromides (ρ = 1.5) was consistent with a concerted-type pathway. The oxidative addition of alkyl halides such as methyl-, isobutyl-, or neopentyl halides was also rapid at room temperature, but substrates with more accessible β-hydrogen positions (e.g., 1-bromobutane) underwent subsequent β-hydride elimination. Cyclization of an alkyl halide containing a radical clock and epimerization of neohexyl iodide-d2 upon oxidative addition to (3,5-Me2MesCNC)Fe(N2)2 are consistent with radical intermediates during C(sp3)-X bond cleavage. Importantly, while C(sp2)-X and C(sp3)-X oxidative addition produces net two-electron chemistry, the preferred pathway for obtaining the products is concerted and stepwise, respectively.

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.

An Iron Pincer Complex in Four Oxidation States

The synthesis and characterization of a series of homoleptic iron complexes [Fe(^benzNHCOCO)₂]^2-/1-/0/1+ supported by the tridentate bis-aryloxide benzimidazolin-2-ylidene pincer ligand ^benzNHCOCO^2- (II) is presented. While the reaction of 2 equiv of free ligand II with a ferrous iron precursor leads to the isolation of the coordination polymer [Fe(^benzNHCOCOK)₂]_n (1), treatment of II with ferric iron salts allows for the synthesis and isolation of the mononuclear, octahedral bis-pincer compound K[Fe(^benzNHCOCO)₂] (2) and its crown-ether derivative [K(18c6)(THF)₂][Fe(^benzNHCOCO)₂] (3). Electrochemical studies of 2 suggested stable products upon further one- and two-electron oxidation. Hence, treatment of 2 with 1 equiv of AgPF₆ yields the charge-neutral species [Fe(^benzNHCOCO)₂] (4). Similarly, the cationic complex [Fe(^benzNHCOCO)₂]PF₆ (5) is obtained by addition of 2 equiv of AgPF₆. The characterization of complexes 1, 3, and 4 reveals iron-centered reduction and oxidation processes; thus, preserving the dianionic, closed-shell structure of both coordinated ^benzNHCOCO pincer chelates, II. This implies a stabilization of a highly Lewis acidic iron(IV) center by four phenolate anions rather than charge distribution across the ligand framework with a lower formal oxidation state at iron. Notably, the overall charge-neutral iron(IV) complex undergoes reductive elimination of the pincer ligand, providing a metal-free compound that can be described as a spirocyclic imidazolone ketal (6). In contrast, the ligand-metal bonds in 5, formally an iron(V) complex, are considerably covalent, rendering the assignment of its oxidation state challenging, if not impossible. All compounds are fully characterized, and the complexes' electronic structures were studied with a variety of spectroscopic and computational methods, including single-crystal X-ray diffraction (SC-XRD), X-band electron paramagnetic resonance (EPR), and zero-field ⁵⁷Fe Mössbauer spectroscopy, variable-field and variable-temperature superconducting quantum interference device (SQUID) magnetization measurements, and multi-reference ab initio (NEVPT2/CASSCF) as well as density functional theory (DFT) studies. Taken altogether, the electronic structure of 5 is best described as an iron(IV) center antiferromagnetically coupled to a ligand-centered radical.

Iron N-heterocyclic carbene complexes in homogeneous catalysis

This review article summarizes recent development of homogeneous iron N-heterocyclic carbene catalysts.

Task-Specific Properties and Prospects of Ionic Liquids in Cross-Coupling Reactions

Ionic liquids (ILs) are considered as highly useful materials for potential diverse uses such as greener and more convenient alternatives to volatile organic solvents, reagents, additives, ligands and co-solvents. Thermal stability, negligible vapor pressure and high polarity with ionic environments have possibly conferred some unique physico-chemical properties and a wider electrochemical window on ILs. More importantly, these properties are tuneable, depending on variations in alkyl chains and counter-anions. On the other hand, various transition-metal-catalyzed cross-coupling reactions constitute an important backbone of contemporary organic synthesis. A vast number of C-C and C-heteroatom cross-coupling reactions are reported in the presence of ILs, often showing better performance. The influence of IL on the action of a given catalyst or on the course of a reaction can be relatively complex, and is not understood well enough to be able to draw succinct conclusions. However, there are a few reports in the literature that help understand the role of actual and active catalytic species stabilized in an IL environment. Stabilization, which can be either helpful or detrimental to catalysis depends on specific circumstances. This review article is aimed primarily at summarizing the various applications of ILs during the past decade, focusing as far as possible on the task-specific properties of ILs in transition-metal-catalyzed C-C and C-heteroatom cross-coupling reactions. Several successful achievements and noteworthy progress in this field of research leads to the sensible conclusion that future prospects in this field of research are not only bright but promise new horizons.

Development and Evolution of Mechanistic Understanding in Iron-Catalyzed Cross-Coupling

Since the pioneering work of Kochi in the 1970s, iron has attracted great interest for cross-coupling catalysis due to its low cost and toxicity as well as its potential for novel reactivity compared to analogous reactions with precious metals like palladium. Today there are numerous iron-based cross-coupling methodologies available, including challenging alkyl-alkyl and enantioselective methods. Furthermore, cross-couplings with simple ferric salts and additives like NMP and TMEDA ( N-methylpyrrolidone and tetramethylethylenediamine) continue to attract interest in pharmaceutical applications. Despite the tremendous advances in iron cross-coupling methodologies, in situ formed and reactive iron species and the underlying mechanisms of catalysis remain poorly understood in many cases, inhibiting mechanism-driven methodology development in this field. This lack of mechanism-driven development has been due, in part, to the challenges of applying traditional characterization methods such as nuclear magnetic resonance (NMR) spectroscopy to iron chemistry due to the multitude of paramagnetic species that can form in situ. The application of a broad array of inorganic spectroscopic methods (e.g., electron paramagnetic resonance, ⁵⁷Fe Mössbauer, and magnetic circular dichroism) removes this barrier and has revolutionized our ability to evaluate iron speciation. In conjunction with inorganic syntheses of unstable organoiron intermediates and combined inorganic spectroscopy/gas chromatography studies to evaluate in situ iron reactivity, this approach has dramatically evolved our understanding of in situ iron speciation, reactivity, and mechanisms in iron-catalyzed cross-coupling over the past 5 years. This Account focuses on the key advances made in obtaining mechanistic insight in iron-catalyzed carbon-carbon cross-couplings using simple ferric salts, iron-bisphosphines, and iron- N-heterocyclic carbenes (NHCs). Our studies of ferric salt catalysis have resulted in the isolation of an unprecedented iron-methyl cluster, allowing us to identify a novel reaction pathway and solve a decades-old mystery in iron chemistry. NMP has also been identified as a key to accessing more stable intermediates in reactions containing nucleophiles with and without β-hydrogens. In iron-bisphosphine chemistry, we have identified several series of transmetalated iron(II)-bisphosphine complexes containing mesityl, phenyl, and alkynyl nucleophile-derived ligands, where mesityl systems were found to be unreliable analogues to phenyls. Finally, in iron-NHC cross-coupling, unique chelation effects were observed in cases where nucleophile-derived ligands contained coordinating functional groups. As with the bisphosphine case, high-spin iron(II) complexes were shown to be reactive and selective in cross-coupling. Overall, these studies have demonstrated key aspects of iron cross-coupling and the utility of detailed speciation and mechanistic studies for the rational improvement and development of iron cross-coupling methods.

Ring-expanded N-heterocyclic carbenes as ligands in iron-catalysed cross-coupling reactions of arylmagnesium reagents and aryl chlorides

The structure-activity relationship of expanded-ring N-heterocyclic carbenes (NHCs) in the iron-catalysed Kumada aryl-aryl coupling reaction was explored. This was achieved by comparing the catalytic performance of Fe-NHC catalysts generated in situ containing NHCs that differ in steric bulk. In particular, the influences of ring sizes (5-8) and N-aryl substituents were explored in terms of spectroscopic and structural features, which affect their %Vbur values. The three best performing ligands were found on a diagonal of a 5 × 4 structural matrix revealing an optimal steric bulk and significant influences of subtle steric variations on the catalytic activities.

NHC and nucleophile chelation effects on reactive iron(ii) species in alkyl-alkyl cross-coupling

While iron-NHC catalysed cross-couplings have been shown to be effective for a wide variety of reactions (e.g. aryl-aryl, aryl-alkyl, alkyl-alkyl), the nature of the in situ formed and reactive iron species in effective catalytic systems remains largely undefined. In the current study, freeze-trapped Mössbauer spectroscopy, and EPR studies combined with inorganic synthesis and reaction studies are utilised to define the key in situ formed and reactive iron-NHC species in the Kumada alkyl-alkyl cross-coupling of (2-(1,3-dioxan-2-yl)ethyl)magnesium bromide and 1-iodo-3-phenylpropane. The key reactive iron species formed in situ is identified as (IMes)Fe((1,3-dioxan-2-yl)ethyl)₂, whereas the S = 1/2 iron species previously identified in this chemistry is found to be only a very minor off-cycle species (<0.5% of all iron). Reaction and kinetic studies demonstrate that (IMes)Fe((1,3-dioxan-2-yl)ethyl)₂ is highly reactive towards the electrophile resulting in two turnovers with respect to iron (k_obs > 24 min^-1) to generate cross-coupled product with overall selectivity analogous to catalysis. The high resistance of this catalytic system to β-hydride elimination of the alkyl nucleophile is attributed to its chelation to iron through ligation of carbon and one oxygen of the acetal moiety of the nucleophile. In fact, alternative NHC ligands such as SIPr are less effective in catalysis due to their increased steric bulk inhibiting the ability of the alkyl ligands to chelate. Overall, this study identifies a novel alkyl chelation method to achieve effective alkyl-alkyl cross-coupling with iron(ii)-NHCs, provides direct structural insight into NHC effects on catalytic performance and extends the importance of iron(ii) reactive species in iron-catalysed cross-coupling.

Iron-Catalyzed C-O Bond Activation: Opportunity for Sustainable Catalysis

Oxygen-based electrophiles have emerged as some of the most valuable cross-coupling partners in organic synthesis due to several major strategic and environmental benefits, such as abundance and potential to avoid toxic halide waste. In this context, iron-catalyzed C-O activation/cross-coupling holds particular promise to achieve sustainable catalytic protocols due to its natural abundance, inherent low toxicity, and excellent economic and ecological profile. Recently, tremendous progress has been achieved in the development of new methods for functional-group-tolerant iron-catalyzed cross-coupling reactions by selective C-O cleavage. These methods establish highly attractive alternatives to traditional cross-coupling reactions by using halides as electrophilic partners. In particular, new easily accessible oxygen-based electrophiles have emerged as substrates in iron-catalyzed cross-coupling reactions, which significantly broaden the scope of this catalysis platform. New mechanistic manifolds involving iron catalysis have been established; thus opening up vistas for the development of a wide range of unprecedented reactions. The synthetic potential of this sustainable mode of reactivity has been highlighted by the development of new strategies in the construction of complex motifs, including in target synthesis. The most recent advances in sustainable iron-catalyzed cross-coupling of C-O-based electrophiles are reviewed, with a focus on both mechanistic aspects and synthetic utility. It should be noted that this catalytic manifold provides access to motifs that are often not easily available by other methods, such as the assembly of stereodefined dienes or C(sp² )-C(sp³ ) cross-couplings, thus emphasizing the synthetic importance of this mode of reactivity.

Iron-Catalyzed C-C Cross-Couplings Using Organometallics

Over the last decades, iron-catalyzed cross-couplings have emerged as an important tool for the formation of C-C bonds. A wide variety of alkenyl, aryl, and alkyl (pseudo)halides have been coupled to organometallic reagents, the most currently used being Grignard reagents. Particular attention has been devoted to the development of iron catalysts for the functionalization of alkyl halides that are generally challenging substrates in classical cross-couplings. The high functional group tolerance of iron-catalyzed cross-couplings has encouraged organic chemists to use them in the synthesis of bioactive compounds. Even if some points remain obscure, numerous studies have been carried out to investigate the mechanism of iron-catalyzed cross-coupling and several hypotheses have been proposed.

Iron(ii) triflate/N-heterocyclic carbene-catalysed cross-coupling of arylmagnesiums with aryl chlorides and tosylates

An iron(ii) triflate/N-heterocyclic carbene catalyst system enables the coupling of arylmagnesiums with aryl chlorides and tosylates with high efficiency.

Recent advances in iron-catalysed cross coupling reactions and their mechanistic underpinning

Advances in iron-catalysed cross coupling from 2010–2015 are critically reviewed.

How low does iron go? Chasing the active species in fe-catalyzed cross-coupling reactions

The catalytic cross-coupling reactions of organic halides or related substrates with organometallic nucleophiles form the cornerstone of many carbon-carbon bond-forming processes. While palladium-based catalysts typically mediate such reactions, there are increasing concerns about the long-term sustainability of palladium in synthesis. This is due to the high cost of palladium, coupled with its low natural abundance, environmentally deleterious extraction (∼6 g of metal are produced per ton of ore), toxicity, and competition for its use from the automotive and consumer electronics sectors. Therefore, there is a growing interest in replacing palladium-based catalysts with those incorporating more earth-abundant elements. With its low cost, high natural abundance, and low toxicity, iron makes a particularly appealing alternative, and accordingly, the development of iron-catalyzed cross-coupling is undergoing explosive growth. However, our understanding of the mechanisms that underpin the iron-based catalytic cycles is still very much in its infancy. Mechanistic insight into catalytic reactions is not only academically important but also allows us to maximize the efficiency of processes or even to develop entirely new transformations. Key to the development of robust mechanistic models for cross-coupling is knowing the lowest oxidation state in the cycle. Once this is established, we can explore subsequent redox processes and build the catalytic manifold. Until we know with confidence what the lowest oxidation state is, any cycles proposed are largely just guesswork. To date, Fe(-II), Fe(-I), Fe(0), Fe(I), and Fe(II) have been proposed as contenders for the lowest-oxidation-state species in the cycle in iron-catalyzed cross-coupling; the aim of this Account is to pull together the various pieces of evidence in support, or otherwise, of each of these suggestions in turn. There currently exists no direct evidence that oxidation states below Fe(0) are active in the catalytic cycle. Meanwhile, the reactivity required of the lowest-oxidation-state species has been observed with model compounds in higher oxidation states, implying that there is no need to invoke such low oxidation states. While subzero-valent complexes do indeed act as effective precatalysts, it is important to recognize that this tells us that they are efficiently converted to an active catalyst but says nothing about the oxidation states of the species in the catalytic cycle. Zero-valent heterogeneous iron nanoparticles can be formed under typical catalytic conditions, but there is no evidence to suggest that homogeneous Fe(0) complexes can be produced under comparable conditions. It seems likely that the zero-valent nanoparticles act as a reservoir for soluble higher-oxidation-state species. Fe(II) complexes can certainly be formed under catalytically relevant conditions, and when bulky nucleophilic coupling partners are exploited, potential intermediates can be isolated. However, the bulky reagents act as poor proxies for most nucleophiles used in cross-coupling, as they give Fe(II) organometallic intermediates that are kinetically stabilized with respect to reductive elimination. When more realistic substrates are exploited, reduction or disproportionation to Fe(I) is widely observed, and while it still has not been conclusively proved, this oxidation state currently represents a likely candidate for the lowest one active in many iron-catalyzed cross-coupling processes.

Practical Iron- and Cobalt-Catalyzed Cross-Coupling Reactions between N-Heterocyclic Halides and Aryl or Heteroaryl Magnesium Reagents

The reaction scope of iron- and cobalt-catalyzed cross-coupling reactions in the presence of isoquinoline (quinoline) in the solvent mixture tBuOMe/THF has been further investigated. Various 2-halogenated pyridine, pyrimidine, and triazine derivatives were arylated under these mild conditions in excellent yields. The presence of isoquinoline allows us to perform Fe-catalyzed cross-coupling reactions between 6-chloroquinoline and aryl magnesium reagents. Furthermore, it was found that the use of 10% N,N-dimethylquinoline-8-amine increases the yields of some Co-catalyzed cross-coupling reactions with chloropyridines bearing electron-withdrawing substituents.

New insights into the mechanism of iron-catalyzed cross-coupling reactions

The computational studies on the mechanism of the iron-catalyzed cross-coupling revealed that the AT pathway requires less energy than the regular OA pathway.